We present a uniquely fast (10 μs) ion temperature measurements in the tokamak edge plasma. Our approach is based on the sweeping of a ball-pen probe, where the ion temperature is obtained by fitting the electron branch of the corresponding I–V characteristic. We have performed measurements on the COMPASS tokamak during L-mode discharge. The temperature histograms reveal a non-Gaussian shape with a high-temperature tail peaking at low values. The fitted values of fast I–V measurements can be used to reconstruct (emulate) the slow swept I–V characteristic of a retarding field analyzer. The resulting ion temperature profile is nearly flat and provides a ratio of ion to electron temperature close to 1–2 in the vicinity of the last closed flux surface during L-mode discharges, as observed on other tokamaks.

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.

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This paper presents measurements of the plasma turbulence with an emphasis on the ion temperature fluctuations in comparison to the electron temperature and the ion saturation current fluctuations in the edge of the COMPASS tokamak. The ion temperature values are obtained by using a fast swept ball-pen probe, with temporal resolution up to 10 µs, accompanied by independent 1 µs measurements of the electron temperature and ion saturation current. The experimental results obtained for a wide range of line-averaged densities show that statistical properties of the fluctuations are in good agreement with a Beta or Gamma distribution function. In addition, the dependence of the averaged ion and electron temperatures on the line-averaged density is studied in the near and main Scrape Off Layer region.