R. L. Hickok

Rensselaer Polytechnic Institute, Troy, New York, United States

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Publications (100)136.93 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Externally applied resonant magnetic fields have been used on TEXT to modify the particle flux and the radial electric field near the plasma edge. Magnetic fields with primary mode numbers m/n = 7/3 and 7/2, and an average radial field amplitude |br|/B 0.1% have been employed. This perturbation produces mixed islands and stochastic regions at the plasma edge (r/a ≥ 0.8) without affecting the interior. Working particle transport is shown to be increased by typically 30% only in the presence of (computed) magnetic islands. The effect is diminished at high perturbing field strength when the islands become stochastic. A novel transport mechanism due to × convection is proposed to explain this. Outward impurity transport is increased as well.
    Nuclear Fusion 01/2011; 30(1):167. · 2.73 Impact Factor
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    ABSTRACT: Experiments were conducted in the ELMO Bumpy Torus (EBT) from 1973 until 1984. A number of papers have been published concerning various aspects of experiments during the final two years of operation. The paper summarizes the final experimental conclusions and discusses those issues which remain unresolved. The main conclusions are as follows: (1) measurements of the width of the hot electron rings showed them to be wider than previously suspected, diluting their diamagnetism and negating their ability to locally reverse the magnetic field gradient; (2) two-dimensional plots of the plasma potential revealed open potential contours in the C-mode and closed potential contours near the plasma centre in the T-mode; (3) an in–out asymmetry in the plasma potential was always observed, giving rise to electric fields which drove convective plasma loss.
    Nuclear Fusion 01/2011; 28(12):2249. · 2.73 Impact Factor
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    ABSTRACT: In the Texas Experimental Tokamak, a discharge regime characterized by persistent peaked profiles was observed to be induced by pellet fuelling, and its transport properties were studied. The hallmark of the regime is the suppression of sawteeth, and the regime was attained by injecting hydrogen pellets to promptly double the plasma density. In each of several pairs of experiments, a pellet fuelled discharge was compared with an edge fuelled discharge with similar averaged electron density, plasma current and toroidal magnetic field in order to characterize the transport change and to look for causal changes in the plasma turbulence. The impurity, radiation and working gas particle profiles were more peaked for the pellet fuelled case. The values of rjt derived from measured ion temperature and density profiles for high density edge fuelled and pellet fuelled discharges indicate that ion pressure gradient driven turbulence should be reduced in the pellet fuelled case. The macroscopic effects were accompanied by microscopic changes. Measurements of turbulent density fluctuations in high density edge fuelled discharges give strong evidence that a component of the turbulence propagates in the ion diamagnetic direction and that this particular mode is reduced in pellet fuelled discharges. The effects of the reduction of an ion mode turbulence were sought in the energy confinement of the discharges, but it was found that for these experiments (tailored for the turbulence diagnostics) the energy flowing in the ion channel was not large enough to affect the energy confinement. Simulations were used to interpret some of the results. Discharge simulations which include the pellet injection can reproduce the sawtooth suppression. This result and known properties of discharges in which sawteeth are suppressed suggest that some of the profile effects (including peaking of the working gas particles) induced by pellet injection are due to sawtooth suppression. The particle peaking may cause the observed reduction in the turbulence which follows pellet injection.
    Nuclear Fusion 01/2011; 30(5):903. · 2.73 Impact Factor
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    ABSTRACT: Wave numbers as functions of frequency for density fluctuations in the interior of the TEXT tokamak are measured in experiments with the heavy ion beam probe by analysing the relative phases of signals originating from nearby points in the plasma. The experiments are carried out in ohmically heated discharges. The adjacent points are typically 2 cm apart, with their relative orientation (δr, δθ) depending on the position (r,θ). For angular frequencies ω ≤ 106 s−1 the signals are quite coherent, leading to reasonably well defined 'dispersion relations'. These do not correspond to known modes of the drift wave type such as ballooning or slab-like electron drift waves or ion temperature gradient modes. The effect of finite sample volume size does not significantly alter this conclusion.
