The objective of thermonuclear fusion consists of producing electricity from the coalescence of light nuclei in high temperature plasmas. The most promising route to fusion envisages the confinement of such plasmas with magnetic fields, whose most studied configuration is the tokamak. Disruptions are catastrophic collapses affecting all tokamak devices and one of the main potential showstoppers on the route to a commercial reactor. In this work we report how, deploying innovative analysis methods on thousands of JET experiments covering the isotopic compositions from hydrogen to full tritium and including the major D-T campaign, the nature of the various forms of collapse is investigated in all phases of the discharges. An original approach to proximity detection has been developed, which allows determining both the probability of and the time interval remaining before an incoming disruption, with adaptive, from scratch, real time compatible techniques. The results indicate that physics based prediction and control tools can be developed, to deploy realistic strategies of disruption avoidance and prevention, meeting the requirements of the next generation of devices.

In 2021 JET exploited its unique capabilities to operate with T and D-T fuel with an ITER-like Be/W wall (JET-ILW). This second major JET D-T campaign (DTE2), after DTE1 in 1997, represented the culmination of a series of JET enhancements-new fusion diagnostics, new T injection capabilities, refurbishment of the T plant, increased auxiliary heating, in-vessel calibration of 14 MeV neutron yield monitors-as well as significant advances in plasma theory and modelling in the fusion community. DTE2 was complemented by a sequence of isotope physics campaigns encompassing operation in pure tritium at high T-NBI power. Carefully conducted for safe operation with tritium, the new T and D-T experiments used 1 kg of T (vs 100 g in DTE1), yielding the most fusion reactor relevant D-T plasmas to date and expanding our understanding of isotopes and D-T mixture physics. Furthermore, since the JET T and DTE2 campaigns occurred almost 25 years after the last major D-T tokamak experiment, it was also a strategic goal of the European fusion programme to refresh operational experience of a nuclear tokamak to prepare staff for ITER operation. The key physics results of the JET T and DTE2 experiments, carried out within the EUROfusion JET1 work package, are reported in this paper. Progress in the technological exploitation of JET D-T operations, development and validation of nuclear codes, neutronic tools and techniques for ITER operations carried out by EUROfusion (started within the Horizon 2020 Framework Programme and continuing under the Horizon Europe FP) are reported in (Litaudon et al Nucl. Fusion accepted), while JET experience on T and D-T operations is presented in (King et al Nucl. Fusion submitted).

To complement wall conditioning research in TOMAS, a characterisation of radio-frequency hydrogen plasmas has been performed using a radially movable triple Langmuir probe. Experimental measurements of electron temperature and density radial profiles at different magnetic field on axis strengths and neutral pressures have been performed. First results of simulations of the radial profiles with the code TOMATOR-1D can qualitatively reproduce the measurements of the diagnostic and may be used to understand the behaviour of the waves inside the plasma.

The plasma electron density and temperature were measured in hydrogen electron-cyclotron resonance (ECR) plasma and combined ECR + radio-frequency (RF) discharges in the TOMAS facility. The results of ECR and combined ECR+RF discharges studies are compared. With an addition of RF, it is possible to vary the plasma parameters around the values provided by the ECR discharge. The propagation of slow (SW) and fast (FW) waves in hydrogen plasma is analyzed. Depending on the plasma density and the parallel component of the wave vector three cases are possible: SW propagation only, SW and FW simultaneous propagation, and FW propagation only.

Plasma edge cooling, ion cyclotron wall conditioning and disruption mitigation techniques involve massive gas injection (by puffs or pellets) to the torus. A certain fraction remains in plasma-facing components (PFC) due to co-deposition and implantation. An uncontrolled release/desorption of such retained species affects the stability of plasma operation. The aim of this work was to determine the lateral and depth distribution of noble (3He, 4He, Ne, Ar), seeded (N2, Ne, Ar) and tracer gases (15N, 18O) in PFC retrieved from the JET tokamak with the ITER-Like Wall (JET-ILW) after three experimental campaigns (ILW-1, ILW-2, ILW-3). Results regarding the retention of those gases are shown as well as a comparison to the deuterium retention in the studied areas. Heavy ion elastic recoil detection analysis was used, being the only technique capable of detection and quantitative assessment of all elements, especially low-Z isotopes. Helium was found on the divertor Tile 5, locally up to 44.1015 3He cm-2 and 12.1015 4He cm-2, and on the limiters as well. Neon was found in two positions on the limiters, with up to 10.1015 Ne cm-2 and the 15N tracer on Be limiters exposed to ILW-3. A correlation of N retention with the N seeding rates for each campaign has also been found.

The paper provides a concise overview of ion beam analysis methods and procedures in studies of materials exposed to fusion plasmas in controlled fusion devices with magnetic confinement. An impact of erosion-deposition processes on the morphology of wall materials is presented. In particular, results for deuterium analyses are discussed. Underlying physics, advantages and limitations of methods are addressed. The role of wall diagnostics in studies of material migration and fuel retention is explained. A brief note on research and handling of radioactive and beryllium-contaminated materials is also given.

In nuclear fusion reactors, plasmas are heated to very high temperatures of more than 100 million kelvin and, in so-called tokamaks, they are confined by magnetic fields in the shape of a torus. Light nuclei, such as deuterium and tritium, undergo a fusion reaction that releases energy, making fusion a promising option for a sustainable and clean energy source. Tokamak plasmas, however, are prone to disruptions as a result of a sudden collapse of the system terminating the fusion reactions. As disruptions lead to an abrupt loss of confinement, they can cause irreversible damage to present-day fusion devices and are expected to have a more devastating effect in future devices. Disruptions expected in the next-generation tokamak, ITER, for example, could cause electromagnetic forces larger than the weight of an Airbus A380. Furthermore, the thermal loads in such an event could exceed the melting threshold of the most resistant state-of-the-art materials by more than an order of magnitude. To prevent disruptions or at least mitigate their detrimental effects, empirical models obtained with artificial intelligence methods, of which an overview is given here, are commonly employed to predict their occurrence—and ideally give enough time to introduce counteracting measures.

