R. L. Hickok

Advanced plasma fluctuation analysis techniques and their impact on fusion research (invited)

This article reviews digital spectral analysis techniques that yield experimental insight into plasma turbulence. Methods to quantify the statistical properties of the fluctuations and to measure the particle and heat flux caused by electrostatic fluctuations are presented. Furthermore, analysis techniques to study the nonlinear coupling process of turbulence and the redistribution of energy among the different modes are discussed. The impact of the analysis techniques on fusion research is demonstrated with experimental results collected with Langmuir probes, heavy‐ion beam probes, and laser scattering in the tokamak TEXT. Special emphasis is given to the characterization of the wavenumber distribution and the correlation lengths in all toroidal directions, including a first measurement of k∥ in a tokamak.This article reviews digital spectral analysis techniques that yield experimental insight into plasma turbulence. Methods to quantify the statistical properties of the fluctuations and to measure the particle and heat flux caused by electrostatic fluctuations are presented. Furthermore, analysis techniques to study the nonlinear coupling process of turbulence and the redistribution of energy among the different modes are discussed. The impact of the analysis techniques on fusion research is demonstrated with experimental results collected with Langmuir probes, heavy‐ion beam probes, and laser scattering in the tokamak TEXT. Special emphasis is given to the characterization of the wavenumber distribution and the correlation lengths in all toroidal directions, including a first measurement of k∥ in a tokamak.

TEXT heavy ion beam probe system

A good signal‐to‐noise ratio has been obtained during initial operation of the 500‐keV TEXT heavy ion beam probe. A number of problems were identified during the start up phase and they have been or are being corrected. Present sensitivity for detecting changes in the space potential is 50 V, but it is expected that this can be reduced to 5 V when additional calibration and alignment procedures are completed.

Heavy‐ion beam probe diagnostic systems (invited)

Heavy‐ion beam probing generally consists of passing a beam of 1+ ions through a plasma imbedded in a magnetic field. Secondary ions with higher ionization levels are produced by ionizing collisions with the plasma electrons. Detection of the secondary ions with a small‐aperture electrostatic energy analyzer allows continuous fluctuation measurements of the plasma density and space potential with both spatial and temporal resolution. Spatial resolution is the order of 0.1 cm3 and temporal resolution is presently electronics limited to ∼1 μs. The energy of the probing beam is determined primarily by the requirement that the secondary ion must escape from the plasma. Typical beam energies extend from 10 to 500 keV. The range of plasma densities that have been investigated is 1012 cm−3<ne<1014 cm−3. At the higher densities, beam attenuation becomes a serious problem. Higher beam energies provide better penetration of the magnetic field, and reduced beam attenuation. Heavy‐ion beam probes were first used to m...

Core turbulence and transport studies on the Texas Experimental Tokamak

Recent experiments on the Texas Experimental Tokamak (TEXT) [Plasma Phys. Controlled Fusion 27, 1335 (1985)] have focused on identifying the drives and transport significance of the turbulence in the interior of discharges in the neo‐Alcator confinement regime. Far‐infrared (FIR) scattering consistently observes density fluctuations indicative of electron drift waves, i.e., ωk∼ωe*+kθvE, where vE≡−Er/B (the electron diamagnetic frequency Doppler‐shifted by the E×B poloidal plasma rotation) and an amplitude that scales inversely with the density scale length Ln. Although consistent with scattering on the power‐weighted frequency ω, heavy‐ion beam probe (HIBP) data typically indicate kθρs<0.1 such that ω≫ωe*+kθvE and n/ne≪1/kθLn. Experiments with a modulated gas feed and/or electron‐cyclotron resonance heating (ECRH) seem to rule out ∇Te as the turbulent drive, although little evidence for ∇ne is apparent either. In fact, the interior fluctuations seen by the HIBP seem to depend more on edge condition...

Internal magnetic and electrostatic fluctuation measurements of magnetohydrodynamic modes in the Texas Experimental Tokamak (TEXT)

A heavy ion‐beam probe has been used to make the first internal measurements of magnetic fluctuations in a hot tokamak. The magnetic vector potential fluctuation, Aφs, is measured during magnetohydrodynamic (MHD) activity and is in reasonable agreement with the prediction of a three‐dimensional, resistive, compressible, nonlinear MHD code. Associated density and potential fluctuation measurements and magnetic island widths are also presented.

Effect of beam‐attenuation modulation on fluctuation measurements by heavy‐ion beam probe

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 ne along the beam path to the local ne 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 ave...

Ballooning characteristics in density fluctuations observed with the 2 MeV heavy ion beam probe on the TEXT-U tokamak

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

Study of density and potential fluctuations in the TEXT tokamak with a heavy ion beam probe

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/spl tilde//n/spl sim//spl phi//spl tilde//kT/sub e/, near the edge of the plasma (0.8 0.9). The turbulent E/spl I.oarr//spl times/B/spl I.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/spl deg/, 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. >

Heavy ion beam probe wavenumber measurements from the TEXT Tokamak edge

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

Recent advances in heavy ion beam probe diagnostics (invited)

Heavy ion beam probes (HIBPs) have proven to be a unique tool for measuring fluctuations and particle transport in tokamaks. They have been used to measure fluctuations in density, electric potential, and magnetic vector potential. The density and potential fluctuation measurements have determined the particle flux due to electrostatic turbulence in the TEXT and ISX‐B tokamaks. In these measurements, the frequency spectra (0–500 kHz) of the phase between density and potential, the wave numbers of the fluctuations, and the fluctuation level are obtained. Three topics are discussed in this paper. We present measurements of magnetic fluctuations during MHD activity using the TEXT HIBP. Analysis of these measurements indicates that the diagnostic is primarily sensitive to the local value of Aφ in the sample volume unless the local Aφ is small. In addition, we discuss instrumental effects associated with wave number measurements. We will discuss the effects of sample volume size on the wave number measurements...