    Nuclear Fusion 01/2011; 31(7):1355. · 2.73 Impact Factor
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    ABSTRACT: Expressions for the quasi-linear electrostatic electron particle and heat fluxes are derived in the wave frequency and electron collisionality regimes appropriate to the interior of the TEXT tokamak. These are expressed in terms of moments of the density fluctuation spectrum accounting for poloidal plasma rotation. Minimal assumptions are made as to the origin of the turbulence. The fluxes are then evaluated for an appropriate discharge condition on TEXT, using the density fluctuation frequency spectrum and equilibrium plasma potential measured by a heavy ion beam probe (HIBP). These are compared with the experimental fluxes for two measurements of the normalized rms poloidal wave number kθ,rmsρs. For kθ,rmsρs ≈ 0.1, which is an upper bound of the HIBP two-point phase shift measurements, the electrostatic fluxes are negligible and the rms frequency of the fluctuations is much larger than the Doppler shifted electron diamagnetic drift frequency. However, for kθ,rmsρs ≈ 0.3, as supported by far-infrared (FIR) scattering and as expected for drift waves, the electrostatic fluxes are sufficient to account for the observed fluxes in the region of low collisionality. In addition, the rms frequency (consistent with FIR scattering) agrees with the Doppler shifted electron diamagnetic drift frequency. However, the computed fluxes fall increasingly short of the observed fluxes with increasing radius.
    Nuclear Fusion 01/2011; 31(4):687. · 2.73 Impact Factor
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    ABSTRACT: The authors presents a plasma density perturbation analysis which closely couples measurements of perturbations in density, temperature and turbulent fluctuation levels with a theoretical model in order to calculate particle fluxes. Non-linearities in the plasma model and the modulation of plasma parameters such as the electron temperature caused by the density perturbation are found to be significant. Some necessary details of the modification of the density fluctuation spectrum, and the plasma potential and Zeff profile remain to be measured. However, the available data allow steady state and perturbed particle fluxes in the low collisionality region of TEXT (5 cm < r < 20 cm) to be calculated using a simplified dissipative trapped electron model. These calculated fluxes do not agree with the measured fluxes
    Nuclear Fusion 11/2002; 32(4):567. · 2.73 Impact Factor
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    ABSTRACT: Spatial correlations and average wavenumbers of fluctuations have been measured in the edge region (0.8 < r/a < 1) of the TEXT tokamak using a heavy ion beam probe. The poloidal correlation length is substantially longer inside the limiter than it is outside. From this result it is concluded that the average wavenumber is smaller inside the limiter than it is outside. The frequency dependence of the average wavenumber also varies with radius, and a shear layer is sometimes observed. The frequency averaged wavenumbers are of the order of 1 cm-1. The statistical phase velocities vary from 2.5 × 105 to 6 × 105 cm/s in this region. The sensitivity to higher wavenumbers was increased by reducing the sample volume size in one experiment, and it was found that the fluctuation level changed by a factor of two while the wavenumber at a given frequency changed by 20%. The measurements are compared with previous results from far infrared scattering and Langmuir probes. The statistical phase velocities obtained with a heavy ion beam probe disagree with those from the other diagnostics in some but not all cases. The observed low frequency statistical phase velocities are comparable to the diamagnetic drift velocity in the laboratory frame in some but not all cases
    Nuclear Fusion 10/2002; 32(8):1295. · 2.73 Impact Factor
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    ABSTRACT: Heavy ion beam probe measurements of the interior of the TEXT-U tokamak plasma have been used to discover a density fluctuation spectrum that exhibits poloidal asymmetry with ballooning characteristics. The asymmetry was eliminated at the smallest radii during the electron cyclotron heating phase, which has a flatter density profile and a higher electron temperature
    Nuclear Fusion 10/2002; 36(3):375. · 2.73 Impact Factor
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    ABSTRACT: Links between turbulent fluctuations and transport processes in tokamaks, particularly in TEXT, are discussed. In the plasma edge probes allow detailed measurements of the quantities required to directly determine the fluctuation driven fluxes. The total flux of particles is well explained by the measured electrostatic fluctuation driven flux. However, a satisfactory model to explain the origin of the fluctuations has not been identified. The processes responsible for determining the edge electron heat flux are less clear. In the confinement region experimental observations are restricted to measurements of density and potential fluctuations and their correlations. Three distinct features have been identified: (1) the ubiquitous broadband turbulence and (2) a quasi-coherent feature, both of which propagate in the electron diamagnetic drift direction, and (3) an ion feature which by definition propagates in the ion drift direction. The characteristics of the measured fluctuations are discussed and compared with the predictions of various models. Comparisons between measured particle, electron heat and ion heat fluxes, and those fluxes predicted to result from the measured fluctuations, are made.
