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Plasma rotation and the radial electric field during off-axis NBI in the DIII-D tokamak

Description: Experiments have been carried out on the DIII-D tokamak to investigate whether off-axis NBI can: (a) drive significant perpendicular flow to lead to increased suppression of turbulence and improved confinement, and (b) be used to control the radial electric field profile. Measurements of both impurity ion poloidal and toroidal rotation profiles were made using charge exchange recombination spectroscopy. These experiments used a low current, low elongation (I{sub p} = 0.5 MA, {kappa} = 1.2) plasma whose magnetic axis was shifted 36 cm vertically upward from the vessel midplane and then shifted downward to be centered on the midplane later in the discharge. 10.7 MW of beam power was applied to maximize NBI effect while operating at low target densities and high temperature to minimize poloidal damping. Results from these experiments show a slight increase in impurity ion poloidal rotation velocity during the vertical shifted phase of off-axis NBI discharge. The toroidal rotation profile is more peaked during off-axis NBI. Both these effects lead to a change in the V x B contribution to the radial electric field during off-axis NBI.
Date: December 1, 1995
Creator: Gohil, P.; Burrell, K.H.; Osborne, T.H. & Hassam, A.B.
Partner: UNT Libraries Government Documents Department

Divertor heat and particle flux due to ELMs in DIII-D and ASDEX-Upgrade

Description: The authors characterize the divertor target plate heat and particle fluxes that occur due to Edge-Localized-Modes (ELMs) during H-mode in DIII-D and ASDEX-Upgrade. During steady-state ELMing H-mode the fraction of main plasma stored energy lost with each ELM varies from 6% to 2% as input power increases above the H-mode power threshold. The ELM energy is deposited near the strikepoints on the divertor target plates in a fast time scale of {le} 1 ms. The spatial profile of the ELM heat pulse is flatter and broader, up to about a factor of 2, than that of the heat flux between ELMs. On ASDEX-Upgrade the inboard strike-point receives the greatest fraction, {ge} 75%, of ELM divertor heat flux, while on DIII-D the in/out split is nearly equal. The toroidal asymmetry of the heat pulse has produced a peaking factor on DIII-D of no more than 1.5. The particle flux, as measured by Langmuir probes, has also been found to be localized near the divertor strike-points. The increased particle flux during ELMs is a significant fraction of the total time-integrated divertor plate particle flux.
Date: October 1, 1996
Creator: Leonard, A.W.; Osborne, T.H. & Suttrop, W.
Partner: UNT Libraries Government Documents Department

H-mode pedestal characteristics in ITER shape discharges on DIII-D

Description: Characteristics of the H-mode pedestal are studied in Type 1 ELM discharges with ITER cross-sectional shape and aspect ratio. The scaling of the width of the edge step gradient region, {delta}, which is most consistent with the data is with the normalized edge pressure, ({beta}{sub POL}{sup PED}){sup 0.4}. Fits of {delta} to a function of temperature, such as {rho}{sub POL}, are ruled out in divertor pumping experiments. The edge pressure gradient is found to scale as would be expected from infinite n ballooning mode theory; however, the value of the pressure gradient exceeds the calculated first stable limit by more than a factor of 2 in some discharges. This high edge pressure gradient is consistent with access to the second stable regime for ideal ballooning for surfaces near the edge. In lower q discharges, including discharges at the ITER value of q, edge second stability requires significant edge current density. Transport simulations give edge bootstrap current of sufficient magnitude to open second stable access in these discharges. Ideal kink analysis using current density profiles including edge bootstrap current indicate that before the ELM these discharges may be unstable to low n, edge localized modes.
Date: September 1, 1998
Creator: Osborne, T.H.; Burrell, K.H. & Groebner, R.J.
Partner: UNT Libraries Government Documents Department

