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  • 51. Ratynskaia, Svetlana V.
    et al.
    Demidov, V. I.
    Rypdal, K.
    Probe measurements of ion energy distribution in magnetized plasmas2002In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 73, no 12, p. 4232-4236Article in journal (Refereed)
    Abstract [en]

    A probe method for measurement of the ion energy distribution in magnetized plasmas is proposed. A cylindrical probe with end plugs is oriented parallel to the magnetic field and used to reduce the electron contribution to the total probe current. The contribution of ions to the second derivative of the total current is identified by experiments with different probe orientation and strength of the magnetic field. The ion density obtained from the measured ion energy distribution is in good agreement with the electron density obtained from measured electron energy distribution.

  • 52. Ratynskaia, Svetlana V.
    et al.
    Demidov, V. I.
    Rypdal, K.
    Probe measurements of low-frequency plasma potential and electric field fluctuations in a magnetized plasma2002In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 9, no 10, p. 4135-4143Article in journal (Refereed)
    Abstract [en]

    A system of two cylindrical probes aligned along the magnetic field, and equipped with insulating end plugs, is proposed for measurement of low-frequency fluctuations of the electrostatic field in a magnetized plasma. It is demonstrated by modeling and experiments that the plug probe floats close to the plasma potential. The electric field component in a given direction is obtained by subtracting the plasma potentials obtained on two spatially separated plug probes. The probe system is applied to low-frequency electrostatic fluctuations in a simple magnetized torus, and reveals the presence of global oscillations, large scale propagating structures (m=1 modes), and developed turbulence with power-law spectra. Two different mode branches for the fluctuations are identified by comparing results from plug probes with results from conventional probes. Sources of errors arising from applying floating potential of conventional probes for electric field measurements are pointed out and discussed.

  • 53. Ratynskaia, Svetlana V.
    et al.
    Khrapak, S.
    Zobnin, A.
    Thoma, M. H.
    Kretschmer, M.
    Usachev, A.
    Yaroshenko, V.
    Quinn, R. A.
    Morfill, G. E.
    Petrov, O.
    Fortov, V.
    Experimental determination of dust-particle charge in a discharge plasma at elevated pressures2004In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 93, no 8Article in journal (Refereed)
    Abstract [en]

    The charge of dust particles is determined experimentally in a bulk dc discharge plasma in the pressure range 20-100 Pa. The charge is obtained by two independent methods: one based on an analysis of the particle motion in a stable particle flow and another on an analysis of the transition of the flow to an unstable regime. Molecular-dynamics simulations of the particle charging for conditions similar to those of the experiment are also performed. The results of both experimental methods and the simulations demonstrate good agreement. The charge obtained is several times smaller than predicted by the collisionless orbital motion theory, and thus the results serve as an experimental indication that ion-neutral collisions significantly affect particle charging.

  • 54. Ratynskaia, Svetlana V.
    et al.
    Knapek, C.
    Rypdal, K.
    Khrapak, S.
    Morfill, G.
    Statistics of particle transport in a two-dimensional dusty plasma cluster2005In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 12, no 2Article in journal (Refereed)
    Abstract [en]

    Statistical analysis is performed on long time series of dust particle trajectories in a two-dimensional dusty plasma cluster. Particle transport is found to be superdiffusive on all time scales until the range of particle displacements approaches the size of the cluster. Analysis of probability distribution functions and rescaled range analysis of the position increments show that the signal is non-Gaussian self-similar with Hurst exponent H=0.6, indicating that the superdiffusion is caused by long-range dependencies in the system. Investigation of temporal and spatial characteristics of persistent particle slips demonstrates that they are associated with collective events present on all time scales and responsible for the non-Gaussianity and long-memory effects.

  • 55. Ratynskaia, Svetlana V.
    et al.
    Kompaneets, R.
    Ivlev, A. V.
    Knapek, C.
    Morfill, G. E.
    Transport in strongly coupled two-dimensional complex plasmas: Role of the interaction potential2007In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 14, no 1Article in journal (Refereed)
    Abstract [en]

    Observations of particle motion in a complex plasma monolayer in rf discharge have revealed anomalous dependence on neutral gas pressure; an immobile system at high gas pressures, then, decreasing the pressure, a state with viscoelastic vortical fluid motions and, hence, dramatically enhanced transport, followed again by an ordered state. While the inhibited transport at higher pressures can be attributed to dissipation on neutrals, the transition of the system back to an ordered state at lower pressures has been explained by an increase of the dust-dust viscous dissipation due to significant modifications of the interaction potential between the particles.

  • 56. Ratynskaia, Svetlana V.
    et al.
    Kretschmer, M.
    Khrapak, S.
    Quinn, R. A.
    Thoma, M. H.
    Morfill, G. E.
    Zobnin, A.
    Usachev, A.
    Petrov, O.
    Fortov, V.
    Dust mode in collisionally dominated complex plasmas with particle drift2004In: IEEE Transactions on Plasma Science, ISSN 0093-3813, E-ISSN 1939-9375, Vol. 32, no 2, p. 613-616Article in journal (Refereed)
    Abstract [en]

    Experiments with flow of microparticles were conducted in a de discharge. A sharp threshold in the neutral gas pressure for the onset of an unstable low-frequency dust wave mode was observed. Highly space- and time-resolved measurements of the microparticle flow combined with probe measurements of the plasma parameters have allowed detailed comparison with a theoretical model. The model demonstrates good qualitative and quantitative agreement with the experimental results providing accurate estimates of the particle charge.

  • 57.
    Ratynskaia, Svetlana V.
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Regnoli, Giorgio
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Rypdal, K.
    Klumov, B.
    Morfill, G.
    Critical fluctuations and anomalous transport in soft Yukawa-Langevin systems2009Article in journal (Refereed)
    Abstract [en]

    Simulation of a Langevin-dynamics model demonstrates emergence of critical fluctuations and anomalous grain transport which have been observed in experiments on "soft" quasi-two-dimensional dusty plasma clusters. Our model does not contain external drive or plasma interactions that serve to drive the system away from thermodynamic equilibrium. The grains are confined by an external potential, interact via static Yukawa forces, and are subject to stochastic heating and dissipation from neutrals. One remarkable feature is emergence of leptokurtic probability distributions of grain displacements xi(tau) on time scales tau <>(1/2) approaches the mean intergrain distance Delta. Others are development of humps in the distributions on multiples of Delta, anomalous Hurst exponents, and transitions from leptokurtic toward Gaussian displacement distributions on time scales tau>tau(Delta). The latter is a signature of intermittency, here interpreted as a transition from bursty transport associated with hopping on intermediate time scales to vortical flows on longer time scales. These intermittency features are quantitatively modeled by a single-particle Itocircumflex-Langevin stochastic equation with a nonlinear drift term.