    Plasma Physics and Controlled Fusion 12/2000; 30(11):1479. · 2.37 Impact Factor
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    ABSTRACT: Broadband density fluctuations and plasma potential profiles are measured with a 2 MeV heavy ion beam probe (HIBP) diagnostic system on the TEXT-U tokamak. Measurements are performed in various locations including the edge and the core, as well as the high and low field side regions of the plasma. Results show a poloidal asymmetry in the core density fluctuation power spectra in which a distinct mode in the frequency range 70–170 kHz is present in the low field side region only. Furthermore, the HIBP provides a direct measurement of the local plasma potential from which the electric field profile across the plasma is inferred. Results showing the poloidal asymmetry as well as the potential and electric field profiles are presented.
    Fusion Engineering and Design 03/1997; 34:613-616. · 0.84 Impact Factor
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    ABSTRACT: An assessment of the feasibility of installing the TEXT 2 MeV heavy-ion beam probe on the DIII-D tokamak has been completed. Detailed drawings of the machine cross section were imported into the CAD application AutoCAD. A set of programs written in AutoLisp were used to generate trajectories. Displays of the accessible cross section of the plasma, scan lines for the entire range of primary beam energy and injection angle ranges, and sample–volume dimensions can be rapidly generated. Because of the large deflection between the primary input beam and the emergent secondary beam, either the analyzer needs to be tracked over a ±20° angle or secondary poloidal deflector plates need to be installed at the exit port. Toroidal deflector plates will be installed at both the injection and exit ports to compensate for toroidal displacements and deflections. The sample volumes generated by this procedure are within a few centimeters of the locations derived from a full three-dimensional calculation.© 1997 American Institute of Physics.
    Review of Scientific Instruments 02/1997; · 1.60 Impact Factor
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    ABSTRACT: Temporal variations in the electric potential have been measured in the interior of the TEXT-Upgrade plasma using a heavy ion beam probe (HIBP) diagnostic. Typical frequencies are on the order of 100 kHz. An earlier 500 keV HIBP experiment on TEXT measured the potential fluctuations and transport in the outer region of the plasma (r/a > 0.65) (J.C. Forster, P.M. Schoch, R.L. Hickok, and W.C. Jennings, IEEE Transactions on Plasma Science 22, 359 (1994)). New measurements extending into the core of the plasma are achieved with higher signal levels due to a higher energy; 2 MeV, HIBP. In the interior of the plasma, correlation between density and potential fluctuations shows small phase shift indicating little transport due to the electrostatic fluctuations. The broadband measurements in the interior are also consistent with the Boltzmann condition, tilde n / n = e tildephi / Te expected for drift waves.
    11/1996;
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    ABSTRACT: Measurements of electric field and plasma potential changes have been made with the 2 MeV HIBP on TEXT-Upgrade for a variety of plasma conditions. Scan lines from plasma center to edge give radial profiles of these characteristics and their temporal evolution. Changes with the addition of 500 kW of ECH power are noted, with an increase in the central potential of 600 Volts. Potential changes with sawtooth cycles are examined showing a variation of up to 100 Volts in the core. Associated with the sawtooth crash are large, rapidly decaying potential oscillations at approximately 100 kHz. Variations in electric field are examined with particular emphasis on the region around the q=1 rational surface.
