29 Matching Results

Search Results

Advanced search parameters have been applied.

Judicial commitment of mental patients in Dallas County

Description: Since mental illness has legal as well as medical aspects, it is the purpose of this thesis to consider one part of the relationship between the courts, the mentally ill person, and the state hospital. This part is concerned with the problem of how mentally ill persons are selected and committed by legal proceedings in the state of Texas for treatment in a mental hospital.
Date: January 1954
Creator: Rhodes, Albert L.
Partner: UNT Libraries

Study of the phase transition dynamics of the L to H transition

Description: A highly radiating zone (MARFE) just above the divertor X-point has been used to access the marginal transition regime P{sub sep} {approx} P{sub thres} to study the existence of a critical point for the L to H transition. Phase transition models predict that at the critical point, the transition duration increases and the plasma parameters vary continuously between L-mode and H-mode. In these experiments, the L to H transition duration increased 50--100 times over fast transitions. However, the evolution of E{sub r} shear, edge density gradient, H-mode pedestal, and fluctuations is essentially unchanged from that in fast transitions. The only difference is in the speed with which and the degree to which the fluctuation amplitudes are transiently reduced. This difference is understandable in terms of the time scales for fluctuation amplitude reduction ({le} 100 {micro}s) and edge pressure gradient increase (several ms), provided the edge fluctuations are pressure-gradient driven.
Date: September 1, 1997
Creator: Moyer, R. A.; Rhodes, T. L. & Rettig, C. L.
Partner: UNT Libraries Government Documents Department

Refractive and Relativistic Effects on ITER Low Field Side Reflectometer Design

Description: The ITER low field side reflectometer faces some unique design challenges, among which are included the effect of relativistic electron temperatures and refraction of probing waves. This paper utilizes GENRAY, a 3- D ray tracing code, to investigate these effects. Using a simulated ITER operating scenario, characteristics of the reflected RF waves returning to the launch plane are quantified as a function of a range of design parameters, including antenna height, antenna size, and antenna radial position. Results for edge/SOL measurement with both O- and X-modes using proposed antennas are reported.
Date: June 1, 2010
Creator: Wang, G.; Rhodes, T. L.; Peebles, W. A.; Harvey, R. W. & Budny, R. V.
Partner: UNT Libraries Government Documents Department

Studies of H-Mode Plasmas Produced Directly by Pellet Injection in the DIII-D Tokamak

Description: A key issue for the physics of H-mode plasmas is to determine which plasma quantities are critical for the formation of the edge transport barrier. One approach is to directly perturb the edge plasma and observe the subsequent changes. In DIII-D, pellet injection has been used to directly change the edge plasma conditions and produce H-mode transitions. One hypothesis for the H-mode transition is that the attainment of a critical edge electron temperature is required for the transition [1-3]. This hypothesis is disproved in this paper. H-mode transitions were produced by injecting frozen deuterium pellets of diameter 2.7 mm from the inner wall of the DIII-D vessel into the high toroidal field side (HFS) and from the outer wall into the low field side (LFS) of the plasma. Both the HFS and LFS pellets produced significant increases in the edge electron density, which led to substantial reductions in the edge electron and ion temperatures. However, H-mode transitions were still produced with the lowered edge temperatures, implying that a critical edge temperature is not necessary for H-mode transitions. The pellet induced H-mode plasma exhibited clear pedestals in electron density and electron and ion temperatures at the plasma edge and persisted for the duration of the applied neutral beam power. The HFS pellet's penetration and deposition profiles were substantially deeper (up to {rho} {approx} 0.2) than that of the LFS pellet (up to {rho} {approx} 0.7). However, since both HFS and LFS pellets produced H-mode transitions, this implies that pellet penetration depth is not important the important factor is the large increase in the electron density right at the plasma edge produced by both types of pellets. The values of the edge plasma quantities at the H-mode transition were expressed in the parametric terms described in several theories and models of the ...
Date: August 1, 2000
Creator: Gohil, P.; Baylor, L.R.; Jernigan, T.C.; Burrell, K.H.; Carlstrom, T.N.; McKee, G.R. et al.
Partner: UNT Libraries Government Documents Department

