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A fast reciprocating Langmuir probe for the DIII-D divertor

Description: A new reciprocating Langmuir probe has been used to measure density and temperature profiles, ion flow, and potential fluctuation levels from the lower divertor floor up to the X-point on the DIII-D tokamak. This probe is designed to make fast (2 kHz swept, 20 kHz Mach, 500 kHz Vfloat) measurements with 2 mm spatial resolution in the region where the largest gradients on the plasma open flux tubes are found and therefore provide the best benchmarks for SOL and divertor numerical models. Profiles are constructed using the 300 ms time history of the probe measurements during the 25 cm reciprocating stroke. Both single and double null plasmas can be measured and compared with a 20 Hz divertor Thomson scattering system. The probe head is constructed of four different kinds of graphite to optimize the electrical and thermal characteristics. Electrically insulated pyrolytic graphite rings act as a heat shield to absorb the plasma heat flux on the probe shaft and are mounted on a carbon/carbon composite core for mechanical strength. The Langmuir probe sampling tips are made of a linear carbon fiber composite. The mechanical, electrical, data acquisition and power supply systems design will be described. Initial measurements will also be presented.
Date: November 1, 1996
Creator: Watkins, J.G.; Hunter, J. & Tafoya, B.
Partner: UNT Libraries Government Documents Department

Enhanced scrape-off layer plasma in DIII-D double-null discharges

Description: In this paper, the authors examine a denser and broader scrape-off layer (SOL) plasma, first seen in VH mode, in the DIII-D tokamak. The enhanced SOL appears in many types of double-null (DN) discharges and is not a property of VH-mode only. The DN enhanced SOL density and temperature profiles exhibit a 5--6 cm broad profile outside the separatrix. For DN and single-null (SN) boundary geometry with similar core plasma conditions, the enhanced SOL is only observed in high triangularity discharges. The origin of the enhanced SOL is, however, not yet understood.
Date: July 1, 1994
Creator: Watkins, J. G.; Jong, R. A. & Moyer, R. A.
Partner: UNT Libraries Government Documents Department


Description: Edge localized modes (ELMs) are commonly observed in high energy confinement, tokamak plasmas and are thought to be caused by magnetohydrodynamic instabilities driven by the steep pressure gradient and the current in the plasma edge region. Our data show that the divertor magnetic balance, i.e., the degree to which the plasma topology resembles a single-null (SN) or a double-null (DN), strongly determines where particle pulses driven by ballooning instabilities at the plasma edge are distributed to surrounding vacuum vessel surfaces. These data also support the conclusions drawn from the stability analysis that ELMs are generated almost entirely on the outboard side of the main plasma.
Date: June 1, 2002
Creator: PETRIE, T.W.; WATKINS, J.G.; LAO, L.L. & SNYDER, P.B.
Partner: UNT Libraries Government Documents Department

Divertor E X B Plasma Convection in DIII-D

Description: Extensive two-dimensional measurements of plasma potential in the DIII-D tokamak divertor region are reported for standard (ion VB{sub T} drift toward divertor X-point) and reversed B{sub T} directions; for low (L) and high (H) confinement modes; and for partially detached divertor mode. The data are consistent with recent computational modeling identifying E x B{sub T} circulation, due to potentials sustained by plasma gradients, as the main cause of divertor plasma sensitivity to B{sub T} direction.
Date: July 1, 1999
Creator: Boedo, J.A.; Schaffer, M.J.; Maingi, M.; Lasnier, C.J. & Watkins, J.G.
Partner: UNT Libraries Government Documents Department


Description: Detailed measurements in two dimensions by probes and Thomson scattering reveal unexpected local electric potential and electron pressure (p{sub e}) maxima near the divertor X-point in L-mode plasmas in the DIII-D tokamak [J.L. Luxon and L.G. Davis, Fusion Technol. 8, 441 (1985)]. The potential drives E x B circulation about the X-point, thereby exchanging plasma between closed and open magnetic surfaces at rates that can be comparable to the total cross-separatrix transport. The potential is consistent with the classical parallel Ohm's law. A simple model is proposed to explain the pressure and potential hills in low power, nearly detached plasmas. Recent two-dimensional edge transport modeling with plasma drifts also shows X-point pressure and potential hills but by a different mechanism. These experimental and theoretical results demonstrate that low power tokamak plasmas can be far from poloidal uniformity in a boundary layer just inside the separatrix. Additional data, though preliminary and incomplete, suggest that E x B circulation across the separatrix might be a common feature of low confinement behavior.
Date: November 1, 2000
Partner: UNT Libraries Government Documents Department

