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ECRH in tandem mirror machines

Description: The thermal barrier concept and the use of ECRH to generate the several electron populations required to establish confinement are discussed. Important physics issues related to the microwave heating are discussed. Fokker-Planck calculations which model the heating processes are presented, followed by recent experimental data on hot electron heating.
Date: May 27, 1983
Creator: Stallard, B.W.
Partner: UNT Libraries Government Documents Department

Experiments on hot-electron ECRH in the Tandem Mirror Experiment-Upgrade

Description: Experiments have begun on the Tandem Mirror Experiment Upgrade (TMX-U) using electron-cyclotron resonant heating (ECRH) to generate the hot electron populations required for thermal barrier operation (Energy E/sub eh/ approx. 50 keV, density n/sub eh/ < 5 x 10/sup 12/, and hot-to-cold fraction n/sub eh/n approx. 0.9). For this operation, rf power produced by 28-GHz gyrotrons is injected with extraordinary mode polarization at both fundamental and second harmonic locations. Our initial experiments, which concentrated on startup of the hot electrons, were carried out at low density (< 1 x 10/sup 12/ cm/sup -3/) where Fokker-Planck calculations predict high heating efficiency when the electron temperature (T/sub e/) is low. Under these conditions, we produced substantial hot electron populations (diamagnetic energy > 400 J, E/sub eh/ in the range of 15 to 50 keV, and n/sub eh//n > 0.5).
Date: February 18, 1983
Creator: Stallard, B.W.
Partner: UNT Libraries Government Documents Department

Electron cyclotron heating in TMX-Upgrade

Description: TMX-Upgrade, an improved tandem mirror experiment under construction at LLNL, will use electron cyclotron resonance heating (ECRH) to create thermal barriers and to increase the center cell ion confining potential. Gyrotron oscillators (200 kW, 28 GHz) supply the heating power for the potential confined electron (fundamental heating) and the mirror-confined electrons (harmonic heating) in the thermal barriers. Important issues are temperature limitation and microstability for the hot electrons. Off-midplane heating can control anisotropy-driven microstability. Spacially restricting heating offers the possibility of temperature control by limiting the energy for resonant interaction.
Date: January 1, 1981
Creator: Stallard, B.W. & Hooper, E.B. Jr.
Partner: UNT Libraries Government Documents Department

Electron heat transport in improved confinement discharges in DIII-D

Description: In DIII-D tokamak plasmas with an internal transport barrier (ITB), the comparison of gyrokinetic linear stability (GKS) predictions with experiments in both low and strong negative magnetic shear plasmas provide improved understanding for ion and electron thermal transport within much of the plasma. As previously reported, the region for improved ion transport seems well characterized by the condition OE~B>Y-, where SERB is the ExB flow shear, calculated from measured quantities, and y,, is the maximum linear growth rate for ion temperature gradient (ITG) modes in the absence of flow shear. Within a limited region just inside the ITB, the electron temperature gradient (ETG) modes appear to control the electron temperature gradient and, consequently, the electron thermal transport. The increase in electron temperature gradient with more strongly negative magnetic shear is consistent with the increase in the ETG mode marginal gradient. Closer to the magnetic axis the Te profile flattens and the ETG modes are predicted to be stable. With additional core electron heating, FIR scattering measurements near the axis show the presence of high k fluctuations (12 cm-l), rotating in the electron diamagnetic drift direction. This turbulence could impact electron transport and possibly also ion transport. Thermal diffusivities for electrons, and to a lesser degree ions, increase. The ETG mode can exist at this wavenumber, but it is computed to be robustly stable near the axis.
Date: November 24, 1998
Creator: Stallard, B. W.
Partner: UNT Libraries Government Documents Department

Fokker-Planck calculations of electron cyclotron resonant heating (ECRH) in mirror geometry