TRANSPORT STUDIES IN DIII-D WITH MODULATED ECH

Description: Experiments have been performed where the T{sub e} profile stiffness was tested at several spatial locations by varying the ECH resonance location. Propagation of the pulses was Fourier analyzed and compared to simulations based on several transport models. The plasma appears to be near the critical T{sub e} gradient for ETG modes and marginally stable to ITG modes. However, the local T{sub e} response to a locally applied heat pulse does not indicate a nonlinear, critical gradient model where T{sub e} is clipped when trying to rise above a critical gradient. The response can be simply understood as the plasma integrating the ECH power, producing an increase in T{sub e} which equilibrates to a new local level with an exponential time constant representing the local confinement time.
Date: July 1, 2002
Creator: DeBOO, J.C.; AUSTIN, M.E.; BRAVENEC, R.V.; KINSEY, J.E; LOHR, J.; LUCE, T.C. et al.
Partner: UNT Libraries Government Documents Department

Turbulent Radial Correlation Lengths in the DIII-D Tokamak

Description: Measurements of the radial correlation length Ar of density fluctuations have been made on the DIII-D tokamak in a variety of L-mode discharges. These measurements span the radial region 0.5 < {rho} < 1 and are found to scale approximately as {rho}{sub {theta},s} or 5-10 {rho}{sub s}. Here {rho}{sub {theta},s} is the poloidal ion Larmor radius calculated using local T{sub e} and poloidal magnetic field and {rho}{sub s} is the same except calculated using the total magnetic field. The {Delta}r data were obtained from a heterodyne reflectometer system. Comparisons to published analytic formulas of Ar have been carried out for a particular discharge condition. The measurements are found to be comparable in magnitude and radial dependence with a slab type formulation of ion temperature gradient (ITG) driven turbulence as well as an electron drift wave turbulence type prediction. Predictions from toroidal ITG and a different slab ITG model were found to be outside the error bars of the measurements. In addition, a detailed comparison to a non-linear gyro-kinetic turbulence code has begun. These and other similar comparisons are believed to be important as they serve to test and benchmark theory and codes as well as to help identify the type(s) of turbulence involved.
Date: August 1, 2000
Creator: Rhodes, T.L.; Leboeuf, J.-N.; Sydora, R.; Doyle, E.J.; Moyer, R.A.; Rettig, C.L. et al.
Partner: UNT Libraries Government Documents Department

Study of Aspect Ratio Effects on Kinetic MHD Instabilities in NSTX and DIII-D

Description: We report general observations of kinetic instabilities on the low aspect-ratio National Spherical Torus Experiment (NSTX) and describe explicit aspect ratio scaling studies of kinetic instabilities using both the NSTX and the DIII-D tokamak. The NSTX and the DIII-D tokamak are nearly ideal for such experiments, having a factor of two difference in major radius but otherwise similar parameters. We also introduce new theoretical work on the physics of kinetic ballooning modes (KBM), toroidal Alfven eigenmodes (TAE), and compressional Alfven eigenmodes (CAE) with applications to NSTX.
Date: October 21, 2004
Creator: Fredrickson, E.D.; Heidbrink, W.W.; Cheng, C.Z.; Gorelenkov, N.N.; Belova, E.; Hyatt, A.W. et al.
Partner: UNT Libraries Government Documents Department

EFFECT OF THE B DRIFT DIRECTION ON PLASMA EDGE PROPERTIES AND THEL-H TRANSITION ON DIII-D

Description: The power threshold for the L-H transition, P{sub TH}, is low when the ion {del}B drift is toward the X-point and increases significantly when it is away from the X-point. In order to study the cause of this effect, we have compared lower single-null (LSN) discharges with upper single-null (USN) discharges where the ion {del}B drift direction is down in both cases. Since many plasma parameters change with input power, we have made comparisons at the same power level (PTOT=2.3 MW). For these experiments, the LSN discharge is just below PTH (2.7 MW) and the USN discharge is far from P{sub TH} (6.8 MW). We have measured various properties of the edge plasma in an attempt to identify changes that may be responsible for the difference in P{sub TH} for the two cases. The equilibrium flux surfaces and diagnostic measurement locations are shown. The most pronounced difference is the reversal and the increased shear in the poloidal group velocity of the density fluctuations near the plasma edge [1]. These results complement a previous study where the plasma configuration was held fixed and the toroidal field was reversed [2]. In that study, differences in the divertor and X-point plasma were measured. In the present work, the X-point could not be located in the range of the divertor Thomson scattering diagnostic due to top/bottom symmetry issues, and those measurements are not available.
Date: July 1, 2002
Creator: CARLSTROM, T.N.; GROEBNER, R.J.; McKEE, G.R.; MOYER, R.A.; RHODES, T.L.; ROST, J.C. et al.
Partner: UNT Libraries Government Documents Department

