K.L. Wong

A review of Alfvén eigenmode observations in toroidal plasmas

In toroidal magnetically confined plasmas, eigenmodes of Alfven waves can be destablized by energetic ions with velocities comparable to the Alfven velocity. With the advent of tokamak experiments in which populations of energetic ions can be introduced by neutral beam injection, radio frequency wave heating or by fusion reactions, major advances have been made in Alfven eigenmode research in the past 10 years. After introducing the basic concepts on the Alfven eigenmode instability, data on this subject from various toroidal devices are described, emphasizing the interplay between experiment and theory. Experimental results on mode identification, instability drive, mode damping and saturation, and energetic ion redistribution are compared with theory.

Plasma-material interactions in TFTR

This paper presents a summary of plasma-material interactions which influence the operation of TFTR with high current (≤ 2.2 MA) ohmically heated, and high-power (∼ 10 MW) neutral-beam heated plasmas. The conditioning procedures which are applied routinely to the first-wall hardware are reviewed. Fueling characteristics during gas, pellet, and neutral-beam fueling are described. Recycling coefficients near unity are observed for most gas fueled discharges. Gas fueled discharges after helium discharge conditioning of the toroidal bumper limiter, and discharges fueled by neutral beams and pellets, show R<1. In the vicinity of the gas fueled density limit (at ne = 5–6 × 1019 m−3) values of Zeff are ≦1.5. Increases in Zeff of ≦1 have been observed with neutral beam heating of 10 MW. The primary low Z impurity is carbon with concentrations decreasing from ∼10% to <1% with increasing ne. Oxygen densities tend to increase with ne, and at the ohmic plasma density limit oxygen and carbon concentrations are comparable. Chromium getter experiments and He2+/D+ plasma comparisons indicate that the limiter is the primary source of carbon and that the vessel wall is a significant source of the oxygen impurity. Metallic impurities, consisting of the vacuum vessel metals (Ni, Fe, Cr) have significant (∼10−4 ne) concentrations only at low plasma densities (ne <1019 m−3). The primary source of metallic impurities is most likely ion sputtering from metals deposited on the carbon limiter surface.

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.

Effects of ion cyclotron harmonic damping on current drive in the lower hybrid frequency range

The ion cyclotron harmonic damping effects on slow and fast waves in the lower hybrid frequency range for tokamak reactor parameters are studied. Inclusion of the higher-order terms in the hot-plasma dielectric tensor introduces ion cyclotron harmonic damping; these terms also contribute to the real part of the dispersion relation and affect the wave trajectories. However, wave absorption by 15 keV deuterium and tritium ions can be avoided by choosing the slow-wave frequency above the lower-hybrid frequency and the fast-wave frequency below the lower hybrid frequency. But preliminary estimates show that energetic alpha particles tend to absorb both the slow and the fast waves. This absorption may become a serious obstacle to fusion-reactor current drive in the lower hybrid frequency range.

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...

Expansion of parameter space for toroidal Alfven eigenmode experiments in TFTR

Several techniques were used to excite toroidal Alfven eigenmodes (TAE) in the Tokamak Fusion Test Reactor (TFTR) (Proc. 13th Int. Conf. on Plasma Physics and Controlled Nuclear Fusion Research Vol 1 (Vienna: IAEA, 1990) 9) at magnetic fields above 10 kG. These involved pellet injection to raise the plasma density, variation of the plasma current to change the energetic ion orbit and the q-profile, and ICRF heating to produce energetic hydrogen ions at velocities comparable with 3.5 MeV alpha particles created in deuterium-tritium (d-t) fusion reactions. The amplitude of the TAE modes observed in ICRF-heated plasmas behaves differently from those driven by neutral beams. This can be explained by the difference in the energetic ion orbits. These experimental results are presented and relevance to fusion reactors are discussed.

SHORT WAVELENGTH FLUCTUATIONS AND ELECTRON HEAT CONDUCTIVITY IN ENHANCED REVERSED SHEAR PLASMAS

Abstract Short wavelength fluctuations with k ∼ ω pe c and kϱ i ∼ 5 are detected by microwave scattering in TFTR plasmas with reversed magnetic shear. They propagate in the ion diamagnetic drift direction with a frequency below the ion diamagnetic drift frequency in the frame of reference where E r = 0. The variation of the fluctuation amplitude in the plasma core correlates with the variation of the local electron heat conductivity, suggesting that these fluctuations may be the cause of anomalous electron heat transport in these plasmas.

Alfven frequency modes at the edge of TFTR plasmas

An Alfven frequency mode (AFM) is very often seen in TFTR neutral beam heated plasmas as well as in ohmic plasmas. This quasi-coherent mode has so far only been seen on magnetic fluctuation diagnostics (Mirnov coils). A close correlation between the plasma edge density and the mode activity (frequency and amplitude) has been observed, which indicates that the AFM is an edge localized mode with r/a>0.85. No direct impact of this mode on the plasma global performance or on fast ion loss (e.g., the alpha particles in DT experiments) has been observed. This mode is not the conventional TAE (toroidicity induced Alfven eigenmode). The present TAE theory cannot explain this observation. Other possible explanations are discussed

Experimental results from detached plasmas in TFTR

Abstract Detached plasmas are formed in TFTR which have the principal property that the boundary to the high temperature plasma core is defined by a radiating layer. This paper documents the properties of TFTR ohmic detached plasmas with a range of plasma densities at two different plasma currents.

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.