Particle fluxes and impurities in EXTRAP T1 from surface probes and spectroscopy

Abstract

EXTRAP T1 is a toroidal device which can be operated as a reversed field pinch, or with an externally applied octupole field as an external ring trap. The device has a stainless steel vessel and has been operated with hydrogen, helium and deuterium plasmas. In the present paper, a series of similar RFP discharges in deuterium have been studied, using surface probes and visible, H α and VUV spectroscopy. Data from H α spectroscopy and surface probes lead to the conclusion that the particle confinement time was τ p ≈ 20 μs and that the total flux density of particles to the wall was ∼2×10 19 D cm −2 s −1 . Moreover, it can be inferred that the flux to the wall consists predominantly of neutrals, with energy 30 eV or less, but includes a high energy component of mainly ions, with energy distribution corresponding to kT i ≈100 eV. The deposition rate of stainless steel components on probes at the wall position is ∼0.5% of the deuterium flux, and is consistent with uniform physical sputtering. The conditions during RFP operation with clean walls are compared with other modes of operation and with operation with contaminated surfaces.