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Vacuum ultraviolet spectroscopic study of plasma impurities in the Tokapole II poloidal divertor experiment

Description: The main diagnostics were two vacuum ultraviolet spectrometers, which were used to identify impurities, observe impurity behavior, and measure the line radiated power. Impurity concentrations and the effects of impurities on gross plasma characteristics were determined with an impurity doping technique. The electron temperature was estimated by a time dependent coronal model describing the time history of the oxygen ions, and the ion temperature was obtaind from Doppler broadening measurements. Energy loss processes were studied with a power balance analysis.
Date: May 1, 1979
Creator: Groebner, R.J.
Partner: UNT Libraries Government Documents Department

An emerging understanding of H-mode discharges in tokamaks

Description: A remarkable degree of consistency of experimental results from tokamaks throughout the world has developed with regard to the phenomenology of the transition from L-mode to H-mode confinement in tokamaks. The transition is initiated in a narrow layer at the plasma periphery where density fluctuations are suppressed and steep gradients of temperature and density form in a region with large first and second radial derivatives in the [upsilon][sub E][sup [yields]] = (E [times] B)/B[sup 2] flow velocity. These results are qualitatively consistent with theories which predict suppression of fluctuations by shear or curvature in [upsilon]E. The required [upsilon]E flow is generated very rapidly when the magnitude of the heating power or of an externally imposed radial current exceed threshold values and several theoretical models have been developed to explain the observed changes in the [upsilon]E flow. After the transition occurs, the altered boundary conditions enable the development of improved confinement in the plasma interior on a confinement time scale. The resulting H-mode discharge has typically twice the confinement of L-mode discharges and regimes of further improved confinement have been obtained in some H-mode scenarios.
Date: December 1, 1992
Creator: Groebner, R.J.
Partner: UNT Libraries Government Documents Department

A global fitting code for multichordal neutral beam spectroscopic data

Description: Knowledge of the heat deposition profile is crucial to all transport analysis of beam heated discharges. The heat deposition profile can be inferred from the fast ion birth profile which, in turn, is directly related to the loss of neutral atoms from the beam. This loss can be measured spectroscopically be the decrease in amplitude of spectral emissions from the beam as it penetrates the plasma. The spectra are complicated by the motional Stark effect which produces a manifold of nine bright peaks for each of the three beam energy components. A code has been written to analyze this kind of data. In the first phase of this work, spectra from tokamak shots are fit with a Stark splitting and Doppler shift model that ties together the geometry of several spatial positions when they are fit simultaneously. In the second phase, a relative position-to-position intensity calibration will be applied to these results to obtain the spectral amplitudes from which beam atom loss can be estimated. This paper reports on the computer code for the first phase. Sample fits to real tokamak spectral data are shown.
Date: May 1, 1992
Creator: Seraydarian, R.P.; Burrell, K.H. & Groebner, R.J.
Partner: UNT Libraries Government Documents Department

Dependence of H-mode power threshold on global and local edge parameters

Description: Measurements of local electron density n{sub e}, electron temperature T{sub e}, and ion temperature T{sub i} have been made at the very edge of the plasma just prior to the transition into H-mode for four different single parameter scans in the DIII-D tokamak. The means and standard derivations of n{sub e}, T{sub e}, and T{sub i} under these conditions for a value of the normalized toroidal flux of 0.98 are respectively, 1.5 {+-} 0.7 x 10{sup 19} m{sup -3}, 0.051 {+-} 0.016 keV, and 0.14 {+-} 0.03 keV. The threshold condition for the transition is more sensitive to temperature than to density. The data indicate that the dependence is not as simple as a requirement for a fixed value of the ion collisionality.
Date: December 1, 1995
Creator: Groebner, R.J.; Carlstrom, T.N. & Burrell, K.H.
Partner: UNT Libraries Government Documents Department

Study of H-mode threshold conditions in DIII-D

Description: Studies have been conducted in DIII-D to determine the dependence of the power threshold P{sub lh} for the transition to the H-mode regime and the threshold P{sub hl} for the transition from H-mode to L-mode as functions of external parameters. There is a value of the line-averaged density n{sub e} at which P{sub lh} has a minimum and P{sub lh} tends to increase for lower and higher values of n{sub e}. Experiments conducted to separate the effect of the neutral density n{sub 0} from the plasma density n{sub e} give evidence of a strong coupling between n{sub 0} and n{sub e}. The separate effect of neutrals on the transition has not been determined. Coordinated experiments with JET made in the ITER shape show that P{sub lh} increases approximately as S{sup 0.5} where S is the plasma surface area. For these discharges, the power threshold in DIII-D was high by normal standards, thus suggesting that effects other than plasma size may have affected the experiment. Studies of H-L transitions have been initiated and hysteresis of order 40% has been observed. Studies have also been done of the dependence of the L-H transition on local edge parameters. Characterization of the edge within a few ms prior to the transition shows that the range of edge temperatures at which the transition has been observed is more restrictive than the range of densities at which it occurs. These results suggest that some temperature function is important for controlling the transition.
Date: October 1996
Creator: Groebner, R. J.; Carlstrom, T. N. & Burrell, K. H.
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