  • 58. Ratynskaia, Svetlana V.
    et al.
    Rypdal, K.
    Demidov, V. I.
    Measurements of plasma potential and electron temperature fluctuation in a low-temperature magnetized plasma2001In: Contributions to Plasma Physics, ISSN 0863-1042, E-ISSN 1521-3986, Vol. 41, no 5, p. 449-454Article in journal (Refereed)
    Abstract [en]

    Measurements of fluctuations of plasma potential and electron temperature in a toroidal magnetized plasma is carried out by applying a cylindrical probe with insulating end plugs oriented parallel to the B-field in conjunction with another cylindrical probe oriented perpendicularly. Coherency and cross-phase between (V) over tilde (f) and (T) over tilde (e) are estimated, and typically have values close to 0.6 and pi respectively. Power-law spectra are found for frequencies well above the poloidal rotation frequency with spectral index typically around 4.0 for (V) over tilde (f) and around 2.5 for (T) over tilde (e). The density gradient is above the threshold for flute interchange instability, and the results are consistent with theory and global numerical simulations of this plasma.

  • 59. Ratynskaia, Svetlana V.
    et al.
    Rypdal, K.
    Knapek, C.
    Khrapak, S.
    Milovanov, A. V.
    Ivlev, A.
    Rasmussen, J. J.
    Morfill, G. E.
    Superdiffusion and viscoelastic vortex flows in a two-dimensional complex plasma2006In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 96, no 10Article in journal (Refereed)
    Abstract [en]

    Viscoelastic vortical fluid motion in a strongly coupled particle system has been observed experimentally. Optical tracking of particle motion in a complex plasma monolayer reveals high grain mobility and large scale vortex flows coexistent with partial preservation of the global hexagonal lattice structure. The transport of particles is superdiffusive and ascribed to Levy statistics on short time scales and to memory effects on the longer scales influenced by cooperative motion. At these longer time scales, the transport is governed by vortex flows covering a wide spectrum of temporal and spatial scales.

  • 60.
    Ratynskaia, Svetlana V.
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    De Angeli, M.
    CNR, Ist Fis Plasma, Via Cozzi 53, I-20125 Milan, Italy..
    Ripamonti, D.
    CNR, Inst Condensed Matter Chem & Energy Technol, Via Cozzi 53, I-20125 Milan, Italy..
    Riva, G.
    CNR, Inst Condensed Matter Chem & Energy Technol, Via Cozzi 53, I-20125 Milan, Italy..
    Aussems, D.
    DIFFER, De Zaale 20, NL-5612 AJ Eindhoven, Netherlands..
    Morgan, T. W.
    DIFFER, De Zaale 20, NL-5612 AJ Eindhoven, Netherlands..
    Interaction of adhered beryllium proxy dust with transient and stationary plasmas2018In: Nuclear Materials and Energy, E-ISSN 2352-1791, Vol. 17, p. 222-227Article in journal (Refereed)
    Abstract [en]

    Tungsten (W) substrates with adhered beryllium (Be) proxy dust-copper, chromium, aluminium -have been exposed in the Magnum-PSI linear device. Their interaction with transient and stationary plasmas has been systematically studied under varying heat fluxes and magnetic field topologies. The dust remobilization activities, macro-morphological changes and chemical modifications induced by the plasma incidence are documented. Aluminium is identified to be the most suitable surrogate material due to the similar binary phase diagram and nearly identical evaporation rates. Extrapolation suggests that Be dust cannot survive on hot W surfaces but it can trigger mixed Be/W effects prior to its plasma removal.

  • 61.
    Ratynskaia, Svetlana V.
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    De Angeli, M.
    Weinzettl, V.
    Matejicek, J.
    Bykov, I.
    Rudakov, D. L.
    Vignitchouk, Ladislas
    KTH.
    Thorén, Emil
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Riva, G.
    Ripamonti, D.
    Morgan, T.
    Panek, R.
    De Temmerman, G.
    Tungsten dust remobilization under steady-state and transient plasma conditions2017In: NUCLEAR MATERIALS AND ENERGY, ISSN 2352-1791, Vol. 12, p. 569-574Article in journal (Refereed)
    Abstract [en]

    Remobilization is one of the most prominent unresolved fusion dust-relevant issues, strongly related to the lifetime of dust in plasma-wetted regions, the survivability of dust on hot plasma-facing surfaces and the formation of dust accumulation sites. A systematic cross-machine study has been initiated to investigate the remobilization of tungsten micron-size dust from tungsten surfaces implementing a newly developed technique based on controlled pre-adhesion by gas dynamics methods. It has been utilized in a number of devices and has provided new insights on remobilization under steady-state and transient conditions. The experiments are interpreted with contact mechanics theory and heat conduction models.

  • 62.
    Ratynskaia, Svetlana
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Vignitchouk, Ladislas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Bykov, Igor
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Bergsåker, Henric
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Litnovsky, A.
    den Harder, N.
    Lazzaro, E.
    Migration of tungsten dust in tokamaks: role of dust-wall collisions2013In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 53, no 12, p. 123002-Article in journal (Refereed)
    Abstract [en]

    The modelling of a controlled tungsten dust injection experiment in TEXTOR by the dust dynamics code MIGRAINe is reported. The code, in addition to the standard dust-plasma interaction processes, also encompasses major mechanical aspects of dust-surface collisions. The use of analytical expressions for the restitution coefficients as functions of the dust radius and impact velocity allows us to account for the sticking and rebound phenomena that define which parts of the dust size distribution can migrate efficiently. The experiment provided unambiguous evidence of long-distance dust migration; artificially introduced tungsten dust particles were collected 120 degrees toroidally away from the injection point, but also a selectivity in the permissible size of transported grains was observed. The main experimental results are reproduced by modelling.

  • 63.
    Ratynskaiy, Svetlana
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    De Angeli, M.
    Lazzaro, E.
    Marmolino, C.
    de Angelis, U.
    Castaldo, C.
    Cremona, A.
    Laguardia, L.
    Gervasini, G.
    Grosso, G.
    Plasma fluctuation spectra as a diagnostic tool for submicron dust2010In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 17, no 4, p. 043703-Article in journal (Refereed)
    Abstract [en]

    It is shown that the measurements of density fluctuation spectra in dusty plasmas can constitute a basis for in situ diagnostic of invisible submicron dust. The self-consistent kinetic theory that includes the charging processes and the natural density fluctuations of the dust particles predicts modifications of the spectra due to the presence of dust. A laboratory experiment was carried out where submicron dust was produced in a gas phase and diagnosed by surface analysis of samples and by measurements of its influence on the plasma density fluctuation spectra. Quantitative comparison of the latter with the theory yields information on dust density, size, and distribution in agreement with the results of the surface analysis. The method can be applied to various plasma environments in laboratory and space.