Description: A time dependent, bounce-averaged Fokker-Planck code, with quasi-linear diffusion at fundamental and second harmonic frequencies, has been used to study cold plasma trapping and heating of hot electrons in mirror geometry. Both electron-electron and electron-ion Coulomb collisions are included. The code can model either cavity heating (electric field throughout cavity as in EBT) or beam controlled heating (electric field spatially restricted as in the TMX-Upgrade tandem mirror). The heating method has implications for the equilibrium energy and anisotropy of the hot electrons. In TMX-Upgrade, off-midplane heating at the second harmonic in the thermal barrier is planned as a means to control anisotropy (T/sub parallel//T/sub perpendicular/. By spatially limiting (limit in B) the microwave beam and with strong single-pass absorption, the mean hot electron energy may also be controlled since the heating rate decreases at high energy due to the relativistic mass shift of the resonance to higher magnetic field.
Date: November 25, 1981
Creator: Stallard, B.W.; Matsuda, Y. & Nevins, W.M.
Partner: UNT Libraries Government Documents Department

Transport and performance in DIII-D discharges with weak or negative central magnetic shear

Description: Discharges exhibiting the highest plasma energy and fusion reactivity yet realized in the DIII-D tokamak have been produced by combining the benefits of a hollow or weakly sheared central current profile with a high confinement (H-mode) edge. In these discharges, low power neutral beam injection heats the electrons during the initial current ramp, and {open_quotes}freezes in{close_quotes} a hollow or flat central current profile. When the neutral beam power is increased, formation of a region of reduced transport and highly peaked profiles in the core often results. Shortly before these plasmas would otherwise disrupt, a transition is triggered from the low (L-mode) to high (H-mode) confinement regimes, thereby broadening the pressure profile and avoiding the disruption. These plasmas continue to evolve until the high performance phase is terminated nondisruptively at much higher {beta}{sub T} (ratio of plasma pressure to toroidal magnetic field pressure) than would be attainable with peaked profiles and an L-mode edge. Transport analysis indicates that in this phase, the ion diffusivity is equivalent to that predicted by Chang-Hinton neoclassical theory over the entire plasma volume. This result is consistent with suppression of turbulence by locally enhanced E x B flow shear, and is supported by observations of reduced fluctuations in the plasma. Calculations of performance in these discharges extrapolated to a deuterium-tritium fuel mixture indicates that such plasmas could produce a DT fusion gain Q{sub DT} = 0.32.
Date: December 1, 1996
Creator: Greenfield, C.M.; Schissel, D.P. & Stallard, B.W.
Partner: UNT Libraries Government Documents Department

Electron heat transport in improved confinement discharges in DIII-D

Description: In DIII-D tokamak plasmas with an internal transport barrier (ITB), the comparison of gyrokinetic linear stability (GKS) predictions with experiments in both low and strong negative magnetic shear plasmas provide improved understanding for electron thermal transport within the plasma. Within a limited region just inside the ITB, the electron temperature gradient (ETG) modes appear to control the electron temperature gradient and, consequently, the electron thermal transport. The increase in the electron temperature gradient with more strongly negative magnetic shear is consistent with the increase in the ETG mode marginal gradient. Closer to the magnetic axis the T{sub e} profile flattens and the ETG modes are predicted to be stable. With additional core electron heating, FIR scattering measurements near the axis show the presence of high k fluctuations (12 cm{sup {minus}1}), rotating in the electron diamagnetic drift direction. This turbulence could impact electron transport and possibly also ion transport. Thermal diffusivities for electrons, and to a lesser degree ions, increase. The ETG mode can exist at this wavenumber, but it is computed to be robustly stable near the axis. Consequently, in the plasmas the authors have examined, calculations of drift wave linear stability do not explain the observed transport near the axis in plasmas with or without additional electron heating, and there are probably other processes controlling transport in this region.
Date: January 1, 1999
Creator: Stallard, B.W.; Greenfield, C.M. & Staebler, G.M.
Partner: UNT Libraries Government Documents Department

Transport in high performance weak and negative central shear discharges in DIII-D

