skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: GAS PUFF FUELED H-MODE DISCHARGES WITH GOOD ENERGY CONFINEMENT ABOVE THE GREENWALD DENSITY LIMIT ON DIII-D

Abstract

A261. GAS PUFF FUELED H-MODE DISCHARGES WITH GOOD ENERGY CONFINEMENT ABOVE THE GREENWALD DENSITY LIMIT ON DIII-D

Authors:
; ; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
GENERAL ATOMICS (US)
Sponsoring Org.:
(US)
OSTI Identifier:
806773
DOE Contract Number:
AC03-99ER54463
Resource Type:
Conference
Resource Relation:
Conference: Conference title not supplied, Conference location not supplied, Conference dates not supplied; Other Information: PBD: 1 Nov 2000
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; DOUBLET-3 DEVICE; H-MODE PLASMA CONFINEMENT; PLASMA DENSITY; LIMITING VALUES

Citation Formats

T.H. OSBORNE, A.W. LEONARD, M.A. MAHDAVI, M.S. CHU, M.E. FENSTERMACHER, R.J. LA HAYE, G.R. McKEE, T.W. PETRIE, E.J. DOYLE, G.M. STAEBLER, and M.R. WADE. GAS PUFF FUELED H-MODE DISCHARGES WITH GOOD ENERGY CONFINEMENT ABOVE THE GREENWALD DENSITY LIMIT ON DIII-D. United States: N. p., 2000. Web.
T.H. OSBORNE, A.W. LEONARD, M.A. MAHDAVI, M.S. CHU, M.E. FENSTERMACHER, R.J. LA HAYE, G.R. McKEE, T.W. PETRIE, E.J. DOYLE, G.M. STAEBLER, & M.R. WADE. GAS PUFF FUELED H-MODE DISCHARGES WITH GOOD ENERGY CONFINEMENT ABOVE THE GREENWALD DENSITY LIMIT ON DIII-D. United States.
T.H. OSBORNE, A.W. LEONARD, M.A. MAHDAVI, M.S. CHU, M.E. FENSTERMACHER, R.J. LA HAYE, G.R. McKEE, T.W. PETRIE, E.J. DOYLE, G.M. STAEBLER, and M.R. WADE. 2000. "GAS PUFF FUELED H-MODE DISCHARGES WITH GOOD ENERGY CONFINEMENT ABOVE THE GREENWALD DENSITY LIMIT ON DIII-D". United States. doi:.
@article{osti_806773,
title = {GAS PUFF FUELED H-MODE DISCHARGES WITH GOOD ENERGY CONFINEMENT ABOVE THE GREENWALD DENSITY LIMIT ON DIII-D},
author = {T.H. OSBORNE and A.W. LEONARD and M.A. MAHDAVI and M.S. CHU and M.E. FENSTERMACHER and R.J. LA HAYE and G.R. McKEE and T.W. PETRIE and E.J. DOYLE and G.M. STAEBLER and M.R. WADE},
abstractNote = {A261. GAS PUFF FUELED H-MODE DISCHARGES WITH GOOD ENERGY CONFINEMENT ABOVE THE GREENWALD DENSITY LIMIT ON DIII-D},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2000,
month =
}

Conference:
Other availability
Please see Document Availability for additional information on obtaining the full-text document. Library patrons may search WorldCat to identify libraries that hold this conference proceeding.