Study of the conditions for spontaneous H-mode transitions in DIII-D

Description: A series of scaling studies attempting to correlate the H(high)-mode power threshold (P{sub TH}) with global parameters have been conducted. Data from these discharges is also being used to look for dependence of P{sub TH} on local edge parameters and to test theories of the transition. Boronization and better operational techniques have resulted in lower power thresholds and weaker density scaling. Neon impurity injection experiments show that radiation also plays a role in determining P{sub TH}. A low density threshold for the L(low)-H(high) transition has been linked with the locked mode low density limit, and can be reduced with the use of an error field correcting coil. Highly developed edge diagnostics, with spatial resolution as low as 5 mm, are used to evaluate how the power threshold depends on local edge conditions. Preliminary analysis of local edge conditions for parameter scans of n{sub e}, B{sub T}, and I{sub p} in single-null discharges, and the X-point imbalance in double-null discharges-show that, just before the transition to H-mode, the edge temperatures near the separatrix are approximately constant at 100 < T{sub i} < 220 eV and 35 < T{sub e} < 130 eV, even though the threshold power varied from 1.5 to 14 MW. During a density scan, the edge ion collisionality, v{sub *i}, varied from 2 to 17, demonstrating that a transition condition as simple as v{sub *i} = constant is inconsistent with the data. The local edge parameters of n{sub e}, T{sub e}, and T{sub i} do not always follow the same global scaling as P{sub TH}. Therefore, theories of the L-H transition need not be constrained by these scalings.
Date: January 1, 1996
Creator: Carlstrom, T.N. & Groebner, R.J.
Partner: UNT Libraries Government Documents Department

Comparison of a {del}B drift effect model with measured H-mode power thresholds

Description: The H-mode power threshold has a weak but positive B{sub T} dependence when the ion {del}B drift is away from the X-point, in contrast to the nearly linear B{sub T} dependence when the ion {del}B drift is toward the X-point. This indicates that geometry plays an important role in the H-mode power threshold scaling. A simple model of the {del}B drift effect failed to predict this behavior, but successfully predicted the sign change of gas puffing and low X-point height on the power threshold. The difference between the threshold power required for sawtooth and nonsawtooth triggered transitions can be substantial. This effect may contribute to the observed B scaling of the H-mode power threshold.
Date: September 1, 1997
Creator: Carlstrom, T.N.; Burrell, K.H. & Groebner, R.J.
Partner: UNT Libraries Government Documents Department

Critical edge parameters for H-mode transition in DIII-D

Description: Measurements in DIII-D of edge ion and electron temperatures (T{sub i} and T{sub e}) just prior to the transition to H-mode are presented. A fitting model based on a hyperbolic tangent function is used in the analysis. The edge temperatures are observed to increase during the L-phase with the application of auxiliary heating. The temperature rise is small if the H-mode power threshold is close to the Ohmic power level in the absence of auxiliary heating and is large if the H-mode threshold is well above the Ohmic power level. The edge temperatures just prior to the transition are approximately proportional to the toroidal magnetic field Bt for the field either in the reversed or forward direction. However, for the reversed magnetic field, the temperatures are at least a factor of two higher than for the forward direction.
Date: November 1, 1997
Creator: Groebner, R.J. & Carlstrom, T.N.
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

Effect of ELMs on the SOL plasma in DIII-D

Description: We have studied the evolution of the edge plasma in VH-mode discharges in DIII-D as the discharge evolves from the ELM-free H-mode phase through the VH-mode phase to the final ELMing H-mode phase, by following the changes in the radial profiles of the density and temperature, in the core plasma near the separatrix and in the scrape-off-layer (SOL) plasma outside the separatrix. The electron density and temperature profiles in the SOL do not show any significant difference between the ELM-free H-mode and VH-mode phases. In the ELMing phase, the, density profile broadens during an ELM, forming a high density (n{sub e} > 1 x 10{sup 19}/m{sup 3}) plateau that extends out into the SOL to the limit of the measurement. This density plateau persists between the ELMs, although the density in the SOL does relax somewhat between the ELMs, with a characteristic time that can be larger than ten milliseconds, much longer than the sonic particle flow time to the divertor plates. The time average density scale length measured at the separatrix increases by about a factor of two after the ELMs begin. This density scale length increases with the ELM background, as measured by the photo-diodes nearest to, but outside the outer strike-point, suggesting that recycling from the divertor targets is supplying particles that maintain the density plateau. The electron temperature profile in the SOL also broadens during an ELM, but relaxes on a much faster time scale and does not persist between ELMs.
Date: May 9, 1995
Creator: Jong, R.A.; Porter, G.D. & Groebner, R.J.
Partner: UNT Libraries Government Documents Department