Scaling studies of the H-mode pedestal

Description: The structure and scaling of the H-mode pedestal are examined for discharges in the DIII-D tokamak. For typical conditions, the pedestal values of the ion and electron temperatures T{sub i} and T{sub e} are comparable. Measurements of main ion and C{sup 6+} profiles indicate that the ion pressure gradient in the barrier is 50%--100% of the electron pressure gradient for deuterium plasmas. The magnitude of the pressure gradient in the barrier often exceeds the predictions of infinite-n ballooning mode theory by a factor of two. Moreover, via the bootstrap current, the finite pressure gradient acts to entirely remove ballooning stability limits for typical discharges. For a large dataset, the width of the pressure barrier {delta} is best described by the dimensionless scaling {delta}/R {proportional_to} ({beta}{sub pol}{sup ped}){sup 0.4} where ({beta}{sub pol}{sup ped}) is the pedestal value of poloidal beta and R is the major radius. Scalings based on the poloidal ion gyroradius or the edge density gradient do not adequately describe overall trends in the data set and the propagation of the pressure barrier observed between edge-localized modes. The width of the T{sub i} barrier is quite variable and is not a good measure of the width of the pressure barrier.
Date: January 1, 1998
Creator: Groebner, R.J. & Osborne, T.H.
Partner: UNT Libraries Government Documents Department

High performance low and high q discharges in DIII-D

Description: The High performance H-mode regime on DIII-D has been extended to both low q and high q (high {Beta}p) and low q operation. In high current operation, VH-mode discharges were obtained for the first time with I{sub P}(MA)/B{sub T} (T) > 1. These discharges had q{sub 95}= 3.4, H = 2.9, {Beta}{sub N}= 3, and {Beta}{sub T}{Tau}{sub E}=3%-sec. {Beta}{sub T}{Tau}{sub E} was improved by approximately 50% over previous results. These discharges were obtained with neutral beam injection during the plasma current ramp up which maintained the axial q above 1. In low current operation, neutral beam heated discharges with 100% of the plasma current from non-inductive sources were obtained at high q, q{sub 95}=15, with {Beta}{sub N}=3.9, H=3.1, and {Beta}{sub p}=4.9. These discharges represent an extension of the high performance regime to q{sub 95} > 7.2, which was made possible by reduction in the locked mode low density limit, as the result of improvements in the magnetic field error correcting coils. These low current discharges do not exhibit some of the standard signatures of VH-mode, but appear to represent a new regime of improved H-mode confinement. Similar, non-VH-mode, high energy confinement discharges were obtained at low density and moderate q.
Date: January 1, 1996
Creator: Osborne, T.H.; Burrell, K.H. & Chu, M.S.
Partner: UNT Libraries Government Documents Department

The role of shaping in achieving high performance in DIII-D

Description: Experiments to study the effect of plasma shape on performance show that the high performance levels characteristic of VH-mode plasmas are only obtained for high triangularity ({delta}). A characteristic difference in central heating efficiency is observed when high and low {delta} plasmas are compared. The authors find this difference to correlate well with the attainment of Mercier stability. The core plasma for the high {delta} cases achieves Mercier stability when q{sub 0} rises above a critical value which depends on {delta}, providing direct access to the second-stable regime. Other attributes of the VH-mode commence after a delay. The rise in q{sub 0} is attributable to the radial gradients in the neutral beam and bootstrap current sources. Simulation indicates that such plasmas would persist for many seconds if the rise in density could be controlled, maintaining a centrally peaked neutral beam deposition profile.
Date: October 1, 1994
Creator: Lazarus, E. A.; Hyatt, A. W. & Osborne, T. H.
Partner: UNT Libraries Government Documents Department

Recent VH-mode results on DIII-D

Description: A regime of improved H-mode energy confinement, VH-mode, is obtained in the DIII-D tokamak with adequate vessel conditioning. The improved confinement in VH-mode is consistent with the extension of the region of high E {times} B velocity shear turbulence suppression zone further in from the plasma boundary. The energy confinement enhancement in VH-mode can be limited by ELMs, localized momentum transfer events, or operation at high heating power or low q. Energy confinement enhancement improves with increasing triangularity of the plasma cross section and is independent of elongation. The termination of VH-mode is associated with an edge localized kink mode which is destabilized by both the large edge pressure gradient and edge current density.
Date: January 1, 1994
Creator: Osborne, T. H.; Burrell, K. H. & Carlstrom, T. N.
Partner: UNT Libraries Government Documents Department

