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  • 1
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    Alfvénic modes excited by the kink instability in PHASMA (2021)
    American Institute of Physics
    Details
    Publication Date: 2021-03-01
    Print ISSN: 1070-664X
    Electronic ISSN: 1089-7674
    Topics: Physics
    Published by American Institute of Physics
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    PAPER CURRENT
  • 2
    Electronic Resource
    Electronic Resource
    Electron temperature measurement by a helium line intensity ratio method in helicon plasmas (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 5303-5314 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electron temperature measurements in helicon plasmas are difficult. The presence of intense rf fields in the plasma complicates the interpretation of Langmuir probe measurements. Furthermore, the non-negligible ion temperature in the plasma considerably shortens the lifetime of conventional Langmuir probes. A spectroscopic technique based on the relative intensities of neutral helium lines is used to measure the electron temperature in the HELIX (Hot hELicon eXperiment) plasma [P. A. Keiter et al., Phys. Plasmas 4, 2741 (1997)]. This nonintrusive diagnostic is based on the fact that electron impact excitation rate coefficients for helium singlet and triplet states differ as a function of the electron temperature. The different aspects related to the validity of this technique to measure the electron temperature in rf generated plasmas are discussed in this paper. At low plasma density (ne≤1011 cm−3), this diagnostic is believed to be very reliable since the population of the emitting level can be easily estimated with reasonable accuracy by assuming that all excitation originates from the ground state (steady-state corona model). At higher density, secondary processes (excitation transfer, excitation from metastable, cascading) become more important and a more complex collisional radiative model must be used to predict the electron temperature. In this work, different helium transitions are examined and a suitable transition pair is identified. For an electron temperature of 10 eV, the line ratio is measured as a function of plasma density and compared to values predicted by models. The measured line ratio function is in good agreement with theory and the data suggest that the excitation transfer is the dominant secondary process in high-density plasmas. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1418020
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  • 3
    Electronic Resource
    Electronic Resource
    Frequency dependent effects in helicon plasmas (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 2741-2747 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Variations in the plasma parameters of a large volume, helicon source as a function of applied rf power (0–2 kW), driving frequency (8–18 MHz), magnetic field (0–1.4 kG) and fill pressure (2–10 mTorr) have been studied. The transitions between the capacitive, inductive, and resonant helicon mode are consistent with previous experiments. Our data indicate that the transition to the helicon mode occurs at a unique magnetic field, independent of the driving frequency. Based on the helicon wave dispersion relation, from which helicon wavelengths can be calculated, the observed variations in plasma density as a function of driving frequency suggest that the wavelength of the helicon wave is a weak function of driving frequency. Calculation of the electron energies which correspond to the phase velocity of the driving wave (i.e., Landau damping) suggest that either Landau damping cannot be responsible for the efficient ionization of helicon sources, or that the helicon portion of the discharge does not extend over the entire radius of the apparatus. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872142
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  • 4
    Electronic Resource
    Electronic Resource
    Ion heating in the HELIX helicon plasma source (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 4767-4772 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Efficient ion heating in a steady-state helicon plasma source is observed with two external loop antennae just above the ion cyclotron frequency. The ion velocity space distribution is measured by laser induced fluorescence in an argon plasma. The measured bulk ion heating is highly anisotropic (the perpendicular temperature increase is ten times the parallel temperature increase) even though the plasma is moderately collisional. Measurements of the perturbed distribution function with laser induced fluorescence suggest that an electrostatic ion cyclotron wave is launched. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873764
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  • 5
    Electronic Resource
    Electronic Resource
    Ion temperature anisotropy limitation in high beta plasmas : The 41st annual meeting of the division of plasma physics of the american physical society (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 2157-2165 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Measurements of parallel and perpendicular ion temperatures in the Large Experiment on Instabilities and Anisotropies (LEIA) space simulation chamber display an inverse correlation between the upper bound on the ion temperature anisotropy and the parallel ion beta (β=8πnkT/B2). Fluctuation measurements indicate the presence of low frequency, transverse, electromagnetic waves with wave numbers and frequencies that are consistent with predictions for Alfvén Ion Cyclotron instabilities. These observations are also consistent with in situ spacecraft measurements in the Earth's magnetosheath and with a theoretical/computational model that predicts that such an upper bound on the ion temperature anisotropy is imposed by scattering from enhanced fluctuations due to growth of the Alfvén ion cyclotron instability. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874036