High spatial and temporal resolution visible spectroscopy of the plasma edge in DIII-D

Description: In DIII-D, visible spectroscopic measurements of the He II 468.6 nm and C VI 529.2 nm Doppler broadened spectral lines, resulting from charge exchange recombination interactions between beam neutral atoms and plasma ions, are performed to determine ion temperatures, and toroidal and poloidal rotation velocities. The diagnostics system comprises 32 viewing chords spanning a typical minor radius of 63 cm across the midplane, of which 16 spatial chords span 11 cm of the plasma edge just within the separatrix. A temporal resolution of 260 {mu}s per time slice can be obtained as a result of using MCP phosphors with short decay times and fast camera readout electronics. Results from this system will be used in radial electric field comparisons with theory at the L-H transition and ion transport analysis. 6 refs., 3 figs.
Date: October 1, 1990
Creator: Gohil, P.; Burrell, K.H.; Groebner, R.J. & Seraydarian, R.P.
Partner: UNT Libraries Government Documents Department

Spectroscopic study of edge poloidal rotation and radial electric fields in the DIII-D tokamak

Description: Doppler-shift spectroscopy has shown that finite values of poloidal rotation velocity {upsilon}{sub {theta}} and of radial electric field E{sub r} exist at the edge of a tokamak plasma and that dramatic increases occur in these parameters at an L-H transition. E{sub r} is negative in the L-mode and becomes more negative in the H-mode; {upsilon}{sub {theta}} increases in magnitude at the transition. In addition, the radial gradients (shear) of {upsilon}{sub {theta}} and E{sub r} are large and these gradients also increase at the L-H transition. These results are based on measurements of Doppler shifts of light emitted by He II ions, located in a region about 1--3 cm inside the separatrix. These observations have been made with horizontally-viewing and vertically-viewing spectrometer systems on the DIII-D tokamak. The nearly orthogonal views of these systems are used to determine the plasma's flow velocity in terms of the orthogonal sets {upsilon}{sub {theta}} and {upsilon}{sub {phi}} or of {upsilon}{sub {perpendicular}} and {upsilon}{sub {parallel}}. Knowledge of {upsilon}{sub {perpendicular}} is used to calculate E{sub r} from the force balance equation for a single ion species. The existing results impose constraints on theories of the L-H transition. More detailed studies of the spatial profiles and temporal evolution of {upsilon}{sub {theta}} and E{sub r} will be made with upgraded instrumentation, which is now coming on-line. 28 refs.
Date: October 1, 1990
Creator: Groebner, R.J.; Burrell, K.H.; Gohil, P. & Seraydarian, R.P.
Partner: UNT Libraries Government Documents Department

Investigation of the effect of large core changes in toroidal plasma rotation and radial electric field on confinement in H-mode discharges in the DIII-D tokamak

Description: The plasma toroidal rotation and the radial electric field in the core ({rho}{approx lt}0.9) of H-mode discharges in DIII-D are greatly altered by the drag produced by application of static, resonant magnetic field perturbations from an external coil ( the n = 1 coil''). Transport loss due to turbulent fluctuations can in theory be reduced by E{sub r} shear stabilization or suppression. This is tested experimentally in DIII-D by using the magnetic breaking'' of rotation (with concomitant change in E{sub r}) as an independent control. The magnetic braking produces reversal of the core radial electric field, E{sub r}, and E{sub r} shear. However, the plasma maintains a negative edge ({rho}{approx lt}0.95) E{sub r} and E{sub r} shear and remains in H-mode with insignificant changes in global confinement, density profile and temperature profiles. The H-mode confinement is remarkably robust to the increasing error fields and the slowed toroidal rotation up to the onset of a locked mode which induces a transition to L-mode, the virtual cessation of plasma rotation and large reconnected islands.
Date: May 1, 1992
Creator: LaHaye, R.J.; Groebner, R.J.; Hyatt, A.W. & Scoville, J.T.
Partner: UNT Libraries Government Documents Department

