A Far Infrared Spectroscopic System Used in Fusion Plasma Diagnostics

A far infrared spectroscopic system, covering the wavelength region between 50 μm and capable of variable spectral resolution between 100 and 1000, has been built. The spectroscopic system is described in some detail. It is comprised of a monochromator, which utilizes five different echelette gratings, each of which covers one octave, of a radiometric system for absolute power calibration, and of various cryogenic and room temperature detectors. The system works in two modes: a continuous mode using the absolute power calibration, the radiometer, and continuous standard radiation sources, and a pulsed mode without the radiometer for plasma radiation studies. In this manner the power spectrum of the observed pulsed plasma radiation is absolutely calibrated. A rather new method in monochromator design utilizing ray transfer matrix method and phase space diagrams is discussed, and various optical errors in the system are evaluated. The method of performing and the evaluation of radiation studies utilizing this system is presented. The system will be used in the very near future to measure the far infrared radiation from several of our fusion experiments: from Texas Tokamak, a large experimental fusion device; from a small, adiabatically heated, symmetric, hot electron stellarator; and from some other hot plasma devices. These experiments have strong magnetic fields, ranging between 30 to 5OkG. Moreover, high electron temperatures--between 1 to 20 keV--are expected, so that the synchrotron radiation in the far infrared will be appreciable. Synchrotron radiation from Tokamak as a function of the electron temperature and density is discussed.