ALBERT

Notes: Experiments involving plasma improvement, confinement scaling, bootstrap currents, and edge fluctuations have been carried out in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)]. Average densities n¯e≤9×1019 m−3 have been obtained, with global energy confinement times τ@B|E≤20 msec. Confinement times generally follow the stellarator/torsatron empirical scaling law, τSL =0.17×P−0.58n0.69eB0.84a2R0.75 (with τSL in seconds, power P in megawatts, density ne in 1020 m−3, and plasma radius a and major radius R in meters). Gas injection during neutral beam injection (NBI) causes increases in ne, so that τ*E does not decrease during NBI. Edge plasma fluctuations are found to exhibit a mode change near the peak of the energy confinement time. Plasma currents observed during electron cyclotron heating have been identified as bootstrap currents.

Notes: Recent experiments in the Advanced Toroidal Facility (ATF) [Fusion Technol. 10, 179 (1986)] have been directed toward investigations of the basic physics mechanisms that control confinement in this device. Measurements of the density fluctuations throughout the plasma volume have provided indications for the existence of theoretically predicted dissipative trapped electron and resistive interchange instabilities. These identifications are supported by results of dynamic configuration scans of the magnetic fields during which the magnetic well volume, shear, and fraction of confined trapped particles are changed continuously. The influence of magnetic islands on the global confinement has been studied by deliberately applying error fields which strongly perturb the nested flux-surface geometry, and the effects of electric fields have been investigated by means of biased limiter experiments.

Notes: Access to the magnetohydrodynamic (MHD) second stability regime has been achieved in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)]. Operation with a field error that reduced the plasma radius and edge rotational transform resulted in peaked pressure profiles and increased Shafranov shift that lowered the theoretical transition to ideal MHD second stability to β0≈1.3%; the experimental β values (β0≤3%) are well above this transition. The measured magnetic fluctuations decrease with increasing β, and the pressure profile broadens, consistent with the theoretical expectations for self-stabilization of resistive interchange modes. Initial results from experiments with the field error removed show that the pressure profile is now broader. These later discharges are characterized by a transition to improved (×2–3) confinement and a marked change in the edge density fluctuation spectrum, but the causal relationship of these changes is not yet clear.

Notes: The Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] has completed experiments focusing on microwave scattering measurements of density fluctuations and transport studies utilizing the modulation of dimensionless parameters. Microwave scattering measurements of electron density fluctuations in the core of low-collisionality electron cyclotron heated (ECH) plasmas show features that might be evidence of trapped electron instabilities. Starting from gyro-Bohm scaling, the additional dependence of confinement on the dimensionless parameters ν* and β (collisionality and beta) has been investigated by modulating each of these parameters separately, revealing the additional favorable dependence, τE∝τgBν*−0.18β+0.3.

Notes: Many of the diagnostics that work on present magnetic fusion experiments will fail badly in the high radiation field of the burning fusion devices which are under conceptual design now. Two quantities of particular interest are the poloidal magnetic field profile versus minor radius, and radial measurements of β near the plasma center. Knowledge of the poloidal magnetic field is sufficient to infer the q profile and/or the current profile which is vital to understanding the MHD stability of the plasma and controlling the current profile. Knowledge of the ratio β of the plasma pressure to the magnetic field pressure radially resolved in the vicinity of the plasma center is also extremely useful for control and diagnosis of the plasma. A proposed new diagnostic uses crossed-sightline correlation of the thermal fluctuations in the electron cyclotron radiation emitted by the plasma to infer the local magnetic field very accurately. The field is measured at the crossing point of two sightlines into the plasma. The sightlines cross at small angles, typically 8° at the plasma center to about 18° at the plasma edge. By scanning the mirrors defining the two sightlines the absolute magnetic field across the plasma minor radius can be measured to an accuracy of better than 1 part in 103, with time resolution of the order of 10 ms per data point. This allows calculation of the q profile over most of the minor radius to an accuracy of better than 10% (given density and temperature measurements), and extremely accurate measurements of the plasma beta from r/a=0 to r/a=0.2.

Notes: The edge density profile can play a significant role in determining the plasma confinement and the coupling of the ion cyclotron resonance frequency (ICRF) heating power to the plasma. To experimentally measure the edge density profile in the Tokamak Fusion Test Facility (TFTR), a two-frequency microwave reflectometer is being built. This reflectometer will operate in a swept two-frequency configuration between 91 and 118 GHz using the extraordinary mode. The frequency separation between the two microwave signals will be held constant while the signals are swept across the frequency band. By measuring the differential phase delay between these two signals, the density profile can be reconstructed. Two-frequency profile reflectometry is discussed and results of modeling of this type of reflectometer measurement for TFTR are shown. Finally, the design of the TFTR edge profile reflectometer microwave system is described.

