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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

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


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
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


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
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

H-mode pedestal characteristics, ELMs, and energy confinement in ITER shape discharges on DIII-D

Description: 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 equally 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.
Date: December 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

Prospects for Edge Current Density Determination Using Li beam on DIII-D

Description: The specific size and structure of the edge current profile has important effects on the MHD stability and ultimate performance of many advanced tokamak (AT) operating modes. This is true for both bootstrap and externally driven currents that may be used to tailor the edge shear. Absent a direct local measurement of j(r), the best alternative is a determination of the poloidal field. Measurements of the precision (0.1-0.01{sup o} in magnetic pitch angle and 1-10 ms) necessary to address issues of stability and control and provide constraints for EFIT are difficult to do in the region of interest ({rho} = 0.9-1.1). Using Zeeman polarization spectroscopy of the 2S-2P lithium resonance line emission from the DIII-D LIBEAM, measurements of the various field components may be made to the necessary precision in exactly the region of interest to these studies. Because of the negligible Stark mixing of the relevant atomic levels, this method of determining j(r) is insensitive to the large local electric fields typically found in enhanced confinement (H-mode) edges, and thus avoids an ambiguity common to Motional Stark Effect (MSE) measurements of B. Key issues for utilizing this technique include good beam quality, an optimum viewing geometry, and a suitable optical pre-filter to isolate the polarized emission line. A prospective diagnostic system for the DIII-D AT program will be described.
Date: August 1, 2000
Creator: Thomas, D.M.; Bozek, A.S.; Carlstrom, T.N.; Finkenthal, D.K.; Jayakumar, R.; Makowski, M.A. et al.
Partner: UNT Libraries Government Documents Department

The Effect of Plasma Shape on H-Mode Pedestal Characteristics on DIII-D

Description: The characteristics of the H-mode are studied in discharges with varying triangularity and squareness. The pressure at the top of the H-mode pedestal increases strongly with triangularity primarily due to an increase in the margin by which the edge pressure gradient exceeds the ideal ballooning mode first stability limit. Two models are considered for how the edge may exceed the ballooning mode limit. In one model [1], access to the ballooning mode second stable regime allows the edge pressure gradient and associated bootstrap current to continue to increase until an edge localized, low toroidal mode number, ideal kink mode is destabilized. In the second model [2], the finite width of the H-mode transport barrier, and diamagnetic effects raise the pressure gradient limit above the ballooning mode limit. We observe a weak inverse dependence of the width of the H-mode transport barrier, {Delta}, on triangularity relative to the previously obtained [3] scaling {Delta} {infinity} ({beta}{sub P}{sup PED}){sup 1/2}. The energy loss for Type I ELMs increases with triangularity in proportion to the pedestal energy increase. The temperature profile is found to respond stiffly to changes in T{sup PED} at low temperature, while at high temperature the response is additive. The response of the density profile is also found to play a role in the response of the total stored energy to changes in the W{sup PED}.
Date: December 1, 1999
Creator: Osborne, T.H.; Ferron, J.R.; Groebner, R.J.; Lao, L.L.; Leonard, A.W.; Maingi, R. et al.
Partner: UNT Libraries Government Documents Department


Description: Developing an understanding of the processes that control the H-mode transport barrier is motivated by the significant impact this small region (typically <2% of the minor radius) can have on overall plasma performance. Conditions at the inner edge of the H-mode transport barrier can strongly influence the overall energy confinement, and the maximum density, and therefore fusion power, that can be achieved with the typically flat H-mode density profiles [1,2]. The ELM instability, which usually regulates the pressure gradient in the H-mode edge, can result in large power loads to, and erosion of, the divertor targets in a reactor scale device [3]. The goal of H-mode pedestal research at DIII-D is to: (1) develop a physics based model that would allow prediction of the conditions at the top of the H-mode pedestal, (2) develop an understanding of processes which control Type I ELM effects in the core and divertor, and (3) explore alternatives to the Type I ELM regime.
Date: July 1, 2001
Partner: UNT Libraries Government Documents Department


Description: Edge conditions in DIII-D are being quantified in order to provide insight into the physics of the H-mode regime. Electron temperature is not the key parameter that controls the L-H transition. Gradients of edge temperature and pressure are much more promising candidates for such parameters. The quality of H-mode confinement is strongly correlated with the height of the H-mode pedestal for the pressure. The gradient of the pressure appears to be controlled by MHD modes, in particular by kink-ballooning modes with finite mode number n. For a wide variety of discharges, the width of the barrier is well described with a relationship that is proportional to ({beta}{sub p}{sup ped}){sup 1/2}. An attractive regime of confinement has been discovered which provides steady-state operation with no ELMs, low impurity content and normal H-mode confinement. A coherent edge MHD-mode evidently provides adequate particle transport to control the plasma density and impurity content while permitting the pressure pedestal to remain almost identical to that observed in ELMing discharges.
Date: October 1, 2000
Partner: UNT Libraries Government Documents Department

Characterization and Modification of Edge-Driven Instabilities in the DIII-D Tokamak

Description: The character of edge localized modes (ELMs) and the height of the edge pressure pedestal in DIII-D tokamak H-mode discharges have been modified by varying the discharge shape (triangularity and squareness) and the safety factor, increasing the edge radiation, and injecting deuterium pellets. Changes in the ELM frequency and amplitude, and the magnitude of the edge pressure gradient, and changes in the calculated extent of the region of access to the ballooning mode second stability regime are observed.
Date: July 1, 1999
Creator: Ferron, J.R.; Lao, L.L.; Osborne, T.H.; Strait, E.J.; Turnbull, A.D.; Miller, R.L. et al.
Partner: UNT Libraries Government Documents Department