ENERGY MEASUREMENT OF FAST IONS TRAPPED IN THE TOROIDAL FIELD RIPPLE OF TORE-SUPRA

During additional heating in Tore Supra [ion cyclotron resonance frequency (ICRF) or neutral beam injection], fast ion losses due to the toroidal field ripple were clearly measured by a set of graphite probes. This detector collects the flow of fast ions entering a vertical port and usually shows a maximum flux for ions originating from the vicinity of surface delta* = 0. During the monster sawteeth regime, achieved with ICRF, a remarkable phenomenon was observed: an ejection of fast ions that were not correlated with any measured magnetohydrodynamic activity. The radial distribution of these ions was quite different from the distribution usually observed exhibiting a peak located in the central section of the plasma. A new diagnostic is being constructed for measurement of the energy distribution of these ions, from 80 keV (energy of the neutral beam injected in Tore Supra) up to 1 MeV (expected during ICRF). The principle of the diagnostic is the identification of the ions through their energy by using their Larmor radius (rho = 1.3 cm for 100 keV --> rho = 3. 6 cm for 700 keV B = 4 T). The detector is made of a hollow graphite cylinder with a small entrance slot, located in a vertical port on the ion drift side. An array of six metallic collectors placed inside the graphite cylinder intercepts the ions. The current on each collector was estimated at 10 --> 100 nA, during ICRF heating. The energy resolution of this diagnostic is expected to be similar to 20 keV for the lowest energy range and 100 keV for the highest energy range. This type of elementary defector might be extrapolated for the measurements of alpha-particle losses in future deuterium-tritium experiments. It should also be suitable for studies of stochastic ripple diffusion.