Energy Distribution Measurements of Fast Particles Generated in Front of the LH Grill Mouth in Tore Supra

A retarding field analyzer (RFA) was used during lower hybrid (LH) current drive experiments in the Tore Supra tokamak to characterize the supra-thermal particles emanating from the region in front of the LH grill. This work is the continuation of our previous efforts (1,2). In addition to fast electrons, we tried to observe fast ions accelerated due to a positive charge accumulated (and measured in the CASTOR tokamak (3)) in front of the LH grill. In the RFA, the plasma particles pass through a slit and two grids to a grounded collector. The slit (30 µm by 5 mm section, cut in a 150 µm nickel plate) is biased negatively (-50 to -100 V) to repel thermal electrons. All ions, as well as electrons with energy greater than the slit voltage, pass through the slit. Both grids are biasable to -1000V, allowing us to scan the particles according to their energy, and separate the ions from the electrons (if their energies are less than 1000 eV). The RFA collects particles that flow along field lines from the outboard side of the tokamak. The measurements were performed when one of the wave-guide rows of the C3 launcher was magnetically connected to the RFA. First, in shot #35613, the top C3 waveguide row was connected to the RFA. We found that -400V applied potential was not large enough to repel all the fast electrons for the launched C3 power of PLH=2 MW. The upper energy boundary of the fast electron beam is therefore higher than 400 eV for the 2 MW launched, which is consistent with modeling (4). For the same magnetic connection, we found in shot #35616 that even +300V is not enough to repel all ions. However, the analysis of the ion current shows no traces of fast ion beams in this shot, and the observed ion current might be produced by the thermal ion acceleration in the near slit sheath voltage. We decided to try operating at lower power levels in order to be able to fully repel even the suprathermal electrons and try to identify any suprathermal ions. In the shot