J. W. Heard

An experimental counter‐example to the local transport paradigm

The response of a tokamak discharge to a sharp drop in edge temperature differs significantly from that expected from typical local transport models in several important respects. Laser ablation of carbon induces large (ΔT/T≤70%), rapid (<200 μs) electron temperature drops in the outermost region of the plasma, r/a≥0.9. This cold pulse proceeds through the outer plasma (r/a≥0.75), rapidly compared with power balance or sawtooth predictions. However, the pulse shrinks markedly thereafter, disappearing near r/a∼0.5. Within r/a∼0.3, the temperature rises promptly. The results are inconsistent with conventional local transport models; a nonlocal phenomenology, in which transport coefficients increase in the edge and decrease in the core, is suggested. The turbulence levels measured with a heavy ion beam probe increase near the edge but are unchanged in the core.

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...

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

Poloidal asymmetry and gradient drive in core electron density and temperature fluctuations on the Texas Experimental Tokamak-Upgrade

Electron temperature and density fluctuations are measured in the core of the Texas Experimental Tokamak‐Upgrade (TEXT‐U) [P. H. Edmonds, E. R. Solano, and A. J. Wootton, in Proceedings of the 15th Symposium on Fusion Technology, Utrecht (Elsevier Science, Amsterdam, 1989), Vol. 1, p. 342] plasma across the poloidal cross section. The high spatial resolution of the heavy‐ion beam probe (HIBP) and correlation radiometry of electron cyclotron emission (CRECE) reveal that both the density and temperature fluctuations are strongly poloidally asymmetric. Temperature fluctuation measurements indicate a broadband drift wave feature localized near the plasma equatorial plane on both the high‐ and low‐field sides, which is consistent with density fluctuation measurements by far infrared (FIR) scattering. In contrast, the HIBP observes this feature localized only to the low‐field side. Excellent spatial resolution allows us to investigate whether changes in the gradient affect the fluctuation amplitudes. We find th...

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...

Path integral effects in heavy ion beam probe density measurements: A comparison of simulation results and experimental data

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 co...

Broadband fluctuation wave number measurement technique using a heavy ion beam probe

The heavy ion beam probe (HIBP) measures localized density fluctuations in the plasma interior. These measurements can be used to calculate two point estimates of interior poloidal wave numbers. The results so far show wave numbers significantly lower than FIR scattering wave numbers and drift wave expectations. Experiments were performed on TEXT to look for possible sources of error in our wave number calculations. The HIBP effects that are considered the possible dominant error terms are finite sample volume size and path effect fluctuations as well as the possible existence of counter‐propagating modes. An aperture experiment showed the presence of a finite sample volume effect, but not large enough to correct the measured wave numbers significantly. Simulations using HIBP data show that our processing techniques detect counter‐propagating modes. Path effects are discussed. The error analysis so far shows the HIBP calculations to be accurate wave number measurements, but more study on path effects is n...

Plasma potential measurements on text‐upgrade with a 2 MeV heavy ion beam probe (abstract)a)

The plasma potential is measured in TEXT‐upgrade tokamak by injection and detection of high energy heavy ions (thallium and cesium with a single charge) using a 2 MeV accelerator and a parallel plate energy analyzer. The change in beam energy, as it crosses the plasma, gives the local plasma potential at the measurement volume. Recent results of high energy beam operations are presented.