    11/1996;
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    ABSTRACT: Summary form only given, as follows. The 2 MeV HIBP on TEXT-Upgrade has been used to measure the plasma potential for a variety of discharge conditions. Small scale, multiple point measurements throughout a cross section of the plasma give variations in electric field with a resolution of better than 10 V/cm. Scan lines through the plasma produce a potential profile along a radial path from plasma center to edge. Changes in electric field and potential are monitored during impurity injection and with 500 kW of ECH heating. Fluctuations in plasma potential have been observed deep within the plasma with 40 V rms sensitivity
    Plasma Science, 1996. IEEE Conference Record - Abstracts., 1996 IEEE International Conference on; 07/1996
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    ABSTRACT: Broad bandwidth (60 kHz â¤{ital f}â¤500 kHz), long wavelength ({ital {bar k}}{sub θ}â¤2 cm⁻¹) density fluctuation measurements were made using a heavy ion beam probe on the Texas Experimental Tokamak (TEXT) [K. W. Gentle, Nucl. Technol./Fusion {bold 1}, 479 (1981)]. Fluctuation amplitudes and correlation length scalings do not correspond to linear or quasilinear drift wave theory. Power-weighted root-mean-square (rms) wave number estimates are much lower ({l_angle}{ital {bar k}}{sub θ}{r_angle}{sub rms}ρ{sub {ital s}}â¤0.1, ρ{sub {ital s}}=[{ital m}{sub {ital iT}}{sub {ital e}}]¹²/{ital eB}) than quasilinear theoretical expectations ({ital {bar k}}{sub â¥}ρ{sub {ital s}}â¼0.3). Core results indicate strong turbulence with a wave number spread, Î{ital k}â¼{ital {bar k}} at fixed frequency {ital f}, consistent with strong turbulence predictions, while edge results can be considered consistent with weak turbulence. In between the core and edge (0.6â¤{ital r}/{ital a}â¤0.8) is a transition region. {copyright} {ital 1995} {ital American} {ital Institute} {ital of} {ital Physics}.
    Physics of Plasmas 01/1995; 2:3360-3367. · 2.38 Impact Factor
  • Source
    R.L. Hickok
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    ABSTRACT: The author describes some aspects of research on controlled thermonuclear fusion as an energy source, starting in the late 1950s. There was a need for new diagnostic techniques for studying high temperature plasmas. The author proposed a diagnostic for measuring plasma density by probing a deuterium plasma with a deuterium beam and measuring the proton production from the D-D nuclear reaction. By the mid 1960s, it was possible to do so. After carrying out a D-D measurement, the author suggested switching over to an H<sub>2</sub><sup>+</sup> beam and looking for an H<sup>+</sup> signal. The H+ was loud and clear and that was the last of the nuclear measurements. One of the first things done using the molecular break-up of the H<sub>2</sub><sup>+</sup> to measure the density of the hollow cathode arc plasma was a study of a coherent instability. The frequency response wasn't sufficient to measure the instability directly, so a Langmuir probe was used to detect the instability. The Langmuir probe gave a signal from a fixed spatial location but the beam probe signal was swept across the plasma giving 2D spatial resolution. This was the first detailed mapping of a plasma instability. Attempts to apply the ion beam probe to measuring the plasma current density in the ST Tokamak are described. Heavy ion beam probing measurements using hollow cathode arcs as target plasmas made it possible to measure T<sub>e</sub> at low temperatures and to identify space potential fluctuations. Installation of a beam probe on the Laser Initiated Target Experiment are described, along with work on the ELMO Bumpy Torus, the VERSATOR Tokamak, TMX, TEXT, ISX-B, the Ergodic Magnetic Limiter and ATF
    IEEE Transactions on Plasma Science 09/1994; · 0.87 Impact Factor
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    ABSTRACT: Two-dimensional electron temperature and density data have been obtained in the midplane of the non-axisymmetric magnetic confinement device ELMO Bumpy Torus (EBT) through the use of a heavy ion beam probe. This beam probe differs from others operated on toroidal or open magnetic geometries in its combination of complete computer control with the steady-state nature of EBT which allows, under normal operating conditions, for extensive calibration of the system in situ, minimizing both alignment and acquisition errors, along with the use of synchronous detection to dramatically improve the quality of the detected signal over what is