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  • 1
    Electronic Resource
    Electronic Resource
    Gyrotron collective Thomson scattering from plasma fluctuations in a Tara axicell (1988)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 59 (1988), S. 1562-1564 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Collective Thomson scattering in the Tara Tandem Mirror axicell at MIT was accomplished with a 137-GHz, ∼0.4-kW, 75-ms pulsed gyrotron. Ion cyclotron waves, ion Bernstein wave harmonics, and other plasma fluctuations possibly due to microinstabilities and magnetohydrodynamic (MHD) activity have been observed during ion cyclotron resonance frequency (ICRF) heating. The observation of ion Bernstein waves may be due to an enhanced ion thermal fluctuation spectrum in an ICRF heated plasma.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1140197
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  • 2
    Electronic Resource
    Electronic Resource
    137-GHz gyrotron diagnostic for instability studies in Tara (1986)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 57 (1986), S. 1983-1985 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A narrow linewidth (〈100 kHz), 1-kW, 137-GHz gyrotron and an efficient TE03 to TE11 cylindrical waveguide mode converter set (〉97% TE11 mode output) have been built for collective Thomson scattering diagnostics. The main goal will be to study instability driven ion density fluctuations in the Tara plug such as the drift cyclotron loss cone (DCLC), the axial loss cone (ALC), harmonics of the DCLC and ALC, and the ion two-stream instability. The heterodyne receiver and signal optics have been installed on Tara. Background electron cyclotron emission (ECE) at 139±1.5 GHz after electron cyclotron resonance heating (ECRH) in the Tara plug corresponded to equivalent blackbody temperatures of 453 and 70 eV for extraordinary and ordinary emission, respectively. The well-collimated receiver field of view completely through the Tara plug has allowed for excellent polarization discrimination of the ECE. The high-power capability of this gyrotron will allow weak fluctuation levels (ñ/n〈10−6) to be detected above this background during ECRH in the plugs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1138813
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  • 3
    Electronic Resource
    Electronic Resource
    High-frequency gyrotron scattering diagnostic for instability studies on TARA (1985)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 56 (1985), S. 914-916 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A 1- to 10-kW,〉30-ms pulsed, narrow linewidth (〈1 MHz), 137-GHz gyrotron is being fabricated for collective Thomson scattering plasma diagnostics on the TARA tandem mirror experiment. The drift cyclotron loss cone, the axial loss cone, harmonics of these instabilities, and the ion two stream instability in the TARA plugs will be studied with this diagnostic.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1138040
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  • 4
    Electronic Resource
    Electronic Resource
    Observation of trapped particle modes in a tandem mirror (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 608-614 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An instability with azimuthal mode number m≥3 localized to an axisymmetric end cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982)] has been observed, most prominently during strong ion cyclotron resonance heating in the end cell. The instability, which causes enhanced radial losses, becomes either more stable or flutelike when the connection (passing fraction) between the central cell and end cell is increased, depending on whether sufficient stabilization is provided in the central cell. The beta is sufficiently low to rule out the possibility of magnetohydrodynamic ballooning modes. Based on the plasma parameters, the instability appears to be a collisionless curvature-driven trapped particle mode that has been predicted to be unstable in linked minimum- and maximum-B mirror devices.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859120
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  • 5
    Electronic Resource
    Electronic Resource
    Analytic solutions and particle simulations of cross-field plasma sheaths (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1069-1082 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Particle simulations have been made of an infinite plasma slab, bounded by absorbing conducting walls, with a magnetic field parallel to the walls. The simulations have been either one dimensional or two dimensional, with the magnetic field normal to the simulation plane. Initially, the plasma has a uniform density between the walls and there is a uniform source of ions and electrons to replace particles lost to the walls. In the one-dimensional (1-D) case, there is no diffusion of the particle guiding centers and the plasma remains uniform in density and potential over most of the slab, with sheaths about a Debye length wide where the potential rises to the wall potential. In the two-dimensional (2-D) case, the density profile becomes parabolic, going almost to zero at the walls, and there is a quasineutral presheath in the bulk of the plasma, in addition to sheaths near the walls. Analytic expressions are found for the density and potential profiles in both cases, including, in the 2-D case, the magnetic presheath resulting from a finite ion Larmor radius, and the effects of the guiding center diffusion rate being either much less than or much greater than the energy diffusion rate. These analytic expressions are shown to agree with the simulations. A 1-D simulation with Monte Carlo guiding center diffusion included gives results that are in good agreement with the much more expensive 2-D simulation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859280
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  • 6
    Electronic Resource
    Electronic Resource
