Atomic Beam Probe diagnostic for plasma edge current measurements at COMPASS

摘要

Introduction The measurement of the plasma edge current density distribution and temporal evolution during the edge localized mode (ELM) cycle is of particular interest in the field of magnetically confinement plasmas, since theoretical models recognize it as a key element for the trigger mechanism of the ELMs. The atomic beam probe (ABP [1]) is an extension of the beam emission spectroscopy (BES) diagnostic [2]. The beam atoms are ionized due to collisions with plasma particles, deflected through a curved path due to the magnetic field and may be detected close to the wall of the machine. The arrival location and the number of ions carry information about the toroidal plasma current distribution, the density profile and the electric potential in the plasma. Figure 1 shows the ion trajectories for a 100 keV Li atomic beam injection. The detector surface, marked with a blue line, has to be placed inside the vessel close to the confined plasma region thus being exposed to high UV radiation and particle impact. Detecting the few microampere ion current close to the plasma edge requires a special detector. The measurements with a preliminary test detector head have been carried out, and a final detector head design was proposed based on these results [3]. The final detector head for the ABP was tested in the lab. The new setup utilizes a shallow Faraday cup matrix, produced with printed-circuit board technology, and a double mask for a secondary electron suppression. Results of the switching time, cross talk and fluctuation sensitivity test in the lab setup will be presented along with the first measurement in a new setup at the COMPASS tokamak [3].