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  • 6
    Electronic Resource
    Electronic Resource
    Beta-dependent upper bound on ion temperature anisotropy in a laboratory plasma (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 779-783 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Laser induced fluorescence measurements of ion temperatures, parallel and perpendicular to the local magnetic field, in the Large Experiment on Instabilities and Anisotropies space simulation chamber (a steady-state, high beta, argon plasma) display an inverse correlation between the upper bound on the ion temperature anisotropy and the parallel ion beta (β=8πnkT/B2). These observations are consistent with in situ spacecraft measurements in the Earth's magnetosheath and with a theoretical/computational model that predicts that such an upper bound is imposed by scattering from enhanced fluctuations due to growth of the ion cyclotron anisotropy instability (the Alfvén ion cyclotron instability). © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873872
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  • 7
    Electronic Resource
    Electronic Resource
    Ion heating and magnetohydrodynamic dynamo fluctuations in the reversed-field pinch (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 4062-4071 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ion temperatures have been measured in the Madison Symmetric Torus (MST) [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed-field pinch (RFP) with a five channel charge exchange analyzer. The characteristic anomalously high ion temperature of RFP discharges has been observed in the MST. The ion heating expected from ion–electron collisions is calculated and shown to be too small to explain the measured ion temperatures. The charge exchange determined ion temperature is also compared to measurements of the thermally broadened Cv 227.1 nm line. The ion temperature, Ti≈250 eV for I=360 kA, increases by more than 100% during discrete dynamo bursts in MST discharges. Magnetic field fluctuations in the range 0.5–5 MHz were also measured during the dynamo bursts. Structure in the fluctuation frequency spectrum at the ion cyclotron frequency suggests that the mechanism of ion heating involves the dissipation of dynamo fluctuations at ion cyclotron frequencies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860313
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  • 8
    Electronic Resource
    Electronic Resource
    Global confinement and discrete dynamo activity in the MST reversed-field pinch (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2241-2246 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results obtained on the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 131 (1991)] after installation of the design poloidal field winding are presented. Values of βθe0≡2μ0ne0Te0/B2θ(a)∼12% are achieved in low-current (I=220 kA) operation; here, ne0 and Te0 are central electron density and temperature, and Bθ(a) is the poloidal magnetic field at the plasma edge. An observed decrease in βθe0 with increasing plasma current may be due to inadequate fueling, enhanced wall interaction, and the growth of a radial field error at the vertical cut in the shell at high current. Energy confinement time varies little with plasma current, lying in the range of 0.5–1.0 msec. Strong discrete dynamo activity is present, characterized by the coupling of m=1, n=5–7 modes leading to an m=0, n=0 crash (m and n are poloidal and toroidal mode numbers). The m=0 crash generates toroidal flux and produces a small (2.5%) increase in plasma current.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859642
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  • 9
    Electronic Resource
    Electronic Resource
    Three-dimensional neutral atom imaging of tokamak plasmas : Proceedings of the tenth topical conference on high temperature plasma diagnostics (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 336-338 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Recent technological advances in neutral atom imaging of space plasmas can be applied to the next generation of long pulse, high density fusion experiments. In particular, the small size of these neutral atom imagers makes them ideal for measurements of charge exchange neutrals in the edge and divertor region of tokamaks. In the edge of tokamak plasmas, the lower energy neutrals are created locally and the higher energy neutrals are generated within a few mean free paths of the edge. This charge exchange neutral flux can be imaged and its energy spectrum analyzed. The energy spectrum measurements provide the radial information necessary for a three-dimensional (3D) image of the convolved ion and neutral populations. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146538
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  • 10
    Electronic Resource
    Electronic Resource
    Design and calibration of a fast-time resolution charge exchange analyzer : Proceedings of the 9th topical conference on high temperature plasma diagnostics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4527-4529 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A five channel, fast time resolution, scanning charge exchange analyzer has been developed for the Madison Symmetric Torus (MST). The analyzer consists of an iron vacuum vessel, a gas stripping cell, an electrostatic bending field, and five continuous electron multiplier detectors. The incident neutral flux and operation of the detectors in current mode limits the time resolution of the analyzer to 10 μs. The analyzer was absolutely calibrated over the energy range of interest (500–2000 eV) with a H+ beam, so that the charge exchange power loss could also be measured. The analyzer can be swiveled on a shot-to-shot basis for measurements of Ti(r), where 0.3〈r/a〈0.7. The mechanical design was driven by the need for a low cost, expandable ion temperature diagnostic.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143710
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