Toroidally Asymmetric Distributions of Hydrocarbon (CD) Emission and Chemical Sputtering Sources in DIII-D

Description: Measurements in DIII-D show that the carbon chemical sputtering sources along the inner divertor and center post are toroidally periodic and highest at the upstream tile edge. Imaging with a tangentially viewing camera and visible spectroscopy were used to monitor the emission from molecular hydrocarbons (CH/CD) at 430.8 nm and deuterium neutrals in attached and partially detached divertors of low-confinement mode plasmas. In contrast to the toroidally periodic CD distribution, emission from deuterium neutrals was observed to be toroidally symmetric along the inner strike zone. The toroidal distribution of the measured tile surface temperature in the inner divertor correlates with that of the CD emission, suggesting larger parallel particle and heat fluxes to the upstream tile edge, either due to toroidal tile gaps or height steps between adjacent tiles.
Date: May 16, 2006
Creator: Groth, M; Brooks, N H; Fenstermacher, M E; Lasnier, C J; McLean, A G & Watkins, J G
Partner: UNT Libraries Government Documents Department

Effect of Magnetic Geometry on ELM Heat Flux Profiles

Description: In this paper we explore how precisely the magnetic up/down symmetry must be controlled to insure sharing of edge localized mode (ELM) heat flux between upper and lower diverters in a double-null tokamak. We show for DIII-D, using infrared thermography, that the spatial distribution of Type-I ELM energy is less strongly affected by variations in magnetic geometry than is the time-averaged peak heat flux in attached discharges. The degree of control necessary to share ELM heat flux deposition equally between diverters was less stringent than the control needed to balance the time averaged heat flux. ELM energy is transported more than four times further into the scrape-off layer than the time-averaged heat flux.
Date: May 15, 2000
Creator: Lasnier, C.J.; Leonard, A.W.; Petrie, T.W. & Watkins, J.G.
Partner: UNT Libraries Government Documents Department

Reciprocating and fixed probe measurements of n{sub e} and T{sub e} in the DIII-D divertor

Description: This paper describes divertor density and temperature measurements using both a new reciprocating Langmuir probe (XPT-RCP) which plunges vertically above the divertor floor up to the X-point height and swept, single, Langmuir probes fixed horizontally across the divertor floor. These types of measurements are important for testing models of the SOL and divertor which then are used to design plasma facing components in reactor size tokamaks. This paper presents an overview of the new divertor probe measurements and how they compare with the new divertor Thomson scattering system. The fast time response of the probe measurements allows detailed study of ELMs.
Date: November 1, 1996
Creator: Watkins, J.G.; Moyer, R.A.; Cuthbertson, J.W.; Buchenauer, D.A.; Carlstrom, T.N.; Hill, D.N. et al.
Partner: UNT Libraries Government Documents Department

Reactivity of Metal Ions Bound to Water-Soluble Polymers

Description: The intent of this work is to determine the effectiveness of catalysts covalently bound to polymers and to understand the consequences of supporting the catalysts on catalyst efficiency and selectivity. Rhodium phosphine complexes with functional groups for coupling to polymers were prepared. These catalyst precursors were characterized using standard techniques including IR, NMR, and elemental analysis. Studies on the modified catalysts showed that they were still active hydrogenation catalysts. However, tethering of the catalysts to polyamines gave systems with low hydrogenation activity. Analogous biphasic systems were also explored. Phosphine ligands with a surfactant-like structure have been synthesized and used to prepare catalytically active complexes of palladium. The palladium complexes were utilized in Heck-type coupling reactions (e.g. coupling of iodobenzene and ethyl acrylate to produce ethyl cinnamate) under vigorously stirred biphasic reaction conditions, and were found to offer superior performance over a standard water-soluble palladium catalyst under analogous conditions.
Date: June 29, 1999
Creator: Sauer, N.N.; Watkins, J.G.; Lin, M.; Birnbaum, E.R.; Robison, T.W.; Smith, B.F. et al.
Partner: UNT Libraries Government Documents Department