typically possible in fast pulse devices. These techniques are important and applicable to long pulse devices where the beam probe may be an ideal diagnostic to measure, for example, parameters of the edge plasma. While the EBT beam probe was implemented to obtain profiles of plasma space potential, we have found that it can also be used effectively to measure the temperature and density profiles in the midplane between magnetic field coils. The data obtained support the contention that the formation of a hollow temperature profile in the T-Mode sufficiently inverts the plasma pressure such that stability of the core plasma would be expected even without diamagnetic effects from the hot electron rings which have previously been considered essential
    IEEE Transactions on Plasma Science 09/1994; · 0.87 Impact Factor
  • G.A. Hallock, R.L. Hickok, R.S. Hornady
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    ABSTRACT: A heavy ion beam probe has been used to measure the radial space potential distribution in the central cell of TMX. This was the first beam probe system to utilize computer control, CAMAC instrumentation, and fast time response for broadband fluctuation capabilities. The fast time response was obtained using off-line processing of the energy analyzer detector signals and wideband transimpedance amplifiers. The on-axis space potential was found to be 300-400 V, with φ<sub>e</sub>/T<sub>ec</sub>~8. The radial potential profile is parabolic when gas box fueling is used. The frequency of observed fluctuations was found to agree with the E×B plasma rotation frequency during the discharge. The measured Tl<sup>++</sup> secondary ion current level is consistent with calculations, given reasonable assumptions for beam attenuation
    IEEE Transactions on Plasma Science 09/1994; · 0.87 Impact Factor
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    ABSTRACT: A heavy ion beam probe was used to study the characteristics of density and potential fluctuations in the TEXT tokamak. Fluctuations of density and space potential are nearly Boltzmann like, n˜/n~φ˜/kT<sub>e</sub>, near the edge of the plasma (0.8<r/a<0.9). Potential fluctuations are proportionally greater in the edge region (r/a>0.9). The turbulent E&oarr;×B&oarr; radial particle flux is sufficient to account for all of the particle loss from the tokamak. No poloidal asymmetries, within a poloidal angle range of about 70°, are observed in the fluctuation levels. The fluctuation spectral shape, the density potential phase angle, and the fluctuation propagation speed show a strong radial dependence
    IEEE Transactions on Plasma Science 09/1994; · 0.87 Impact Factor
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    ABSTRACT: The heavy ion beam probe (HIBP) signal used to measure local density fluctuations in a plasma is also sensitive to modulation due to density fluctuations along the entire beam trajectory. A modulation model of the HIBP experiment on the Texas experimental tokamak (TEXT) is presented. The model includes profile information for equilibrium and fluctuating parameters, allows for differences in the radial and poloidal characteristics of the fluctuations, and uses realistic beam trajectories. It is shown that profile effects are important in understanding HIBP modulation and that modulation does not simply increase with line average density in TEXT. In addition, calculations of the modulation effects show that only the terms which correspond to in‐phase signals at the two sample volumes are significant. Therefore, the modulation effects can be approximated with a real parameter. Under these assumptions, it is shown that only long correlation length, low wave number modes will contribute significantly to the corruption of the measured signal. The calculation of the modulation effects are consistent with the experiment. It is illustrated herein how the measured data can be used to set limits on the modulation signal without doing extensive model calculations. These limits show that there must be long wavelength fluctuations in the plasma.
    Review of Scientific Instruments 05/1993; · 1.60 Impact Factor

Publication Stats

706 Citations
136.93 Total Impact Points

Institutions

  • 1975–2011
    • Rensselaer Polytechnic Institute
      • Department of Electrical, Computer, and Systems Engineering
      Troy, New York, United States
  • 1988–2002
    • The University of Texas at Austin
      • Fusion Research Center
      Texas City, TX, United States
  • 1990
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Angeles, California, United States