MECHANISMS FOR ELECTRON TRANSPORT BARRIER FORMATION IN THE DIII-D TOKAMAK

Description: The E x B shear stabilization paradigm explains much of the phenomenology of ion thermal transport in tokamaks. Behavior in the electron channel, however, has continued to challenge our understanding. Recent experiments in DIII-D and elsewhere produce regions where electron thermal transport is almost completely eliminated with intense, localized, direct electron heating. Simulations of DIII-D discharges identify {alpha}-stabilization, local magnetic shear stabilization due to the Shafranov shift, as the dominant turbulence reduction mechanism in these experiments and may point the way toward regimes with simultaneous electron and ion internal transport barriers.
Date: February 1, 2001
Creator: GREENFIELD, C.M.; PRATER, R.; STAEBLER, G.M.; KINSEY, J.E.; BURRELL, K.H.; BOO, J.C. De et al.
Partner: UNT Libraries Government Documents Department

PROGRESS IN QUANTIFYING THE EDGE PHYSICS OF H-MODE REGIME IN DIII-D

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
Creator: GROEBNER, R.J.; BAKER, D.R.; BOEDO, J.A.; BURRELL, K.H.; CARLSTROM, T.N.; DERANIAN, R.D. et al.
Partner: UNT Libraries Government Documents Department

Nanoindentation and nanoscratching of hard coating materials for magnetic disks

Description: Nanoindentation and nanoscratching experiments have been performed to assess the mechanical and tribological behavior of three thin film materials with potential application as wear resistant coatings for magnetic disk storage: (1) hydrogenated-carbon (CHx); (2) nitrogenated-carbon (CNx); and (3) boron suboxide (BOx). The hardness and elastic modulus were measured using nanoindentation. Ultra-low load nanoscratching tests were performed to assess the relative scratch resistance of the films and measure their friction coefficients. The mechanical and tribological performance of the three materials are discussed and compared.
Date: December 31, 1994
Creator: Tsui, T.Y.; Pharr, G.M.; Oliver, W.C.; Chung, Y.W.; Cutiongco, E.C.; Bhatia, C.S. et al.
Partner: UNT Libraries Government Documents Department

Optimization of negative central shear discharges in shaped cross sections

Description: Magnetohydrodynamic (MHD) stability analyses of Negative Central Shear (NCS) equilibria have revealed a new understanding of the limiting MHD instabilities in NCS experiments. Ideal stability calculations show a synergistic effect between cross section shape and pressure profile optimization; strong shaping and broader pressure independently lead to moderately higher {Beta} limits, but broadening of the pressure profile in a strongly dee-shaped cross- section leads to a dramatic increase in the ideal {Beta} limit. Localized resistive interchange (RI) modes can be unstable in the negative shear region and are most restrictive for peaked pressure profiles. Resistive global modes can also be destabilized significantly below the ideal P limit. Experiments largely confirm the general trends, and diagnostic measurements and numerical stability calculations are found to be in good qualitative agreement. Observed disruptions in NCS discharges with L-mode edge and strongly peaked pressure, appear to be initiated by interactions between the RI, and the global ideal and resistive modes.
Date: October 1, 1996
Creator: Turnbull, A.D., Chu, M.S., Taylor, T.S., Casper, T.A., Rice, B.W.; Greene, J.M., Greenfield, C.M., La Haye, R.J., Lao, L.L., Lee, B.J.; Miller, R.L., Ren, C., Strait, E.J., Tritz, K.; Rettig, C.L., Rhodes, T.L. & Sauter, O.
Partner: UNT Libraries Government Documents Department