  • 64.
    Ratynskaya, Svetlana
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    De Angeli, M.
    Riva, G.
    Ripamonti, D.
    Daminelli, G.
    Bykov, Igor
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Masselin, Matthieu
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Reggiori, A.
    Grosso, G.
    Dust-surface collisions in adhesion regime for tokamak relevant materials2013In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 438, no Suppl., p. S796-S799Article in journal (Refereed)
    Abstract [en]

    Empirical studies of dust-surface collisions for metal and carbon projectiles impacting on metal targets in the velocity range from a few m/s to those in excess of 1 km/s have been carried out with the use of a modified pellet injection system. The selected projectile/target shapes, sizes and materials are mimicking the scenario of dust colliding with plasma facing components (PFCs) of a metal machine. The low velocities (10's m/s) reported here, characteristic for dust motion in tokamak scrape-off layer plasmas, are in the range of sticking phenomenon; the critical velocity and size for bouncing off are essential and timely inputs for the dust dynamics codes and statistical models aiming to predict dust transport and redepo-sition on PFCs.

  • 65. Riva, G.
    et al.
    Tolias, Panagiotis
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Daminelli, G.
    Donde, R.
    De Angeli, M.
    Vassallo, E.
    Pedroni, M.
    Adhesion measurements for tungsten dust deposited on tungsten surfaces2017In: NUCLEAR MATERIALS AND ENERGY, ISSN 2352-1791, Vol. 12, p. 593-598Article in journal (Refereed)
    Abstract [en]

    The first experimental determination of the pull-off force for tungsten dust adhered to tungsten surfaces is reported. Dust deposition is conducted with gas dynamics methods in a manner that mimics sticking as it occurs in the tokamak environment. Adhesion measurements are carried out with the electrostatic detachment method. The adhesion strength is systematically characterized for spherical micron dust of different sizes and planar surfaces of varying roughness. The experimental pull-off force is nearly two orders of magnitude smaller than the predictions of contact mechanics models, but in strong agreement with the Van der Waals formula. A theoretical interpretation is provided that invokes the effects of nanometer-scale surface roughness for stiff materials such as tungsten.

  • 66. Rudakov, D. L.
    et al.
    Yu, J. H.
    Boedo, J. A.
    Hollmann, E. M.
    Krasheninnikov, S. I.
    Moyer, R. A.
    Muller, S. H.
    Pigarov, A. Yu.
    Rosenberg, M.
    Smirnov, R. D.
    West, W. P.
    Boivin, R. L.
    Bray, B. D.
    Brooks, N. H.
    Hyatt, A. W.
    Wong, C. P. C.
    Roquemore, A. L.
    Skinner, C. H.
    Solomon, W. M.
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Fenstermacher, M. E.
    Groth, M.
    Lasnier, C. J.
    McLean, A. G.
    Stangeby, P. C.
    Dust measurements in tokamaks (invited)2008In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 79, no 10, p. 10F303-Article in journal (Refereed)
    Abstract [en]

    Dust production and accumulation present potential safety and operational issues for the ITER. Dust diagnostics can be divided into two groups: diagnostics of dust on surfaces and diagnostics of dust in plasma. Diagnostics from both groups are employed in contemporary tokamaks; new diagnostics suitable for ITER are also being developed and tested. Dust accumulation in ITER is likely to occur in hidden areas, e.g., between tiles and under divertor baffles. A novel electrostatic dust detector for monitoring dust in these regions has been developed and tested at PPPL. In the DIII-D tokamak dust diagnostics include Mie scattering from Nd:YAG lasers, visible imaging, and spectroscopy. Laser scattering is able to resolve particles between 0.16 and 1.6 mu m in diameter; using these data the total dust content in the edge plasmas and trends in the dust production rates within this size range have been established. Individual dust particles are observed by visible imaging using fast framing cameras, detecting dust particles of a few microns in diameter and larger. Dust velocities and trajectories can be determined in two-dimension with a single camera or three-dimension using multiple cameras, but determination of particle size is challenging. In order to calibrate diagnostics and benchmark dust dynamics modeling, precharacterized carbon dust has been injected into the lower divertor of DIII-D. Injected dust is seen by cameras, and spectroscopic diagnostics observe an increase in carbon line (CI, CII, C(2) dimer) and thermal continuum emissions from the injected dust. The latter observation can be used in the design of novel dust survey diagnostics.

  • 67. Rypdal, K.
    et al.
    Kozelov, B.
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Klumov, B.
    Knapek, C.
    Rypdal, M.
    Scale-free vortex cascade emerging from random forcing in a strongly coupled system2008In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 10Article in journal (Refereed)
    Abstract [en]

    We elucidate the unifying aspects of self-organized criticality (SOC) and turbulence through analysis of data from a laboratory dusty plasma monolayer. We compare analysis of experimental data with simulations of a two-dimensional (2D) many-body system, of 2D chaotic fluid flow, and two different SOC-models, the Zhang and the Bak-Tang-Wiesenfeld (BTW) models, all subject to steady random forcing at small scales. The scale-free vortex cascade is apparent from structure functions as well as spatio-temporal avalanche analysis. We find similar scaling exponents for the experiment, the many-body simulation, and the fluid simulation, indicating some common dynamical features. However, the exponents of the Zhang model are different from those of the BTW model, and they are all different from those of the dust and fluid systems. Thus, we conclude that the dust monolayer dynamics can be viewed as turbulent as well as avalanching, but a fluid model is a better representation of the dust dynamics for this particular experiment than the sandpile models considered. The experiment exhibits global fluctuation statistics consistent with a recent hypothesis predicting universal non-Gaussian probability density functions, but the model systems yield this result only in a restricted range of forcing conditions.

  • 68. Rypdal, K.
    et al.
    Paulsen, J. V.
    Garcia, O. E.
    Ratynskaia, Svetlana V.
    Demidov, V. I.
    Non-equilibrium quasi-stationary states in a magnetized plasma2003In: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 10, no 02-jan, p. 139-149Article in journal (Refereed)
    Abstract [en]

    Non-equilibrium quasi-stationary states resulting from curvature driven interchange instabilities and driftwave instabilities in a low beta, weakly ionized, magnetized plasma are investigated in the context of laboratory experiments in a toroidal configuration. Analytic modelling, numerical simulations and experimental results are discussed with emphasis on identifying the unstable modes and understanding the physics of anomalous particle and energy fluxes and their linkage to self-organized pressure profiles.

  • 69. Rypdal, K.
    et al.
    Ratynskaia, Svetlana V.
    Fluctuation threshold and profile resilience in weakly ionized plasma in a curved, unsheared magnetic field2004In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 11, no 10, p. 4623-4633Article in journal (Refereed)
    Abstract [en]

    A study of electrostatic fluctuations in a weakly ionized plasma confined by a purely toroidal magnetic field B is presented. At low B field the plasma is quiescent and transport is provided by stationary ExB flows along open equipotential surfaces. For B above a certain threshold, strong electrostatic fluctuations are excited, and the electron pressure profile on the low-field side is resilient with scale length much shorter than the threshold scale length for flute instability. At threshold the fluctuations exhibit the signatures of a monochromatic drift mode, and above threshold they are dominated by flute modes growing on the low-field side. Threshold behavior and profile resilience are explained as a two-stage process, where drift waves act as a seed for the unstable flute modes, and where a negative feedback mechanism involving anomalous transport regulates the electron pressure profile to maintain the imposed particle flux.

