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  • 1
    Electronic Resource
    Electronic Resource
    Effect of beam-attenuation modulation on fluctuation measurements by heavy-ion beam probe (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 2232-2240 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Beam-attenuation modulation arising from density fluctuations along the orbit of the heavy-ion beam probe (HIBP) in a plasma can distort the local amplitude, coherence, and phase derived from one- and two-point correlation measurements. Path-integral expressions for these effects are derived and applications to the TEXT tokamak are discussed. The work is part of an effort to account for previously reported wave-number data. However, the analysis is general and bears on any correlation measurement in turbulent media that depends on beam propagation or might otherwise be affected by spurious common-mode signals. In the HIBP case the effects depend critically on the ratio of the average fluctuation amplitude ñe along the beam path to the local ñe at the sample volume. Because the fluctuation amplitude is small in the core and rises sharply toward the plasma edge, the contamination effect is negligible in a radial zone near the edge but rises sharply to the interior of a critical radius. With increasing average plasma density n¯e the interior contamination increases strongly and the critical radius moves outward. The conclusion is that beam-modulation fluctuations affect the data but do not fully explain the disagreement with theoretical predictions of drift waves. The effects are expected to decrease with increasing beam energy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143144
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  • 2
    Electronic Resource
    Electronic Resource
    Path integral effects in heavy ion beam probe density measurements: A comparison of simulation results and experimental data (1993)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 1001-1009 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1144169
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  • 3
    Electronic Resource
    Electronic Resource
    Initial operation of the TEXT-Upgrade 2-MeV heavy ion beam probe (abstract) : 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. 4582-4582 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The new heavy ion beam probe diagnostic system on TEXT is presently operational at reduced energy. Thallium and cesium ion beams with energy up to 590 keV have been obtained using a single injection beamline and an energy analyzer that can be operated with low-energy beams only (≤590 keV). At full energy (up to 2 MeV) two injection beamlines and two energy analyzers (presently under construction) will allow access to 90% of the plasma cross section. Stable 2-MeV test beams with currents up to 10 μA have been obtained but not injected into the tokamak. The capacitive liner feedback control system allowed stable operations with ripple less than 100 V. Results of initial operations and preliminary measurements of density fluctuations are presented. This work is supported by the U. S. Department of Energy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143676
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  • 4
    Electronic Resource
    Electronic Resource
    Evolution of the 2-MeV electrostatic energy analyzer for TEXT-Upgrade (abstract) : 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. 4582-4582 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The development of an electrostatic energy analyzer to be used with the 2-MeV heavy ion beam probe on the TEXT-Upgrade tokamak has presented several very interesting voltage breakdown phenomena. In order to maintain a 400-kV potential on large surface area plates (46.2 cm×152 cm) various configurations of support dielectrics, voltage feedthrough, plate materials, and plate geometry have been evaluated. Experimental evaluation of several designs and the resulting modifications leading to a working analyzer will be presented. This work was supported by the U.S. Department of Energy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143675
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  • 5
    Electronic Resource
    Electronic Resource
    Broadband density fluctuation measurements using a heavy ion beam probe on the Texas Experimental Tokamak (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3360-3367 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Broad bandwidth (60 kHz ≤f≤500 kHz), long wavelength (k¯θ≤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 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 (〈k¯θ〉rmsρs≤0.1, ρs=[miTe]1/2/eB) than quasilinear theoretical expectations (k¯⊥ρs∼0.3). Core results indicate strong turbulence with a wave number spread, Δk∼k¯ at fixed frequency f, consistent with strong turbulence predictions, while edge results can be considered consistent with weak turbulence. In between the core and edge (0.6≤r/a≤0.8) is a transition region. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871170
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  • 6
    Electronic Resource
    Electronic Resource
    Experimental investigation of a coherent flute instability using a heavy ion beam probe (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 2704-2709 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A coherent, low-frequency instability found in a cylindrical, hollow cathode arc plasma has been investigated by using a heavy ion beam probe (HIBP). The energy density of the plasma was high enough to render it inaccessible to Langmuir probes, but the HIBP was able to provide measurements throughout the plasma cross section. The data clearly show that azimuthal symmetry does not exist. Radial profiles of steady-state density and space potential and of simultaneous ñ, φ˜ amplitude and phase were obtained to allow detailed comparison between theory and experiment. Predictions from a cylindrically symmetric, small-perturbation theoretical model provide reasonably conclusive identification of the instability as a Kelvin–Helmholtz flute driven by and localized in a region of fluid shear. The most serious discrepancy was with regard to the oscillation frequency, which was consistently predicted to be three to four times lower than that observed experimentally. The reason for the discrepancy is not understood, but it is probably related to inadequacies in the theory caused by assumptions of azimuthal symmetry and of small linear perturbations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866549