    Stability of plasmas sustained by ion cyclotron wave excitation in the central cell of the Tara tandem mirror (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 851-861 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The stability of plasmas produced by radio-frequency heating in the ion cyclotron frequency range (ICRF) has been studied in the central cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion Research 1986, Proceedings of the 11th International Conference, Kyoto (IAEA, Vienna, 1987), Vol. II, p. 251]. Ion cyclotron wave excitation by a slot antenna provided stability against macroscopic plasma motions in an axisymmetric configuration. The maintenance of macroscopic stability depended on the ICRF power, gas fueling rate, ion cyclotron resonance location, and ω/ωci at the antenna location. The ICRF ponderomotive force model is consistent with many of the observed stability features and predicts that the E+ component of the ion cyclotron wave was responsible for the stabilization. The Alfvén ion cyclotron microinstability was observed when the plasma β⊥ and anisotropy were sufficiently high. Magnetic probe measurements of the unstable mode identified it as an ion cyclotron wave and the instability threshold was within a factor of 2 of the theoretical value.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859010
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  • 7
    Electronic Resource
    Electronic Resource
    Access to the second stability region in a high-shear, low-aspect-ratio tokamak (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 2674-2682 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is known that tokamaks display a second region of stability to ideal magnetohydrodynamic (MHD) internal modes. An important determining factor for MHD properties is the radial profile of toroidal current. Here it is shown that in a low-aspect-ratio tokamak with high on-axis safety factor (q0(approximately-equal-to)2) and high shear, a path to high beta can be obtained that remains completely stable against ideal MHD modes. By maintaining high shear this scenario avoids fixed boundary instabilities for both high and low toroidal mode numbers for beta values well above the Troyon limit (stability was tested up to εβp=1.4, β=10.8%). For a close fitting wall (awall/aplasma(approximately-equal-to)1.2) this configuration is also stable to low toroidal mode number balloon-kink modes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866545
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  • 8
    Electronic Resource
    Electronic Resource
    Plasma production and heating in a tandem mirror central cell by radio-frequency waves in the ion cyclotron frequency range (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 3744-3753 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Plasma production and heating in the central cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion Research, 1986, Proceedings of the 11th International Conference, Kyoto, Japan (IAEA, Vienna, 1987), Vol. 2, p. 251] have been studied. Using radio-frequency excitation by a slot antenna in the ion cyclotron frequency range (ICRF), plasmas with a peak β⊥ of 3%, density of 4×1012 cm−3, ion temperature of 800 eV, and electron temperature of 75–100 eV were routinely produced. The plasma radius decreased with increasing ICRF power, causing reduced ICRF coupling and saturation of the plasma beta. About 70% of the applied ICRF power can be accounted for in direct heating of both ions and electrons. Wave field measurements have identified the applied ICRF to be the slow, ion cyclotron wave. In operation without end plugging, the plasma parameters were limited by poor axial confinement and the requirements for maintenance of magnetohydrodynamic stability and microstability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866893
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  • 9
    Electronic Resource
    Electronic Resource
    Line-tying of interchange modes in a hot electron plasma (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 2214-2230 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The dispersion relation of low-frequency (ω(very-much-less-than)ωci) electrostatic flute-like interchange modes in a mirror cell with a fraction α of hot bi-Maxwellian electrons, with bulk line-tying to cold (nonemitting) end walls, has been solved using a slab model and the local approximation. In the absence of line-tying, hot-electron interchange modes are never completely stabilized (in contrast to the conventional theory [Phys. Fluids 9, 820 (1966); Phys. Fluids 19, 1255 (1976)], which assumes monoenergetic hot electrons and has little relevance to real plasmas). In the presence of line-tying, hot-electron interchange modes are more effectively stabilized than magnetohydrodynamic (MHD) interchange modes, because (1) the line-tying is enhanced by a factor of (ω/νe)1/2 when the wave frequency ω is greater than the cold-electron collision frequency νe; and (2) hot-electron interchange modes can be completely stabilized, rather than merely having their growth rates reduced, if there is a spread of hot-electron-curvature drift velocities. Predictions of the minimum α needed for instability and of the first azimuthal mode number m to go unstable, and of the scaling of these quantities with neutral gas pressure, are in good quantitative agreement with observations of hot-electron interchange instabilities in the Tara tendem mirror experiment [Bull. Am. Phys. Soc. 30, 1581 (1985)], provided a correction is made for the fact that the modes in Tara are not flute-like, but should have higher amplitudes in the plug than in the central cell. The theory may also explain observations in other experiments [Phys. Fluids 27, 1019 (1984); Phys. Fluids 19, 1203 (1976)]. Increasing the ion temperature Ti should have a modest stabilizing effect. In addition to the hot-electron interchange modes, there are also ion-driven interchange modes, which are unstable even in the absence of hot electrons, but generally have low growth rates, much less than MHD growth rates. Even these modes may be completely stabilized by finite Larmor radius and line-tying when Ti is sufficiently great.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865560
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