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New concepts for drift pumping a thermal barrier with rf

Description: Pump neutral beams, which are directed into the loss cone of the TMX-U plugs, are normally used to pump ions from the thermal barriers. Because these neutral beams introduce cold gas that reduces pumping efficiency, and require a straight line entrance and exit from the plug, alternate methods are being investigated to provide barrier pumping. To maintain the thermal barrier, either of two classes of particles can be pumped. First, the collisionally trapped ions can be pumped directly. In this case, the most promising selection criterion is the azimuthal drift frequency. Second, the excess sloshing-ion density can be removed, allowing the use of increased sloshing-beam density to pump the trapped ions. The selection mechanism in this case is the Doppler-shifted ion-cyclotron resonance of the high-energy sloshing-ions (3 keV less than or equal to U/sub parallel/ less than or equal to 10 keV).
Date: May 9, 1985
Creator: Barter, J.D.; Baldwin, D.; Chen, Y. & Poulsen, P.
Partner: UNT Libraries Government Documents Department

ICRF heating of passing ions in a thermal barrier tandem mirror

Description: Ion heating is used in the central cells of tandem mirrors to reduce the collisional trapping of passing ions in the end cell thermal barriers. In this paper, we reevaluate ICRF heating of the TMX-U central cell in two limits. The first we term isotropic, because we impose the condition that ions heated in the perpendicular direction be confined for at least one 90/sup 0/ scattering time, thereby heating the passing ions. The second we call anisotropic heating. It uses higher ICRF power to mirror trap a majority of the ions near the midplane, thereby reducing the density and collisionality of passing ions. Anisotropic heating has the advantage of increasing with ICRF power, whereas isotropic heating is limited by ion collisionality. Both techniques require gas fueling near the central cell midplane, with an ion cyclotron resonance toward each end cell to heat the cold ions.
Date: May 1, 1985
Creator: Molvik, A.W.; Dimonte, G.; Campbell, R.; Barter, J.; Cummins, W.F.; Falabella, S. et al.
Partner: UNT Libraries Government Documents Department

Ion cyclotron resonant heating 2 x 170/sup 0/ loop antenna for the Tandem Mirror Experiment-Upgrade

Description: This paper reviews the mechanical design and improvements that have taken place on the loop type ion cyclotron resonance heating (ICRH) antennas that are located in the center cell region of the Tandem Mirror Experiment-Upgrade (TMX-U).
Date: November 14, 1985
Creator: Brooksby, C.A.; Ferguson, S.W.; Molvik, A.W. & Barter, J.
Partner: UNT Libraries Government Documents Department

X-band microwave backscattering from ocean waves

Description: Backscattering experiments at microwave frequencies were conducted off the west coast of Scotland in the summer of 1991. Using a dual-polarization, 8-frequency X-band coherent scatterometer mounted on the bow of a boat, we measured time-resolved backscattering from ocean waves at a range of grazing angles from 10{degrees} to 70{degrees}. From the grazing-angle-dependent signals and their Doppler spectra, we differentiate Bragg scattering from non-Bragg scattering and resolve ``peak separation`` between the vertical and horizontal polarizations. We observe instances of ``super`` events, i.e., instances when the horizontal polarization return power equals or exceeds the vertical polarization power. We find that ``super`` events occur not only at low grazing angles but at any grazing angle for against-wind viewing directions. Statistics for such occurrences as a function of grazing angle are obtained. We study the coherence properties of scatterers and find strong evidence that at low grazing angles, lifetime-dominated, non-Bragg scattering contributes noticeably to returns of both polarizations, but is dominant in providing returns for the horizontal polarization. We examine ``spiking`` events and find that they can be related to, but need not be limited to, breaking wave events. By comparing the data of against-wind runs with cross-wind and circle runs, we obtain wind-direction dependence of Doppler spectra which further assists in the identification of scattering mechanisms.
Date: January 4, 1994
Creator: Lee, P. H. Y.; Barter, J. D. & Beach, K. L.
Partner: UNT Libraries Government Documents Department

ICRF heating of passing ions in TMX-U

Description: By placing ion-cyclotron resonant frequency (ICRF) antennas on both sides of a midplane gas-feed system in the central cell of the Tandem Mirror Experiment-Upgrade (TMX-U), our results have improved in the following areas: (a) The end losses out both ends show a factor of 3 to 4 increase in passing-ion temperatures and a factor of 2 to 3 decrease in passing-ion densities. (b) The passing-ion heating is consistent with Monte Carlo predictions. (c) The plasma density can be sustained by ICRF plus gas fueling as observed on other experiments.
Date: April 1, 1986
Creator: Molvik, A.W.; Dimonte, G.; Barter, J.; Campbell, R.; Cummins, W.F.; Falabella, S. et al.
Partner: UNT Libraries Government Documents Department

ICRF heating in the Tandem Mirror Experiment-Upgrade (TMX-U)

Description: Central cell plasma in Tandem Mirror Experiment-Upgrade (TMX-U) are heated with 100 kW of ICRF transmitter power to ion temperatures of 1.5 keV at densities of 2 x 10/sup 12/ cm/sup -3/. We have used two Faraday-shielded antennas: the first had one 90/sup 0/ loop; and the second, in current use, has two 170/sup 0/ loops connected in an m = 1 configuration. We are also installing a slot antenna. Optimum heating for wave launching occus below the cyclotron frequency, consistent with slow wave heating. In TMX-U, we observed a power threshold, which is consistent with computed end-loss power balance. The measured loading resistance varies with density and frequency in agreement with McVey's antenna-plasma coupling code.
Date: March 1, 1984
Creator: Molvik, A.W.; Cummins, W.F.; Falabella, S.; Poulsen, P.; Barter, J.; Dimonte, G. et al.
Partner: UNT Libraries Government Documents Department