  • 70. Rypdal, K.
    et al.
    Ratynskaia, Svetlana V.
    Onset of turbulence and profile resilience in the Helimak configuration2005In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 94, no 22Article in journal (Refereed)
    Abstract [en]

    An experimental study of the onset of drift wave and flute interchange instabilities in the Helimak configuration is presented. It is shown that the Helimak offers the opportunity to separate the regions where these instabilities are active and to assess their relative role in cross-field anomalous transport and in the self-organization of exponential plasma density profiles with resilient scale length. Some results indicating a period doubling route to turbulence are also presented.

  • 71. Rypdal, K.
    et al.
    Ratynskaia, Svetlana V.
    Plasma profiles, waves and anomalous transport in a purely toroidal plasma modified by a biased internal anode2006In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T122, p. 52-65Article in journal (Refereed)
    Abstract [en]

    Two-dimensional (2D) plasma profiles, gradient-driven low frequency electrostatic waves and anomalous particle transport are studied experimentally in a purely toroidal plasma configuration. The plasma is produced by a negatively biased, emissive cathode in conjunction with a variably biased anode plate localized on the same field line. Nearly circularly symmetric potential profiles, positive or negative, can be obtained by varying the anode plate bias. Confinement loss in the form of plasma ejection in the major radius direction is only observed when the anode is biased close to ground potential. For the cases of circularly symmetric plasma flow electrostatic flute modes with poloidal mode number m = 1 and m = 2 are identified as unstable on the low- field side of the density maximum, but indications are also given that they coexist with spatially quasi-uniform oscillations and, for low frequencies, with an ion acoustic mode with toroidal mode number n = 1 propagating parallel to the magnetic field. The anomalous particle flux density is found to be non- uniformly distributed on a toroidal surface, and the main flux is passed through two lobes directed almost vertically upwards and downwards, while almost no flux takes place along the major radius direction.

  • 72. Rypdal, K.
    et al.
    Ratynskaia, Svetlana V.
    Statistics of low-frequency plasma fluctuations in a simple magnetized torus2003In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 10, no 7, p. 2686-2695Article in journal (Refereed)
    Abstract [en]

    Statistical analysis is performed on time series of plasma potential and electron pressure fluctuations for two particular plasma states in a simple magnetized torus. In spite of the occurrence of cyclic trends in the signals, the analysis demonstrates the existence of long-range dependence on much longer time-scales than the plasma confinement time, and both Gaussian and non-Gaussian self-affinity is demonstrated in the probability distribution functions (PDFs) of increasingly coarse grained time series. Non-Gaussian PDFs for electron pressure fluctuations p are believed to arise from a nonlinear relationship between p and the plasma potential V. Non-Gaussian PDFs for V are well approximated by distributions derived from extreme value statistics.

  • 73. Shalpegin, A.
    et al.
    Brochard, F.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    De Angeli, M.
    Vignitchouk, Ladislas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Bykov, Igor
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Bardin, S.
    Bystrov, K.
    Morgan, T.
    De Temmerman, G.
    Highly resolved measurements of dust motion in the sheath boundary of magnetized plasmas2015In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 55, no 11, article id 112001Article in journal (Refereed)
    Abstract [en]

    Dust trajectories have been recorded with an unprecedented, under fusion-relevant plasma conditions, spatial resolution of 9 mu m/pixel in Pilot-PSI. The optical setup allowed the use of fast cameras as a basic microscope. It is demonstrated that such a resolution is essential for the correct interpretation of experiments on several aspects of dust-surface interactions. Highly resolved tungsten dust dynamics measurements are presented from dedicated experiments on dust collisions with plasma facing components, motion in the vicinity of castellated samples and remobilization from planar samples.

  • 74.
    Ström, Petter
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Rubel, Marek
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Bergsåker, Henric
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Bykov, Igor
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Garcia Carrasco, Alvaro
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Menmuir, Sheena
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Tholerus, Simon
    Weckmann, Armin
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics. KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Rachlew, Elisabeth
    KTH, Superseded Departments (pre-2005), Physics. KTH, School of Engineering Sciences (SCI), Physics, Atomic and Molecular Physics.
    Vallejos, Pablo
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Johnson, T.
    Stefanikova, E.
    Zhou, Y.
    Zychor, I.
    et al.,
    Analysis of deposited layers with deuterium and impurity elements on samples from the divertor of JET with ITER-like wall2019In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 516, p. 202-213Article in journal (Refereed)
    Abstract [en]

    Inconel-600 blocks and stainless steel covers for quartz microbalance crystals from remote corners in the JET-ILW divertor were studied with time-of-flight elastic recoil detection analysis and nuclear reaction analysis to obtain information about the areal densities and depth profiles of elements present in deposited material layers. Surface morphology and the composition of dust particles were examined with scanning electron microscopy and energy-dispersive X-ray spectroscopy. The analyzed components were present in JET during three ITER-like wall campaigns between 2010 and 2017. Deposited layers had a stratified structure, primarily made up of beryllium, carbon and oxygen with varying atomic fractions of deuterium, up to more than 20%. The range of carbon transport from the ribs of the divertor carrier was limited to a few centimeters, and carbon/deuterium co-deposition was indicated on the Inconel blocks. High atomic fractions of deuterium were also found in almost carbon-free layers on the quartz microbalance covers. Layer thicknesses up to more than 1 micrometer were indicated, but typical values were on the order of a few hundred nanometers. Chromium, iron and nickel fractions were less than or around 1% at layer surfaces while increasing close to the layer-substrate interface. The tungsten fraction depended on the proximity of the plasma strike point to the divertor corners. Particles of tungsten, molybdenum and copper with sizes less than or around 1 micrometer were found. Nitrogen, argon and neon were present after plasma edge cooling and disruption mitigation. Oxygen-18 was found on component surfaces after injection, indicating in-vessel oxidation. Compensation of elastic recoil detection data for detection efficiency and ion-induced release of deuterium during the measurement gave quantitative agreement with nuclear reaction analysis, which strengthens the validity of the results.

  • 75. Thoma, M. H.
    et al.
    Hoefner, H.
    Kretschmer, M.
    Ratynskaia, Svetlana V.
    Morfill, G. E.
    Usachev, A.
    Zobnin, A.
    Petrov, O.
    Fortov, V.
    Parabolic flight experiments with PK-42006In: Microgravity, science and technology, ISSN 0938-0108, E-ISSN 1875-0494, Vol. 18, no 04-mar, p. 47-50Article in journal (Refereed)
    Abstract [en]

    PK-4 is an experiment designed to investigate complex plasmas (low-temperature plasmas containing microparticles, e.g. dust grains) in a combined dc/rf discharge under microgravity conditions on board of the International Space Station. Within the 35th and 36th ESA parabolic flight campaigns first experiments under microgravity conditions in a specially designed experiment set-up have been performed. The particle flow inside the tube, the appearance of dust waves, and lane formation in interpenetrating particle clouds have been observed.

