L-H transition in tokamak plasma: study of local parameters in the edge plasma

Oscilace v průběhu L-H přechodu z módu nízkého do vysokého udržení a profily Rey- noldsova napětí byly zkoumány pomocí dvou mnoho-hrotových sondových hlavic navrhnutých a použitých na tokamaku COMPASS. Tyto sondové hlavice obsahující Langmuirovy i ball-pen sondy umožňují rychlé, současné měření radiálních a poloidálních elektrických polí oběma typy sond a elektronové teploty až 5 mm uvnitř posledního uzavřeného povrchu. Na radiálních profilech Reynoldsova napětí byl nalezen významný vliv fluktuací elektronové teploty na meření s Langmuirovými sondami v provnání s ball-pen sondami. Oscilace během L-H přechodu byly identifikovány sondovými měřeními jako limitní cykolvé oscilace typu J, během kterých intenzita turbulence roste po snížení střihu rychlosti hnané gradientem tlaku. Tyto oscilace byly odlišeny od nestabilit lokalizovaných na okraji (ELM).Oscillations during the L-H transition from the low to high confinement modes and Reynolds stress profiles were investigates using two multi-pin probe heads designed and used on the COMPASS tokamak. The probe heads consisting of both Langmuir and ball-pen probes enable fast, simultaneous measurements of the radial and poloidal electric fields with either probe type and the electron temperature up to several mm inside the last closed flux surface. The radial Reynoldss stress profiles show a significant impact of the electron temperature fluctuations on the Langmuir probe measurements in comparison to the ball-pen probe measurements. The oscillations during the L-H transition were identified through probe measurements as type-J limit cycle oscillations during which the turbulence intensity grows after the velocity shear driven by the pressure gradient decreases. The oscillations were found to be different from edge-localized modes (ELM)

Enhanced Plasma Confinement in Tokamaks with Focus on the Turbulence-Flow Interaction

Zlepšené udržení je klíčové pro úspěšné operování budoucích fúzních elektráren. Práce se zaměřuje především na tzv. H-mód (mód vysokého udržení) a související úkazy jako např. limitní cyklové oscilace, studovány především v tokamaku COMPASS. Výzkum těchto oscilací vedl k vytvoření a zobecnění modelu založeného na prvních principech, který systematicky předpovídá pozorované frekvence tlakem uvolněných limitních cyklových oscilací v blízkosti přechodu do H-módu ve 4 tokamacích (JET, ASDEX Upgrade, COMPASS, Globus-M).Enhanced confinement is a key ingredient for successfully operating future fusion power plants. The main focus of the thesis is the H-mode (high confinement mode) and associated phenomena such as limit cycle oscillations, primarily studied in the COMPASS tokamak. Studies of of these oscillations led to the development and generalization of a first-principles-based model which systematically predicts the observed frequency of pressure relaxation limit cycle oscillations in the vicinity of the transition to H-mode in 4 tokamaks (JET, ASDEX Upgrade, COMPASS, Globus-M)

Overview of the COMPASS results

International audienc

Overview of the COMPASS results

COMPASS addressed several physical processes that may explain the behaviour of important phenomena. This paper presents results related to the main fields of COMPASS research obtained in the recent two years, including studies of turbulence, L–H transition, plasma material interaction, runaway electron, and disruption physics: • Tomographic reconstruction of the edge/SOL turbulence observed by a fast visible camera allowed to visualize turbulent structures without perturbing the plasma. • Dependence of the power threshold on the X-point height was studied and related role of radial electric field in the edge/SOL plasma was identified. • The effect of high-field-side error fields on the L–H transition was investigated in order to assess the influence of the central solenoid misalignment and the possibility to compensate these error fields by low-field-side coils. • Results of fast measurements of electron temperature during ELMs show the ELM peak values at the divertor are around 80% of the initial temperature at the pedestal. • Liquid metals were used for the first time as plasma facing material in ELMy H-mode in the tokamak divertor. Good power handling capability was observed for heat fluxes up to 12 MW m−2 and no direct droplet ejection was observed. • Partial detachment regime was achieved by impurity seeding in the divertor. The evolution of the heat flux footprint at the outer target was studied. • Runaway electrons were studied using new unique systems—impact calorimetry, carbon pellet injection technique, wide variety of magnetic perturbations. Radial feedback control was imposed on the beam. • Forces during plasma disruptions were monitored by a number of new diagnostics for vacuum vessel (VV) motion in order to contribute to the scaling laws of sideways disruption forces for ITER. • Current flows towards the divertor tiles, incl. possible short-circuiting through PFCs, were investigated during the VDE experiments. The results support ATEC model and improve understanding of disruption loads