Asymmetric catalysis in organic synthesis

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The goal of the project was to prepare new catalyst systems, which would perform chemical reactions in an enantioselective manner so as to produce only one of the possible optical isomers of the product molecule. The authors have investigated the use of lanthanide metals bearing both diolate and Schiff-base ligands as catalysts for the enantioselective reduction of prochiral ketones to secondary alcohols. The ligands were prepared from cheap, readily available starting materials, and their synthesis was performed in a ''modular'' manner such that tailoring of specific groups within the ligand could be carried out without repeating the entire synthetic procedure. In addition, they have developed a new ligand system for Group IV and lanthanide-based olefin polymerization catalysts. The ligand system is easily prepared from readily available starting materials and offers the opportunity to rapidly prepare a wide range of closely related ligands that differ only in their substitution patterns at an aromatic ring. When attached to a metal center, the ligand system has the potential to carry out polymerization reactions in a stereocontrolled manner.
Date: November 1, 1998
Creator: Reilly, S.D.; Click, D.R.; Grumbine, S.K.; Scott, B.L. & Watkins, J.G.
Partner: UNT Libraries Government Documents Department

Observation of SOL Current Correlated with MHD Activity in NBI-heated DIII-D Tokamak Discharges

Description: This work investigates the potential roles played by the scrape-off-layer current (SOLC) in MHD activity of tokamak plasmas, including effects on stability. SOLCs are found during MHD activity that are: (1) slowly growing after a mode-locking-like event, (2) oscillating in the several kHz range and phase-locked with magnetic and electron temperature oscillations, (3) rapidly growing with a sub-ms time scale during a thermal collapse and a current quench, and (4) spiky in temporal behavior and correlated with spiky features in Da signals commonly identified with the edge localized mode (ELM). These SOLCs are found to be an integral part of the MHD activity, with a propensity to flow in a toroidally non-axisymmetric pattern and with magnitude potentially large enough to play a role in the MHD stability. Candidate mechanisms that can drive these SOLCs are identified: (a) toroidally non-axisymmetric thermoelectric potential, (b) electromotive force (EMF) from MHD activity, and (c) flux swing, both toroidal and poloidal, of the plasma column. An effect is found, stemming from the shear in the field line pitch angle, that mitigates the efficacy of a toroidally non-axisymmetric SOLC to generate a toroidally non-axisymmetric error field. Other potential magnetic consequences of the SOLC are identified: (i) its error field can introduce complications in feedback control schemes for stabilizing MHD activity and (ii) its toroidally non-axisymmetric field can be falsely identified as an axisymmetric field by the tokamak control logic and in equilibrium reconstruction. The radial profile of a SOLC observed during a quiescent discharge period is determined, and found to possess polarity reversals as a function of radial distance.
Date: March 26, 2004
Creator: Takahashi, H.; Fredrickson, E.D.; Schaffer, M.J.; Austin, M.E.; Evans, T.E.; Lao, L.L. et al.
Partner: UNT Libraries Government Documents Department


Description: The evolution of 2-D emission profiles of D{sub {alpha}} and C III during type-I ELMs has been investigated in DIII-D using a tangentially viewing gated, intensified charge-injected device (CID) camera. The measured CIII emission profiles indicate transient inner leg attachment with the arrival of the ELM heat pulse. The measured D{sub {alpha}} emission profiles during an ELM cycle show enhanced deuterium recycling during the deposition of the ELM particle pulse at the target, which suggests the detachment of the divertor plasma from the target plates. Measurements taken in ELMy H-mode discharges at densities of 50% and 90% of the Greenwald density limit are compared utilizing the CID camera system and a comprehensive set of other divertor diagnostics. An ELM model based on fluid and PIC simulations is used to discuss the observation on the response of the divertor plasma to the ELM heat and particle pulse.
Date: June 1, 2002
Partner: UNT Libraries Government Documents Department

Halo recycler for the Tandem Mirror Experiment Upgrade

Description: The halow recycler is a mechanical device on the Tandem Mirror Experiment Upgrade (TMX-U) that is designed to increase the density at the plasma edge providing enhanced shielding of the core plasma from background neutrals. Each recycler body, which consists of coaxial annular cylinders and a adjacent plenum closely follows the magnetic flux tubes in the transition region of TMX-U. Each plenum is provided with gas feed and pressure measurement systems as well as Langmuir probes. This paper describes the design, fabrication, installation, and operation of these devices.
Date: November 14, 1985
Creator: Brooksby, C.A.; Allen, S.L.; Pedrotti, L.R.; Simonen, T.C.; Wood, B.E.; Hsu, W.L. et al.
Partner: UNT Libraries Government Documents Department

