Overview of the COMPASS results ^*

Abstract

COMPASS addressed several physical processes that may explain the behaviour of important phenomena. This paper presents results related to 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 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.

Estimation of wall forces solely from magnetic measurements: an application to RFX-mod experiment

“…In tokamaks, the wall force due to disruptions may lead to a global horizontal displacement of the whole torus as reported in JET experiment [1]. Such wall force is called sideways force and it is related to asymmetric magnetic perturbations due to the symmetry-breaking plasma deformations [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] or wall non-uniformity along the toroidal direction (asymmetric wall) [21,22]. In a Cartesian system of coordinates-where the z axis corresponds to the torus axis-sideways force corresponds to F x and F y components of the integral wall force while the remaining F z component is the vertical force.…”

On the applicability of three and four parameter fits for analysis of swept embedded Langmuir probes in magnetised plasma

“…In this work we extend the modelling study on flush mounted probes [7] to the case of protruding probes which are employed in the divertor of the COMPASS tokamak. The divertor of the COMPASS tokamak [8] has been equipped with a combined probe array, which comprises 55 probe triplets, each triplet consisting of two LPs and one ballpen probe [9]. Under normal operating conditions, one of the LPs in each triplet is biased to constant large negative voltage (−270 V) in order to measure the ion saturation current I sat .…”

Numerical modelling for beam duct heat loads calculations and application to the new 1 MW neutral beam injector in the COMPASS tokamak

“…We present the workflow of simulation codes that was developed to predict hot-spot position and the heat flux onto the beam-duct. During the last COMPASS tokamak (major radius R 0 = 0.56 m, vessel midplane minor radius a = 0.23 m, toroidal field B t = 0.9-2.1 T [3]) experimental campaign, a new NBI system was installed. Injection energies of up to 68 keV were achieved at the nominal power of 1 MW.…”