Plasma shape experiments for an optimized tokamak

Description: In this paper we present results from recent experiments at DIII-D which measured the plasma stability and confinement performance product, {beta}{tau}{sub E}, in one previously studied and three new plasma shapes. One important goal of these experiments was to identify performance vs shape trends which would identify a shape compatible with both high performance and the planned effort to decrease the power flux to the divertor floor using a closed ``slot`` divertor geometry. power flux to the divertor floor using a closed ``slot`` divertor geometry. The closed divertor hardware must be designed for a reduced set of plasma shapes, so care must be taken to choose the shape that optimizes {beta}{tau}{sub E} and divertor performance. The four shapes studied form a matrix of moderate and high elongations ({kappa} {congruent} 1.8 and 2.1) and low and high triangularities ({delta} {congruent} 0.3 and 0.9). All configurations were double-null diverted (DND), held fixed during a shot, with neutral beam heating. The shapes span a range of X-point locations compatible with the envisioned closed divertor. We find that from shape to shape, a shot`s transient normalized performance, {beta}{sub N}H, where {beta}{sub N} {equivalent_to} {beta}/(I{sup p})/aB{sub T} and H {equivalent_to} {tau}{sub E}/{tau}{sub E}{sup ITER-89P}, increases strongly with triangularity, but depends only weakly on elongation. However, the normalized performance during quasi stationary ELMing H-mode, to which these discharges eventually relax, is insensitive to both triangularity and elongation. The moderate elongation, high triangularity DND shape is shown to be near optimum for future studies on DIII-D.
Date: July 1, 1994
Creator: Hyatt, A. W.; Osborne, T. H. & Lazarus, E. A.
Partner: UNT Libraries Government Documents Department

A quantitative analysis of the effect of ELMs on H-mode thermal energy confinement in DIII-D

Description: The desire to reach ignition in future tokamaks the energy confinement time critical parameter. The most promising enhanced (over L-mode) confinement regime is the H-mode, discovered on ASDEX with neutral beam heating, and then confirmed with various auxiliary heating sources on numerous machines. The knowledge of how H-mode {tau}{sub E} depends on different parameters is of chemical importance to the performance predictions for next generation devices. Inter-machine H-mode total and thermal energy confinement ({tau}{sub th}) scalings, which are being utilized to predict ITER thermal energy confinement, have been created for discharges where the Edge Localized Mode (ELM) instability has not been present. Confinement scaling research hm concentrated on this ELM-free H-mode phase mostly owing to the difficulty of characterizing ELM behavior. To date, long pulse H-mode operation has only been achieved by utilizing ELMs to flush out unpurities and prevent radiative collapse of the discharge. Unfortunately, accompanying the ELMS is a decrease of the plasma stored energy due to the expulsion of particles near the edge of the discharge resulting in a reduction of the steep edge electron density gradient. In order to predict ITER`s H-mode {tau}{sub th} in the presence of ELMS, an estimated 25% confinement degradation factor has been applied to the ELM-free predictions. Our work, summarized in this paper, indicates that this 25% reduction factor is too large and instead a value of approximately 15% would be more appropriate.
Date: June 1, 1992
Creator: Schissel, D. P.; Osborne, T. H.; Carlstrom, T. N. & Zohm, H.
Partner: UNT Libraries Government Documents Department

Ballooning mode stability for self-consistent pressure and current profiles at the H-mode edge

Description: The edge pressure gradient (H-mode pedestal) for computed equilibria in which the current density profile is consistent with the bootstrap current may not be limited by the first regime ballooning limit. The transition to second stability is easier for: higher elongation, intermediate triangularity, larger ratio, pedestal at larger radius, narrower pedestal width, higher q{sub 95}, and lower collisionality.
Date: November 1, 1997
Creator: Miller, R.L.; Lin-Liu, Y.R.; Osborne, T.H. & Taylor, T.S.
Partner: UNT Libraries Government Documents Department

