R.J. Colchin

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).

Heat pulse propagation studies in TFTR

The time-scales for sawtooth repetition and heat pulse propagation are much longer (tens of milliseconds) in the large tokamak TFTR than in previous, smaller tokamaks. This extended time-scale, coupled with more detailed diagnostics, has led us to revisit the analysis of heat pulse propagation as a method to determine the electron heat diffusivity χe in the plasma. A combination of analytic and computer solutions of the electron heat diffusion equation is used to clarify previous work and to develop new methods for determining χe. Direct comparison of the predicted heat pulses with soft-X-ray and ECE data indicates that the space-time evolution is diffusive. However, the χe determined from heat pulse propagation usually exceeds that determined from background plasma power balance considerations by a factor ranging from two to ten. Some hypotheses for resolving this discrepancy are discussed.

Solid hydrogen pellet injection into the Ormak tokamak

Solid hydrogen spheres were injected into the ORMAK tokamak as a test of pellet refueling for tokamak fusion reactors. Pellets 70 ..mu..m and 210 ..mu..m in diameter were injected with speeds of 91 m/sec and 100 m/sec, respectively. Each of the 210-..mu..m pellets added about 1% to the number of particles contained in the plasma. Excited neutrals, ablated from these hydrogen spheres, emitted light which was monitored either by a photomultiplier or by a high speed framing camera. From these light signals it was possible to measure pellet lifetimes, ablation rates, and the spatial distribution of hydrogen atoms in the ablation clouds. The average measured lifetime of the 70-..mu..m pellets was 422 ..mu..sec, and the 210-..mu..m spheres lasted 880 ..mu..sec under bombardment by the plasma. These lifetimes and measured ablation rates are in good agreement with a theoretical model which takes into account shielding of plasma electrons by hydrogen atoms ablated from spherical hydrogen ice.

HIGH PERFORMANCE H-MODE PLASMAS AT DENSITIES ABOVE THE GREENWALD LIMIT

Densities of up to 40% above the Greenwald limit are reproducibly achieved in high confinement (HITER89P = 2) ELMing H mode discharges. Simultaneous gas fuelling and divertor pumping were used to obtain these results. Confinement of these discharges, similar to moderate density H mode, is characterized by a stiff temperature profile, and is therefore sensitive to the density profile. A particle transport model is presented that explains the roles of divertor pumping and geometry for access to high densities. The energy loss per ELM at high density is a factor of five lower than the predictions of an earlier scaling, based on data from lower density discharges.

Physics of confinement improvement of plasmas with impurity injection in DIII-D

External impurity injection into L mode edge discharges in DIII-D has produced clear confinement improvement (a factor of 2 in energy confinement and neutron emission), reduction in all transport channels (particularly ion thermal diffusivity to the neoclassical level), and simultaneous reduction of long wavelength turbulence. Suppression of the long wavelength turbulence and transport reduction are attributed to synergistic effects of impurity induced enhancement of E × B shearing rate and reduction of toroidal drift wave turbulence growth rate. A prompt reduction of density fluctuations and local transport at the beginning of impurity injection appears to result from an increased gradient of toroidal rotation enhancing the E × B shearing. Transport simulations carried out using the National Transport Code Collaboration demonstration code with a gyro-Landau fluid model, GLF23, indicate that E × B shearing suppression is the dominant transport suppression mechanism.

Plasma properties in the ELMO bumpy torus

Experiments with 200 kW of applied electron cyclotron heating (ECH) power have demonstrated electron temperatures of about 1 keV in the ELMO Bumpy Torus-Scale (EBT-S) device. Electron densities are in the range of (0.5-1.5)*1018 m-3 and increase as the square root of the applied ECH power. A potential well is present, and its depth in V closely follows the electron temperature expressed in electron volts. Only low charge states of impurities are found, and Zeff approximately=1.0. Data from the electrons can be compared with simple scaling laws when scale lengths are held constant. These comparisons indicate that electron densities, temperatures, and confinement times scale according to neoclassical expectations.

Observation of hot electron ring instabilities in ELMO Bumpy Torus

A high‐frequency hot electron instability is observed in ELMO Bumpy Torus (EBT) plasmas when the hot electron‐to‐ion density ratio exceeds 0.4. Both the real frequency and the imaginary frequency are larger than the ion cyclotron frequency. The azimuthal mode number (m) is 7, and the instability rotates in the hot electron curvature drift direction. This instability is identified as a curvature‐driven mode. When it is strongly excited, the equilibrium of the hot electron annuli and confined plasmas are destroyed (disruption). Ion heating and neutron bursts are associated with this instability.

Feedback control for plasma equilibrium in ORMAK

Absolute feedback control of the horizontal plasma position has been implemented on the ORMAK tokamak. Position control was accomplished by combining a pre-programmed vertical field current with a corrector current. The pre-programmed current was that predicted from optimum control theory, based on an approximate knowledge of plasma conditions. The corrector current, which usually is different for each shot, was derived from an on-line computation of the position of the outer poloidal magnetic flux surface. As a result of these controls, this surface can be held constant to within a few millimetres throughout the bulk of the discharge. In practice, the outer poloidal flux surface is deliberately shifted inward so that the centre of the magnetic field it produces is at the geometric centre of the torus.

Evolution and Stabilization of Flutes in a Magnetic Mirror

This paper deals with flutes as observed in a hot‐ion, low‐β plasma. This plasma was established in an adiabatic, simple mirror trap by Lorentz or gas‐collisional ionization of an energetic H0 beam. The studies emphasize: (1) the detailed evolution of the flute mode and comparisons with linear theory and (2) the stabilizing effects of increased electrical connection of the plasma to conducting end walls (line tying). The evolution is traced as a function of density from a stable drift wave at low density to threshold for a growing, unstable wave which quickly exhibited nonlinear frequency behavior. Evolution of the stable drift wave is much as predicted by the linear theories. Instability threshold seems to occur at a somewhat higher density than that calculated even from theories treating bounded plasmas. The line‐tying experiments used movable end walls as a control on the electrical connection. Threshold was raised with end walls close to the hot plasma, in a fashion that is in qualitative agreement wi...

Drift‐wave‐like density fluctuations in the Advanced Toroidal Facility (ATF) torsatron

Density fluctuations in low‐collisionality, low‐beta (β∼0.1%), currentless plasmas produced with electron cyclotron heating (ECH) in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] have been studied using a 2 mm microwave scattering diagnostic. Pulsed gas puffing is used to produce transient steepening of the density profile from its typically flat shape; this leads to growth in the density fluctuations when the temperature and density gradients both point in the same direction in the confinement region. The wave number spectra of the fluctuations that appear during this perturbation have a maximum at higher k⊥ρs (∼1) than is typically seen in tokamaks. The in–out asymmetry of the fluctuations along the major radius correlates with the distribution of confined trapped particles expected for the ATF magnetic field geometry. During the perturbation, the relative level of the density fluctuations in the confinement region (integrated over normalized minor radii ρ from 0.5 to 0...