Description: In recent experiments in the DIII-D tokamak, the previously reported enhanced performance regime with negative central magnetic shear (NCS) has been extended to further improve fusion performance. This was done by using controlled L-H transitions to further broaden the pressure profile, thereby delaying the onset of MHD activity which would lead to the termination of the high performance phase. Such discharges have achieved record parameters for DIII-D, including D-D fusion power up to 28 kW and stored energy in excess of 4 MJ.
Date: July 1, 1996
Creator: Greenfield, C.M.; Schissel, D.P. & Stallard, B.W.
Partner: UNT Libraries Government Documents Department

Milestone report: Status report on high {beta}p experiments at high plasma current

Description: This report summarizes LLNL`s involvement in recent high {beta}{sub p} experiments on the DIII-D tokamak at General Atomics. These experiments were done in collaboration with several members of the DIII-D physics staff from GA and from other collaborating institutions and could not have succeeded without this joint effort. In this report, the authors summary a specific, limited set of experiments to extend high {beta}{sub p} operation with enhanced core confinement to higher plasma currents. The interest in these experiments stems from the non-inductive current drive requirement for steady-state advanced tokamak regimes which can most reasonably be met by operation with a high bootstrap current fraction.
Date: July 1, 1995
Creator: Casper, T.A.; James, R.A.; Rice, B.W. & Stallard, B.W.
Partner: UNT Libraries Government Documents Department

Fundamental mode rectangular waveguide system for electron-cyclotron resonant heating (ECRH) for tandem mirror experiment-upgrade (TMX-U)

Description: We present a brief history of TMX-U's electron cyclotron resonant heating (ECRH) progress. We emphasize the 2-year performance of the system, which is composed of four 200-kW pulsed gyrotrons operated at 28 GHz. This system uses WR42 waveguide inside the vacuum vessel, and includes barrier windows, twists, elbows, and antennas, as well as custom-formed waveguides. Outside the TMX-U vessel are directional couplers, detectors, elbows, and waveguide bends in WR42 rectangular waveguide. An arc detector, mode filter, eight-arm mode converter, and water load in the 2.5-in. circular waveguide are attached directly to the gyrotron. Other specific areas discussed include the operational performance of the TMX-U pulsed gyrotrons, windows and component arcing, alignment, mode generation, and extreme temperature variations. Solutions for a number of these problems are described.
Date: December 1, 1983
Creator: Rubert, R.R.; Felker, B.; Stallard, B.W. & Williams, C.W.
Partner: UNT Libraries Government Documents Department

Electron cyclotron resonance heating (ECRH) in the TMX-Upgrade tandem-mirror experiment

Description: Results are described of engineering tests of operation of two gyrotrons from one power supply, tests of waveguide components and antennae, antenna design, and the x-ray shielding concept. Finally, we describe considerations of interaction with other system components.
Date: October 1, 1981
Creator: Griffin, D.H.; Rubert, R.R.; Simonen, T.C.; Stallard, B.W. & Wieskamp, T.F.
Partner: UNT Libraries Government Documents Department

Microwave measurement test results of circular waveguide components for electron cyclotron resonant heating (ECRH) of the Tandem Mirror Experiment-Upgrade (TMX-U)

Description: Development of high-power components for electron cyclotron resonant heating (ECRH) applications requires extensive testing. In this paper we describe the high-power testing of various circular waveguide components designed for application on the Tandem Mirror Experiment-Upgrade (TMX-U). These include a 2.5-in. vacuum valve, polarizing reflectors, directional couplers, mode converters, and flexible waveguides. All of these components were tested to 200 kW power level with 40-ms pulses. Cold tests were used to determine field distribution. The techniques used in these tests are illustrated. The new high-power test facility at Lawrence Livermore National Laboratory (LLNL) is described and test procedures are discussed. We discuss the following test results: efficiency at high power of mode converters, comparison of high power vs low power for waveguide components, and full power tests of the waveguide system. We also explain the reasons behind selection of these systems for use on TMX-U.
Date: December 1, 1983
Creator: Williams, C.W.; Rubert, R.R.; Coffield, F.E.; Felker, B.; Stallard, B.W. & Taska, J.
Partner: UNT Libraries Government Documents Department