THE ROLE OF NEUTRALS IN H-MODE PEDESTAL FORMATION

Description: An analytic model, derived from coupled continuity equations for the electron and neutral deuterium densities, is consistent with many features of edge electron density profiles in the DIII-D tokamak. For an assumed constant particle diffusion coefficient, the model shows that particle transport and neutral fueling produce electron and neutral density profiles that have the same characteristic scale lengths at the plasma edge. For systematic variations of density in H-mode discharges, the model predicts that the width of the electron density transport barrier decreases and the maximum gradient increases, as observed in the experiments. The widths computed from the model agree quantitatively with the experimental widths for conditions in which the model is valid. These results support models of transport barrier formation in which the H-mode particle barrier is driven by the edge particle flux and the width of the barrier is approximately the neutral penetration length.
Date: November 1, 2001
Creator: GROEBNER, R.J.; MAHDAVI, M.A.; LEONARD, A.W.; OSBORNE, T.H.; PORTER, G.D.; COLCHIN, R.J. et al.
Partner: UNT Libraries Government Documents Department

Comparison of L-H transition measurements with physics models

Description: A technique of fitting a modified hyperbolic tangent to the edge profiles has improved the localization of plasma edge parameters. Non-dimensional edge parameters are broadly consistent with several theories of the L-H transition that use edge gradients in their formulation of a critical threshold parameter. The ion {del}B drift direction has only a small effect on the edge plasma conditions measured near the plasma midplane but a large effect on the divertor plasma. The dramatic change of power threshold with the direction of the ion {del}B drift implies that phenomena in the divertor region may be critical for the L-H transition.
Date: December 1998
Creator: Carlstrom, T. N.; Burrell, K. H.; Groebner, R. J.; Leonard, A. W.; Osborne, T. H. & Thomas, D. M.
Partner: UNT Libraries Government Documents Department

The impact of ELMs on the ITER divertor

Description: Edge-Localized-Modes (ELMs) are expected to present a significant transient flux of energy and particles to the ITER divertor. The threshold for ablation of the graphite target will be reached if the ELM transient exceeds Q/t{sup 1/2} {approximately} 45 MJ-m{sup {minus}2}-s{sup {minus}1/2} where Q is the ELM deposition energy density and t is the ELM deposition time. The ablation parameter in ITER can be determined by scaling four factors from present experiments: the ELM energy loss from the core plasma, the fraction of ELM energy deposited on the divertor target, the area of the ELM profile onto the target, and finally the time for the ELM deposition. Review of the ELM energy loss of Type 1 ELM data suggests an ITER ELM energy loss of 2--6% of the stored energy or 25--80 MJ. The fraction of heating power crossing the separatrix due to ELMs is nearly constant (20--40%) resulting in an inverse relationship between ELM amplitude and frequency. Measurements on DIII-D and ASDEX-Upgrade indicate that 50--80% of the ELM energy is deposited on the target. There is currently no evidence for a large fraction of the ELM energy being dissipated through radiation. Profiles of the ELM heat flux are typically 1--2 times the width of steady heat flux between ELMs, with the ELM amplitude usually larger on the inboard target. The ELM deposition time varies from about 0.1 ms in JET to as high as 1.0 ms in ASDEX-Upgrade and DIII-D. The ELM deposition time for ITER will depend upon the level of conductive versus convective transport determined by the ratio of energy to particles released by the ELM. Preliminary analysis suggests that large Type 1 ELMs for low recycling H-mode may exceed the ablation parameter by a factor of 5. Promising regimes with smaller ELMS have been found at other ...
Date: July 1, 1998
Creator: Leonard, A.W.; Osborne, T.H.; Suttrop, W.; Hermann, A.; Itami, K.; Lingertat, J. et al.
Partner: UNT Libraries Government Documents Department

