Study of impurities in the Tandem Mirror Experiment using extreme-ultraviolet spectroscopy

Abstract

Impurities in the Tandem Mirror Experiment (TMX) have been studied using extreme ultraviolet spectroscopy. Three time-resolving absolutely-calibrated normal-incidence monochromators, one on each section of TMX, were used to study the impurity emissions in the wavelength range of 300 A -1600 A. The instruments on the east end cell and central cell were each capable of obtaining spatially-resolved profiles from 22 chords of the plasma simultaneously while the instrument on the west end cell monitored the central chord. The impurities identified in TMX were carbon, nitrogen, oxygen, and titanium. Emphasis was placed upon determining the impurity densities and radiated power losses of the central cell; results indicate that the impurity concentrations were low-less than 0.4% for each species-and that less than 10% of the total net trapped neutral beam power was lost to radiation. The use of titanium gettering on the central cell walls was observed to decrease the brightnesses of singly-and doubly-ionized carbon and oxygen in the central cell plasma. In the end cells, oxygen was the main impurity with a concentration of about 1.5% and was injected by the neutral beams; the other impurities had concentrations of about 0.5%. Radiated losses from the end cells were negligible.Evidence has been obtained which indicates that impurities in the central cell are subject to a radially outward transport. The density ratios of 0 V to 0 VI indicate a short (0.3-3 ms) central cell confinement time for 0 VI ions, even though calculations predict that axial confinement times should be longer than the TMX shot duration. Impurity puffing experiments performed in the central cell of TMX support this conclusion. Oxygen and neon puffed into the central cell during the TMX shot were observed to accumulate in the outer boundary of the plasma, while only a small fraction penetrated into the central regions. None of the injected impurity was observed in the end cell plasmas; instrumental sensitivity puts an upper limit of 0.03 on the ratio of injected impurity density in the end cells to that in the central cell. Oxygen puffed before the TMX shot was observed to decay from all regions of the plasma during the shot. These results provide an explanation for the observed low concentrations of impurities in the central cell of TMX.

Experimental evidence for outward radial transport of impurities from the central cell of TMX

“…In the central cell of TMX, the condition of the vacuum chamber walls was important in controlling impurity levels in the central-cell plasma [5,6]. Previous experiments showed that the brightnesses of carbon and oxygen emissions decreased as a result of titanium gettering.…”

“…(The ambient level of each oxygen ionization state was determined during an identical shot without injecting oxygen, and was substracted.) Most of the injected oxygen remains near the plasma edge, while only a small fraction penetrates inward; the penetration rate is approximately 3 cm-ms" 1 [6]. Examining a radial profile near the end of the shot (Fig.…”

“…Further evidence that impurity ions are transported radially outward is provided by comparing the observed radial brightness profiles of the impurities with calculated profiles. These calculated profiles were obtained with a confinement-modified coronalequilibrium model [6] that solves a set of timeindependent simultaneous equations similar to Eq. ( 1).…”

“…This paper describes the measurements of the sources and confinement times of impurities in the central cell of TMX. The spectroscopic instruments have been described in detail elsewhere [5,6]. Briefly, two monochromators were used to measure time-resolved spatial profiles of the impurity emissions from the central-cell and east-end-cell plasmas.…”

“…Briefly, two monochromators were used to measure time-resolved spatial profiles of the impurity emissions from the central-cell and east-end-cell plasmas. These instruments were equipped with spatial imaging detector systems (SIDS) [6,7] that each measured 22 chords of the plasma simultaneously. A third monochromator [8] was used to measure the impurity emissions as a function of time from the central chord of the westend cell.…”