Radiation from high-temperature collision-dominated plasmas

Abstract

Almost completely ionized hydrogen and helium plasmas with electron densities ranging from 1017 to 1019 cm-3 are produced in electromagnetic T-tubes. The shock-heated plasmas attain local thermal equilibrium in times shorter than cooling times and act therefore as useful spectroscopic light sources. Since it is necessary for many investigations to build the entrance slit of the vacuum monochromator spectrograph directly into the shock tube wall to minimize absorption, a rugged detector was developed in which only the photosensitive surface is in the differentially pumped volume. Photoelectrons are then accelerated to 20 or 30 kV and finally detected by a standard plastic scintillator-photomultiplier combination. This method alleviates normal deterioration of dynodes in open photomultiplier tubes and scattered light problems encountered with usual scintillator-photomultiplier combinations.Three vacuum u.v.-experiments with T-tubes are in progress. First an attempt is being made to measure the Stark profiles of Ly-α and Ly-β at high electron densities where Doppler broadening is only a small correction. Since Ly-α is mainly broadened by electron impacts and Ly-β by the quasi-static action of ions, it would be possible to separate the two broadening mechanisms experimentally, thus providing a more direct check on the theory than by the measurement of Balmer line profiles.In another experiment the oscillator strength of the neutral helium resonance line is being measured. A 10 per cent determination might be possible so that a comparison of the measured value with calculated oscillator strengths would help to decide which of the various approximations employed is the more appropriate. Again the answer to this theoretical question will be clearer than that obtainable from measurements of visible line intensities since the determination of the ground state wave function is most sensitive to the approximation scheme.In the third experiment the radiation from a high pressure T-tube is utilized for absolute intensity calibration in the vacuum u.v.-region. This is possible because for collision-dominated hydrogen and helium plasmas the emitted spectrum can be calculated from temperature and electron density, which are obtained from measured line and continuum intensities in the visible.