R&D

Abstract

In recent years there has been an increasing interest in the production of energetic plasmas at temperatures and densities relevant to the study of controlled thermonuclear reactions. Experiments of this type at AWRE Aldermaston use rapidly rising axial magnetic fields to ionize and compress deuterium gas to particle densities in excess of 1017 cm-3 and temperatures approaching 107 °K. A megajoule condenser bank under construction will increase these densities and temperatures, and extend the duration of the discharge to tens of μsec. Such a plasma provides a controlled laboratory source of electromagnetic radiation whose energy peaks in the vacuum ultraviolet and soft X-ray spectral regions. Production of such a plasma stimulates the study of radiative processes in this energy range, and requires the development of vacuum ultraviolet and X-ray instruments for measuring its properties.This paper reviews the characteristics of radiation emitted by hydrogenic plasma at a particle density of 1017 cm-3 and temperature of 107 °K containing a small admixture of oxygen and permeated with an axial magnetic field of some 50 kG. The distribution of the ionic species in steady state conditions is discussed and the contribution of free-free, free-bound and bound-bound transitions to the radiation spectrum is evaluated. Phenomena such as Stark broadening, Doppler shifts, and Zeeman splitting are considered as physical processes which can be used to diagnose the state of the plasma.The final section of the paper describes the experimental programme of vacuum ultraviolet radiation measurements in progress at AWRE. Soft X-ray pinhole camera photographs have been used to establish the dimensions of the emitting plasma and absorption measurements using aluminium, beryllium, carbon and nickel foils have provided estimates of the electron temperature from the short wavelength spectral cut-off. Several new instruments constructed at the establishment will shortly be available for studying the vacuum spectrum of the plasma. These include: a versatile X-ray crystal spectrometer; a grazing incidence spectrograph available with photoelectric recording; and a photographic instrument which uses a concave grating at near normal incidence and which is time-resolved by reflection at 70° from a high speed rotor. Preliminary results obtained with this latter instrument are briefly discussed.