B. D. Blackwell

H-1 Design and Construction

H-1 is a three-field period medium-sized heliac (R{sub 0} = 1 m, r = 0.22 m, and B{sub 0} = 1 T) nearing completion. It is intended for basic plasma physics studies, including finite systems. The authors discuss how its design allows a wide range of current-free magnetic configurations to be explored for their properties of equilibrium and stability.

Ion Detachment in the Helicon Double-Layer Thruster Exhaust Beam

This study is devoted to simulating the orbits of the ions in the supersonic beam observed experimentally in the laboratory development of the helicon double-layer thruster. Given the cylindrical symmetry of the problem, the particle orbits are generated in cylindrical coordinates (r, Φ, z), thereby enabling the analysis of magnetic detachment (in which the particles free themselves from the magnetic field of the source) to be confined to the analysis of the curvature of trajectories on the (r, z) plane. Because the trajectory of a magnetized particle in space is helical, detachment can be established if the curvature of r(z) on the (r, z) plane asymptotically approaches zero. The simulation shows that the detachment surface is a paraboloid opening in the direction of the expanding magnetic field, with its base along the radial (r) axis oscillating around the axial value z 0.38 m. The radius of the ion-beam detachment surface is in agreement with laboratory measurements of the beam density profile in the detachment region.

Physics Design of a High-beta Quasi-axisymmetric Stellarator

Note: 8th Toki 11th International Stellarator Conference, Toki-City, Japan, September/October 1997, Proc. published in J. Plasma and Fusion Res., SERIES, Vol. 1, 429 - 432 (1998) Reference CRPP-CONF-1998-055 Record created on 2008-05-13, modified on 2016-08-08

Linear ideal MHD predictions for n?=?2 non-axisymmetric magnetic perturbations on DIII-D

An extensive examination of the plasma response to dominantly n = 2 non-axisymmetric magnetic perturbations (MPs) on the DIII-D tokamak shows the potential to control 3D field interactions by varying the poloidal spectrum of the radial magnetic field. The plasma response is calculated as a function of the applied magnetic field structure and plasma parameters, using the linear magnetohydrodynamic code MARS-F (Liu et al 2000 Phys. Plasmas 7 3681). The ideal, single fluid plasma response is decomposed into two main components: a local pitch-resonant response occurring at rational magnetic flux surfaces, and a global kink response. The efficiency with which the field couples to the total plasma response is determined by the safety factor and the structure of the applied field. In many cases, control of the applied field has a more significant effect than control of plasma parameters, which is of particular interest since it can be modified at will throughout a shot to achieve a desired effect. The presence of toroidal harmonics, other than the dominant n = 2 component, is examined revealing a significant n = 4 component in the perturbations applied by the DIII-D MP coils; however, modeling shows the plasma responses to n = 4 perturbations are substantially smaller than the dominant n = 2 responses in most situations.

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.

Impurity generation during ICRF heating experiments on Alcator C

Observations of impurity behaviour are presented from ICRF heating experiments at 180 MHz performed for a variety of conditions on the Alcator C tokamak, using graphite limiters and stainless steel antenna Faraday shields. Spectroscopic observations revealed significant increases in metal impurity concentrations during the RF pulse, with iron levels increasing by as much as a factor of twelve at the highest RF powers (about 350–400 kW). Analysis of the inferred iron source rates shows an approximately linear dependence on RF power up to 400 kW, with no clear dependence on resonance conditions or bulk plasma parameters. However, a sharp temperature increase in the limiter shadow region was observed during the ICRF pulse, which was well correlated with the iron influx rate. From this and other evidence it is concluded that physical sputtering of the Faraday shield due to an elevated sheath potential is the primary source of metal impurities during ICRF heating on Alcator C. The same process, occurring at the graphite limiter, is believed to be the dominant source of carbon and oxygen. The calculated sputtering yields obtained from an edge erosion code demonstrate the plausibility of this model.

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

A simple miniature magnetic probe with inherent electrostatic rejection

A modification of a basic center‐tapped magnetic probe design has been developed wherein subtraction of capacitive signals is carried out by the probe itself. The electrostatic rejection of this probe is compared with other typical designs.

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.

Effects of resistivity and rotation on the linear plasma response to non-axisymmetric magnetic perturbations on DIII-D

SRH wishes to thank AINSE Ltd. for providing financial assistance to enable this work to be conducted.