  • 76. Thoma, Markus H.
    et al.
    Fink, Martin A.
    Hoefner, Herwig
    Kretschmer, Michael
    Khrapak, Sergey A.
    Ratynskaia, Svetlana V.
    Yaroshenko, Victoria V.
    Morfill, Gregor E.
    Petrov, Oleg F.
    Usachev, Alexander D.
    Zobnin, Andrey V.
    Fortov, Vladimir E.
    PK-4: Complex plasmas in space - The next generation2007In: IEEE Transactions on Plasma Science, ISSN 0093-3813, E-ISSN 1939-9375, Vol. 35, no 2, p. 255-259Article in journal (Refereed)
    Abstract [en]

    PK-4 is an experiment designed to investigate complex plasmas in a combined dc/RF discharge under microgravity conditions on board of the International Space Station. The dc,discharge is produced in a glass tube with a length of 35 cm and, a diameter of 3 cm. In addition, an RF discharge can be applied by external RF coils. The setup is especially suited for studying the liquid phase of the complex plasmas, e.g., flow phenomena such as turbulence or nozzles, and forces acting on the microparticles. Experiments in the laboratory and in parabolic flights have been used to determine the charge of the microparticles as well as the ion drag force acting on them.

  • 77.
    Thorén, Emil
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Bazylev, B.
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Krieger, K.
    Pitts, R. A.
    Pestchanyi, S.
    Komm, M.
    Sieglin, B.
    Simulations with current constraints of ELM-induced tungsten melt motion in ASDEX Upgrade2017In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T170, article id 014006Article in journal (Refereed)
    Abstract [en]

    Melt motion simulations of recent ASDEX Upgrade experiments on transient-induced melting of a tungsten leading edge during ELMing H-mode are performed with the incompressible fluid dynamics code MEMOS 3D. The total current flowing through the sample was measured in these experiments providing an important constraint for the simulations since thermionic emission is considered to be responsible for the replacement current driving melt motion. To allow for a reliable comparison, the description of the space-charge limited regime of thermionic emission has been updated in the code. The effect of non-periodic aspects of the spatio-temporal heat flux in the temperature distribution and melt characteristics as well as the importance of current limitation are investigated. The results are compared with measurements of the total current and melt profile.

  • 78.
    Thorén, Emil
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Pitts, R. A.
    ITER Org, Route Vinon Sur Verdon,CS 90 046, F-13067 St Paul Les Durance, France..
    Krieger, K.
    Max Planck Inst Plasma Phys, D-85748 Garching, Germany..
    Komm, M.
    Inst Plasma Phys CAS, Za Slovankou 3, Prague 18200 8, Czech Republic..
    Baken, M.
    Max Planck Inst Plasma Phys, D-85748 Garching, Germany..
    MEMOS 3D modelling of ELM-induced transient melt damage on an inclined tungsten surface in the ASDEX Upgrade outer divertor2018In: Nuclear Materials and Energy, E-ISSN 2352-1791, Vol. 17, p. 194-199Article in journal (Refereed)
    Abstract [en]

    The first MEMOS 3D simulations of liquid metal motion on an inclined bulk tungsten sample transiently molten by edge-localized modes (ELMs) are reported. The exposures took place at the outer ASDEX-Upgrade divertor with the tungsten surface tangent intersecting the magnetic field at similar to 18 degrees. Simulations confirm that the observed poloidal melt motion is caused by the volumetric J x B force with J the bulk replacement current triggered by thermionic emission. The final erosion profile and total melt build up are reproduced by employing the escaping thermionic current dependence on the incident heat flux derived from dedicated particle-in-cell simulations. Modelling reveals that melt dynamics is governed by the volumetric Lorentz force, capillary flows due to thermal surface tension gradients and viscous deceleration. The effect of the evolving surface deformation, that locally alters the field-line inclination modifying the absorbed power flux and the escaping thermionic current, in the final surface morphology is demonstrated to be significant.

  • 79.
    Thorén, Emil
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Pitts, R. A.
    ITER Org, Route Vinon Sur Verdon,CS 90 046, F-13067 St Paul Les Durance, France..
    Krieger, K.
    Max Planck Inst Plasma Phys, D-85748 Garching, Germany..
    Self-consistent description of the replacement current driving melt layer motion in fusion devices2018In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 58, no 10, article id 106003Article in journal (Refereed)
    Abstract [en]

    The bulk replacement current density triggered by surface charge loss owing to thermionic emission leads to a volumetric Lorentz force which has been observed to drive macroscopic melt layer motion in transient tungsten melting tokamak experiments in which components of different geometries (deliberate leading edges and sloped surfaces) have been exposed to edge localized mode (ELM) pulsed heat loads in high power H-mode discharges. A self-consistent approach is formulated for the replacement current which is based on the magnetostatic limit of the resistive thermoelectric magnetohydrodynamic description of the liquid metal and results in a well-defined boundary value problem for the whole conductor. A new module is incorporated into the incompressible fluid dynamics code MEMOS-3D, which numerically solves the finite difference representation of the problem. The phenomenological approach, employed thus far to describe the replacement current, is demonstrated to be accurate for the sloped geometry but inadequate for the leading edge. MEMOS-3D simulations of very recent ASDEX-Upgrade leading edge experiments with the rigorous as well as the simplified approach are reported. For these simulations, the self-consistent approach predicts a fivefold reduction of the displaced material volume, a sevenfold reduction of the maximum peak height of displaced material and a different eroded surface morphology in comparison with the previously applied simplified approach.

  • 80.
    Tierens, W.
    et al.
    Max Planck Inst Plasma Phys, Garching, Germany..
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Fridström, Richard
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Garcia Carrasco, Alvaro
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Jonsson, Thomas
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Vallejos Olivares, Pablo
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Vignitchouk, Ladislas
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Zuin, M.
    Consorzio RFX, Padua, Italy..
    et al.,
    Validation of the ICRF antenna coupling code RAPLICASOL against TOPICA and experiments2019In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 59, no 4, article id 046001Article in journal (Refereed)
    Abstract [en]

    In this paper we validate the finite element code RAPLICASOL, which models radiofrequency wave propagation in edge plasmas near ICRF antennas, against calculations with the TOPICA code. We compare the output of both codes for the ASDEX Upgrade 2-strap antenna, and for a 4-strap WEST-like antenna. Although RAPLICASOL requires considerably fewer computational resources than TOPICA, we find that the predicted quantities of experimental interest (including reflection coefficients, coupling resistances, S- and Z-matrix entries, optimal matching settings, and even radiofrequency electric fields) are in good agreement provided we are careful to use the same geometry in both codes.

