Experiments on drift wave launching: II. The launching process

Abstract

We present a theoretical and experimental study of the launching impedance Z L , which describes the effectiveness of drift wave launching into the plasma in the quadrupole GOLUX. The theory, valid only for damped modes, incorporates the plasma response function (the inverse of the dielectric function) and measurements of the impedance can, therefore, lead to an experimental test of theoretical forms for this function. Using a lock-in amplifier technique, Z L has been measured for four different branches of the drift wave dispersion curve, two expected to be linearly stable and damped and two believed to be linearly unstable but in practice apparently damped. For the stable modes theory and experiment agree within a factor of two, as good as can be expected given experimental error and approximations in the theory. For the unstable mode, a well defined value of Z L is obtained, but it exceeds the theoretical estimate by two orders of magnitude. Using a different technique we have been able to detect growth of the unstable mode after launching, provided the launched amplitude is sufficiently small; this explains the failure of the theory. It appears, however, that in most cases the wave launched into the plasma is already saturated and does not grow further. We propose that the launched wave amplitude is limited by the power available from the external circuit, and on this basis we have derived an upper limit for the impedance.

Experiments on drift wave launching: I. Dispersion curves and damping rates

“…We, therefore, accept an ion mass of 2. For the ion temperature, Carter et al (1981) used an estimate of about 0.17 eV, obtained by Nossair (1980) using the Katsumata probe technique; however, Qader (1993) has used a technique based on the comparison of cylindrical and spherical probes to obtain a lower value of 0.07 eV, and gives reasons for thinking that this is a more reliable technique in the region of interest. He has also shown that the lower value is more nearly consistent with the ion energy balance, on the assumption that the ions are heated only by the energy delivered to them by ion Landau damping of the drift wave spectrum, as suggested by Crossley et al (1992).…”

Non-local theory for trapped electron modes in a magnetic quadrupole

“…When separability ceases to be a valid approximation at high k's, the electrodes, used in recent wave launching experiments to excite drift waves at smaller k's [17], are not necessarily well matched to the potential wave form. Prof. M. G. Rusbridge has pointed out that this may explain the collapse of the launching impedance in the large k end of the spectrum.…”