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We have obtained the first measurements of n e and T e in the DIII-D divertor region with a multi-pulse (20 Hz) divertor Thomson scattering (DTS) system. Eight measurement locations are distributed vertically up to 21 cm above the divertor plate. Two-dimensional distributions have been obtained by sweeping the divertor plasma across the DTS measurement location. Several operating modes have been studied, including Ohmic, L-mode, ELMing H-mode, and radiative divertor operation with puffing of D 2 and impurities. Mapping of the data to either the ( L pol , φ ) or ( R, Z ) planes with the EFIT equilibrium is used to analyze the 2D profiles. We find that in ELMing H-mode: n e , T e , and P e are relatively constant along field lines from the X-point to the divertor plate, especially near the separatrix field line. With D 2 puffing, the DTS profiles indicate that T e in a large part of divertor region below the X-point is dramatically reduced from ∼ 30-40 eV in ELMing H-mode to 1-2 eV. This results in a fairly uniform low-T e divertor, with an increased electron density in the range of 2 to 4 × 10 m . Detailed comparisons of the spatial profiles of n e , T e , and electron pressure P e , are presented for several operating modes. In addition, these data are compared with initial calculations from the UEDGE fluid code.

Neutral-beam-heated expanded-boundary (XB) divertor discharges have been obtained in Doublet III with high heating efficiency for wide ranges of plasma parameters (Ip: 300-800 kA; BT: 8-24kG; e:(2-10) × 1013cm?3; Pb< 4.5 MW, ?p? 1.6, ?t? 2.8%). The improved heating efficiency is well correlated with a configurational change from limiter discharges to XB discharges. The beam-heated, fully diverted expanded-boundary discharges with a limiter/separatrix distance greater than 1.5 cm exhibit an improvement of up to a factor of two in energy confinement time. The ?Eincreases approximately linearly with Ip, but is insensitive to variations of a factor of two in eand BT. Over the inner two-thirds of the plasma radius (r/a ? 0.7), the shape of the Teprofile for XB discharges is similar to that for limiter discharges. Hence, the improvement of the global energy confinement is consistent with a reduction of thermal conductivity over most of the plasma radius. With 2 MW of neutral-beam injection into a high-current (Ip= 750 kA) XB discharge, energy confinement times ?E? 115 ms and e?E? 1 × 1013cm?3· s?1have been obtained. At high beam power (Pb> 3 MW), a mild deterioration of the energy confinement time has been observed.

Results are presented for multiple deuterium pellet injection into both the Ohmic phase and the beam heated phase of Doublet III discharges. Pellet injection experiments were carried out using an injector, built at Oak Ridge National Laboratory, which has the capability of firing up to 40 hydrogen or deuterium pellets per second at a speed of 800 m·s?1. For the deuterium pellets used in this experiment, one pellet raises the line average density to 1.3 × 1013cm?3. In the Ohmic phase, the pellets penetrate deep into the plasma, but with 2.2 MW of neutral beam heating they penetrate only half as deep. This difference is mostly due to the increased ablation in the outer 25% of the plasma during beam heating. Ablation calculations using the neutral shielding model agree well with experimental results during Ohmic heating, but they predict a shallower pellet penetration than is actually observed during neutral beam heating. About 90-100% of the pellet mass can be accounted for after injection. Electron temperature profile relaxation accompanies pellet injection, but on a time-scale that is shorter than the pellet lifetime. A transient increase in the energy confinement time ?Eis possible, provided pellet penetration is sufficient to sustain a peaked density profile. In discharges with one or more multifractured pellets, which ablate rapidly at the plasma edge and thus broaden the density profile, no increase in ?Eis seen. These results indicate that continuous ?Eimprovement may be possible if a peaked density profile can be maintained by sustained deep pellet penetration.

γ-ray burst spectra have often been fit by optically thin thermal bremsstrahlung. However, at the high temperatures implied by such fits (kT∼300 keV), the free–free cross-section is so much smaller than the Compton cross-section that Compton scattering might dominate the spectral formation processes. We have investigated the possibility that emission mechanisms based on Compton scattering can also fit the data. In particular, Monte Carlo calculations have been used to compare the γ-ray burst spectral data with black-body spectra which have undergone inverse comptonization by a much hotter, overlying plasma. The best-fit parameters suggest that the underlying black body is an X-ray source (kTBB∼2.4 keV) while the overlying, comptonizing plasma has kTe∼150 keV and a column density of 4 × 1024electrons cm−2. Such a model would also naturally explain some of the unusual γ-ray burst spectra. In particular, a low energy (∼400 keV) cutoff in the spectrum of GB781119 can be explained as a Wien peak produced by a higher column density in the comptonizing region, and two-component spectra (for example, GB790329) can be explained by a lower column density.