Development and operation of a thin foil Faraday collector as a lost ion diagnostic for high yield d-t tokamak fusion plasmas

We are continuing our development of a radiation-hard, charged-particle detector consisting of a series of thin parallel conducting foils as a lost ion diagnostic for high yield d-t tokamak fusion plasmas. Advantages of this detector concept include economy, ability to operate in relatively intense neutron/gamma ray radiation backgrounds and at moderately high temperatures, and a modest degree of energy resolution. A detector consisting of four parallel foils of Ni, each of thickness 2.5 μm, was operated in the Joint European Torus during the recent DTE-1 experiment. During the highest yield pulses of this campaign, (16 MW), the flux of energetic alpha particles at the detector was measured to be less than about 2 nA/cm². This upper limit is significantly greater than the expected flux assuming classical losses and given the geometry of the detector. During most of the nearly 2500 pulses of the DTE-1 experiment for which the detector response has been inspected, a relatively intense (up to 200 nA/cm²) flux of low energy positively charged particles was observed which appears related to the D-α photon flux at the plasma edge. Similar detector designs have been recently evaluated using monoenergetic helium ion beams from the tandem accelerator at Sandia National Laboratories. One such detector, consisting of six foils of 6 μm thick Al demonstrated an energy resolution of about 7% for 7 MeV alpha particles. Possible improvements to this detector concept include the fabrication of an "integrated circuit" like design consisting of alternately deposited layers of insulator and conductor. Future applications of the detector concept include first wall lost ion diagnostics for the ignition device to test engineering concepts and NSTX.