Plasma Heating and Losses in Toroidal Multipole Fields

Description: The heating and loss of plasmas have been studied in three pulsed, toroidal multipole devices: a large levitated octupole, a small supported octupole and a very .small supported quadrupole. Plasmas are produced by gun injection and heated by electron and ion cyclotron resonance heating and ohmic heating. Electron cyclotron heating rates have been measured over a wide range of parameters, and the results are in quantitative agreement with stochastic heating theory. Electron cyclotron resonance heating produces ions with energies larger than predicted by theory. With the addition of a toroidal field, ohmic heating gives densities as high as 10{sup 13}cm{sup -3} in the toroidal quadrupole and 10{sup 12}cm{sup -3} in the small octupole. Plasma losses for n=5 x 10{sup 9}cm{sup -3} plasmas are inferred from Langmuir probe and Fabry-Perot interferometer measurements, and measured with special striped collectors on the wall and rings. The loss to a levitated ring is measured using a modulated light beam telemeter. The confinement is better than Bohm but considerably worse than classical. Low frequency convective cells which are fixed in space are observed. These cells around the ring are diminished when a weak toroidal field is added, and loss collectors show a vastly reduced flux to the rings. Analysis of the spatial density profile shows features of B-independent diffusion. The confinement is sensitive to some kinds of dc field errors, but surprisingly insensitive to perturbations of the ac confining field.
Date: September 1, 1974
Creator: Armentrout, C. J.; Barter, J. D.; Breun, R. A.; Cavallo, A. J.; Drake, J. R.; Etzweiler, et al.
Partner: UNT Libraries Government Documents Department

ECRH and ICRH in the TMX-U tandem mirror

Description: In the Tandem Mirror Experiment Upgrade (TMX-U), the formation of a thermal barrier and the potential plugging of ion end loss were achieved at central-cell densities up to 2 x 10/sup 12/ cm/sup -3/. The presence of a thermal barrier was confirmed by direct measurement, and ion axial-confinement times in the range 50 to 100 ms were measured. The ECRH in the end cells (a) initiates plasma startup, (b) generates hot, mirror-confined electrons to form thermal barriers, and (c) creates the plugging potential for central-cell ions. The ECRH system consists of four 200 kW, 28 GHz gyrotrons each feeding power to a separate heating location (two in each end plug). Fundamental heating is used at the potential plug, and second harmonic is used in the thermal barrier. Hot-electron plasmas are produced at total end-cell antenna power levels up to 300 kW. Strong single-pass absorption and net hot-electron heating efficiencies exceeding 40% are observed. Hot-electron parameters achieved are: n/sub eh//n/sub et/ up to 0.8, volume-average beta <..beta..> approx. = 0.15, and T/sub x/ (x-ray tail above 40 keV) in the range 75 to 200 keV.
Date: March 15, 1984
Creator: Stallard, B.W.; Cummins, W.F.; Molvik, A.W.; Poulsen, P.; Simonen, T.C.; Falabella, S. et al.
Partner: UNT Libraries Government Documents Department

Thomson scattering diagnostic for the Microwave Tokamak Experiment

Description: The Thomson-scattering diagnostic system (TSS) on the Microwave Tokamak Experiment (MTX) at LLNL routinely monitors electron temperature (T{sub e}) and density. Typical measured values at the plasma center under clean conditions are 900 {plus minus} 70 eV and 1 to 2 {times} 10{sup 14} ({plus minus}30%) cm{sup {minus}3}. The TSS apparatus is compact, with all elements mounted on one sturdy, two-level optics table. Because of this, we maintain with minimum effort the alignment of both the ruby-laser input optics and the scattered-light collecting optics. Undesired background signals, e.g., plasma light as well as ruby-laser light scattered off obstacles and walls, are generally small compared with the Thomson-scattered signals we normally detect. In the MTX T{sub e} region, the TSS data are definitely fitted better when relativistic effects are included in the equations. Besides determining the temperature of the Maxwellian electron distribution, the system is designed to detect electron heating from GW-level free-electron laser (FEL) pulses by measuring large wavelength shifts of the scattered laser photons. TSS data suggest that we may indeed by able to detect these electrons, which can have energies up to 10 keV, according to computer simulation. 7 refs., 4 figs.
Date: May 4, 1990
Creator: Foote, J.H.; Barter, J.D.; Sewall, N.R.; Jolly, J.J. & Schlander, L.F.
Partner: UNT Libraries Government Documents Department

TMX-U (Tandem Mirror Experiment-Upgrade) tandem-mirror thermal-barrier experiments

Description: Thermal-barrier experiments have been carried out in the Tandem Mirror Experiment-Upgrade (TMX-U). Measurements of nonambipolar and ambipolar radial transport show that these transport processes, as well as end losses, can be controlled at modest densities and durations. Central-cell heating methods using ion-cyclotron heating (ICH) and neutral-beam injection have been demonstrated. Potential mesurements with recently developed methods indicate that deep thermal barriers can be established.
Date: October 29, 1986
Creator: Simonen, T.C.; Allen, S.L.; Baldwin, D.E.; Barter, J.D.; Berzins, L.V.; Carter, M.R. et al.
Partner: UNT Libraries Government Documents Department