R.J. Taylor

Trapping and removal of oxygen in tokamaks

Various discharge cleaning techniques have been studied in Microtor. It was found that high temperature discharges are not only ineffective in reducing the oxygen on the wall, and hence in the plasma, but may even induce trapping. Oxygen can be removed from the wall by a low-temperature, partially ionized hydrogen plasma by conversion to water vapour. The amount of loosely bound oxygen on the surface is thus reduced below 1% of a monolayer. The residual amount of the oxygen in the plasma is near 1010 atom cm−3, and represents a significant improvement over previous tokamak results.

Dynamics of H-mode-like edge transitions brought about by external polarization

By setting up radial electric fields in the edge of a tokamak plasma by means of electrodes it is possible to trigger edge rearrangements which are similar to L-H transitions. The radial currents due to the finite orthogonal conductivity provide a poloidal torque which spins up the plasma against the viscous force opposing the poloidal rotation. It is found, in agreement with theoretical predictions, that this force first increases with the rotation speed and then, above a critical rotation speed, decreases. Under conditions in which the applied torque exceeds the maximum opposing force, a bifurcation from low field to high field is inevitable and results in many of the typical L-H transition signatures. The measured radial conductivity is compared with neoclassical estimates.

Phenomenological comparison of magnetic and electrostatic fluctuations in the Macrotor tokamak

Probe measurements of magnetic and electrostatic fluctuations in Macrotor are compared. Attention is concentrated on small-scale fluctuations which lie in the frequency range above the usual low-frequency Mirnov oscillations. In this range, 20 kHz 125 kHz, the spectra of and are similar; the radial profile of /n is, however, peaked toward the outside while that of r is peaked toward the centre, and no local cross-correlation between and has been detected. The radial correlation length of is smaller than that for r (both being much smaller than the minor radius), but the poloidal correlation lengths of p and are similar. Both and fluctuations rotate in the electron diamagnetic drift direction. The spectra are roughly invariant to an increase in density; however, the magnitude of /n decreases with density while that of r remains unchanged. No connection between these fluctuations and anomalous transport has been established.

Transition from Bohm to classical diffusion due to edge rotation of a cylindrical plasma

The outer region of the plasma column of a large, linear plasma device is rotated in a controlled fashion by biasing a section of the vacuum chamber wall positive with respect to the cathode (Er<0). The magnitude and temporal dependence of the observed cross-field current, produced when the bias voltage is applied, is consistent with ion current arising from ion-neutral collisions. Flow speeds in the outer regions of the plasma column exceed the local sound speed. In the nonrotating plasma column, cross-field, radial particle transport proceeds at the Bohm diffusion rate. Rotation, above a threshold bias voltage, reduces cross-field transport from Bohm to classical rates, leading to steeper radial density profiles. Reduction of particle transport is global and not isolated to the region of flow shear. The transition from the nonrotating to the rotating plasma edge in the linear plasma column is similar to the low confinement to high confinement mode transition observed in tokamaks when Er<0.

Edge-plasma properties of the UCLA tokamaks

Probe measurements of the edge plasmas of the Macrotor and Microtor tokamaks are described. Limiter scrape-off layer thicknesses as measured with Langmuir probes are for Macrotor λ7–10 cm and for Microtor λ ~ 1 cm, both values being consistent with the Bohm diffusion rate. Heat deposition measured with thermocouples attached to small probes shows an anomalously high heat flux, particularly from the electron drift direction. It is argued that the anomalous particle diffusion is most likely associated with the large edge density fluctuations, while the anomalous heat flux is most likely due to a directed high-energy electron population.

Poloidally asymmetric response of turbulence to the H mode on the CCT tokamak

A distinct poloidal asymmetry is observed in the response of the density fluctuations at the H mode transition on the CCT tokamak. The fluctuations are located on the same density layer and are monitored by using reflectometers at four poloidal positions (θ = 0°, 90°, 180°, 270°). On the outboard midplane the fluctuations decrease significantly at the transition, while those on the inboard midplane generally show little or no reduction. The fluctuations located at the top and bottom of the tokamak exhibit an intermediate response. All poloidal positions show different time histories even for the same shot. These observations may indicate a poloidal symmetrization of the fluctuations at the H mode transition. A poloidal variation in the E × B velocity induced Doppler shift is also observed, indicating a poloidally asymmetric E × B velocity profile. The significance of these observations is discussed, and comparison is made with the transition properties on the larger neutral beam heated DIII-D tokamak

Mechanical divertor experiments in Macrotor

The feasibility of pumped limiters has been demonstrated on the Macrotor tokamak using both small neutral collection probes (2.5 cm diameter) and a large-area limiter (20 cm × 60 cm). Neutral pressures due to particle accumulation have reached as high as 8 microns. The particle removal efficiency of the neutral collection probes was measured to be 17% and estimated to be greater than 50% in the large-area limiter case. A quadratic dependence of the pressure on plasma density is observed.

Macrotor physics and technology results

The fusion research result in Macrotor has been focused on the basic issues of impurity generation (O, C), the role of the radial electric field in particle and energy transport, magnetic turbulence, particle pumping with a limiter, and the development of a large-area ICRF coupler for phased arrays for current drive and plasma heating. The total impurity line radiation has been reduced below the value contributed by the hydrogen edge radiation in ohmically heated plasmas. In RF-heated discharges, edge electron heating and the corresponding influx of impurities have been reduced by k-parallel shaping using large-area phased ICRF couplers. The Faraday shields have been used as limiters, and are used to remove the plasma heat. Current drive due to the lower hybrid fast wave has been observed with these couplers.

Relativistic electron beam current drive in the macrotor tokamak

Relativistic electron beam current drive experiments in the Macrotor tokamak achieved current drive levels (increases in the toroidal current) which were consistent with the zero-dimensional theory. Injection of a 2–4 kJ relativistic electron beam into Macrotor produced an additional toroidal current of 4.5–7.5 kA, which persisted for more than 800 μs. The beam-induced increase in toroidal current was observed to be independent, to first order, of the initial tokamak plasma current, Ip, and the number density, np, over the entire range of plasma parameters available in Macrotor (3 kA > Ip > 60 kA; 5 × 1012 > np > 2 × 1013). The 1 μs rise-time of the current implied an enhanced plasma collision frequency of 102 – 103 over classical. Comparison of co- and counter-injection data demonstrated that the initial increase in current was entirely due to beam-driven currents while later in time the increased current was sustained by the OH system.

Fast-wave multifrequency diagnostic of tokamak plasmas

An exploratory investigation, both computational and experimental, is made of the diagnostic possibilities of a fast wave-based method for measuring the ion density and temperature profiles of tokamak plasmas. The concept consists of simultaneously launching several discrete frequencies and detecting their amplitude and phase at various toroidal locations. This study focuses on an array of frequencies that match the second harmonic of the ion gyrofrequency at various radial locations in the plasma profile. The method is explored with simple, wave-propagation codes for parameters corresponding to the Electric Tokamak (ET) [R. J. Taylor, et al., Nucl. Fusion 42, 46 (2002)] and ITER [R. Aymar, P. Barabaschi, and Y. Shimomura, Plasma Phys. Controlled Fusion 44, 519 (2002)]. An experimental investigation of the concept has been performed in a large tokamak (ET) in which six frequencies are launched and detected at four toroidal locations. Positive results obtained indicate that the concept and other variants d...