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  • 7
    Electronic Resource
    Electronic Resource
    Comparison of drift wave models with fluctuation data from the interior of the TEXT tokamak (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2251-2260 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental evidence for and against drift waves as the origin of the observed fluctuations and anomalous transport in the plasma interior is reviewed. Fluctuation spectra observed by far-infrared (FIR) scattering and a heavy ion beam probe (HIBP) are compared. The FIR system observes broad S(k,ω), which are spatially resolved at large k and readily identified with electron drift waves. At higher densities a clear ion feature, which may be associated with ion temperature gradient (ITG) modes, appears as well. A quasicoherent feature in the drift wave range of phase velocities is also found at the inner midplane. But interior HIBP measurements using a thallium beam have, by a two-point correlation method, measured wave numbers far too small (i.e., phase velocities far too high) to be those of drift waves of either variety. Some recent measurements with a cesium beam have produced phase velocities more closely in accord with drift waves at low frequencies. It is suggested that a new mode may be present, which is not drift wavelike. However, the phase velocity and coherence of the HIBP data exhibit qualitative features that suggest possible instrumental effects. Several of these are investigated, which individually do not appear to reconcile the data with a pure drift wave model. They are (1) sample volume size, (2) common mode effects such as cross-talk or path integrals of beam attenuation, and (3) two-stream instabilities. Reconciliation of these measurements is important to transport studies. That is, calculated particle and energy fluxes depend sensitively on the frequency and wave-number spectrum employed. The theoretical considerations about two-point correlations and possible instrumental effects are also relevant to other diagnostics using this technique.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859644
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  • 8
    Electronic Resource
    Electronic Resource
    Impurity rotation in hydrogen and helium plasmas (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 917-923 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Rotation velocities were measured in two Ohmically heated discharges and compared to neoclassical predictions evaluated with measurements of Ti, ni, and Er. Hydrogen fueled one of the discharges, and helium was the principal fuel for the other. For both discharges, poloidal rotation is consistent with the momentum balance equation. In contrast, current neoclassical theory predicts the toroidal rotation velocity only in the case of negligible ion–neutral charge exchange and neutral ionization.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860108
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  • 9
    Electronic Resource
    Electronic Resource
    The space potential in the tokamak text (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 3448-3461 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A heavy ion beam probe has been used to measure the plasma space potential profiles in the tokamak TEXT [Nucl. Fusion Technol. 1, 479 (1981)]. The Ohmic discharges studied were perturbed by externally produced resonant magnetic fields (an ergodic magnetic limiter or EML). Without these perturbations the plasma central potential is generally consistent with the value calculated from radial ion momentum balance, using experimental values of density and ion temperature and assuming a neoclassical poloidal rotation velocity. Exceptions to the agreement are found when operating with reduced plasma parameters. Possible reasons for this discrepancy are explored, in particular, the effects of intrinsic magnetic field fluctuations, and modifications to the self-consistent radial electric sheath. With the application of the EML fields the edge electric field and potential increase during periods of magnetic island overlap. A test particle calculation of electron transport shows increases in diffusivity also occur during periods of magnetic island overlap. These calculated changes in diffusivity are interpreted in terms of a stochastic layer width, which is itself used to predict a potential change for comparison with the experimental results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859774
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  • 10
    Electronic Resource
    Electronic Resource
    Electric field studies of a 2 MeV electrostatic energy analyzer : Eight topical conference on high-temperature plasma diagnostics (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 2955-2957 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An energy analyzer based on the Proca and Green parallel-plate design is being developed for use with the 2 MeV heavy ion beam probe on TEXT. In a departure from the conventional configuration, guard ring electrodes will not be used. Instead, a shaped top plate will provide for comparable, or improved, uniformity of the analyzer electric field region. To quantify this effect, and to characterize the electrostatic field, numerical solution methods have been utilized. Simulations have included effects of top plate shape, wire screens, vacuum chamber design, and dielectric support structures. The modeling has permitted us to design an analyzer electrode structure that is an integral part of a uniquely shaped vacuum vessel. The design electric field is 20 kV/cm with less than 1% error in uniformity within the parallel plate region. To examine the electric field structure experimentally, a quarter-scale prototype analyzer has been constructed and tested. The electric field characteristics are examined by varying the path of a heavy ion beam through the analyzer and examining the resulting analyzer performance. A simulated vacuum wall can be positioned to examine the effects of different vessel configurations and to determine the sensitivity of the analyzer to this boundary condition. The experimental results show excellent agreement with the numerically predicted fields and confirm the validity of the shaped top plate electrode concept.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1141788
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