ELM heat flux in the ITER divertor

Description: Edge-Localized-Modes (ELMs) have the potential to produce unacceptable levels of erosion of the ITER divertor. Ablation of the carbon divertor target will occur if the surface temperature rises above about 2,500 C. Because a large number of ELMs, {ge}1000, are expected in each discharge it is important that the surface temperature rise due to an individual ELM remain below this threshold. Calculations that have been carried out for the ITER carbon divertor target indicate ablation will occur for ELM energy {ge}0.5MJ/m{sup 2} if it is deposited in 0.1 ms, or 1.2 MJ/m{sup 2} if the deposition time is 1.0 ms. Since {Delta}T{proportional_to}Q{Delta}t{sup {minus}1/2}, an ablation threshold can be estimated at Q{Delta}t{sup {minus}1/2}{approx}45 MJm{sup {minus}2} s{sup {minus}1/2} where Q is the divertor ELM energy density in J-m{sup {minus}2} and {Delta}t is the time in seconds for that deposition. If a significant fraction of ELMs exceed this threshold then an unacceptable level of erosion may take place. The ablation parameter in ITER can be determined by scaling four factors from present experiments: the ELM energy loss from the core plasma, the fraction of ELM energy deposited on the divertor target, the area of the ELM profile onto the target, and the time for the ELM deposition. ELM data from JET, ASDEX-Upgrade, JT-60U, DIII-D and Compass-D have been assembled by the ITER Divertor Modeling and Database expert group into a database for the purpose of predicting these factors for ELMs in the ITER divertor.
Date: July 1, 1998
Creator: Leonard, A.W.; Osborne, T.H.; Hermann, A.; Suttrop, W.; Itami, K.; Lingertat, J. et al.
Partner: UNT Libraries Government Documents Department

State transitions, hysteresis, and control parameters on DIII-D

Description: The theory of turbulence decorrelation by ExB velocity shear is the leading candidate to explain the changes in turbulence and transport that are seen at the plasma edge at the L to H transition. Based on this, a key question is: What are the conditions or control parameters needed to begin the formation of the E{sub r} shear layer and thus trigger the L to H transition? On the DIII-D tokamak, the authors are attacking this question both through direct tests of the various theories and by trying to gain insight into the fundamental physics by investigating the control parameters which have a major effect on the power threshold. In this paper the authors describe results of studies on oscillating discharges where the plasma transitions continuously between L and H states. By following the dynamics of the plasma state through the forward and back transitions, they can represent the evolution of various control parameter candidates as a trajectory in various parametric spaces. The shape of these control curves can illustrate the specific nonlinearities governing the L-H transition problem, and under the proper conditions may be interpreted in the context of various phase-transition based models. In particular, the hysteresis exhibited in the various curves may help to clarify causality (what are the critical parameters) and may serve as tests of the models, given sufficient experimental accuracy. At present they are looking at T{sub e}, E{sub r} and ballooning/diamagnetic parameters as possible control parameter candidates.
Date: July 1998
Creator: Thomas, D. M.; Groebner, R. J.; Carlstrom, T. N.; Osborne, T. H. & Petrie, T. W.
Partner: UNT Libraries Government Documents Department

High Density H-Mode Discharges with Gas Fueling and Good Confinement on DIII-D

Description: 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 results 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.
Date: August 1, 2000
Creator: Leonard, A.W.; Osborne, T.H.; Mahdavi, M.A.; Fenstermacher, M.E.; Lasnier, C.J.; Petrie, T.W. et al.
Partner: UNT Libraries Government Documents Department

The back transition and hysteresis effects in DIII-D

Description: The back transition from H-mode to L-mode has been studied on DIII-D as a part of the investigation of the L-H transition power threshold scaling. Based on a density-dependent scaling for the H-mode power threshold, ITER will require substantial hysteresis in this parameter to remain in H-mode as n{sub e} rises. Defining the hysteresis in terms of the ratio of sustaining to threshold power, P{sub HL}/P{sub LH} may need to be as small as 50% for ITER. Operation of DIII-D at injection powers slightly above the H-mode threshold results in an oscillatory behavior with multiple forward-backward transitions in the course of a discharge. These discharges represent a unique system for studying various control parameters that may influence the H{leftrightarrow}L state transition. Careful analysis of the power flow through the edge gives values for the sustaining power which are well below the corresponding threshold powers (P{sub HL}/P{sub LH} = 35--70%), indicating substantial hysteresis can be achieved in this parameter. Studies of other control parameter candidates such as edge temperature during the back transitions are less clear: the amount of hysteresis seen in these parameters, if any, is primarily dependent on the nature (ELMing, ELM-free) of the parent H-state.
Date: September 1, 1997
Creator: Thomas, D.M.; Groebner, R.J.; Burrell, K.H.; Osborne, T.H. & Carlstrom, T.N.
Partner: UNT Libraries Government Documents Department