  • 81.
    Tolias, Panagiotis
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    De Angeli, M.
    CNR, Ist Fis Plasma, Via Corsi 53, I-20125 Milan, Italy..
    Riva, G.
    CNR, Inst Condensed Matter Chem & Energy Technol, Via Cozzi 53, I-20125 Milan, Italy..
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Daminelli, G.
    CNR, Inst Condensed Matter Chem & Energy Technol, Via Cozzi 53, I-20125 Milan, Italy..
    Laguardia, L.
    CNR, Ist Fis Plasma, Via Corsi 53, I-20125 Milan, Italy..
    Pedron, M.
    CNR, Ist Fis Plasma, Via Corsi 53, I-20125 Milan, Italy..
    Ripamonti, D.
    CNR, Inst Condensed Matter Chem & Energy Technol, Via Cozzi 53, I-20125 Milan, Italy..
    Uccello, A.
    CNR, Ist Fis Plasma, Via Corsi 53, I-20125 Milan, Italy..
    Vassallo, E.
    CNR, Ist Fis Plasma, Via Corsi 53, I-20125 Milan, Italy..
    The adhesion of tungsten dust on plasma-exposed tungsten surfaces2019In: Nuclear Materials and Energy, E-ISSN 2352-1791, Vol. 18, p. 18-22Article in journal (Refereed)
    Abstract [en]

    The adhesion of tungsten dust is measured on plasma-exposed and non-exposed tungsten substrates with the electrostatic detachment method. Tungsten substrates of comparable surface roughness have been exposed to the deuterium plasmas of the GyM linear device and the argon plasmas of rf glow discharges under conditions which invariably modify the surface composition due to physical sputtering. The adhesion has been systematically characterized for different spherical nearly monodisperse dust populations. Independent of the dust size, an approximate 50% post-exposure reduction of the average and spread of the adhesive force has been consistently observed and attributed to surface chemistry modifications.

  • 82.
    Tolias, Panagiotis
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Scattering of radiation in collisionless dusty plasmas2013In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, no 4, p. 043706-Article in journal (Refereed)
    Abstract [en]

    Scattering of electromagnetic waves in collisionless dusty plasmas is studied in the framework of a multi-component kinetic model. The investigation focuses on the spectral distribution of the scattered radiation. Pronounced dust signatures are identified in the coherent spectrum due to scattering from the shielding cloud around the dust grains, dust acoustic waves, and dust-ion acoustic waves. The magnitude and shape of the scattered signal near these spectral regions are determined with the aid of analytical expressions and its dependence on the dust parameters is investigated. The use of radiation scattering as a potential diagnostic tool for dust detection is discussed.

  • 83.
    Tolias, Panagiotis
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Screening in weakly ionized dusty plasmas; effect of dust density perturbations2013In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, no 2, p. 023702-Article in journal (Refereed)
    Abstract [en]

    The screening of the charge of a non-emitting dust grain immersed in a weakly ionized dusty plasma is studied on the basis of a self-consistent hydrodynamic description. The dust number density is considered large enough so that the test grain is not isolated from other grains and dust collective effects are important. Not only dust charge perturbations but also dust density perturbations are taken into account, the latter are shown to have a strong effect on both the short and long range part of the potential. The realization of collective attraction via the newly obtained potential is discussed, a mechanism that could be central to the understanding of phase-transitions and self-organization processes in dusty plasmas.

  • 84.
    Tolias, Panagiotis
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    De Angeli, M.
    De Temmerman, G.
    Ripamonti, D.
    Riva, G.
    Bykov, Igor
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Shalpegin, A.
    Vignitchouk, Ladislas Tancrède Raymond
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Brochard, F.
    Bystrov, K.
    Bardin, S.
    Litnovsky, A.
    Dust remobilization in fusion plasmas under steady state conditions2016In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 58, no 2, article id 025009Article in journal (Refereed)
    Abstract [en]

    The first combined experimental and theoretical studies of dust remobilization by plasma forces are reported. The main theoretical aspects of remobilization in fusion devices under steady state conditions are analyzed. In particular, the dominant role of adhesive forces is highlighted and generic remobilization conditions-direct lift-up, sliding, rolling-are formulated. A novel experimental technique is proposed, based on controlled adhesion of dust grains on tungsten samples combined with detailed mapping of the dust deposition profile prior and post plasma exposure. Proof-of-principle experiments in the TEXTOR tokamak and the EXTRAP-T2R reversed-field pinch are presented. The versatile environment of the linear device Pilot-PSI allowed for experiments with different magnetic field topologies and varying plasma conditions that were complemented with camera observations.

  • 85.
    Tolias, Panagiotis
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    de Angelis, U.
    Kinetic models of partially ionized complex plasmas in the low frequency regime2011In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 18, no 7, p. 073705-Article in journal (Refereed)
    Abstract [en]

    The results from three kinetic models of complex plasmas taking into account collisions with neutrals are compared in the low-frequency regime: The "full" model which considers the absorption of plasma fluxes on dust particles and dust charge fluctuations, the "multi-component" model where both these effects are neglected, and the "standard" model which takes into account the dust charge perturbations but not the absorption of fluxes. We derive and numerically evaluate expressions of the low frequency responses of these models, also taking into account the modification of the capture cross-sections due to the effect of neutrals. The role of plasma sources and collisions with neutrals is assessed by computing the plasma permittivities and static permittivities for all the three models.

  • 86.
    Tolias, Panagiotis
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    de Angelis, U.
    Regimes for experimental tests of kinetic effects in dust acoustic waves2010In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 17, no 10, p. 103707-Article in journal (Refereed)
    Abstract [en]

    The low frequency responses of a kinetic model of dusty plasma, taking into account the absorption of the plasma particles on the dust particles and fluctuations of dust charge, are evaluated numerically in order to identify realistic experimental parameter regimes where charging effects are dominant. A range of parameters in terms of dust number density and size as well as plasma densities, where significant differences with the multicomponent approach can be found, is proposed. General expressions for the imaginary parts of the low frequency responses are derived and numerical results for the damping rate in the corresponding ranges are provided to ensure that observations of dust-acoustic waves in those conditions are plausible.

  • 87.
    Tolias, Panagiotis
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. Sezione di Napoli, Italy.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    de Angelis, U.
    Soft mean spherical approximation for dusty plasma liquids: Level of accuracy and analytic expressions2015In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 22, no 8, article id 083703Article in journal (Refereed)
    Abstract [en]

    The soft mean spherical approximation is employed for the study of the thermodynamics of dusty plasma liquids, the latter treated as Yukawa one-component plasmas. Within this integral theory method, the only input necessary for the calculation of the reduced excess energy stems from the solution of a single non-linear algebraic equation. Consequently, thermodynamic quantities can be routinely computed without the need to determine the pair correlation function or the structure factor. The level of accuracy of the approach is quantified after an extensive comparison with numerical simulation results. The approach is solved over a million times with input spanning the whole parameter space and reliable analytic expressions are obtained for the basic thermodynamic quantities.

