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Visible spectroscopic diagnostics: application and development in fusion plasmas
KTH, School of Engineering Sciences (SCI), Physics, Atomic and Molecular Physics.
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Diagnostic measurements play a vital role in experiments. Without them we would be in the dark with no way of knowing what was happening; of understanding the processes and behaviour occurring; or even of judging the success or failure of our experiments. The development of fusion plasma devices is no different. In this thesis we concentrate on visible spectroscopy based diagnostics: examining the techniques for measurement and analysis; the breadth of plasma parameters that can be extracted from the spectroscopic data; and how the application of these diagnostic techniques gives us a broader picture of the plasma and the events taking place within. Techniques are developed and applied to plasmas in three fusion experiments, EXTRAP T2R, ASDEX Upgrade and JET. The diagnostic techniques exploit different features of the measurements of the emitted photons to obtain various useful plasma parameters.

Determination of the ion temperature and rotation velocity of oxygen impurity ions in the EXTRAP T2R plasma is achieved through measurement and analysis of, respectively, the Doppler broadening and the Doppler wavelength shift of visible wavelength atomic spectral lines. The evolution of the temperature and rotation is studied as a function of the discharge parameters, in particular looking at the effect of applying active feedback control schemes to the resistive wall modes and/or pulsed poloidal current drive. Measurements of multiple ionisation stages are used to estimate radial profiles of the toroidal rotation and the ion temperature and correlations between the ion rotations and the rotation velocities of tearing modes are also established.

Radial profiles of the emissivity and density (or concentration) of the oxygen ions are obtained by means of measurements of the spectral line intensities on a small array of linesof- sight through the plasma. Changes to the profiles for different plasma schemes and the implications for particle transport are investigated. The derived emissivity profiles are used in the analysis for some of the other spectroscopic diagnostics. Spectral line intensity measurements (in this case of neutral ions) are also the basis for calculations of both the electron temperature and the particle fluxes at the plasma edge. The latter is an indicator of the degree and type of interaction between the plasma and the surrounding surfaces. Particle fluxes of the operating gas hydrogen and of chromium and molybdenum impurities are investigated in EXTRAP T2R for different operating scenarios, in particular changes in the metallic influx with the application of active feedback mode control are examined along with the correspondence between spectroscopic and collector probe results. In the ASDEX Upgrade divertor estimates of the particle flux of the deuterium operating gas are also made through analysis of spectral intensities. Molecular D2 band structure is explored in addition to the Balmer Dα spectral line intensity to acquire both atomic and molecular particle fluxes, investigate the contribution of the dissociating D2 to the Dα line and study the effect of changes in the divertor.

Analysis of the D2 molecular band structure (the relative intensities of the rotational lines and vibrational bands) also enables calculation of the upper state rotational and ground state vibrational temperatures. The locations of emitting atomic ions in JET are estimated from Zeeman splitting analysis of the structure of their spectral lines.

The measurement and analysis of visible wavelength light is demonstrated to be a sensitive diagnostic tool in the quest for increased knowledge about fusion plasmas and their operating scenarios.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , x, 52 p.
Series
Trita-FYS, ISSN 0280-316X ; 2007:72
Keyword [en]
visible spectroscopy, fusion, Doppler shift, Doppler broadening
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:kth:diva-4541ISBN: 978-91-7178-771-2 (print)OAI: oai:DiVA.org:kth-4541DiVA: diva2:12758
Public defence
2007-12-07, Sal FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100810Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2010-08-10Bibliographically approved
List of papers
1. Impurity identifications, concentrations and particle fluxes from spectral measurements of the EXTRAP T2R plasma
Open this publication in new window or tab >>Impurity identifications, concentrations and particle fluxes from spectral measurements of the EXTRAP T2R plasma
2006 (English)In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 74, no 4, 439-448 p.Article in journal (Refereed) Published
Abstract [en]

An absolute intensity calibrated 0.5m spectrometer with optical multi-channel analyser detector was used to observe the visible-UV radiation from the plasma in the EXTRAP T2R reversed field pinch experiment. Spectral lines were identified indicating the presence of oxygen, chromium, iron and molybdenum impurities in the hydrogen plasma. Certain regions of interest were examined in more detail and at different times in the plasma discharge. Impurity concentration calculations were made using the absolute intensities of lines of OIV and OV measured at 1-2 ms into the discharge generating estimates of the order of 0.2% of ne in the central region rising to 0.7% of ne at greater radii for OIV and 0.3% rising to 0.6% for OV. Edge electron temperatures of 0.5-5 eV at electron densities of 5-10 x 10(11) cm(-3) were calculated from the measured relative intensities of hydrogen Balmer lines. The absolute intensities of hydrogen lines and of multiplets of neutral chromium and molybdenum were used to determine particle fluxes (at 4-5 ms into the plasma) of the order 1 x 10(16), 7 x 1013 and 3 x 10(13) particles cm(-2) s(-1), respectively.

