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 δ* = 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 (ρ = 1.3 cm for 100 keV → ρ = 3.6 cm for 700 keV, B = 4T). 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 ∼20 keV for the lowest energy range and 100 keV for the highest energy range. This type of elementary detector 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.