Formation and control of plasma potentials in TMX upgrade

Description: The methods to be employed to form and control plasma potentials in the TMX Upgrade tandem mirror with thermal barriers are described. ECRH-generated mirror -confined electron plasmas are used to establish a negative potential region to isolate the end-plug and central-cell celectrons. This thermal isolation will allow a higher end-plug electron temperature and an increased central-cell confining potential. Improved axial central-cell ion confinement results since higher temperature central-cell ions can be confined. This paper describes: (1) calculations of the sensitivity of barrier formation to vacuum conditions and to the presence of impurities in the neutral beams, (2) calculations of microwave penetration and absorption used to design the ECRH system, and (3) techniques to limit electron runaway to high energies by localized microwave beams and by relativistic detuning.
Date: May 6, 1981
Creator: Simonen, T.C.; Orzechowski, T.J.; Porkolab, M. & Stallard, B.W.
Partner: UNT Libraries Government Documents Department

Dispersion relations for a plasma-filled helix-loaded-waveguide

Description: The propagation of waves on bounded, magnetized plasma columns arises in connection with a variety of applications. To this end dispersion relations axe developed for a variety of multi-region circularly symmetric configurations. These include, a sheath helix in free space, a plasma column in free space, a plasma filled conducting tube, a plasma filled sheath-helix in free space, a sheath helix within a conducting cylinder, a plasma filled sheath-helix within a conducting cylinder, and a plasma column within a sheath-helix contained within a conducting cylinder. The latter configuration is of the most interest for whistler wave excitation for plasma thruster applications, since it includes the effect of a vacuum region separating the plasma column from the helical excitation structure.
Date: January 1, 1994
Creator: Makowski, M. A.; Hooper, E. B. & Stallard, B. W.
Partner: UNT Libraries Government Documents Department

q Profile evolution and enhanced core confinement of high {beta}{sub p} plasmas in DIII-D

Description: In DIII-D the authors have investigated the long pulse evolution of high poloidal beta ({sub beta}{sub p}), double-null diverted H-mode discharges, which exhibit high bootstrap current fractions attractive for a reactor. At low currents I{sub p}, the current profile evolved over several seconds and the on-axis safety factor (q{sub 0}) increased. When q{sub 0} increased above {approximately}2, the MHD character changed from an m/n = 2/1 to an m/n = 3/1 internal kink mode, where m(n) are poloidal (toroidal) mode numbers, which then disappeared with further increases in q{sub 0}. Coincident with a strong reduction of fluctuations, the authors observed enhanced core confinement, leading to strong density peaking, a further rise in {beta}{sub p}, and a bootstrap current increasing to I{sub boot}/I{sub p} {approx} 0.8, peaked within the core. Ideal MHD calculations showed access to second stability during the density rise. During the enhanced performance phase core particle lifetime ({tau}{sub p}) and global energy lifetime ({tau}{sub E}) increased by factors of 2 and 1.2 respectively. Transport analysis showed that core particle and thermal diffusivities D{sub e} and {chi}{sub eff} approached neoclassical values. During the low current experiments, large losses of fast ions (typically {approximately}50% at 0.4 MA) were observed; at higher currents these losses are much smaller. The authors have also investigated discharges with current rampdown to high {beta}{sub p}. For a 5 to 6 {tau}{sub E} duration following rampdown, enhanced values of {beta}{sub p}, normalized toroidal beta ({beta}{sub N}), and {tau}{sub E} were obtained at high internal inductance ({ell}{sub i}). During both the ramp and the high confinement phases, fast ion losses were low ({approximately}10%). The loss then increased, correlated with an increase in the anisotropy ratio of perpendicular and parallel plasma pressure that suggests a fast ion loss mechanism coupled to the fast ion parallel energy.
Date: October 1, 1994
Creator: Stallard, B. W.; Casper, T. A. & Fenstermacher, M. E.
Partner: UNT Libraries Government Documents Department

Engineering design of the interaction waveguide for high-power accelerator-driven microwave free-electron lasers