A Comparison of Plasma Performance Between Single-Null and Double-Null Configurations During Elming H-Mode

Description: Tokamak plasma performance generally improves with increased shaping of the plasma cross section, such as higher elongation and higher triangularity. The stronger shaping, especially higher triangularity, leads to changes in the magnetic topology of the divertor. Because there are engineering and divertor physics issues associated with changes in the details of the divertor flux geometry, especially as the configuration transitions from a single-null (SN) divertor to a marginally balanced double-null (DN) divertor, we have undertaken a systematic evaluation of the plasma characteristics as the magnetic geometry is varied, particularly with respect to (1) energy confinement, (2) the response of the plasma to deuterium gas fueling, (3) the operational density range for the ELMing H-mode, and (4) heat flux sharing by the diverters. To quantify the degree of divertor imbalance (or equivalently, to what degree the shape is double-null or single-null), we define a parameter DRSEP. DRSEP is taken as the radial distance between the upper divertor separatrix and the lower divertor separatrix, as determined at the outboard midplane. For example, if DRSEP=O, the configuration is a magnetically balanced DN; if DRSEP = +1.0 cm, the divertor configuration is biased toward the upper divertor. Three examples are shown in Fig. 1. In the following discussions, VB drift is directed toward the lower divertor.
Date: July 1, 1999
Creator: Petrie, T.W.; Fenstermacher, M.E.; Allen, S.L.; Carlstrom, T.N.; Gohil, P.; Groebner, R.J. et al.
Partner: UNT Libraries Government Documents Department

RECENT EXPERIMENTAL STUDIES OF EDGE AND INTERNAL TRANSPORT BARRIERS IN THE DIII-D TOKAMAK

Description: Results from recent experiments on the DIII-D tokamak have revealed many important details on transport barriers at the plasma edge and in the plasma core. These experiments include: (a) the formation of the H-mode edge barrier directly by pellet injection; (b) the formation of a quiescent H-mode edge barrier (QH-mode) which is free from edge localized modes (ELMs), but which still exhibits good density and radiative power control; (c) the formation of multiple transport barriers, such as the quiescent double barrier (QDB) which combines a internal transport barrier with the quiescent H-mode edge barrier. Results from the pellet-induced H-mode experiments indicate that: (a) the edge temperature (electron or ion) is not a critical parameter for the formation of the H-mode barrier, (b) pellet injection leads to an increased gradient in the radial electric field, E{sub r}, at the plasma edge; (c) the experimentally determined edge parameters at barrier transition are well below the predictions of several theories on the formation of the H-mode barrier, (d) pellet injection can lower the threshold power required to form the H-mode barrier. The quiescent H-mode barrier exhibits good density control as the result of continuous magnetohydrodynamic (MHD) activity at the plasma edge called the edge harmonic oscillation (EHO). The EHO enhances the edge particle transport while maintaining a good energy transport barrier. The ability to produce multiple barriers in the QDB regime has led to long duration, high performance plasmas with {beta}{sub NH{sub 89}} values of 7 for up to 10 times the confinement time. Density profile control in the plasma core of QDB plasmas has been demonstrated using on-axis ECH.
Date: August 1, 2002
Creator: GOHIL, P.; BAYLOR, L.R.; BURRELL, K.H.; CASPER, T.A.; DOYLE, E.J.; GREENFIELD, C.M. et al.
Partner: UNT Libraries Government Documents Department

ATF (Advanced Toroidal Facility) edge plasma turbulence studies using a fast reciprocating Langmuir probe

