Thin foil Faraday collectors as a radiation hard fast lost-ion diagnostic

We are investigating thin foil Faraday collectors as a diagnostic for lost fast ions from tokamak fusion plasmas. Prototype devices have been recently installed in the National Spherical Torus Experiment and DIII-D. Initial results from these devices indicate a loss of energetic ions from a variety of plasma conditions. Results from a device installed immediately outside a thin Be window on ALCATOR C-mod, as a test on the response to moderately intense fluxes of soft x rays indicate an upper limit of about 2×10−22 A/photon/cm2 at a plasma electron temperature of 1.8 keV. An important property of the diagnostic is the expected ability to operate under fairly high neutron/gamma radiation backgrounds. We have tested this expectation by measuring the current from a thin (2.5 μm) Ni foil placed in the core of a TRIGA fission reactor. At a maximum steady-state power of 950 kW (1013 n/cm2/s), a current of 1.2 nA/cm2 was measured.