V. Arunasalam

Overview of TFTR transport studies

A review of TFTR plasma transport studies is presented. Parallel transport and the confinement of suprathermal ions are found to be relatively well described by theory. Cross-field transport of the thermal plasma, however, is anomalous with the momentum diffusivity being comparable to the ion thermal diffusivity and larger than the electron thermal diffusivity in neutral beam heated discharges. Perturbative experiments have studied nonlinear dependencies in the transport coefficients and examined the role of possible nonlocal phenomena. The underlying turbulence has been studied using microwave scattering, beam emission spectroscopy and microwave reflectometry over a much broader range in k perpendicular to than previously possible. Results indicate the existence of large-wavelength fluctuations correlated with enhanced transport.

Correlations of heat and momentum transport in the TFTR tokamak

Measurements of the toroidal rotation speed vφ(r) driven by neutral beam injection in tokamak plasmas and, in particular, simultaneous profile measurements of vφ, Ti, Te, and ne, have provided new insights into the nature of anomalous transport in tokamaks. Low‐recycling plasmas heated with unidirectional neutral beam injection exhibit a strong correlation among the local diffusivities, χφ≊χi>χe. Recent measurements have confirmed similar behavior in broad‐density L‐mode plasmas. These results are consistent with the conjecture that electrostatic turbulence is the dominant transport mechanism in the tokamak fusion test reactor tokamak (TFTR) [Phys. Rev. Lett. 58, 1004 (1987)], and are inconsistent with predictions both from test‐particle models of strong magnetic turbulence and from ripple transport. Toroidal rotation speed measurements in peaked‐density TFTR ‘‘supershots’’ with partially unbalanced beam injection indicate that momentum transport decreases as the density profile becomes more peaked. In hi...

Study of drift-wave turbulence by microwave scattering in a toroidal plasma

A study of drift wave turbulence by microwave scattering technique was carried out in a toroidal plasma confinement device, the FM-1 spherator. The principal results are; (a) Observation of the linear dispersion relation of drift waves in the high magnetic shear condition. The linear dispersion relation is followed up to weak turbulence state (..gamma.. approximately less than ..omega..). (b) An experimental demonstration of the stabilization of the drift waves with an increase of the shear strength. In particular, the ion mass dependence of the observed marginal stability criterion for drift waves (..omega.. approximately ..omega../sub */ was in reasonably good agreement with theoretical expectations. Further, very low frequency (..omega.. much less than ..omega../sub */) fluctuations were enhanced when the shear strength was lowered. (c) A detailed study of the behavior of drift waves in a weak shear condition where a state of isotropic turbulence was observed. The turbulent density fluctuations showed a strong dependence on frequency, but no dependence on wave number (somewhat in sharp contrast to theoretical expectations). The possible dependence of the anomalous loss process on the observed drift wave turbulence is also discussed.

High power neutral beam heating experiments on TFTR with balanced and unbalanced momemtum input

New long-pulse ion sources have been employed to extend the neutral beam pulse on TFTR from 0.5 sec to 2.0 sec. This made it possible to study the long-term evolution of supershots at constant current and to perform experiments in which the plasma current was ramped up during the heating pulse. Experiments were conducted with co and counter injection as well as with nearly balanced injection of deuterium beams up to a total power of 20 MW. The best results, i.e., central ion temperatures Tio > 25 keV and neo τE Tio values of 3 × 1020 keV sec m-3, were obtained with nearly balanced injection. The central toroidal plasma rotation velocity scales in a linear-offset fashion with beam power and density. The scaling of the inferred global momentum confinement time with plasma parameters is inconsistent with the predictions of the neoclassical theory of gyroviscous damping. An interesting plasma regime with properties similar to the H-mode has been observed for limiter plasmas with edge qa just above 3 and 2.5.

Parametric instabilities and plasma heating in an inhomogeneous plasma

Experimental studies of plasma heating due to microwave irradiation of the magnetically confined plasma column in the Princeton L-3 device is presented. X-band (10.4 GHz) microwave power, both in the ordinary and the extraordinary modes of propagation, is used in these experiments. Plasma heating is observed to occur simultaneously with the occurrence of parametric decay instabilities. The mode structure of the pump wave and the decay ion wave dispersion have been measured with high frequency probes. Detailed measurements of electron heating rates are presented and compared with collisional heating rates. In addition, production of suprathermal electrons and ions is also observed and measured. A comparison is made with recent laser-pellet interaction experiments.

Quenching of the current-driven ion-wave instability in the trapped-electron regime in a toroidal plasma

An analysis is presented of an experimental study of the scattering of microwaves by density fluctuations due to the current‐driven ion acoustic wave instability in a toroidal plasma confinement device, the FM‐1 spherator. It is found that the instability exists only in the collisional regime (where νef≳ωbe) and it is inhibited (i.e., quenched by two orders of magnitude) in the trapped‐electron banana regime (where νef<ωbe). Further, it is found that for two different values of the ratio of the trapped to untrapped electron number density (i.e., for both 70% and 40% mirror trapping) this inhibition always starts to occur when νef ≈ωbe. Finally, it is shown that the nature of this inhibition is in general agreement with the recent theoretical notion that in the banana regime the entire Ohmic heating current is carried only by the freely circulating electrons.

Carbon influx flow rate in an Ohmically heated plasma in the FM-1 spherator

The flow rate of neutral carbon atoms during the initial start-up phase of an Ohmically heated plasma in th e FM-1 spherator was determined from the spatially resolved measurements of the carbon line radiation and independent measurements of spatial distribution of plasma density and electron temperature. This was accomplished by comparing the measured carbon line intensity with a numerical solution of time-dependen t couple d differential rate equations for several carbon ionization states. The differential equations used in the present model correspond to a spatially homogeneous medium. However, th e effects of plasma inhomogeneity were taken into account by calculating the plasma emission at each location using the experimentally measured electron density and temperature (i.e. the measured radial distributions of density and temperature were folded into the numerical calculations). The empirical flow rate of carbon atoms into the discharge. Γflow(t),was found to be Γflow(t)ne(t)/τflow. where τflow was 30–100 ms depending upon discharge conditions

Experimental results from the TFTR tokamak

Recent experiments on TFTR have extended the operating regime of TFTR in both ohmic- and neutral-beam -heated discharges. The TFTR tokamak has reached its original machine-design specifications (Ip = 2.5 MA and BT = 5.2 T). Initial neutral-beam -heating experiments used up to 6.3 MW of deuterium beams. With the recent installation of two additional beamlines, the power has been increased up to 11 MW. A deuterium pellet injector was used to increase the central density to 2.5 x 1020 m-3 in high-current discharges. At the opposite extreme, by operating at low plasm a current (Ip ~ 0.8 MA) and low density (ne~ 1 x 1019 m-3), high ion temperatures (9 + 2 keV) and rotation speeds (7 x 105 m s-1) have been achieved during injection. In addition, plasma-compression experiments have demonstrated acceleration of beam ions from 82 to 150 keV, in accord with expectations. The wide operating range of TFTR, together with an extensive set of diagnostics and a flexible control system, has facilitated transport and scaling studies of both ohmic- and neutral-beam -heated discharges. The result of these confinement studies are presented.

Energy loss rates of energetic ions injected into the FM-1 spherator

Energy loss rates of energetic ions were studied during neutral beam injection experiments on the FM‐1 spherator. Under typical experimental conditions, the injected beam ions slowed‐down on the plasma electrons resulting in electron heating. The measured slowing‐down rate was found to be in agreement with classical predictions.