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  • 1
    Electronic Resource
    Electronic Resource
    Emission of electron Bernstein waves in plasmas (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 409-418 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In previous publications [A. K. Ram and S. D. Schultz, Phys. Plasmas 7, 4084 (2000); A. Bers, A. K. Ram, and S. D. Schultz, in Proceedings of the Second Europhysics Topical Conference on RF Heating and Current Drive of Fusion Devices, edited by J. Jacquinot, G. Van Oost, and R. R. Weynants (European Physical Society, Petit-Lancy, 1998), Vol. 22A, pp. 237–240] it has been shown that, in overdense plasmas of the type encountered in spherical tori, electron Bernstein waves can be excited in a plasma by mode conversion of either an externally launched X mode or an O mode. The electron Bernstein waves are strongly absorbed by electrons in the region where the wave frequency matches the Doppler broadened electron cyclotron resonance frequency or its harmonics. The strong absorption also implies that electron Bernstein waves are emitted by a thermal plasma. These waves can then mode convert to the X mode and to the O mode and be observed external to the plasma. In this paper an approximate kinetic model describing the coupling between the X mode, the O mode, and the electron Bernstein waves is derived. This model is used to study the mode conversion properties of electron Bernstein wave emission from the plasma interior. It is shown, analytically and numerically, that the energy flow conversion efficiencies of the electron Bernstein wave to the X mode and to the O mode are the same as the energy flow conversion efficiency of the X mode to electron Bernstein waves and of the O mode to the electron Bernstein waves, respectively. This has important experimental consequences when designing experiments to heat overdense plasmas by electron Bernstein waves. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1429634
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  • 2
    Electronic Resource
    Electronic Resource
    Charged-particle acceleration and energy loss in laser-produced plasmas (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 5106-5117 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Spectral measurements have been made of charged fusion products produced in deuterium + helium-3 filled targets irradiated by the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Comparing the energy shifts of four particle types has allowed two distinct physical processes to be probed: Electrostatic acceleration in the low-density corona and energy loss in the high-density target. When the fusion burn occurred during the laser pulse, particle energy shifts were dominated by acceleration effects. Using a simple model for the accelerating field region, the time history of the target electrostatic potential was found and shown to decay to zero soon after laser irradiation was complete. When the fusion burn occurred after the pulse, particle energy shifts were dominated by energy losses in the target, allowing fundamental charged-particle stopping-power predictions to be tested. The results provide the first experimental verification of the general form of stopping power theories over a wide velocity range. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1320467
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  • 3
    Electronic Resource
    Electronic Resource
    Ion dynamics in multiple electrostatic waves in a magnetized plasma. I. Coherent acceleration (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 3224-3232 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new phenomenon of coherent acceleration of ions by a discrete spectrum of electrostatic waves propagating perpendicularly to a uniform magnetic field is described. It allows the energization of ions whose initial energies correspond to a region of phase space that is below the chaotic domain. The ion orbits below the chaotic domain are described very accurately using a perturbation analysis to second order in the wave amplitudes. This analysis shows that the coherent acceleration takes place only when the wave spectrum contains at least two waves whose frequencies are separated by an amount close to an integer multiple of the cyclotron frequency. The way the ion energization depends on the wave numbers and wave amplitudes is also presented in detail using the results of the perturbation analysis. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872989
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  • 4
    Electronic Resource
    Electronic Resource
    Ion dynamics in multiple electrostatic waves in a magnetized plasma. II. Enhancement of the acceleration (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 3233-3241 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The maximal energy an ion can gain from a discrete spectrum of electrostatic waves propagating perpendicularly to a uniform magnetic field is investigated. In the case when the wave spectrum contains at least two on-resonance waves, the ion is shown to reach energies which are much higher than in the case of one wave. When the ion energization is enhanced, even when the ion motion is not coherent, the ion orbit remains close to orbits found from a first order perturbation analysis. This implies that, unlike in the case of a single wave, the ion can reach high energies regardless of how small the wave amplitudes are. The dependence of the ion energization on the wave spectrum characteristics is described in great detail by deriving the extent, in action, of the first order orbits, and the way these orbits may connect. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872990
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  • 5
    Electronic Resource
    Electronic Resource
    Observations of fast protons above 1 MeV produced in direct-drive laser-fusion experiments (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 606-610 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Fast protons (approximately-greater-than)1 MeV have been observed on the 60-beam, 30 kJ OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at an intensity I(similar, equals)1015 W/cm2 and a wavelength λ=0.35 μm. These energies are more than 5 times greater than those observed on previous, single-beam experiments at the same Iλ2. The total energy in the proton spectrum above 0.2 MeV is ∼0.1% of the laser energy. Some of the proton spectra display intense, regular lines which may be related to ion acoustic perturbations in the expanding plasma. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1335831
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  • 6
    Electronic Resource
    Electronic Resource