Evidence for modified transport due to sheared E x B flows in high-temperature plasmas

Description: Sheared mass flows are generated in many fluids and are often important for the dynamics of instabilities in these fluids. Similarly, large values of the E x B velocity have been observed in magnetic confinement machines and there is theoretical and experimental evidence that sufficiently large shear in this velocity may stabilize important instabilities. Two examples of this phenomenon have been observed in the DIII-D tokamak. In the first example, sufficient heating power can lead to the L-H transition, a rapid improvement in confinement in the boundary layer of the plasma. For discharges with heating power close to the threshold required to get the transition, changes in the edge radial electric field are observed to occur prior to the transition itself. In the second example, certain classes of discharges with toroidal momentum input from neutral beam injection exhibit a further improvement of confinement in the plasma core leading to a regime called the VH-mode. In both examples, the region of improved confinement is characterized by an increase of shear in the radial electric field E{sub r}, reduced levels of turbulence and increases in gradients of temperatures and densities. These observations are consistent with the hypothesis that the improved confinement is caused by an increase in shear of the E x B velocity which leads to a reduction of turbulence. For the VH-mode, the dominant term controlling E{sub r} is the toroidal rotation v{sub {phi}}, indicating that the E{sub r} profile is controlled by the source and transport of toroidal momentum.
Date: November 1, 1994
Creator: Groebner, R. J.; Burrell, K. H. & Austin, M. E.
Partner: UNT Libraries Government Documents Department

A fast CCD detector for charge exchange recombination spectroscopy on the DIII-D tokamak

Description: Charge Exchange Recombination (CER) spectroscopy has become a standard diagnostic for tokamaks. CER measurements have been used to determine spatially and temporally resolved ion temperature, toroidal and poloidal ion rotation speed, impurity density and radial electric field. Knowledge of the spatial profile and temporal evolution of the electric field shear in the plasma edge is crucial to understanding the physics of the L to H transition. High speed CER measurements are also valuable for Edge Localized Mode (ELM) studies. Since the 0.52 ms minimum time resolution of our present system is barely adequate to study the time evolution of these phenomena, we have developed a new CCD detector system with about a factor of two better time resolution. In addition, our existing system detects sufficient photons to utilize the shortest time resolution only under exceptional conditions. The new CCD detector has a quantum efficiency of about 0.65, which is a factor of 7 better than our previous image intensifier-silicon photodiode detector systems. We have also equipped the new system with spectrometers of lower f/number. This combination should allow more routine operation at the minimum integration time, as well as improving data quality for measurements in the divertor-relevant region outside of the separatrix. Construction details, benchmark data and initial tokamak measurements for the new system will be presented.
Date: May 1996
Creator: Thomas, D. M.; Burrell, K. H.; Groebner, R. J. & Gohil, P.
Partner: UNT Libraries Government Documents Department

H-mode threshold power scaling and the {gradient}B drift effect

Description: One of the largest influences on the H-mode power threshold (P{sub TH}) is the direction of the ion {gradient}B drift relative to the X-point location, where factors of 2--3 increase in P{sub TH} are observed for the ion {gradient}B drift away from the X-point. It is proposed that the threshold power scaling observed in single-null configurations with the ion {gradient}B drift toward the X-point location (P{sub TH} {approximately} nB, where n is the plasma density, and B is the toroidal field) is due to the scaling of the magnitude of the {gradient}B drift effect. Hinton and later Hinton and Stebler have modeled this effect as neoclassical cross field fluxes of both heat and particles driven by poloidal temperature gradients on the open field lines in the scrape-off layer (SOL). The {gradient}B drift effect influences the power threshold by affecting the edge conditions needed for the L-H transition. It is not essential for the L-H transition itself since transitions are observed with either direction of B. Predictions of this model include saturation of the B scaling of P{sub TH} at high field, 1/B scaling of P{sub TH} with reverse B, and no B scaling of P{sub TH} in balanced double-null configurations. This last prediction is consistent with the observed scaling of p{sub TH} in double-null plasma sin DIII-D.
Date: June 1, 1997
Creator: Carlstrom, T.N.; Burrell, K.H.; Groebner, R.J. & Staebler, G.M.
Partner: UNT Libraries Government Documents Department

