Broadening of the parallel and perpendicular ion energy spectrum and correlation with turbulent potential fluctuations in a linear magnetized plasma

Abstract

We previously reported electrostatic energy analyzer (GEA) measurements of the parallel energy distribution of ions incident on a floating target in PISCES-A [J. Cuthbertson, R. Lehmer, M. Jakubowski, S.C. Luckhardt, Bull. Am. Phys. Soc. 41 (1996) 1575]. The principal finding was that ion energy spectrum was significantly broader (6–8 eV) than the ion temperature expected from classical collision processes ( T i ≅ 2–3 eV). In this paper, we present further gridded energy analyzer measurements, and Doppler broadening measurements of ion spectral lines. Moreover, we have varied plasma conditions such that the turbulent plasma potential fluctuation amplitudes covered the range 3 eV < 2 δφ < 16 eV. Interestingly, during this variation, the width of the measured ion energy distributions varies over a corresponding range of 7 eV < Δ E i < 28 eV; that is, the parallel ion temperature is comparable to our previous measurements of the parallel energy spread. These experiments are modeled using a 3D, time-dependent Fokker–Planck code [K. Kupfer, R.W. Harvey, O. Sauter, G. Staebler, M.J. Schaffer, Phys. Plasmas 3 (1996) 3644] for the ions, giving Coulomb collision effects parallel and perpendicular to B, and spatial variation parallel to the magnetic field. We suggest that the observed spectral broadening is consistent with rapid parallel and perpendicular fluctuations in the ion flow speed caused by turbulent plasma potential fluctuations and associated E × B velocity fluctuations. Direct ion heating by these potential fluctuations may also be of importance.