Keyword
Carrier concentration, Impurities, Optical devices, Spectrometers, Spectrum analyzers, Ultraviolet radiation, Field pinch experiment, Multi-channel analysers, Plasma discharges, Plasmas
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7654 (URN)10.1088/0031-8949/74/4/005 (DOI)000241682600005 ()2-s2.0-33846118810 (Scopus ID)
Note
QC 20100810Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2017-12-14Bibliographically approved
2. Particle flux and surface interaction in EXTRAP T2R
Open this publication in new window or tab >>Particle flux and surface interaction in EXTRAP T2R
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2007 (English)In: 34th EPS Conference on Plasma Physics 2007, EPS 2007 - Europhysics Conference Abstracts, European Physical Society , 2007, 347-350 p.Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
European Physical Society, 2007
Series
Europhysics Conference Abstracts, ISSN 0378-2271 ; 31
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7655 (URN)2-s2.0-84873822225 (Scopus ID)978-162276334-4 (ISBN)
Conference
34th European Physical Society Conference on Plasma Physics 2007, EPS 2007; Warsaw; Poland; 2 July 2007 through 6 July 2007
Note

QC 20100810

Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2014-10-28Bibliographically approved
3. Molecular contribution to the D alpha emission in the divertor of the ASDEX Upgrade experiment
Open this publication in new window or tab >>Molecular contribution to the D alpha emission in the divertor of the ASDEX Upgrade experiment
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2007 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 105, no 3, 425-437 p.Article in journal (Refereed) Published
Abstract [en]

Spectral line emission from the deuterium Balmer alpha line and from molecular deuterium in the Fulcher band, d(3)Pi(u)-> a(3)Sigma(+)(g), was studied in the outer divertor region of the ASDEX Upgrade plasma with a pair of relatively calibrated spectrometers. A dependence of the signals on the proximity of the plasma strike point and on divertor gas puffing was noted. The rotational spectrum of the lowest diagonal vibrational molecular bands indicated rotational temperatures of ca. 1000 K and hence, through calculation of relative upper state populations, a ground state vibrational temperature of ca. 4000 K. Calculating the particle fluxes, the molecular deuterium was found to contribute up to 1% of the D alpha intensity, the relative importance of the molecules at some divertor heights was affected by the position of the strike point.

Keyword
ASDEX Upgrade, Calibration, Deuterium molecules, Divertor spectroscopy
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7656 (URN)10.1016/j.jqsrt.2006.12.004 (DOI)000245564400006 ()2-s2.0-33847233592 (Scopus ID)
Note
QC 20100810Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2017-12-14Bibliographically approved
4. Rotation in a reversed field pinch with active feedback stabilization of resistive wall modes
Open this publication in new window or tab >>Rotation in a reversed field pinch with active feedback stabilization of resistive wall modes
2006 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 48, no 9, 1311-1331 p.Article in journal (Refereed) Published
Abstract [en]

Active feedback stabilization of multiple resistive wall modes (RWMs) has been successfully proven in the EXTRAP T2R reversed field pinch. One of the features of plasma discharges operated with active feedback stabilization, in addition to the prolongation of the plasma discharge, is the sustainment of the plasma rotation. Sustained rotation is observed both for the internally resonant tearing modes (TMs) and the intrinsic impurity oxygen ions. Good quantitative agreement between the toroidal rotation velocities of both is found: the toroidal rotation is characterized by an acceleration phase followed, after one wall time, by a deceleration phase that is slower than in standard discharges. The TMs and the impurity ions rotate in the same poloidal direction with also similar velocities. Poloidal and toroidal velocities have comparable amplitudes and a simple model of their radial profile reproduces the main features of the helical angular phase velocity. RWMs feedback does not qualitatively change the TMs behaviour and typical phenomena such as the dynamo and the `slinky' are still observed. The improved sustainment of the plasma and TMs rotation occurs also when feedback only acts on internally non- resonant RWMs. This may be due to an indirect positive effect, through non- linear coupling between TMs and RWMs, of feedback on the TMs or to a reduced plasma- wall interaction affecting the plasma flow rotation. Electromagnetic torque calculations show that with active feedback stabilization the TMs amplitude remains well below the locking threshold condition for a thick shell. Finally, it is suggested that active feedback stabilization of RWMs and current profile control techniques can be employed simultaneously thus improving both the plasma duration and its confinement properties.