Save / Share:
  • H-mode operation at high density is an attractive regime for future reactor-grade tokamaks [1]. High density maximizes fusion power output while the high confinement of H-mode keeps the plasma energy loss below the alpha heating power. One concern though is the energy released due to individual ELMs must be kept small to protect the diverter target from excess ablation. We report on discharges in DIII-D with electron densities as high as 1.45 times the Greenwald density, n{sub GW}(10{sup 20}m{sup -3})=I{sub p}(MA)/[{pi}{sup 2}(m)], with good confinement, H{sub ITER89P}=1.9, and ELMs with energy amplitude small enough to protect the divertor. These resultsmore » were achieved at low triangularity single-null divertor, {delta}{approx}0.0 with a plasma current of 1.2 MA, q{sub 95} {approx} 3-4, and moderate neutral beam heating power of 2-4 MW. The density was controlled by moderate gas puffing and private flux pumping. A typical discharge is shown in Fig. 1 where upon gas puffing the pedestal density, n{sub e,epd}, quickly rises to {approx}0.8 x n{sub GW}. The confinement initially drops with the gas puff, on a longer timescale the central density rises, peaking the profile and increasing the confinement until an MHD instability terminates the high density and high confinement phase of the discharge. In this report we describe in detail edge pedestal changes and its effect on confinement as the density is increased. We then describe peaking of the density profile that offsets degradation of the pedestal at high density and restores good confinement. Finally we describe the small benign ELMs that result at these high densities.« less
  • The H-mode confinement enhancement factor, H, is found to be strongly correlated with the height of the edge pressure pedestal in ITER shape discharges. In discharges with Type I ELMs the pedestal pressure is set by the maximum pressure gradient before the ELM and the width of the H-mode transport barrier. The pressure gradient before Type I ELMs is found to scale as would be expected for a stability limit set by ideal ballooning modes, but with values significantly in excess of that predicted by stability code calculations. The width of the H-mode transport barrier is found to scale equallymore » well with pedestal P(POL)(2/3) or B(POL)(1/2). The improved H value in high B(POL) discharges may be due to a larger edge pressure gradient and wider H-mode transport barrier consistent with their higher edge ballooning mode limit. Deuterium puffing is found to reduce H consistent with the smaller pedestal pressure which results from the reduced barrier width and critical pressure gradient. Type I ELM energy loss is found to be proportional to the change in the pedestal energy.« less
  • Measurements of the radial electric field, E{sub r}, with high spatial and high time resolution in H-mode and VH-mode discharges in the DIII-D tokamak have revealed the significant influence of the shear in E{sub r} on confinement and transport in these discharges. These measurements are made using the DIII-D Charge Exchange Recombination (CER) System. At the L-H transition in DIII-D plasmas, a negative well-like E{sub r} profile develops just within the magnetic separatrix. A region of shear in E{sub r} results, which extends 1 to 2 cm into the plasma from the separatrix. At the transition, this region of shearedmore » E{sub r} exhibits the greatest increase in impurity ion poloidal rotation velocity and the greatest reduction in plasma fluctuations. A transport barrier is formed in this same region of E {times} B velocity shear as is signified by large increases in the observed gradients of the ion temperature, the carbon density, the electron temperature and electron density. The development of the region of sheared E{sub r}, the increase in impurity ion poloidal rotation, the reduction in plasma turbulence, and the transport barrier all occur simultaneously at the L-H transition. Measurements of the radial electric field, plasma turbulence, thermal transport, and energy confinement have been performed for a wide range of plasma conditions and configurations. The results support the supposition that the progression of improving confinement at the L-H transition, into the H-mode and then into the VH-mode can be explained by the hypothesis of the suppression of plasma turbulence by the increasing penetration of the region of sheared E {times} B velocity into the plasma interior.« less
  • The role of neutrals in triggering the H-L back transition in high density ELMing H-mode plasmas is explored in double-null (DN) and single-null (SN) divertors. The authors propose that the neutral particle buildup below the X-point may play an important role in triggering the H-L transition at high density. Neutral pressure in the private flux region is, in fact, significant near the H-L backtransition. High density formation inside the separatrix near the X-point may also be a factor in triggering the H-L backtransition. They have observed that the ELMing H-mode density limit in SN divertors normally occurred at or nearmore » the H-L back transition. The radiated power coming from inside the separatrix at the H-L transition did not appear sufficient by itself to produce this back transition, since it is only {approximately}15--30% of P{sub in}. Poloidally-localized neutrals may explain two important differences in SN and double-null (DN) plasmas near their respective H-L backtransitions. First, electron pressure along the separatrix between the X-point and the outboard strike point decreased only modestly for DN divertors, even at densities comparable to the Greenwald density limit {bar n}{sub e,G}, in contrast to SN plasmas. Second, no divertor (or core) MARFEs were detected in the DNs as {bar n}{sub e} approaches {bar n}{sub e,G}, in contrast to SNs, where divertor MARFEs can form at {bar n}{sub e}/{bar n}{sub e,G} as low as {approximately}0.6. High X-point DNs achieved density limits well above those of comparably-prepared SNs, e.g., {bar n}{sub e}/n{sub e,G} {approx} 0.9--1.0 for DNs versus 0.75--0.80 for SNs. These differences result from a lower neutral pressure in the private flux regions of DNs than in comparable SNs at the same {bar n}{sub e}, since neutrals impact both pressure balance and MARFEing behavior.« less
  • A regime of very high confinement (VH-mode) has been observed in neutral beam-heated deuterium discharges in the DIII-D tokamak with thermal energy confinement times up to [approx]3.6 times that predicted by the ITER-89P L-mode scaling and 2 times that predicted by ELM-free H-mode thermal confinement scalings. This high confinement has led to increased plasma performance, n[sub D] (0)T[sub i](0)[tau][sub E] = 2 [times] 10[sup 20] m[sup [minus]3] keV sec with I[sub p] = 1.6 MA, B[sub T] = 2.1 T, Z[sub eff] [le] 2. Detailed transport analysis shows a correspondence between the large decrease in thermal diffusivity in the regionmore » 0.75 [le] [rho] [le] 0.9 and the development of a strong shear in the radial electric field in the same region. This suggests that stabilization of turbulence by sheared E [times] B flow is responsible for the improved confinement in VH-mode. A substantial fraction of the edge plasma entering the second regime of stability may also contribute to the increase in confinement. The duration of the VH-mode phase has been lengthened by feedback controlling the input power to limit plasma beta.« less