    Excitation, propagation, and damping of electron Bernstein waves in tokamaks (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 4084-4094 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The conventional ordinary O-mode and the extraordinary X-mode in the electron cyclotron range of frequencies are not suitable for core heating in high-β spherical tokamak plasmas, like the National Spherical Torus Experiment [M. Ono, S. Kaye, M. Peng et al., in Proceedings of the 17th International Atomic Energy Agency Fusion Energy Conference (International Atomic Energy Agency, Vienna, 1999), Vol. 3, p. 1135], as they are weakly damped at high harmonics of the electron cyclotron frequency. However, electron Bernstein waves (EBW) can be effective for heating and driving currents in spherical tokamak plasmas. Power can be coupled to EBWs via mode conversion of either the X-mode or the O-mode. The two mode conversions are optimized in different regions of the parameter space spanned by the parallel wavelength and wave frequency. The conditions for optimized mode conversion to EBWs are evaluated analytically and numerically using a cold plasma model and an approximate kinetic model. From geometric optics ray tracing it is found that the EBWs damp strongly near the Doppler-broadened resonance at harmonics of the electron cyclotron frequency. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1289689
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  • 7
    Electronic Resource
    Electronic Resource
    Mode conversion of fast Alfvén waves at the ion–ion hybrid resonance (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1976-1982 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Substantial radio-frequency power in the ion-cyclotron range of frequencies can be effectively coupled to a tokamak plasma from poloidal current strap antennas at the plasma edge. If there exists an ion–ion hybrid resonance inside the plasma, then some of the power from the antenna, delivered into the plasma by fast Alfvén waves, can be mode converted to ion-Bernstein waves. In tokamak confinement fields the mode-converted ion-Bernstein waves can damp effectively and locally on electrons [A. K. Ram and A. Bers, Phys. Fluids B 3, 1059 (1991)]. The usual mode-conversion analysis that studies the propagation of fast Alfvén waves in the immediate vicinity of the ion–ion hybrid resonance is extended to include the propagation and reflection of the fast Alfvén waves on the high magnetic-field side of the ion–ion hybrid resonance. It is shown that there exist plasma conditions for which the entire fast Alfvén wave power incident on the ion–ion hybrid resonance can be converted to ion-Bernstein waves. In this extended analysis of the mode conversion process, the fast Alfvén waves can be envisioned as being coupled to an internal plasma resonator. This resonator extends from the low magnetic-field cutoff near the ion–ion hybrid resonance to the high magnetic-field cutoff. The condition for 100% mode conversion corresponds to a critical coupling of the fast Alfvén waves to this internal resonator. As an example, the appropriate plasma conditions for 100% mode conversion are determined for the Tokamak Fusion Test Reactor (TFTR) [R. Majeski et al., Proceedings of the 11th Topical Conference on RF Power in Plasmas, Palm Springs (American Institute of Physics, New York, 1995), Vol. 355, p. 63] experimental parameters. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871677
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  • 8
    Electronic Resource
    Electronic Resource
    Mode conversion and electron damping of the fast Alfvén wave in a tokamak at the ion–ion hybrid frequency (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 1637-1647 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Mode conversion of the fast Alfvén wave (FAW) at the ion-hybrid frequency in the ion cyclotron range of frequencies (ICRF) is studied in the presence of ion cyclotron absorption and direct electron damping in a tokamak plasma. The usual Budden model is extended to include the effect of electron damping and of the high-field-side cutoff, and is solved analytically and numerically. The mode-conversion efficiency is given as a function of the Budden transmission coefficient and of a phase integral, which describes interference between the incoming and outgoing waves. In incidence from the low-field side, a discrete spectrum of phases exists for which complete absorption (i.e., combined mode conversion and direct electron damping) of the FAW for a single transit of the resonance region can be achieved. This permits efficient electron heating and/or current drive via mode conversion of FAWs. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871312
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  • 9
    Electronic Resource
    Electronic Resource
    Damping of the fast Alfvén wave in ion-cyclotron resonance heating (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 2018-2026 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The conditions are determined under which the damping of the fast Alfvén wave, used in the heating of tokamak plasmas, can be described by the geometrical optics approximations. Also, for these conditions, an analytic formula is derived, which explicitly determines the amount of energy that is damped onto the plasma particles by the fast wave. The conditions for the validity of the geometrical optics approximations cover a wide range of k(parallel)'s (the component of the wave vector parallel to the total magnetic field). There is a narrow range of k(parallel)'s over which this is not valid; this is the regime where mode conversion is dominant. The results from this theory are in good agreement with those obtained from numerical solutions of fourth- and sixth-order differential equations that are commonly used to describe this type of heating. However, this theory has a distinct advantage of lending itself to detailed scaling studies as explicit expressions for the damping of the fast Alfvén wave are obtained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859065
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  • 10
    Electronic Resource
    Electronic Resource
    Finite temperature effects on the space-time evolution of two-stream instabilities (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 255-261 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Pinch-point instability analysis is used to study the effects of finite temperature on the time-asymptotic pulse shapes of electrostatic and electromagnetic two-stream instabilities. Their absolute or convective instability nature is established over the entire regime from instability threshold at finite temperatures to the cold-plasma hydrodynamic limit. The analysis is based upon nonrelativistic Vlasov theory dispersion relations for streams and plasmas with Maxwellian thermal distributions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865984
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