Keyword
Plasma discharges; Plasma rotation; Resistive wall modes; Electric currents; Electromagnetism; Torque measurement; Velocity measurement; Plasma flow
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7657 (URN)10.1088/0741-3335/48/9/004 (DOI)000240120200006 ()2-s2.0-33749510126 (Scopus ID)
Note
QC 20100810Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2017-12-14Bibliographically approved
5. Large periodic fluctuations of plasma signals in EXTRAP T2R
Open this publication in new window or tab >>Large periodic fluctuations of plasma signals in EXTRAP T2R
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2007 (English)In: Proceedings of the 34th European Physical Society Conference on Plasma Physics, 2007Conference paper, Published paper (Refereed)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7658 (URN)2-s2.0-84873858869 (Scopus ID)
Conference
34th European Physical Society Conf. on Plasma Physics, Warsaw (2007)
Note
QC 20100810 Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2011-12-19Bibliographically approved
6. Reversed field pinch operation with intelligent shell feedback control in EXTRAP T2R
Open this publication in new window or tab >>Reversed field pinch operation with intelligent shell feedback control in EXTRAP T2R
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2006 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 46, no 11, 904-913 p.Article in journal (Refereed) Published
Abstract [en]

Discharges in the thin shell reversed field pinch (RFP) device EXTRAP T2R without active feedback control are characterized by growth of non-resonant m = 1 unstable resistive wall modes (RWMs) in agreement with linear MHD theory. Resonant m = 1 tearing modes (TMs) exhibit initially fast rotation and the associated perturbed radial fields at the shell are small, but eventually TMs wall-lock and give rise to a growing radial field. The increase in the radial field at the wall due to growing RWMs and wall-locked TMs is correlated with an increase in the toroidal loop voltage, which leads to discharge termination after 3-4 wall times. An active magnetic feedback control system has been installed in EXTRAP T2R. A two-dimensional array of 128 active saddle coils (pair-connected into 64 independent m = 1 coils) is used with intelligent shell feedback control to suppress the m = 1 radial field at the shell. With feedback control, active stabilization of the full toroidal spectrum of 16 unstable m = 1 non-resonant RWMs is achieved, and TM wall locking is avoided. A three-fold extension of the pulse length, up to the power supply limit, is observed. Intelligent shell feedback control is able to maintain the plasma equilibrium for 10 wall times, with plasma confinement parameters sustained at values comparable to those obtained in thick shell devices of similar size.

Keyword
Correlation methods; Feedback control; Perturbation techniques; Pinch effect; Plasma confinement; Resonance; Rotation; Discharge termination; Perturbed radial fields; Reversed field pinch operation; Tearing modes (TM); Shells (structures)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-6558 (URN)10.1088/0029-5515/46/11/004 (DOI)000242664000006 ()2-s2.0-33846062292 (Scopus ID)
Note
QC 20100810Available from: 2006-12-11 Created: 2006-12-11 Last updated: 2017-12-14Bibliographically approved
7. Current profile modifications with active feedback stabilization of resistive wall modes in a reversed field pinch
Open this publication in new window or tab >>Current profile modifications with active feedback stabilization of resistive wall modes in a reversed field pinch
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2006 (English)In: Proceedings of the 33rd European Physical Society Conference on Plasma Physics, 2006, 1680-1683 p.Conference paper, Published paper (Refereed)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7660 (URN)2-s2.0-84872659265 (Scopus ID)978-162276333-7 (ISBN)
Conference
The 33rd European Physical Society Conference on Plasma Physics, Rome, 19 - 23 June 2006
Note

QC 20100810

Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2014-10-30Bibliographically approved
8. Measurements and modeling of transport and impurity radial profiles in the EXTRAP T2R reversed field pinch
Open this publication in new window or tab >>Measurements and modeling of transport and impurity radial profiles in the EXTRAP T2R reversed field pinch
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2006 (English)In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 13, no 9, 092506- p.Article in journal (Refereed) Published
Abstract [en]

Radial impurity profiles of oxygen in the rebuilt reversed field pinch EXTRAP T2R [P. R. Brunsell , Plasma Phys. Control. Fusion 43, 1457 (2001)] have been measured with a multichannel spectrometer. Absolute ion densities for oxygen peak between 1-4x10(10) cm(-3) for a central electron density of 1x10(13) cm(-3). Transport simulations with the one-dimensional transport code STRAHL with a diffusion coefficient of 20 m(2) s(-1) yield density profiles similar to those measured. Direct measurement of the ion profile evolution during pulsed poloidal current drive suggests that the diffusion coefficient is reduced by a factor similar to 2 in the core but remains unaffected toward the edge. Core transport is not significantly affected by the radial magnetic field growth seen at the edge in discharges without feedback control. This indicates that the mode core amplitude remains the same while the mode eigenfunction increases at the edge.

Keyword
Carrier concentration; Diffusion; Eigenvalues and eigenfunctions; Feedback control; Magnetic fields; Radial flow; Spectrometers; Diffusion coefficients; Multichannel spectrometers; Radial impurity; Impurities
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7661 (URN)10.1063/1.2349304 (DOI)000240877800033 ()2-s2.0-33749334196 (Scopus ID)
Note
QC 20100810. Titel ändrad, tidigare titel: "Measurements and modelling of transport and impurity profiles in the EXTRAP T2R reversed field pinch" 20100810.Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2017-12-14Bibliographically approved

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