Description: Linear induction accelerators (LIAs) operating at beam energies of a few million electron volts and currents of a few thousand amperes are suitable drivers for free-electron lasers (FELs). Such lasers are capable of producing gigawatts of peak power and megawatts of average power at microwave frequencies. Such devices are being studied as possible power sources for future high-gradient accelerators and are being constructed for plasma heating applications. At high power levels, the engineering design of the interaction waveguide presents a challenge. This paper discusses several concerns, including electrical breakdown and metal fatigue limits, choice of material, and choice of operating propagation mode. 13 refs., 3 figs.
Date: March 16, 1989
Creator: Hopkins, D.B.; Clay, H.W.; Stallard, B.W.; Throop, A.L.; Listvinsky, G. & Makowski, M.A.
Partner: UNT Libraries Government Documents Department

Focusing twist reflector for electron-cyclotron resonance heating in the Tandem Mirror Experiment-Upgrade

Description: A twist reflector plate is described that linearly polarizes and focuses the TE/sub O/sub 1// circular waveguide mode for heating hot electrons in the thermal barrier of the Tandem Mirror Experiment-Upgrade (TMX-U). The plate polarizing efficiency is 95%, and it has operated satisfactorily at 150 kW power level.
Date: May 1, 1984
Creator: Stallard, B.W.; Coffield, F.E.; Felker, B.; Taska, J.; Christensen, T.E.; Gallagher, N.C. Jr. et al.
Partner: UNT Libraries Government Documents Department

Astron Program final report

Description: This report describes important experimental results obtained in the last two years of the Astron Program, an LLL controlled nuclear fusion program which terminated in 1973. Little theoretical work is included, but an extensive bibliography is given. (auth)
Date: August 25, 1975
Creator: Briggs, R.J.; Hester, R.E.; Porter, G.D.; Sherwood, W.A.; Spoerlein, R.; Stallard, B.W. et al.
Partner: UNT Libraries Government Documents Department

Polarizing holographic reflector for electron cyclotron resonant heating (ECRH) on the Tandem Mirror Experiment Upgrade (TMX-U)

Description: A reflector for electron cyclotron resonant heating on the Tandem Mirror Experiment Upgrade has been designed to convert the high-power TE/sub 01/ output of the circular waveguide system into a linearly polarized gaussian intensity pattern in the plasma. The reflector is a computer-generated holographic optical element with a twist polarizer. Its design, fabrication, and performance are discussed. Results of the low- and high-power tests are presented. Low-power tests were used to determine the beam pattern and the degree of cross-polarization. High-power tests verified that arcing across the grooves of the twist polarizer does not occur.
Date: November 14, 1983
Creator: Coffield, F.E.; Felker, B.; Gallagher, N.C. Jr.; Pedrotti, L.R.; Stallard, B.W.; Sweeney, D.W. et al.
Partner: UNT Libraries Government Documents Department

Central Thomson scattering upgrade on DIII-D

Description: The existing 36 channel Thomson scattering system on the DIII-D tokamak measures the plasma temperature and density in the core region. However, with the recent interest in core peaked density profiles, coverage needs to be extended into the magnetic axis. This paper addresses the technical issues involved with extending the viewing region from a major radius of 194 cm to 165 cm. At least one of the existing seven core laser beams will be rerouted to probe the plasma horizontally instead of vertically. To do this, a rigid extension of the existing laser/collection optics tower will be built to route the laser to a nearby tangential port. A fiber bundle array from one of the two existing core plasma collection optics sets will be rotated to allow up to 10 of the 36 core channels to view along this new beam path. A new in-vessel absorbing glass laser dump must be developed since there are no appropriate laser beam exit ports. The close proximity of this laser dump to the viewing region presents stray light issues that must be resolved to allow for an accurate density calibration using Rayleigh scattering in argon gas.
Date: November 1, 1997
Creator: Nilson, D.G.; Stallard, B.W.; Carlstrom, T.N.; Hsieh, C.L. & Stockdale, R.E.
Partner: UNT Libraries Government Documents Department