ROLE OF NEUTRALS IN CORE FUELING AND PEDESTAL STRUCTURE IN H-MODE DIII-D DISCHARGES

Description: OAK A271 ROLE OF NEUTRALS IN CORE FUELING AND PEDESTAL STRUCTURE IN H-MODE DIII-D DISCHARGES. The 2-D fluid code UEDGE was used to analyze DIII-D experiments to determine the role of neutrals in core fueling, core impurities, and also the H-mode pedestal structure. The authors compared the effects of divertor closure on the fueling rate and impurity density of high-triangularity, H-mode plasmas. UEDGE simulations indicate that the decrease in both deuterium core fueling ({approx} 15%-20%) and core carbon density ({approx} 15%-30%) with the closed divertor compared to the open divertor configuration is due to greater divertor screening of neutrals. They also compared UEDGE results with a simple analytic model of the H-mode pedestal structure. The model predicts both the width and gradient of the transport barrier in n{sub e} as a function of the pedestal density. The more sophisticated UEDGE simulations of H-mode discharges corroborate the simple analytic model, which is consistent with the hypothesis that fueling processes play a role in H-mode transport barrier formation.
Date: October 2002
Creator: Wolf, N. S.; Petrie, T. W.; Porter, G. D.; Rognlien, T. D.; Groebner, R. J. & Makowski, M. A.
Partner: UNT Libraries Government Documents Department

Extraction of Poloidal Velocity from Charge Exchange Recombination Spectroscopy Measurements

Description: A novel approach has been implemented on DIII-D to allow the correct determination of the plasma poloidal velocity from charge exchange spectroscopy measurements. Unlike usual techniques, the need for detailed atomic physics calculations to properly interpret the results is alleviated. Instead, the needed atomic physics corrections are self-consistently determined directly from the measurements, by making use of specially chosen viewing chords. Modeling results are presented that were used to determine a set of views capable of measuring the correction terms. We present the analysis of a quiescent H-mode discharge, illustrating that significant modifications to the velocity profiles are required in these high ion temperature conditions. We also present preliminary measurements providing the first direct comparison of the standard cross-section correction to the atomic physics calculations.
Date: July 16, 2004
Creator: Solomon, W.M.; Burrell, K.H.; Gohil, P.; Groebner, R.J. & Baylor, L.R.
Partner: UNT Libraries Government Documents Department

Comparison of Edge Plasma Perturbation During ELM Control Using One vs Two Toroidal Rows of RMP Coils in ITER Similar Shaped Plasmas on DIII-D

Description: Large Type-I edge localized modes (ELMs) were suppressed by n = 3 resonant magnetic perturbations (RMPs) from a set of internal coils (I-coil) in plasmas with an ITER similar shape at the ITER pedestal collisionality, {nu}*{sub e} {approx} 0.1 and low edge safety factor (q{sub 95} {approx} 3.6), with either a single toroidal row of the internal RMP coils or two poloidally separated rows of coils. ELM suppression with a single row of internal coils was achieved at approximately the same q{sub 95} surface-averaged perturbation field as with two rows of coils, but required higher current per coil. Maintaining complete suppression of ELMs using n = 3 RMPs from a single toroidal row of internal coils was less robust to variations in input neutral beam injection torque than previous ELM suppression cases using both rows of internal coils. With either configuration of RMP coils, maximum ELM size is correlated with the width of the edge region having good overlap of the magnetic islands from vacuum field calculations.
Date: May 21, 2008
Creator: Fenstermacher, M E; Evans, T E; Osborne, T H; Schaffer, M J; deGrassie, J S; Gohil, P et al.
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

TOROID ROTATION IN NEUTRAL BEAM HEATED DISCHARGES IN DIII-D

Description: It is known that the toroidal angular momentum and the ion thermal energy are correlated in tokamak discharges heated by neutral beam injection. Here, data from ten years of measurements on DIII-D are considered, for representative discharges from all types and all conditions. The ratio of simple replacement times for momentum and energy is found to order this correlation indicating that these times are approximately equal, across the minor radius. Representative discharges of several types are discussed in more detail, as well as transport analysis results for the momentum and thermal ion diffusivities.
Date: August 1, 2002
Creator: deGRASSIE, J.S.; BAKER, D.R.; BURRELL, K.H.; GOHIL, P.; GREENFIELD, C.M.; GROEBNER, 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

Modeling of Effects of Neutrals on the L-H Transition Power Threshold in DIII-D

Description: In order to study possible effects of neutrals on the power threshold for transition from L mode to the H mode confinement in DIII-D, the problem of reconstructing neutral particle distributions inside the separatrix from available edge, scrape-off layer, and divertor plasma diagnostic data is addressed.
Date: December 31, 1997
Creator: Owen, L.W.; Carreras, B.A.; Maingi, R.; Mioduszewski, P.K.; Carlstrom, T.N. & Groebner, R.J.
Partner: UNT Libraries Government Documents Department