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An impedance model is formulated and tested for the ~1 kV, 1 kA/cm2, arc-plasma cathode electron source used for local helicity injection tokamak startup. A double layer sheath is established between the high-density arc plasma (narc ≈ 1021 m-3) within the electron source, and the less dense external tokamak edge plasma (nedge ≈ 1018 m-3) into which current is injected at the applied injector voltage, Vinj. Experiments on the Pegasus spherical tokamak show that the injected current, Iinj, increases with Vinj according to the standard double layer scaling Iinj ~ V3/2 inj at low current and transitions to Iinj ~ V1/2 inj at high currents. In this high current regime, sheath expansion and/or space charge neutralization impose limits on the beam density nb ~ Iinj=V1/2 inj . For low tokamak edge density nedge and high Iinj, the inferred beam density nb is consistent with the requirement nb ~ nedge imposed by space-charge neutralization of the beam in the tokamak edge plasma. At sufficient edge density, nb ~ narc is observed, consistent with a limit to nb imposed by expansion of the double layer sheath. These results suggest that narc is a viable control actuator for the source impedance. [ABSTRACT FROM AUTHOR]

The power required to induce a bifurcation from a low-confinement mode to a high-confinement mode in DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] plasmas is found to depend sensitively on the injected neutral beam torque and consequent toroidal rotation. Plasmas exhibit a factor of 2–4 reduction in this power threshold, dependent on ion ∇B drift direction. Correlated with this change, turbulence velocity measurements near 0.9

The divertor magnetic geometry has a significant effect on the poloidal velocity and resulting velocity shear of turbulent density fluctuations in the outer region of L-mode tokamak plasmas, as determined via two-dimensional measurements of density fluctuations with beam emission spectroscopy on DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)]. Plasmas with similar parameters, except that in one case the ion ∇B drift points towards the divertor X point (lower-single-null, LSN), and in the other case, the ion ∇B drift points away from the divertor X point (upper-single-null, USN), are compared. Inside r/a=0.9, the turbulence characteristics (density fluctuation amplitude, flow direction, correlation lengths) are similar in both cases, while near r/a=0.92, a dramatic reversal of the poloidal velocity of turbulent eddies relative to the core flow direction is observed in plasmas with the ion ∇B drift pointing towards the divertor X point. No such velocity reversal is observed in plasmas with the ion ∇B drift pointing away from the divertor X point. This poloidal velocity reversal results in a significantly larger local shear in the poloidal velocity of density fluctuations in plasmas with the ion ∇B drift pointing towards the divertor X point. Additionally, these plasmas locally exhibit significant dispersion with two distinct and counterpropagating turbulence modes. Likewise, the radial correlation length of the density fluctuations is reduced in these plasmas, consistent with biorthogonal decomposition measurements of dominant turbulence structures. The naturally occurring density fluctuation poloidal velocity shear in these LSN plasmas may facilitate the L-H transition that occurs at an input power of roughly one-half to one-third that of corresponding plasmas with the ion ∇B drift pointing away from the X point. [ABSTRACT FROM AUTHOR]

The application of time-delay-estimation techniques to two-dimensional measurements of density fluctuations, obtained with beam emission spectroscopy in DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] plasmas, has provided temporally and spatially resolved measurements of the turbulence flow-field. Features that are characteristic of self-generated zonal flows are observed in the radial region 0.85 &les;r/a &les;1.0. These features include a coherent oscillation (approximately 15 kHz) in the poloidal flow of density fluctuations that has a long poloidal wavelength, possibly m = 0, narrow radial extent (k[SUBr]ρ[SUBI] < 0.2), and whose frequency varies monotonically with the local temperature. The approximate effective shearing rate, dv[SUBθ/dr, of the flow is of the same order of magnitude as the measured nonlinear decorrelation rate of the turbulence, and the density fluctuation amplitude is modulated at the frequency of the observed flow oscillation. Some phase coherence is observed between the higher wavenumber density fluctuations and low frequency poloidal flow fluctuations, suggesting a Reynolds stress contribution. These characteristics are consistent with predicted features of zonal flows, specifically identified as geodesic acoustic modes, observed in 3-D Braginskii simulations of core/edge turbulence. [ABSTRACT FROM AUTHOR]

Intermittent plasma objects (IPOs), featuring higher pressure than the surrounding plasma, are responsible for ∼50% of the E × B[SUBT] radial transport in the scrape off layer (SOL) of the Doublet III D (DIII-D) tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] 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 × B[SUBT]/B[SUP2] velocities of ∼2600 m/s near the last closed flux surface (LCFS), and ∼ 330 m/s near the wall. The IPOs slow down as they shrink in size from 2 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 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 ∼ 3 cm into the SOL while their temperature decays much faster (∼1 cm). [ABSTRACT FROM AUTHOR]

