R. J. Fonck

Transport by intermittency in the boundary of the DIII-D tokamak

A271 TRANSPORT BY INTERMITTENCY IN THE BOUNDARY OF THE DIII-D TOKAMAK. Intermittent plasma objectives (IPOs) featuring higher pressure than the surrounding plasma, are responsible for {approx} 50% of the E x B{sub T} radial transport in the scrape off layer (SOL) of the DIII-D tokamak in L- and H-mode discharges. Conditional averaging reveals that the IPOs are positively charged and feature internal poloidal electric fields of up to 4000 V/m. The IPOs move radially with E x B{sub T}/B{sup 2} velocities of {approx} 2600 m/s near the last closed flux surface (LCFS), and {approx} 330 m/s near the wall. The IPOs slow down as they shrink in radial size from 4 cm at the LCFS to 0.5 cm near the wall. The skewness (i.e. asymmetry of fluctuations from the average) of probe and beam emission spectroscopy (BES) data indicate IPO formation at or near the LCFS and the existence of positive and negative IPOs which move in opposite directions. The particle content of the IPOs at the LCFS is linearly dependent on the local density and decays over {approx} 3 cm into the SOL while their temperature decays much faster ({approx} 1 cm).

Plasma fluctuation measurements in tokamaks using beam‐plasma interactions

High‐frequency observations of light emitted from the interactions between plasma ions and injected neutral beam atoms allow the measurement of moderate‐wavelength fluctuations in plasma and impurity ion densities. To detect turbulence in the local plasma ion density, the collisionally excited fluorescence from a neutral beam is measured either separately at several spatial points or with a multichannel imaging detector. Similarly, the role of impurity ion density fluctuations is measured using charge exchange recombination excited transitions emitted by the ion species of interest. This technique can access the relatively unexplored region of long‐wavelength plasma turbulence with k⊥ρi≪1, and hence complements measurements from scattering experiments. Optimization of neutral beam geometry and optical sightlines can result in very good localization and resolution (Δx≤1 cm) in the hot plasma core region. The detectable fluctuation level is determined by photon statistics, atomic excitation processes, and b...

The beam emission spectroscopy diagnostic on the DIII-D tokamak

A beam emission spectroscopy system has been installed on DIII-D to provide localized density fluctuation measurements for long-wavelength turbulent modes with k⩽3 cm−1 which are typically associated with anomalous radial transport. High signal-to-noise fluctuations measurements are accomplished through use of high speed electronics to maintain a frequency response of over 500 KHz and cryogenically cooled amplifiers and detectors to reduce electronic noise. The optics and neutral beam-sightline geometry have been optimized to allow for spatial resolution of Δr⩽1 cm. In addition, a half-scale two-dimensional (2D) fiber array to measure the 2D turbulent density field, necessary to measure the full S(kr,kθ) wavenumber spectra, has been implemented and initial results obtained.

Experimental progress on zonal flow physics in toroidal plasmas

The present status of experiments on zonal flows in magnetic confinement experiments is examined. The innovative use of traditional and modern diagnostics has revealed unambiguously the existence of zonal flows, their spatio-temporal characteristics, their relationship to turbulence and their effects on confinement. In particular, a number of observations have been accumulated on the oscillatory branch of zonal flows, named geodesic acoustic modes, suggesting the necessity for theories to give their proper description. In addition to these basic properties of zonal flows, several new methods have elucidated the processes of zonal flow generation from turbulence. Further investigation of the relationship between zonal flows and confinement is strongly encouraged as cross-device activity including low temperature, toroidal and linear devices.

Convergence, electrostatic potential, and density measurements in a spherically convergent ion focus

Unique measurements of the basic plasma-flow characteristics in a low pressure (⩽53 mPa H2) spherically convergent ion focus are obtained using high-voltage (⩽5 kV) emissive and double probes. The radial plasma potential distribution agrees with a collisionless, recirculating, space-charge-limited current model. Flow convergence increases with voltage and neutral pressure and decreases with cathode grid wire spacing and current. Core radii within 4–5 times the ideal geometric limit are measured, and the observed core sizes are consistent with predictions from a multipass orbit model which includes asymmetries in the accelerating potential well. A virtual anode is observed in the converged core region, and no evidence for multiple potential well structures in the core is found. Measurements of the core ion density (nic∼1015 m−3) are consistent with simple flow convergence models.

