Recent developments in millimeter wave diagnostics at JET (invited) (abstract)

Abstract

There are presently two substantial millimeter wave diagnostic systems on JET in different stages of development: an established, comprehensive electron cyclotron emission (ECE) system and a recently installed multichannel reflectometer. The ECE system comprises a ten-channel antenna array in the vacuum vessel, an oversized waveguide transmission system, and an assembly of ECE instrumentation—Michelson and Fabry Perot interferometers, grating polychromator, and heterodyne radiometer. From measurements made with the Michelson interferometers the time and spatial dependencies of the electron temperature in the poloidal cross section are determined with resolutions of ∼15 cm and 15 ms, respectively. From measurements made with the other instruments, the time dependence of the temperature at specific locations is determined with resolutions ≳2 cm and ≳100 μs. The sensitivity is such that changes in temperature down to approximately a few eV can be clearly resolved. The multichannel reflectometer will probe the plasma in the ordinary mode at 12 frequencies in the range 18–80 GHz. It will have two modes of operation: narrow-band swept and fixed frequency. In the swept mode the spatial profile of the electron density will be measured with a time resolution ≳200 μs and a spatial resolution ∼1 cm, while in the fixed frequency mode of operation movements in the different density layers will be measured with a sensitivity approximately a few mm and bandwidth ≳100 kHz. In addition, a possible collective Thomson scattering diagnostic for measuring the velocity distribution of the confined α particles in the DT phase of JET operation is currently being studied. This system would use a high-power millimeter wave (140 GHz) gyrotron and heterodyne detection.