In the Saskatchewan Torus-Modified (STOR-M) tokamak [Phys. Fluids B 4, 3277 (1992)], application of a negative bias results in large negative radial electric field, Er, at the plasma edge, reduced plasma toroidal rotation velocity, and a large poloidal rotation in the electron diamagnetic drift direction. Conversely, a positive bias leads to a relatively small negative Er at the plasma edge, a positive Er in the scrape-off layer, increased toroidal rotation, and an increased poloidal rotation speed in the ion diamagnetic drift direction. Increases in edge plasma density and steepening of its radial profile have also been observed for both polarities.
We observe an increase in plasma density and a significant reduction in fluctuations with bias electrode in a pure toroidal plasma having no poloidal magnetic field. The improved toroidal plasma behavior is found to depend upon the radial currents driven by positive biasing of a ring electrode. Although biasing establishes a radial electric field at the edge, we see suppression of fluctuations and density rise in the central core region of the plasma.
A high-brightness electron beam produced by a six-gap pseudospark chamber operated in 5–25 kV voltage and 30–70 mTorr pressure ranges is experimentally investigated. The electron beam of current 150 A at an average energy of 20 keV and pulse duration of 10 nsec is extracted with a repetitive frequency of ∼10 Hz. The electron-beam current scales linearly with the breakdown voltage and about 20% of the stored energy is converted into the total beam energy. The time-integrated rms emittance of the electron beam is measured, and a typical value is found to be 65 mm mrad, yielding a normalized brightness of the beam Bn≊2×1010 A/(m2 rad2).
The potential of electron cyclotron emission (ECE) diagnostic to determine plasma parameters in spheromak experiments and spheromak reactor is examined in detail. When a linearly polarized electromagnetic wave propagates through a sheared magnetic field in dense plasma, it undergoes rotation. The amount of rotation depends on plasma parameters, magnetic field amplitude and its orientation as well as on spatial variation. We have calculated the value of the rotation angle for different anisotropy conditions. It is observed that the amount of the rotation angle is negligible for a weak anisotropy. That means the wave is propagated as if in vacuum and local information carried by the wave is preserved. We show that, by proper choice of harmonics frequency of ECE radiation, a weak anisotropy condition can be satisfied. Hence measurement of ECE radiation for those range of frequencies (for which the weak anisotropy condition is satisfied), plasma parameters and magnetic field orientation can be obtained easily. As an example, the use of this diagnostic method in existing sustained spheromak physics experiment is described. We also mention that this diagnostic can measure electron temperature with about 10 cm or less spatial resolution in a 'two-component type spheromak reactor' having electron temperature 7 keV. The measurement aspects of ECE for a spheromak are discussed.
A multigap pseudospark device filled with hydrogen or argon gas is operated up to 20 kV and ejected ion beam characteristics are experimentally investigated. The ion beam of peak current density in excess of ∼500 A/cm2 and pulse duration ∼50 ns is extracted at repetitive frequency of several tens of Hz. Results of the scaling study for the ion beam current with breakdown voltage, external capacitance, and number of insulator gaps are presented.
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