Experiments on drift wave launching: II. The launching process

We present a theoretical and experimental study of the launching impedance , 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, 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 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.