  • 88.
    Tolias, Panagiotis
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    de Angelis, U.
    Soft mean spherical approximation for dusty plasma liquids: One- component Yukawa systems with plasma shielding2014In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 90, no 5, p. 053101-Article in journal (Refereed)
    Abstract [en]

    The structure and thermodynamics of strongly coupled dusty plasmas are investigated with the soft mean spherical approximation. This integral theory approach is analytically solvable for Yukawa pair interactions yielding a closed-form solution for the direct correlation function. The pair correlation function, the structure factor, and basic thermodynamic quantities are calculated for a wide range of parameters. Exact consistency between the "energy"-"virial" thermodynamic routes and approximate consistency between the "energy" "compressibility" paths is demonstrated. Comparison with extensive molecular dynamics results is carried out and a remarkable agreement from the Coulomb limit to the strongly screened limit is revealed. The soft mean spherical approximation is concluded to be particularly well suited for the study of dusty plasma liquids, uniquely combining simplicity and accuracy.

  • 89.
    Tolias, Panagiotis
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. Istituto Nazionale di Fisica Nucleare (INFN), Italy.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    De Angelis, U.
    Lazzaro, E.
    The finite probe size effect in fluctuation measurements; application to dusty plasmas2016In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 82, no 2, article id 615820201Article in journal (Refereed)
    Abstract [en]

    The effect of the finite probe size in plasma fluctuation measurements is revisited for dusty plasmas, where it has been argued that dust leads to a significant low-frequency enhancement in the spectral densities of ion density fluctuations, which can constitute the physical basis of a dust diagnostic technique. Theoretical predictions for the spectral modifications are presented and the dust acoustic mode contribution is analysed. The finite probe size effect is treated within the volume average approach, which introduces geometry dependent form factors that are calculated for spherical and cylindrical probes. The volume average approach is compared with the typically employed cutoff wavenumber approximation for various dust and plasma parameters. The contribution of temperature fluctuations to the spectral density of current fluctuations is also evaluated.

  • 90.
    Tolias, Panagiotis
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    de Angelis, Umberto
    Spectra of ion density and potential fluctuations in weakly ionized plasmas in the presence of dust grains2012In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 85, no 2, p. 026408-1-026408-9Article in journal (Refereed)
    Abstract [en]

    The spectral densities of ion density and electrostatic potential fluctuations are derived in the framework of a self-consistent kinetic model of partially ionized dusty plasmas in the low-frequency regime. Neutral gas density can be responsible for significant modifications of the fluctuation level, hence the inclusion of the effect of neutrals is essential for a more realistic comparison with experiments, especially if spectral measurements are intended for dust diagnostic purposes. Comparison with the multicomponent model, attractive due to its simplicity as compared to the self-consistent one, is carried out to establish its limits of validity. Numerical calculations are performed for parameters typical of low-temperature plasma discharges. A criterion is derived for the omission of plasma discreteness in the low-frequency regime.

  • 91.
    Tolias, Panagiotis
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Panarese, A.
    Longo, S.
    de Angelis, U.
    Natural fluctuations in un-magnetized and magnetized plasmas2015In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 81, article id 905810314Article in journal (Refereed)
    Abstract [en]

    There are still open issues within the fluctuation theory of plasmas, in view of the difficulty of formulating adequate theoretical approaches and solving the related equations in particular regimes. A promising alternative approach is direct microphysical modeling based on first principles, as successfully applied to neutral rarefied fluids. Within this approach, the equations of motion of a large ensemble of charged particles are solved numerically while correlations are obtained from statistical analysis of the ensemble at different times. As a first step, in this work we validate the data analysis technique adopted in this numerical scheme for the case of an electron ensemble neglecting Coulomb interactions. The simulation results are compared with the analytical theory of 'natural' fluctuations for both un-magnetized and magnetized plasmas. For the latter, the derivations for arbitrary average distribution functions are presented.

  • 92. Tolias, Panagiotis
    et al.
    Ratynskaia, Svetlana V.
    Shalpegin, A.
    Vignitchouk, Ladislas
    KTH.
    Brochard, F.
    De Angeli, M.
    van der Meiden, H.
    Experimental validation of the analytical model for tungsten dust - wall mechanical impacts incorporated in the MIGRAINe dust dynamics code2017In: NUCLEAR MATERIALS AND ENERGY, ISSN 2352-1791, Vol. 12, p. 524-529Article in journal (Refereed)
    Abstract [en]

    Mechanical dust-wall collisions are unavoidable in fusion devices and their accurate modeling is essential for the understanding of dust transport. The MIGRAINe dust dynamics code features analytical models addressing all facets of dust-surface impacts, some aspects of which have not been experimentally validated thus far. Dedicated dust injection experiments have been carried out in Pilot-PSI resulting to the visualization of in-plasma tungsten dust-surface impacts with an unprecedented resolution. They allowed for a calibration of key quantities of the MIGRAINe impact model.

  • 93.
    Tolias, Panagiotis
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Riva, G.
    De Angeli, M.
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Daminelli, G.
    Lungu, C. P.
    Porosnicu, C.
    Adhesive force distributions for tungsten dust deposited on bulk tungsten and beryllium-coated tungsten surfaces2018In: Nuclear Materials and Energy, E-ISSN 2352-1791, Vol. 15, p. 55-63Article in journal (Refereed)
    Abstract [en]

    Comprehensive measurements of the adhesive force for tungsten dust adhered to tungsten surfaces have been performed with the electrostatic detachment method. Monodisperse spherical dust has been deposited with gas dynamics techniques or with gravity mimicking adhesion as it naturally occurs in tokamaks. The adhesive force is confirmed to follow the log-normal distribution and empirical correlations are proposed for the size-dependence of its mean and standard deviation. Systematic differences are observed between the two deposition methods and attributed to plastic deformation during sticking impacts. The presence of thin beryllium coatings on tungsten surfaces is demonstrated to barely affect adhesion.

  • 94.
    Trier, E.
    et al.
    Max Planck Inst Plasma Phys, D-85748 Garching, Germany..
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Fridström, Richard
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Garcia Carrasco, Alvaro
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Vallejos, Pablo
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Vignitchouk, Ladislas
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Zuin, M.
    Consorzio RFX, Corso Stati Uniti 4, I-35127 Padua, Italy..
    ELM-induced cold pulse propagation in ASDEX Upgrade2019In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 61, no 4, article id 045003Article in journal (Refereed)
    Abstract [en]

    In ASDEX Upgrade, the propagation of cold pulses induced by type-I edge localized modes (ELMs) is studied using electron cyclotron emission measurements, in a dataset of plasmas with moderate triangularity. It is found that the edge safety factor or the plasma current are the main determining parameters for the inward penetration of the T-e perturbations. With increasing plasma current the ELM penetration is more shallow in spite of the stronger ELMs. Estimates of the heat pulse diffusivity show that the corresponding transport is too large to be representative of the inter-ELM phase. Ergodization of the plasma edge during ELMs is a possible explanation for the observed properties of the cold pulse propagation, which is qualitatively consistent with non-linear magneto-hydro-dynamic simulations.

