Active Spectroscopic Diagnostics for ITER Utilizing Neutral Beams

Abstract

The use of neutral beams, combined with visible spectroscopy, is widely applied to the diagnosis of tokamak plasmas. Local plasma parameters which have been measured with these techniques include low Z impurity densities (e. g. Isler, 1977; Boileau, et al., 1989; von Hellerman, et al., 1992) and impurity transport coefficients (e. g. Synakowski, et al., 1990), ion temperature and rotation (e. g. Fonck, et al., 1984), internal magnetic field, (and thus current density and safety factor (Levinton, et al., 1989; Wrobleski and Lao, 1992; Wolf, et al., 1993)), and electron density fluctuations (eg. Durst, et al., 1992). Very recently, the first measurements of the energy distribution of slowing-down D-T fusion α particles have been reported (McKee, et al., 1995). An examination of the proposed measurement requirements for ITER (ITER 1995), as developed by the ITER JCT in consultation with the ITER Diagnostic Experts Group and diagnosticians from around the world, reveals several obvious candidates for application of diagnostic neutral beams (DNB’s). Of these, the one for which there appears to be no alternative whatsoever, is measurement of the thermalized helium density profile in the confinement region. Since helium ash accumulation is one of the key physics issues to be tested in long pulse (> 1000 second) operation, the inability to measure this quantity accurately in real time would be a significant handicap and could conceivably compromise some of the main ITER missions. A diagnostic beam, combined with charge exchange recombination spectroscopy (CXRS) will be able to provide this measurement.