Statistical analysis of correlations and intermittency of a turbulent rotating column in a magnetoplasma device

12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France)A detailed statistical analysis of density fluctuations in the cylindrical non-fusion device "Mistral" is performed. The experimental set-up is implemented in order to reach turbulent behavior in the rotating plasma column. Two different turbulent regimes are obtained corresponding to two selected sets of values for the anode potential and the biasing of the collecting plate. The first regime displays a slowly-rotating column characterized by the presence of a shear layer separating the plasma bulk from the scrape-off layer (SOL), the latter showing a strong intermittent behavior. The second regime corresponds to a weakly-rotating column in which coherence is lost in the plasma bulk and a standard diffusive process takes place in the SOL. These findings are supported by the calculation of the Hurst exponent of the turbulent signals using Wavelet-analysis techniques

Formation of convective cells in the scrape-off layer of the CASTOR tokamak

Understanding of the scrape-off layer (SOL) physics in tokamaks requires diagnostics with sufficient temporal and spatial resolution. This contribution describes results of experiments performed in the SOL of the CASTOR tokamak (R=40 cm, a = 6 cm) by means of a ring of 124 Langmuir probes surrounding the whole poloidal cross section. The individual probes measure either the ion saturation current of the floating potential with the spatial resolution up to 3 mm. Experiments are performed in a particular magnetic configuration, characterized by a long parallel connection length in the SOL, L_par ~q2piR. We report on measurements in discharges, where the edge electric field is modified by inserting a biased electrode into the edge plasma. In particular, a complex picture is observed, if the biased electrode is located inside the SOL. The poloidal distribution of the floating potential appears to be strongly non-uniform at biasing. The peaks of potential are observed at particular poloidal angles. This is interpreted as formation of a biased flux tube, which emanates from the electrode along the magnetic field lines and snakes q times around the torus. The resulting electric field in the SOL is 2-dimensional, having the radial as well as the poloidal component. It is demonstrated that the poloidal electric field E_pol convects the edge plasma radially due to the E_pol x B_T drift either inward or outward depending on its sign. The convective particle flux is by two orders of magnitude larger than the fluctuation-induced one and consequently dominates.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France

Contrasting H-mode behaviour with fuelling and nitrogen seeding in the all-carbon and metallic versions of JET

An all-metal ITER-Like Wall (JET-ILW), consisting of beryllium in the main chamber and tungsten surfaces in the divertor, has now been installed in JET to pursue low retention of fuel species and to explore the impact on next-step-relevant plasmas. Its implementation has offered a unique opportunity to compare behaviour with that in the previous all-Carbon lining (JET-C), notably for high-triangularity Type I H-modes with impurity seeding. This technique is recognised to be necessary for power handling both in ITER and in JET at full performance. Contrasting results are reported for closely-matched deuterium-fuelling plus nitrogen-seeding scans in each JET environment. Attention is focused upon neutral-beam-heated plasmas with total input power 15­17MW at 2.65T, 2.5MA, q95 3.5 , average triangularity d 0.4 , elongation k 1.7 and gas inputs spanning ranges 0.75 FD 3.3 , 0 FD 4.7 (1022 electrons / s assuming full ionisation). JET-C cases also included 1­2MW of central ion-cyclotron-resonance-frequency heating, so far absent from JET-ILW pulses, with possible consequences for respective core sawtooth and impurity-concentration results.Preprint of Paper to be submitted for publication in Proceedings of the 40th EPS Conference on Plasma Physics, Espoo, Finland 1st July 2013 - 5th July 201