Non-dimensional scaling of turbulence characteristics and turbulent diffusivity

Plasma turbulence characteristics, including radial correlation lengths, decorrelation times, amplitude profile and flow velocity, have been measured during a ρ* scan on DIII-D while all other transport relevant dimensionless quantities (e.g., β, ν*, κ, q, Te/Ti) are held nearly constant. The turbulence is measured by examining the correlation properties of the local long wavelength (k⊥ρi ≤ 1) density fluctuations, measured with beam emission spectroscopy. The radial correlation length of the turbulence Lc,r is shown to scale with the local ion gyroradius, Lc,r ≈ 5ρi, while the decorrelation times scale with the local acoustic velocity as τc~a/cs. The turbulent diffusivity parameter, D~(Lc,r2/τc), scales in a roughly gyro-Bohm-like fashion, as predicted by the gyrokinetic equations governing turbulent transport. The experimental one fluid power balance heat diffusivity scaling and that from GLF23 modelling compare reasonably well.

Structure and scaling properties of the geodesic acoustic mode

Characteristics and scaling properties of the geodesic acoustic mode (GAM), a coherent, radially-sheared high frequency (~15?kHz) zonal flow oscillation, are studied systematically using time-delay-estimation techniques applied to localized, multi-point density fluctuation measurements obtained by beam emission spectroscopy on DIII-D. The GAM amplitude is shown to increase strongly with increasing safety factor, q95, and to likewise become undetectably small for q95 < 4.2, qualitatively consistent with theoretical predictions based on collisional damping as well as simulations. The radial structure of the GAM exhibits peak amplitude in the radial range 0.88 < r/a < 0.95 with a rapid amplitude reduction inside and outside this region. The measured frequency is close to the predicted frequency, though some deviation to higher frequency is observed at lower q. The GAM amplitude is also shown to increase with plasma elongation, ?, while its frequency decreases.

Density fluctuation measurements via beam emission spectroscopy (invited)

Previous studies of plasma microturbulence have indicated that the fluctuation power scales with radial wave number, k⊥ , like k⊥−2→k⊥−3.5 for k⊥ ≥2 cm−1. This implies that low k fluctuations may dominate the spectrum. Beam emission spectroscopy (BES) has been developed to provide spatially localized measurements of density fluctuations in this low k region of the spectrum (k⊥ ≤2 cm−1). A 20‐channel system has been installed on TFTR which images one of the heating neutral beams (via fiber optics) onto a set of photoconductive photodiode detectors. Fluctuations in the fluorescent Dα emission from the beam can be related to the local plasma density fluctuations via a model of the atomic excitation processes. The analysis of BES data utilizes many of the standard statistical analysis techniques such as power spectra, coherency and cross phase, and correlation analysis which are also used in the analysis of, for example, Langmuir probe data. In the case of BES however, these techniques require some special mo...

Turbulence imaging and applications using beam emission spectroscopy on DIII-D (invited)

Two-dimensional measurements of density fluctuations are obtained in the radial and poloidal plane of the DIII-D tokamak with the Beam Emission Spectroscopy (BES) diagnostic system. The goals are to visualize the spatial structure and time evolution of turbulent eddies, as well as to obtain the 2D statistical properties of turbulence. The measurements are obtained with an array of localized BES spatial channels configured to image a midplane region of the plasma. 32 channels have been deployed, each with a spatial resolution of about 1 cm in the radial and poloidal directions, thus providing measurements of turbulence in the wave number range 0<k⊥⩽3 cm−1. A 5 (radial)×6 (poloidal) channel grid provides time-resolved images near the outer midplane at the sampling frequency of 1 MHz, thus providing a modest spatial resolution, high throughput, high time resolution turbulence imaging system. The images and resulting movies have broad application to a wide variety of fundamental turbulence studies: imaging of...

FUSION REACTIVITY CHARACTERIZATION OF A SPHERICALLY CONVERGENT ION FOCUS

The deuterium-deuterium (D-D) fusion reaction rate in a spherically convergent ion focus is observed to significantly exceed the rate predicted by a collisionless flow model. However, a careful consideration of ion-neutral collisions and the trapped neutral density in the cathode account for the extra reactivity without invoking anomalous ion trapping in the converged core region. This conclusion is supported by proton collimation measurements, which indicate that the bulk of the observed reactivity originates outside the core region. In addition, a classical flow model, where charge exchange collisional effects on the ion and fast neutral distributions are included, provides fusion rate estimates that are quantitatively consistent with the observed D-D fusion neutron production rate.