Microwave reflectometry: Observing density layer motion in the Microwave Tokamak Experiment.

Abstract

The Microwave Tokamak Experiment (MTX) at Lawrence Livermore National Laboratory investigated new means of electron cyclotron heating of high magnetic field, high density plasmas. The JAERI (Japan Atomic Energy Research Institute) microwave reflectometer is one of many diagnostics installed to study plasma parameters in MTX. It operates in O-mode with a variable frequency range of 75-110 GHz. This frequency range provides for density measurements at cutoff layers between 0.7 and 1.5 $\times$ 10$\sp{20}$ m$\sp{-3}.$ We use an optical relay system to transport microwaves from the transceiver located in a low magnetic field near the tokamak and to provide beam focusing through the narrow MTX ports. The reflectometer is used in MTX to study density perturbations. Results which are displayed in this work illustrate the reflectometer's sensitivity to the cutoff density layer. The reflectometry data shown track plasma motion, compare well to other established diagnostics, and appear to have the advantage of being very sensitive. The reflectometer measurements in this work identify bursts and plasma disruption. The reflectometer appears to be extremely sensitive to bursts and density oscillations in the edge region of the plasma. It also supersedes the Far-Infrared (FIR) interferometer in observing the termination process of the discharge. In general, this work is intended to show that the reflectometer is a promising diagnostic for future tokamaks.