Fast ion ripple losses during neutral injection in TFR

Abstract

Fast ion ripple losses during neutral beam injection in TFR are measured near-quantitatively with an array of Faraday collectors mounted between two toroidal field coils. The collected currents show multiple evidence for fast ion ripple losses and correspond to global losses of 30-40% of the injected current. The measurements are complemented by a simple Fokker-Planck model, itself checked against Monte Carlo simulations on several cases. These calculations clarify many experimental features but predict only about half of the observed ripple lost current. This discrepancy could be due in part to various experimental and theoretical uncertainties although there are indications of some enhanced ripple losses in the experiments. Such losses could arise from other processes than toroidal field ripple not included in the calculations and may suggest, along with other diagnostics, an enhanced depletion of central fast ions, the origin of which remains unclear. Fast ion ripple losses are calculated and observed to be fairly insensitive to factors of two variations in most plasma parameters. Substantial losses are predicted only when injecting near perpendicularly into a tokamak plasma of edge ripple ≥1%. Fast ion ripple losses in several devices are considered and calculations for TORE SUPRA are presented.