The propagation of the fast magnetosonic wave in a tokamakplasma has been investigated at low power, between 10 and 300 watts,as a prelude to future heating experiments.
The attention of the experiments has been focused on the understandingof the coupling between a loop antenna and a plasma-filledcavity. Special emphasis has been given to the measurement of the complexloading impedance of the plasma. The importance of this measurementis that once the complex loading impedance of the plasma is known,a matching network can be designed so that the r.f. generator impedancecan be matched to one of the cavity modes, thus delivering maximumpower to the plasma. For future heating experiments it will be essentialto be able to match the generator impedance to a cavity mode inorder to couple the r.f. energy efficiently to the plasma.
As a consequence of the complex impedance measurements, it wasdiscovered that the designs of the transmitting antenna and the impedancematching network are both crucial. The losses in the antenna andthe matching network must be kept below the plasma loading in order tobe able to detect the complex plasma loading impedance. This is evenmore important in future heating experiments, because the fundamentalbasis for efficient heating before any other consideration is to delivermore energy into the plasma than is dissipated in the antenna system.
The characteristics of the magnetosonic cavity modes are confirmedby three different methods. First, the cavity modes are observedas voltage maxima at the output of a six-turn receiving probe.Second, they also appear as maxima in the input resistance of the transmittingantenna. Finally, when the real and imaginary parts of themeasured complex input impedance of the antenna are plotted in thecomplex impedance plane, the resulting curves are approximately circles,indicating a resonance phenomenon.
The observed plasma loading resistances at the various cavitymodes are as high as 3 to 4 times the basic antenna resistance (~ .4 Ω).The estimated cavity Q’s were between 400 and 700. This means thatefficient energy coupling into the tokamak and low losses in the antennasystem are possible.