Notes: The local value of the magnetic field B(r) inside the plasma in magnetic fusion experiments is a quantity of considerable interest. Given information about plasma density and temperature it can be used to calculate the poloidal magnetic field, or given a zero current device (or the current profile) the local value of beta. The time history B(,t) can be used to infer the level of magnetic fluctuations in the plasma. The poloidal magnetic field and the level of magnetic fluctuations are particularly interesting quantities, since they are difficult to measure, and are both important for understanding particle and energy transport in magnetic fusion experiments. In fact, to date, no measurement of the local value of the total magnetic field or its time variation has been made in a high temperature plasma. Previous measurements have been confined to relatively cold plasmas where probes could be used, or have inadequate resolution to distinguish the value of the field with plasma present from its vacuum value. It has been recently suggested that crossed-sightline correlation of electron-cyclotron emission might be used to infer B(rx,t) at the crossing point rx in the plasma of the two sightlines. The equations and techniques necessary to simulate this proposed diagnostic are developed below, and the results of a numerical simulation of the diagnostic resolution for both TEXT and CIT are presented.

Notes: The Advanced Toroidal Facility (ATF) density fluctuation reflectometer system consists of two individual reflectometers operating in the 30–40 GHz band. Each reflectometer consists of a tunable microwave source and a quadrature phase detector connected to the same antenna system. This arrangement allows two-frequency operation along the same radial chord for radial coherence measurements. The technique used in making radial coherence measurements is discussed and the results of such experiments are given. Initial experiments have shown high coherence when the frequencies of the two reflectometers are tuned close together and a clear loss of coherence as the radial separation of the cutoff layers is increased by increasing the frequency separation of the two reflectometers. Recent results have shown that local measurements of density fluctuations in plasmas with electron cyclotron heating (ECH) are possible and that detailed structure can be seen in the fluctuation spectra. In addition, radial correlation lengths have been found to be from 0.5 to 1.0 cm in ECH plasmas, with some frequency structures having correlation lengths up to 3 cm. In plasmas with neutral beam injection (NBI), the radial correlation lengths in the edge region have been found to be approximately 0.1–0.2 cm.

Notes: Abstract The late Archean, north-south trending Kolar Schist Belt in south India, 4 km wide by 80 km long, is thought to be a suture between two gneiss terranes (Krogstad et al. 1989). Within this volcanics-dominated belt are recognized both tholeiitic and high Mg (komatiitic and picritic) amphibolites, which make up some 70% and 5% respectively of the exposed outcrops. A massive tholeiitic amphibolite separates the belt into western and eastern parts. A Pb-Pb whole-rock age of 2732±155 Ma on samples from a single outcrop of massive tholeiite is a minimum age for this rock. Samples of this rock have ɛNd values at 2700 Ma that range between +3.8 and +6.8, μ1 (initial 238U/204Pb) of 7.5 and K 1 (initial 232Th/238U) of about 4. Two different types of high-Mg amphibolites are recognized from the western part of the belt: a picritic or P-type, and a komatiitic or K-type. The P-type have highly variable Ce/Al ratios all greater than chondritic, Nd/Yb ratios greater than chondritic, ɛNd at 2700 Ma of +1.5 to +8, and Pb isotope compositions variable in 207Pb/204Pb with μ1 of about 8.0 and k 1 of about 4. The trace-element data suggest that the light-REE enrichment is a character of the mantle source and is not due to residual garnet. The K-type amphibolites have near chondritic Ce/Al and Nd/Yb ratios, ɛNd at 2700 Ma of +1.5 to +8, and μ1 of about 8 and k 1 of about 4. Although the P-type is light-REE enriched compared to the K-type, both types have similar Ce/Nd ratios as well as initial Pb and Nd isotopes. If the 2696±136 Ma age for the Sm-Nd isochron which includes both types of high-Mg amphibolite has any significance it dates the time of light-REE enrichment of the mantle source for the P-type komatiitic amphibolites. The high-Mg amphibolites in the eastern part of the belt are light-REE enriched, have Pb isotopic compositions that are variable in 207Pb/204Pb with a μ1 about 8.5 and ɛNd at 2700 Ma of +1.8 to +4.5. Hydrothermal fluids associated with metamorphism and shearing prior to about 2400 Ma ago were responsible for the introduction of gold-quartz-carbonate veins into the Kolar Schist Belt. The Pb isotope composition of galena in these veins suggests that these fluids may have also introduced extraneous Pb from adjacent older granitoid gneisses into the amphibolites, which could be responsible for the variability in the 207Pb/204Pb ratios of the samples. This extraneous Pb probably is not responsible for the distinct Pb isotope character of each type of amphibolite.