S.M. Hamberger

Helicon wave propagation in the SHEILA heliac

The behaviour and structure of waves launched from a novel double-loop antenna in the SHEILA heliac are presented. Radial wave field and azimuthal phase measurements are compared with cylindrical helicon wave theory after an appropriate magnetic coordinate transformation. This comparison provides strong evidence for the involvement of helicon waves. Damping of the waves is investigated by comparing measured spatial damping rates with theoretical estimates. Comparison of measured density with predictions from the helicon dispersion relation reveals how geometry influences propagation of the waves in SHEILA.

Experimental investigation of the magnetic structure in the H-1 heliac

The results of an experimental study of the magnetic structure in the H-1 heliac are presented. Electron beam magnetic mapping has confirmed the existence of closed nested flux surfaces, in good agreement with a computer model. Measurements over a wide range of helical winding currents demonstrated a variety of attainable magnetic configurations within a rotational transform range of 0.6 ≤ t0 ≤ 1.8. The observed islands can be attributed to deduced small errors in the coil alignment in H-1. A magnetic island study and a correction in the model to fit the experimental observations revealed the error sources in the magnetic field

First studies of plasma confined in a toroidal heliac

The first experimental results obtained on plasma confined in a toroidal heliac are reported. A simple method of generating highly ionized, weakly collisional plasma is described. It is found that the geometry of the plasma in general and the measured pressure profiles in particular conform closely with the calculated helical axis magnetic surfaces. Preliminary indications of plasma confinement are favourable.

Experimental investigation of different configurations in a flexible heliac

The effect of varying the magnetic field configuration by adding an l = 1 helical winding to the standard heliac has been studied experimentally. Equilibrium plasma configurations in the range 0.7 ≤ t(0) ≤ 1.86 have been obtained. Analyses of the plasma pressure profiles measured by Langmuir probes in this range show good agreement between the plasma isobars and the computed vacuum magnetic surfaces; for configurations with a rotational transform t(0) close to unity it is necessary to take known error fields into account. When low-order rational surfaces are present, a deterioration of the plasma confinement is clearly observed. Magnetic islands, resulting from the resonance between the low-order rational surface ι = 3/2 and the m = 2, n = 3 vacuum field harmonics inherent in the geometry, are identified with features observed in both the plasma pressure and the floating potential profiles.

PRODUCTION OF FAST ELECTRONS IN A PROTOTYPE HELIAC BY HELICON WAVES

The generation of plasma by helicon waves has been investigated in the Small Heliac Experimental Apparatus [Nucl. Fusion 25, 1485 (1985)]. Evidence for wave–particle interaction in both argon and helium is presented in the form of observations of the electron distribution function. A high energy bump‐on‐tail feature is shown to coincide with the wave phase velocity and with an energy corresponding to a large ionization cross section. It is shown that the bump‐on‐tail feature may be related to the ionization capability of helicon waves and may be a result of some form of radio frequency current drive.

Pre-disruption MHD activity in the LT-4 tokamak

In the LT-4 tokamak, the precursor activity for disruption at q(a)≤2.5 does not exhibit the oscillatory low order mode MHD behaviour usually seen elsewhere. Instead, each thermal quench is preceded by a fast transient perturbation with dominant Fourier components m/n = 2/1 and 3/2. The corresponding islands grow so rapidly, at rates consistent with the Rutherford non-linear tearing mode theory, that they reach estimated sizes sufficient to cause disruption in less than a typical Mirnov oscillation period.

Studies of drift waves in a toroidal heliac

Low frequency, coherent density fluctuations have been studied for three typical magnetic configurations in the helical axis stellarator SHEILA using Langmuir probe techniques. The parametric dependence, the threshold magnetic field, the frequency spectrum and the spatial structure of the fluctuations are measured experimentally. Mode analyses are made in a magnetic coordinate system. Both the mode numbers thus obtained and the smallness of the directly measured values of k/sub ///(k perpendicular to /k/sub /// approximately 100) indicate a close correspondence between the helicity of the fluctuations and the field lines. These experimental results are consistent with a collisional drift wave model, derived from a linearized two-fluid theory, related to the heliac geometry. Density reduction associated with the fluctuations is clearly observed and is consistent with rough estimates of the cross-field particle flux due to the fluctuations.

The H-1 radio frequency system and an initial study of plasma formation

Abstract Eight commercial broadcast transmitters rated at 30 kW PEP 4–26 MHz are being commissioned for plasma formation and heating in H-1. These may be either excited separately or combined into two antennas by a set of six power combiners. In this paper we describe the various components of the r.f. power transmission system. We present preliminary results using this system for plasma formation in argon and hydrogen with magnetic fields

Scintillation measurement of density fluctuations in a tokamak plasma (abstract)

Recent theoretical results for line integrated measurements of phase and amplitude perturbations introduced on a laser beam probing a randomly fluctuating plasma are compared with phase scintillation data obtained on the LT‐4 tokamak. It is shown that an estimate of the transverse velocity profile along the line‐of‐sight can be obtained from single shot measurements of the phase perturbations.

Optimum design of helical windings for a toroidal ℓ = 3 stellarator with large separatrix

The paper presents the results of a detailed computational study of possible l = 3 stellarator winding configurations in which the separatrix radius and position are maintained constant. It is shown that toroidal effects on both the rotational transform properties and the mechanical forces can be reduced by suitably modulating the winding pitch, which results in considerable savings in the required current and mechanical support. The existence of significant magnetic wells is demonstrated. The results generally favour the use of shallow rather than steep windings and fairly large aspect ratios.