Silicon plates were installed above the inner and outer divertor of the JET with the ITER-like wall (ILW) after the second and third ILW campaigns to monitor dust generation and deposition with the aim to determine the morphology and content of individual particles and co-deposits, including deuterium content. Particular interest was in metal-based particles: Be, W, steel, Cu. Ex-situ examination after two ILW campaigns was performed by a set of microscopy and ion beam methods including micro-beam nuclear reaction analysis and particle-induced X-ray emission. Different categories of Be-rich particles were found: co-deposits peeled-off from plasma-facing components (PFC), complex multi-element spherical objects, and solid metal splashes and regular spherical droplets. The fuel content on the two latter categories was at the level of 1 x 10(16) at/cm(-2) indicating that Be melting and splashing occurred in the very last phase of the second experimental campaign. The splashes adhere firmly to the substrate thus not posing risk of Be dust mobilisation. No tungsten droplets were detected. The only W-containing particles were fragments of tungsten coatings from the divertor tiles.

Alpha particles with energies on the order of megaelectronvolts will be the main source of plasma heating in future magnetic confinement fusion reactors. Instead of heating fuel ions, most of the energy of alpha particles is transferred to electrons in the plasma. Furthermore, alpha particles can also excite Alfvénic instabilities, which were previously considered to be detrimental to the performance of the fusion device. Here we report improved thermal ion confinement in the presence of megaelectronvolts ions and strong fast ion-driven Alfvénic instabilities in recent experiments on the Joint European Torus. Detailed transport analysis of these experiments reveals turbulence suppression through a complex multi-scale mechanism that generates large-scale zonal flows. This holds promise for more economical operation of fusion reactors with dominant alpha particle heating and ultimately cheaper fusion electricity.

Understanding of material migration and control of fuel retention are essential for the safe operation of a reactor-class fusion machine. Work presented in the thesis focuses on erosion-deposition processes which are decisive for the formation and properties of co-deposited fuel-containing layers on plasma-facing and diagnostic components, and for the dust formation. The thesis is based on experiments carried out in plasma devices such as JET-ILW, KSTAR, EXTRAP-T2R, TOMAS and, in materials research laboratories where comprehensive analyses of the plasma-exposed materials were performed by a large number of complementary ion, electron and optical methods. The major objectives were to determine: (a) plasma impact on test mirrors; (b) properties of metal dust generated under operation with metal walls in JET with the ITER-Like Wall; (c) material transport to areas shadowed from the direct plasma line-of-sight; (d) neutral particle fluxes in wall conditioning discharges. All these topics are inter-related and, they are in line with the ITER needs in areas of diagnostic development, mitigation of fuel inventory and detailed knowledge of dust particles generated in the tokamak with metal walls. The novelty in research is demonstrated by several elements. Plasma impact on diagnostic mirrors was determined by exposure of test mirrors in JET two types of holders including the ITER-like assembly resembling a diagnostic duct in a reactor. Dust studies allowed for the determination of particles’ properties (size, weight) and, for the classification of various detected objects. The impact of tile shaping and intentional misalignment on fuel retention was revealed in a dedicated experiment in KSTAR. A neutral particle analyser was first tested at EXTRAP-T2R and then installed at the TOMAS facility. Particle fluxes were characterized in wall conditioning discharges heated by electron- and ion cyclotron systems.

Att förstå förflyttning av material och ha kontroll över hur bränsle fastnar i väggplattor ärviktigt för säker användning av fusionsreaktorer. Denna avhandling fokuserar på de erosionsoch deponerings processer som är avgörande för skapandet och egenskaperna hos dedeponerade lagren på plasma mötande komponenter och diagnostik samt för skapandet omdamm. Arbetet bygger på material som exponerats i plasma experimenten JET-ILW, KSTAR,EXTRAP-T2R och TOMAS samt i materiallaboratorier där omfattande analyser avplasmautsatta material genomförts med joner, elektroner och optiska metoder. Huvudmålenhar varit att bestämma: (a) plasma påverkan på speglar (b) egenskaper hos metalldamm sombildats i JET med plasmautsatta metallväggar (c) materialtransport till områden utanför direktsynhåll från plasmat (d) flödet av neutrala partiklar i speciella urladdningar för attkonditionera väggarna. All dessa områden är sammankopplade med varandra samt medITERs behov av utveckling av diagnostik, begräsning och detaljerad kunskap om bränsle påväggar och damm i tokamaker med metall väggar. Nyskapandet i forskningen kans ses inomflera områden: Påverkan på speglar för diagnostik av plasma i JET inkluderat speglarmonterade i en hållare som liknar diagnostik kanaler ITER har bestämts. Damms egenskaperhar bestämts (storlek, vikt) och olika objekt har katalogiserats. Effekten och medvetenfellinjering av profilen på väggplattor på bränsleansamling i särskilt avsedda experiment iKSTAR har undersöktes. Ett detektorsystem för neutrala partiklar testades först i EXTRAPT2R och flyttades därefter till TOMAS och partikelflödet under plasma värmda medelektron- och jon-cyklotron system för att genomföra konditionering väggar.