Dynamics of the edge transport barrier at plasma biasing on the CASTOR tokamak

A clear and reproducible transition to a regime with an improved particle confinement is routinely observed on the CASTOR tokamak, if the biasing electrode is inserted deep enough into the plasma (r/a ~ 0.5) and biased up to +250V. The steepening of the radial profiles of the plasma density and potential demonstrate the formation of a transport barrier just inside the last closed flux surface. Fast relaxations of the edge plasma parameters, with a frequency of about 10 kHz, are observed when the average radial electric field within the barrier prevails values of about 20 kV/m. A detailed analysis of the spatial-temporal behavior of these relaxations is presented.На токамаке CASTOR устойчиво наблюдается отчетливый и воспроизводимый переход в режим с улучшенным удержанием частиц при достаточно глубоком введении в плазму (r/a - 0,5) электрода, на который подаётся смещение до +250 В. У кручение радиальных профилей плотности плазмы и потенциала свидетельствуют об образовании транспортного барьера сразу внутри последней замкнутой магнитной поверхности. Наблюдаются быстрые релаксации параметров краевой плазмы с частотой около 10 кГц, когда среднее радиальное электрическое поле внутри барьера превышает значение около 20 кВ/м. Приводится обстоятельный анализ пространственно-временного поведения этих релаксаций.На токамаці CASTOR стійко спостерігається чіткий та відтворений перехід до режиму з поліпшеним утриманням частинок при достатньо глибокому введенні у плазму (r/a ~ 0,5) електрода, на який подано зміщення до +250 В. Радіальні профілі густини плазми та потенціалу стають більш крутими, що свідчить про створення потенціального бар'єру зразу усередині останньої замкненої магнітної поверхні. Спостерігаються швидкі релаксації параметрів крайової плазми з частотою порядку 10 кГц, коли середнє радіальне електричне поле усередині бар'єра перевищує значення приблизно 20 кВ/м. Надається докладний аналіз просторово-часової поведінки цих релаксацій

Model for screening of resonant magnetic perturbations by plasma in a realistic tokamak geometry and its impact on divertor strike points

This work addresses the question of the relation between strike-point splitting and magnetic stochasticity at the edge of a poloidally diverted tokamak in the presence of externally imposed magnetic perturbations. More specifically, ad-hoc helical current sheets are introduced in order to mimic a hypothetical screening of the external resonant magnetic perturbations by the plasma. These current sheets, which suppress magnetic islands, are found to reduce the amount of splitting expected at the target, which suggests that screening effects should be observable experimentally. Multiple screening current sheets reinforce each other, i.e. less current relative to the case of only one current sheet is required to screen the perturbation.Comment: Accepted in the Proceedings of the 19th International Conference on Plasma Surface Interactions, to be published in Journal of Nuclear Materials. Version 2: minor formatting and text improvements, more results mentioned in the conclusion and abstrac

Operating a full tungsten actively cooled tokamak: overview of WEST first phase of operation

WEST is an MA class superconducting, actively cooled, full tungsten (W) tokamak, designed to operate in long pulses up to 1000 s. In support of ITER operation and DEMO conceptual activities, key missions of WEST are: (i) qualification of high heat flux plasma-facing components in integrating both technological and physics aspects in relevant heat and particle exhaust conditions, particularly for the tungsten monoblocks foreseen in ITER divertor; (ii) integrated steady-state operation at high confinement, with a focus on power exhaust issues. During the phase 1 of operation (2017–2020), a set of actively cooled ITER-grade plasma facing unit prototypes was integrated into the inertially cooled W coated startup lower divertor. Up to 8.8 MW of RF power has been coupled to the plasma and divertor heat flux of up to 6 MW m−2 were reached. Long pulse operation was started, using the upper actively cooled divertor, with a discharge of about 1 min achieved. This paper gives an overview of the results achieved in phase 1. Perspectives for phase 2, operating with the full capability of the device with the complete ITER-grade actively cooled lower divertor, are also described

Modelling of the effect of ELMs on fuel retention at the bulk W divertor of JET

Effect of ELMs on fuel retention at the bulk W target of JET ITER-Like Wall was studied with multi-scale calculations. Plasma input parameters were taken from ELMy H-mode plasma experiment. The energetic intra-ELM fuel particles get implanted and create near-surface defects up to depths of few tens of nm, which act as the main fuel trapping sites during ELMs. Clustering of implantation-induced vacancies were found to take place. The incoming flux of inter-ELM plasma particles increases the different filling levels of trapped fuel in defects. The temperature increase of the W target during the pulse increases the fuel detrapping rate. The inter-ELM fuel particle flux refills the partially emptied trapping sites and fills new sites. This leads to a competing effect on the retention and release rates of the implanted particles. At high temperatures the main retention appeared in larger vacancy clusters due to increased clustering rate