  • 95. Tuccillo, A. A.
    et al.
    Alekseyev, A.
    Angelini, B.
    Annibaldi, Silvia Valeria
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Overview of the FTU results2009In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 49, no 10Article in journal (Refereed)
    Abstract [en]

    Spontaneous increases in plasma density, up to similar to 1.6 times the Greenwald value, are observed in FTU with lithized walls. These plasmas are characterized by profile peaking up to the highest obtained densities. The transport analysis of these discharges shows a 20% enhancement of the energy confinement time, with respect to the ITER97 L-mode scaling, correlated with a threshold in the peaking factor. It has been found that 0.4 MW of ECRH power, coupled at q = 2 surface, are sufficient to avoid disruptions in 0.5 MA discharges. Direct heating of magnetic islands produced by MHD modes determines current quench delay or avoidance. Supra-thermal electrons generated by 0.5 MW of lower hybrid power are sufficient to trigger precursors of the electron-fishbone instability. Evidence of spatial redistribution of fast electrons, on the similar to 100 mu s typical mode timescale, is shown by the fast electrons bremsstrahlung diagnostic. From the presence of new magnetic island induced accumulation points in the continuous spectrum of the shear Alfven wave spectrum, the existence of new magnetic island induced Alfven eigenmodes (MiAE) is suggested. Due to the frequency dependence on the magnetic island size, the feasibility of utilizing MiAE continuum effects as a novel magnetic island diagnostic is also discussed. Langmuir probes have been used on FTU to identify hypervelocity (10 km s(-1)), micrometre size, dust grains. The Thomson scattering diagnostic was also used to characterize the dust grains, present in the FTU vacuum chamber, following a disruption. Analysis of the broad emitted light spectrum was carried out and a model taking into account the particle vaporization is compared with the data. A new oblique ECE diagnostic has been installed and the first results, both in the presence of lower hybrid or electron cyclotron waves, are being compared with code predictions. A time-of-flight refractometer at 60 GHz, which could be a good candidate for the ITER density feedback control system, has also been tested.

  • 96. Usachev, A.
    et al.
    Zobnin, A.
    Petrov, O.
    Fortov, V.
    Thoma, M.
    Kretschmer, M.
    Ratynskaia, Svetlana V.
    Quinn, R.
    Hoefner, H.
    Morfill, G.
    The project Plasmakristall-4 (PK-4) - a dusty plasma experiment in a combined dc/rf(i) discharge plasma under microgravity conditions2004In: Czechoslovak Journal of Physics, ISSN 0011-4626, E-ISSN 1572-9486, Vol. 54, p. C639-C647Article in journal (Refereed)
    Abstract [en]

    The PK-4 experiment is a continuation of the successful dusty plasma experiments PK-1, PK-2 and PK-3 conducted on board of the orbital space stations Mir and IS S (International Space Station). The aim of the PK-4 experiment is an investigation of physical processes in complex (dusty) plasmas under microgravity conditions in a combined dc/rf discharge plasma.

  • 97.
    Vignitchouk, L.
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Ratynskaia, S.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Kantor, M.
    Ioffe Inst, St Petersburg 194021, Russia..
    Tolias, Panagiotis
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    De Angeli, M.
    CNR, Ist Fis Plasma Piero Caldirola, Via Cozzi 53, I-20125 Milan, Italy..
    van der Meiden, H.
    Dutch Inst Fundamental Energy Res, FOM Inst DIFFER, De Zaale 20, NL-5612 AJ Eindhoven, Netherlands..
    Vernimmen, J.
    Dutch Inst Fundamental Energy Res, FOM Inst DIFFER, De Zaale 20, NL-5612 AJ Eindhoven, Netherlands..
    Brochard, F.
    Univ Lorraine, Inst Jean Lamour, UMR 7198, CNRS, F-54506 Vandoeuvre Les Nancy, France..
    Shalpegin, A.
    Univ Lorraine, Inst Jean Lamour, UMR 7198, CNRS, F-54506 Vandoeuvre Les Nancy, France..
    Thorén, E.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Banon, J-P
    NTNU Norwegian Univ Sci & Technol, Dept Phys, NO-7491 Trondheim, Norway..
    Validating heat balance models for tungsten dust in cold dense plasmas2018In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 60, no 11, article id 115002Article in journal (Refereed)
    Abstract [en]

    The first comparison of dust radius and surface temperature estimates, obtained from spectroscopic measurements of thermal radiation, with simulations of dust heating and vaporization by the MIGRAINe dust dynamics code is reported. The measurements were performed during controlled tungsten dust injection experiments in the cold and dense plasmas of Pilot-PSI, reproducing ITER divertor conditions. The comparison has allowed us to single out the dominating role of the work function contribution to the dust heating budget. However, in the plasmas of interest, dust was found to enter the strong vaporization regime, in which its temperature is practically insensitive to plasma properties and the various uncertainties in modeling. This makes the dust temperature a poor figure of merit for model validation purposes. On the other hand, simple numerical scalings obtained from orbital-motion-limited estimates were found to be remarkably robust and sufficient to understand the main physics at play in such cold and dense plasmas.

  • 98.
    Vignitchouk, Ladislas
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Delzanno, G. L.
    Los Alamos Natl Lab, Theoret Div, Los Alamos, NM 87545 USA..
    Tolias, Panagiotis
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Electron reflection effects on particle and heat fluxes to positively charged dust subject to strong electron emission2018In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 25, no 6, article id 063702Article in journal (Refereed)
    Abstract [en]

    A new model describing dust charging and heating in unmagnetized plasmas in the presence of large electron emission currents is presented. By accounting for the formation of a potential well due to trapped emitted electrons when the dust is positively charged, this model extends the so-called OML+ approach, thus far limited to thermionic emission, by including electron-induced emission processes, and in particular low-energy quasi-elastic electron reflection. Revised semi-analytical formulas for the current and heat fluxes associated with emitted electrons are successfully validated against particle-in-cell simulations and predict an overall reduction of dust heating by up to a factor of 2. When applied to tungsten dust heating in divertor-like plasmas, the new model predicts that the dust lifetime increases by up to 80%, as compared with standard orbital-motion-limited estimates.

  • 99.
    Vignitchouk, Ladislas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    De Temmerman, Gregory
    Lehnen, Michael
    Lisgo, Steve
    Beryllium droplet cooling and distribution in the ITER vessel after a disruption2016In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326Article in journal (Refereed)
  • 100.
    Vignitchouk, Ladislas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ratynskaia, Svetlana V.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Analytical model of particle and heat flux collection by dust immersed in dense magnetized plasmas2017In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 59, no 10, article id 104002Article in journal (Refereed)
    Abstract [en]

    A comprehensive analytical description is presented for the particle and heat fluxes collected by dust in dense magnetized plasmas. Compared to the widely used orbital motion limited theory, the suppression of cross-field transport leads to a strong reduction of the electron fluxes, while ion collection is inhibited by thin-sheath effects and the formation of a potential overshoot along the field lines. As a result, the incoming heat flux loses its sensitivity to the floating potential, thereby diminishing the importance of electron emission processes in dust survivability. Numerical simulations implementing the new model for ITER-like detached divertor plasmas predict a drastic enhancement of the dust lifetime.

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