The Pegasus Toroidal Experiment [R. Fonck et al., Bull. Am. Phys. Soc. 41, 1400 (1996)] is a spherical torus designed to study the limits of plasma behavior as the aspect ratio A approaches unity. Access to near-unity A is achieved through the use of a novel high-stress reinforced solenoid magnet. High toroidal beta β[SUBt] is obtained in ohmically-heated plasmas by operation at low field with densities up to the Greenwald limit. Values of β[SUBt] up to 20% and normalized beta up to 5 have been obtained. The ratio of plasma current to toroidal field rod current, known as the toroidal field utilization, reaches values as large as 1 but appears to approach a "soft" boundary at that level related to both ohmic flux limitations and the onset of resistive magnetohydrodynamic (MHD) activity. The m/n = 2/1 and 3/2 modes are most frequently observed, in agreement with the inferred safety factor profiles. Experiments are beginning to access the external kink stability boundary at edge safety factor q[SUB95] = 5, which is significantly higher than that observed in conventional tokamaks. Calculations using the DCON code [A. H. Glasser and M. S. Chance, Bull. Am. Phys. Soc. 42, 1848 (1997)] confirm instability to the ideal kink. [ABSTRACT FROM AUTHOR]

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

A compact 15-channel bolometer array composed of discrete thermistors has been installed on the Princeton Beta Experiment tokamak. The response time, fastest for detectors with low thermal capacities, has been optimized by using "free-standing" detectors. The detector is supported only by thin electrical leads and no thermal substrate is employed. Overheating due to limited heat dissipation is controlled with careful detector biasing. The sensitivity is limited by the systematic errors in the model of the heat transfer from the detector to the electrodes and leads. A two-pole cooling model was found to characterize the detector's thermal decay far better than a simple exponential and resulted in an NEP≈3×10[sup 7] W√Hz with a 300-Hz bandwidth. [ABSTRACT FROM AUTHOR]

Previous studies of plasma microturbulence have indicated that the fluctuation power scales with radial wave number, k[sub ⊥], like k[sup -2, sub ⊥ → k[sup -3.5, sub ⊥] for k[sub ⊥] > 2 cm[sup -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[sub ⊥] <2 cm[sup -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 D[sub α] 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 modifications to account for systematic effects such as photon statistics and fluctuations in the neutral beam density induced by the strong fluctuations near the plasma edge. [ABSTRACT FROM AUTHOR]

Several experiments have been devised to measure plasma fluctuations in an effort to help elucidate a possible connection between plasma microturbulence and anomalous transport. Results from microwave scattering on the Tokamak Fusion Test Reactor (TFTR) [Nucl. Fusion 18, 1089 (1978)] show that the level of plasma fluctuations increases toward the long-wavelength region (k⊥ρs≤0.2), at which point the fluctuations cannot be spatially resolved. The desire to measure long-wavelength fluctuations has motivated the development of two fluctuation diagnostics, beam emission spectroscopy (BES), and microwave reflectometry on TFTR. BES measures long-wavelength density fluctuations (k⊥≤2 cm-1) by observing the fluorescence emitted from collisionally excited atoms in a TFTR heating beam. In L-mode discharges with relatively flat density profiles, the fluctuations measured with BES are concentrated in the low-frequency region (≤30 kHz). In the laboratory frame, the fluctuations have a poloidal propagation velocity that is approximately equal to that of the plasma rotation, and the frequency spectra are broadened by Doppler effects. Measured fluctuation levels are 5%–10% at the edge of the plasma. In the core, the level falls to less than 1%, which is comparable to observations made with microwave reflectometry and scattering. The fluctuation level in the core (r/a=0.7) is seen to increase with injected beam power, as is observed with microwave scattering at shorter wavelengths (k⊥≊2–10 cm-1). In contrast, the fluctuation level in the edge region does not change significantly with neutral beam power. [ABSTRACT FROM AUTHOR]