Edge Stability and Transport Control with Resonant Magnetic Perturbations in Collisionless Tokamak Plasmas

Description: A critical issue for fusion plasma research is the erosion of the first wall of the experimental device due to impulsive heating from repetitive edge magneto-hydrodynamic (MHD) instabilities known as 'edge-localized modes' (ELMs). Here, we show that the addition of small resonant magnetic field perturbations completely eliminates ELMs while maintaining a steady-state high-confinement (H-mode) plasma. These perturbations induce a chaotic behavior in the magnetic field lines, which reduces the edge pressure gradient below the ELM instability threshold. The pressure gradient reduction results from a reduction in particle content of the plasma, rather than an increase in the electron thermal transport. This is inconsistent with the predictions of stochastic electron heat transport theory. These results provide a first experimental test of stochastic transport theory in a highly rotating, hot, collisionless plasma and demonstrate a promising solution to the critical issue of controlling edge instabilities in fusion plasma devices.
Date: June 13, 2006
Creator: Evans, T E; Moyer, R A; Burrell, K H; Fenstermacher, M E; Joseph, I; Leonard, A W et al.
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 and profiles of heat flux during partial detachment in DIII-D

Description: The authors examine the scaling of the peak divertor heat flux and total divertor plate power in partially detached divertor (PDD) discharges in DIII-D, as a function of input power and radiated power. The peak divertor heat flux in the attached plasma increases linearly with input power, but saturates in the detached cases. The total divertor plate power remains linear with input power in both the attached and detached plasmas. This is consistent with the fact that the heat flux peak is reduced from the attached case but other areas receive increased radiant heating from the detached plasma. The divertor plate radiant heating is linear with input power because the total radiated power from the entire plasma is a linear function of input power in both attached and detached plasma. In the private flux region, radiated heat flux absorbed on the target plate calculated from bolometer data is enough to account for the measured plate heating. Approximately half of the overall plate heating power in detached plasma is due to absorbed radiation. By mapping the divertor heat flux before and during the PDD to flux coordinates, and comparing with a flux mapping of inserted bolometer and tangential TV data, they have verified that the radiated power is emitted from the same flux surfaces on which heat flux is reduced.
Date: August 1998
Creator: Lasnier, C. J.; Hill, D. N.; Allen, S. L.; Fenstermacher, M. E.; Porter, G. D.; Leonard, A. W. et al.
Partner: UNT Libraries Government Documents Department

An evaluation of kinetic effects in the DIII-D divertor

Description: Under certain conditions, high temperature electrons diffusing across the separatrix near the midplane can propagate to the divertor without collision and produce a high energy tail in the electron distribution function near the separatrix. The authors evaluate a kinetic criterion along the field using measurements from divertor Thomson scattering and compare temperature measurements from target plate Langmuir probes to estimate the magnitude of the high energy tail. A Fokker-Planck kinetic model, which includes electron-electron, electron-ion, electron-neutral, and charge exchange interactions is used to estimate the size of the high energy tail. For the cases considered, the authors found less than 1% of non-thermal particles near the target plate at typical densities and power levels for the DIII-D scrape-off layer plasma. ELMs violate the kinetic criterion.
Date: August 1, 1998
Creator: Watkins, J.G.; Batishchev, O.; Boedo, J.A.; Lehmer, R.; Moyer, R.A.; Hill, D.N. et al.
Partner: UNT Libraries Government Documents Department

Measurements of flows in the DIII-D divertor by Mach probes

Description: First measurements of Mach number of background plasma in the DIII-D divertor are presented in conjunction with temperature T{sub e} and density n{sub e} using a fast scanning probe array. To validate the probe measurements, the authors compared the T{sub e}, n{sub e} and J{sub sat} data to Thomson scattering data and find good overall agreement in attached discharges and some discrepancy for T{sub e} and n{sub e} in detached discharges. The discrepancy is mostly due to the effect of large fluctuations present during detached plasmas on the probe characteristic; the particle flux is accurately measured in every case. A composite 2-D map of measured flows is presented for an ELMing H-mode discharge and they focus on some of the details. They have also documented the temperature, density and Mach number in the private flux region of the divertor and the vicinity of the X-point, which are important transition regions that have been little studied or modeled. Background parallel plasma flows and electric fields in the divertor region show a complex structure.
Date: June 1, 1998
Creator: Boedo, J.A.; Lehmer, R.; Moyer, R.A.; Watkins, J.G.; Porter, G.D.; Evans, T.E. 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