Description: Electrostatic turbulence on the edge of the Advanced Torodial Facility (ATF) torsatron is investigated experimentally with a fast reciprocating Langmuir probe (FRLP) array. Initial measurements of plasma electron density n{sub e} and temperature T{sub e} and fluctuations in density ({tilde n}{sub e}) and plasma floating potential ({tilde {phi}}{sub f}) are made in ECH plasmas at 1 T. At the last closed flux surface (LCFS, r/{bar a} {approximately}1), T{sub e} {approx} 20--40 eV and n{sub e} {approx} 10{sup 12} cm{sup {minus}3} for a line-averaged electron density {bar n}{sub e} = (3--6) {times} 10{sup 12} cm{sup {minus}3}. Relative fluctuation levels, as the FRLP is moved into core plasma where T{sub e} > 20 eV, are {tilde n}{sub e}/n{sub e} {approx} 5%, and e {tilde {phi}}{sub f}/T{sub e} {approx} 2{tilde n}{sub e}/n{sub e} about 2 cm inside the LCFS. The observed fluctuation spectra are broadband (40--300 kHz) with {bar k}{rho}{sub s} {le} 0.1, where {bar k} is the wavenumber of the fluctuations and {rho}{sub s} is the ion Larmor radius at the sound speed. The propagation direction of the fluctuations reverses to the electron diamagnetic direction around r/{bar a} < 1. The phase velocity and the electron drift velocity are comparable (v{sub ph} {approximately} v{sub de}). The fluctuation-induced particle flux is comparable to fluxes estimated from the particle balance using the H{sub {alpha}} spectroscopic measurements. Many of the features seen in these experiments resemble the features of ohmically heated plasmas in the Texas Experimental Tokamak (TEXT). 17 refs., 10 figs.
Date: January 1, 1990
Creator: Uckan, T.; Hidalgo, C.; Bell, J.D.; Harris, J.H.; Dunlap, J.L.; Dyer, G.R. et al.
Partner: UNT Libraries Government Documents Department

ELM Suppression in Low Edge Collisionality H-Mode Discharges Using n=3 Magnetic Perturbations

Description: Using resonant magnetic perturbations with toroidal mode number n = 3, we have produced H-mode discharges without edge localized modes (ELMs) which run with constant density and radiated power for periods up to about 2550 ms (17 energy confinement times). These ELM suppression results are achieved at pedestal collisionalities close to those desired for next step burning plasma experiments such as ITER and provide a means of eliminating the rapid erosion of divertor components in such machines which could be caused by giant ELMs. The ELM suppression is due to an enhancement in the edge particle transport which reduces the edge pressure gradient and pedestal current density below the threshold for peeling-ballooning modes. These n = 3 magnetic perturbations provide a means of active control of edge plasma transport.
Date: July 11, 2005
Creator: Burrell, K H; Evans, T E; Doyle, E J; Fenstermacher, M E; Groebner, R J; Leonard, A W et al.
Partner: UNT Libraries Government Documents Department

QUIESCENT DOUBLE BARRIER H-MODE PLASMAS IN THE DIII-D TOKAMAK

Description: High confinement (H-mode) operation is the choice for next-step tokamak devices based either on conventional or advanced tokamak physics. This choice, however, comes at a significant cost for both the conventional and advanced tokamaks because of the effects of edge localized modes (ELMs). ELMs can produce significant erosion in the divertor and can affect the beta limit and reduced core transport regions needed for advanced tokamak operation. Experimental results from DIII-D [J.L. Luxon, et al., Plasma Phys. and Contr. Nucl. Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987) Vol. I, p. 159] this year have demonstrated a new operating regime, the quiescent H-mode regime, which solves these problems. We have achieved quiescent H-mode operation which is ELM-free and yet has good density and impurity control. In addition, we have demonstrated that an internal transport barrier can be produced and maintained inside the H-mode edge barrier for long periods of time (&gt;3.5 seconds or &gt;25 energy confinement times {tau}{sub E}), yielding a quiescent double barrier regime. By slowly ramping the input power, we have achieved {beta}{sub N} H{sub 89} = 7 for up to 5 times the {tau}{sub E} of 150 ms. The {beta}{sub N} H{sub 89} values of 7 substantially exceed the value of 4 routinely achieved in standard ELMing H-mode. The key factors in creating the quiescent H-mode operation are neutral beam injection in the direction opposite to the plasma current (counter injection) plus cryopumping to reduce the density. Density and impurity control in the quiescent H-mode is possible because of the presence of an edge magnetic hydrodynamic (MHD) oscillation, the edge harmonic oscillation, which enhances the edge particle transport while leaving the energy transport unaffected.
Date: November 1, 2000
Creator: BURRELL, K.H.; AUSTIN, M.E.; BRENNAN, D.P.; DeBOO, J.C.; DOYLE, E.J.; FENZI, C. et al.
Partner: UNT Libraries Government Documents Department