Showing papers by "R.D. Stambaugh published in 1989"

Confinement physics of H-mode discharges in DIII-D

Abstract: The authors' data indicate that the L-mode to H-mode transition in the DIII-D tokamak is associated with the sudden reduction in anomalous, fluctuation-connected transport across the outer midplane of the plasma. In addition to the reduction in edge density and magnetic fluctuations observed at the transition, the edge radial electric field becomes more negative after the transition. They have determined the scaling of the H-mode power threshold with various plasma parameters; the roughly linear increase with plasma density and toroidal field are particularly significant. Control of the ELM frequency and duration by adjusting neutral beam input power has allowed us to produce H-mode plasmas with constant impurity levels and durations up to 5 s. Energy confinement time in ohmic H-mode plasmas and in deuterium H-mode plasmas with deuterium beam injection can exceed saturated ohmic confinement times by at least a factor of two. Energy confinement times above 0.3 s have been achieved in these beam-heated plasmas with plasma currents in the range of 2.0 to 2.5 MA. Local transport studies have shown that electron and ion thermal diffusivities and angular momentum diffusivity are comparable in magnitude and all decrease with increasing plasma current.

Energy confinement properties of H-mode discharges in the DIII-D tokamak

Abstract: Neutral beam heated DIII-D expanded boundary divertor discharges have exhibited ASDEX-like H-mode behaviour over a wide parameter range. The deuterium H-mode energy confinement of 120 ms remained near the Ohmic value for up to 6 MW of neutral beam heating, where it was 2-2.5 times higher than the L-mode value at a plasma current of 1 MA. The hydrogen and helium H-mode energy confinement times were similar and substantially below the deuterium H-mode confinement time. The H-mode confinement times decreased with increasing neutral beam power and were only 30% better than the L-mode confinement times at 5 MW. In an H-mode with a mixture of hydrogen and deuterium ([H]/[H+D] 40%), the confinement time was in between the values obtained in the pure hydrogen and deuterium cases, increased linearly with plasma current for q95 > 3.2, and decreased with increasing neutral beam power. The confinement quality in these plasmas was 85 ms per MA at a heating power of 5.6 MW. The lower energy confinement in the non-deuterium H-modes and the degradation of energy confinement with neutral beam power were both accompanied by an increase in the edge localized mode (ELM) amplitude and frequency. The changing ELM characteristics make a determination of the intrinsic isotopic and neutral beam effect on confinement difficult. For values of BT < 0.9 T and q95 < 3, the confinement quality in the deuterium and hydrogen/deuterium H-modes deteriorated to values near the L-mode level. This deterioration in energy confinement was not related to operation at high beta but instead appears to be due to a combined action between sawteeth and ELMs that becomes more pronounced at low q and low BT. L-mode energy confinement was independent of ion species and in good agreement with Kaye-Goldston scaling. Odajima-Shimomura scaling disagreed with the present L-mode τE data in terms of isotopic mass dependence; their prediction for the hydrogen L-mode exceeds the present measurements by a factor of two.

MHD instabilities near the beta limit in the doublet III-D tokamak.

Abstract: Saturation or slow collapse of ..beta.. is observed in DIII-D divertor discharges at high-values of normalized ..beta.. ((..beta../(I/aB)greater than or equal to2.4). The sequential appearance of MHD instabilities with poloidal/ toroidal mode numbers m/n = 5/4, 4/3, 3/2, and 2/1 at progressively larger minor radii leads to the saturation or collapse of ..beta... Only n = 2 and n = 1 modes affect global energy confinement. A model is suggested in which the instabilities originate as high-n ballooning modes near the center of the discharge, progressing to lower-n modes by modification of pressure and current profiles.

MHD stability in high βT DIII-D divertor discharges

Abstract: Stability against ideal low n kink and infinite n ballooning modes in high beta T H-mode single null DIII-D divertor plasmas is examined. The equilibrium plasma pressure and current profiles are reconstructed self-consistently from experimentally measured magnetic data and kinetic profile data. The results show that the plasma is marginally stable to the ballooning mode near the central region and is stable to the ballooning mode over most of the plasma volume. Although the total volume averaged toroidal beta beta T in these discharges has not reached the limit imposed by the ideal ballooning mode, the pressure gradient near the edge region approaches the ballooning limit locally, particularly just preceding the occurrence of a giant edge localized mode (ELM). The results also indicate that plasmas with high normalized toroidal beta beta N>or=2.7-3.0 marginally stable to the ideal n=1 pressure driven kink mode without a conducting wall.

Features of edge magnetic turbulence in DIII-D expanded boundary divertor discharges

Abstract: It is found that the magnetic fluctuation amplitudes abruptly decrease at L-H transitions and increase at H-ELM transitions (ELM is the edge localized mode). No other plasma parameter change precedes the magnetic signals, suggesting that the fluctuations are responsible for the increased edge transport in regimes with poor confinement. The fluctuation amplitude during L- and ELM-phases is largest at the points where the separatrix intersects the divertor plates. This is interpreted as evidence for the existence of line tied image currents in the scrape-off plasma.

Achievement of reactor-relevant β in low-q divertor discharges in the doublet III-D tokamak

Abstract: A volume-average toroidal /3 of 6.2% has been obtained with 10 M W of hydrogen-neutral-beam injection into deuterium divertor discharges in DIII-D. High-^ discharges were maintained quiescently for many energy confinement times in elongated, K= 2 , divertor discharges where the safety factor at the 95% flux surface was near 2. The maximum p calculated to be stable against ideal kinks and ideal ballooning modes is 8.5%-9.5%, confirming that these I/aB^^2.5 M A / m T discharges are some distance below the stability limit.

DIII-D Latest Results and Implications

Abstract: The DIII-D tokamak has demonstrated the feasibility of the divertor configuration for future reactor relevant devices. H-mode (good) confinement has been demonstrated for deuterium discharges in both single-null and double-null divertor configurations and under a variety of heating scenarios including neutral beam injection, electron cyclotron heating, and ohmic heating. Plasma beta values (..beta.. = 6.8%) near expected theoretical limits (3.5 I/aB) have been demonstrated. Plasma currents are limited by safety factors q > 2 near the surface of the plasma. Energy confinement improves with increasing plasma current until q decreases below 3. Divertor heat flow has been documented and initial boundary physics studies carried out. Non-inductive current drive has been demonstrated using neutral beam injection. 7 refs., 8 figs., 1 tab.