Coupled MHD and transport analysis of improved confinement DIII-D discharges

Description: Transiently high internal inductance discharges are used to check the validity of evolvement of current profiles obtained with the MHD/transport coupled scheme presented below. Both a current ramp and an elongation ramp discharge are examined. To simulate energy confinement the Rebut-Lallia-Watkins model is used for the current ramp discharge. In our simulation of experimental discharges external circuit behavior is accounted for by using measured, time dependent, flux loops, magnetic probes, field shaping and ohmic coil currents. The small effect of eddy currents in the plasma vessel are neglected. We use measured, time dependent electron and ion temperature profiles, electron density, Z{sub eff}, toroidal rotation and radiated power profiles to constrain our analyses. For the RLW simulation measured values of T{sub e} and T{sub i} at the plasma edge are used as boundary conditions in the electron and ion energy equations.
Date: May 1, 1993
Creator: St. John, H.; Ferron, J. R.; Lao, L. L.; Osborne, T. H.; Thompson, S. J. & Wroblewski, D.
Partner: UNT Libraries Government Documents Department

The role of the radial electric field in confinement and transport in H-mode and VH-mode discharges in the DIII-D tokamak

Description: 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 sheared 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.
Date: August 1, 1993
Creator: Gohil, P.; Burrell, K. H.; Groebner, R. J.; Osborne, T. H.; Doyle, E. J. & Rettig, C. L.
Partner: UNT Libraries Government Documents Department

TRANSPORT OF ELM ENERGY AND PARTICLES INTO THE SOL AND DIVERTOR OF DIII-D

Description: We report on DIII-D data that reveal the underlying processes responsible for transport of energy and particles from the edge pedestal to the divertor target during edge-localized modes (ELMs). The separate convective and conductive transport of energy due to an ELM is determined by Thomson scattering measurements of electron density and temperature in the pedestal. Conductive transport is measured as a drop in pedestal temperature and decreases with increasing density. The convective transport of energy, measured as a loss of density from the pedestal, however, remains constant as a function of density. From the SOL ELM energy is quickly carried to the divertor target. An expected sheath limit to the ELM heat flux set by the slower arrival of pedestal ions is overcome by additional ionization of neutrals generated from the divertor target as evidenced by a fast, {approx}100 {micro}s, rise in divertor density. A large in/out asymmetry of the divertor ELM heat flux is observed at high density, but becomes nearly symmetric at low density.
Date: June 1, 2002
Creator: LEONARD, A.W.; BOEDO, J.A.; FENSTERMACHER, M.E.; GROEBNER, R.J.; GROTH, M.; LASNIER, C.J. et al.
Partner: UNT Libraries Government Documents Department

Scaling of ELM and H-mode pedestal characteristics in ITER shape discharges in the DIII-D tokamak

Description: The authors have shown a correlation between the H-mode pressure pedestal height and the energy confinement enhancement in ITER shape discharges on DIII-D which is consistent with the behavior of H in different ELM classes. The width of the steep gradient region was found to equally well fit the scalings {delta}/R {proportional_to} ({rho}{sub POL}/R){sup 2/3} and {delta}/R {proportional_to} ({beta}{sub POL}{sup PED}/R){sup 1/2}. The normalized pressure gradient {alpha}{sub MHD} was found to be relatively constant just before a type I ELM. An estimate of T{sup PED} for ITER gave 1 to 5 keV. They also estimate {Delta}E{sub ELM} {approx_equal} 26 MJ for ITER. They identified a distinct class of type III ELM at low density which may play a role in setting H at powers near the H-mode threshold power.
Date: July 1, 1997
Creator: Osborne, T.H.; Groebner, R.J.; Lao, L.L.; Leonard, A.W.; Miller, R.L.; Thomas, D.M. et al.
Partner: UNT Libraries Government Documents Department