Toroidal Alfvén eigenmodes (TAE) were excited by the energetic neutral beam ions tangentially injected into plasmas at low magnetic field in the Tokamak Fusion Test Reactor (TFTR) [Proceedings of the 11th International Conference on Plasma Physics and Controlled Fusion Research (IAEA, Vienna, 1987), Vol. 1, p. 51]. The injection velocities were comparable to the Alfvén speed. The modes were identified by measurements from Mirnov coils and beam emission spectroscopy (BES). TAE modes appear in bursts whose repetition rate increases with beam power. The neutron emission rate exhibits sawtoothlike behavior and the crashes always coincide with TAE bursts. This indicates ejection of fast ions from the plasma until these modes are stabilized. The dynamics of growth and stabilization were investigated at various plasma currents and magnetic fields. The results indicate that the instability can effectively clamp the number of energetic ions in the plasmas. The observed instability threshold is discussed in light of recent theories. In addition to these TAE modes, intermittent oscillations at three times the fundamental TAE frequency were observed by Mirnov coils, but no corresponding signal was found in BES. It appears that these high-frequency oscillations do not have a direct effect on the plasma neutron source strength. [ABSTRACT FROM AUTHOR]

Magnetohydrodynamic (MHD) activity within three zones (core, half-radius, and edge) of TFTR [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. 1, p. 51 ] tokamak plasmas are discussed. Near the core of the plasma column, sawteeth are often observed. Two types of sawteeth are studied in detail; one with complete, and the other with incomplete, magnetic reconnection. Their characteristics are determined by the shape of the q profile. Near the half-radius the m/n = 3/2 and 2/1 resistive ballooning modes are found to correlate with a beta collapse. The pressure and the pressure gradient at the mode rational surface are found to play an important role in stability. MHD activity is also studied at the plasma edge during limiter H modes. The edge localized modes (ELM's) are found to have a precursor mode with a frequency between 50-500 kHz and a mode number m/n = 1/0. The mode does not show a ballooning structure. While these instabilities have been studied on many other machines, on TFTR the studies have been extended to high pressure (plasma i e pressure greater than 4 × 10[sup 5] Pa) and low collisionality [v[sup i, sub *] (a/2) < 0.002, v[sup e, sub *] (a/2) < 0.01 ]. [ABSTRACT FROM AUTHOR]

A key challenge for the development of practical fusion power is to understand how particle and thermal transport are controlled by plasma turbulence. Of particular interest is the turbulent velocity field, which drives the “anomalous” fluxes. Current measurements of the velocity fluctuations have generally been limited to the outer edges of the plasma, but the recently proposed time-delay estimation method offers the possibility of measuring velocity fluctuations further into the plasma, motivating a comprehensive investigation of this method. The first step of this investigation, reported here, is to develop a greater degree of confidence in the method, by testing the technique on known signals and velocity fields. Issues such as optimal sampling rate, noise sensitivity, and the impact of signal processing techniques such as data windowing and interpolation are discussed. Future steps of the investigation will also be presented. [ABSTRACT FROM AUTHOR]

For the next generation, burning tokamak plasmas such as in the International Thermonuclear Experimental Reactor (ITER), diagnostic neutral beams and beam spectroscopy will continue to be used to determine a variety of plasmas parameters such as ion temperature, rotation, fluctuations, impurity content, current density profile, and confined alpha particle density and energy distribution. Present-day low-current, long-pulse beam technology will be unable to provide the required signal intensities because of higher beam attenuation and background bremsstrahlung radiation in these larger, higher-density plasmas. To address this problem, we are developing a short-pulse, intense diagnostic neutral beam. Protons or deuterons are accelerated using magnetic-insulated ion-diode technology, and neutralized in a transient gas cell. A prototype 50-kA, 100-kV, 1-µs accelerator is under construction at Los Alamos. Initial experiments will focus on ITER-related issues of beam energy distribution, current density, pulse length, divergence, propagation, impurity content, reproducibility, and maintenance. [ABSTRACT FROM AUTHOR]

A compact 15-channel bolometer array that views plasma emission tangentially across the midplane has been installed on the PBX tokamak to supplement a 19-channel radially viewing poloidat array. By comparing these diagnostics, poloidal asymmetries in the emission profile can be determined. The detector array consists of 15 discrete 2-mm-diam thinistors, a mixed semiconductor material, which has a high temperature coefficient of resistance. Each active detector is operated in tandem with an identical blind detector in a Wheatstone bridge configuration. The variation in voltage owing to the change in resistance is amplified and stored on a CAMAC transient recorder. The detectors are sensitive to accumulated heat and software has been written to smooth and differentiate the signals so that instantaneous power can be calculated, in addition, the cooling time constants are determined in vacuo (3.1 s) so that the loss of heat from the detector during the course of a shot can be compensated. The detectors were relatively calibrated with a tungsten-halogen filament lamp and were found to vary by ± 7%. The irradiance profiles are inverted and the power density is typically in the 0.1 to 0.5-W/cm³ range. [ABSTRACT FROM AUTHOR]