A. W. DeSilva

Spectroscopic Investigation of Enhanced Plasma Oscillations in a High‐Voltage Theta Pinch

Intensity and frequency of fluctuating electric fields are obtained from a “plasma satellite” on the 21P ‐ 41D line of helium observed in the piston field of a collisionless shock. In contrast to second‐order perturbation theory only one satellite is seen. The perturbation theory is developed, therefore, to higher orders. Agreement between this theory and experiment is achieved. From the observed level of electric field fluctuations an effective collision frequency of about one‐fourth of the ion plasma frequency can be deduced.

Experimental Study of the Magnetic Piston‐Shock Wave Problem in a Collisionless Plasma

The time‐dependent problem of piston and shock wave formation in a low density (collision‐free) magnetized plasma has been experimentally studied in a high voltage theta pinch. The applied field strongly interacts with the plasma: either exciting instabilities that cause a fast resistive penetration and very low force density, or exerting a sufficiently large force density to produce high Mach number (5‐20) shock waves. The data on piston formation is discussed in terms of a magnetic field‐dependent resistivity such as might be expected from an ion plasma drift wave instability. The piston ejects ions possibly by the interaction of ions with ion plasma waves. The shock wave is seen to evolve from an initially one‐dimensional disturbance.

Experimental studies of spheromak formation

Studies in the PS‐1 spheromak configuration can be effectively formed by a combined z‐ and θ‐pinch technique on both a fast (τformation≂τAlfven) and a much slower timescale. The gross tilt and shift instability of the toroid can be suppressed by a combination of conduction walls, shaping the separatrix by externally applied fields, and the use of ‘‘figure‐eight’’ coils. Optimum stabilty is obtained for almost spherical toroids. Maximum field‐reversal times for stable, well‐confined toroids are ≥40 /μsec, consistent with resistive decay. Temperatures during the stable decay are 5–10 eV; impurity radiation is an important energy‐loss mechanism.

Role of turbulence in the coupling of a magnetic pulse to a collisionless plasma

The coupling of a magnetic pulse to an initially magnetized collisionless plasma is studied experimentally using a high voltage theta pinch. The plasma parameters studied include: the turbulent electric fields, particle heating, magnetic and electric field profiles, and the plasma resistivity. The results indicate that the interaction of the plasma with the magnetic pulse can be understood using a fluid model which includes anomalous resistivity and particle heating. The source of the anomalous resistivity is investigated based on the presently available instability theories.

Time-Resolved Spectroscopic Measurements of Plasmas after Pulsed Discharges in Liquid Helium

The densities and electron temperatures in transient plasmas produced by high-voltage pulsed discharges in liquid helium below 4.2 K, at 1 atm, and cryogenic helium gas near 4.2 K, at 1 atm, have been measured by time-resolved emission spectroscopy up to 3.8 µs after the discharges. The densities are measured using Stark broadening of line profiles. The electron temperatures are deduced from the ratios of total spectral line intensities to continuum intensities. Gaseous plasmas with relatively high density and low electron temperature are obtained in cryogenic environments. Typical density and electron temperature at 0.2 µs after breakdown are 1018 cm-3 and 35,000 K, respectively. Mechanisms of the dominant spectral line broadening at an early stage after the discharges in liquid helium are discussed in detail.

The effects of impurity ions on the scattered light spectrum of a plasma

Measurement of the scattered light spectrum from hydrogen plasmas containing small amounts of heavy impurity ions confirms the predictions of theory that the low‐frequency ion‐acoustic scattered light feature consists of the normal profile due to scattering from a pure hydrogen plasma, with a narrow peak superimposed that is attributed to the impurity. The intensity of the impurity peak scales as the square of a mean charge Z*. Measurements are reported for electron densities of 1–3×1018 cm−3 and electron temperatures ranging from 10 to 80 eV.

Experimental Study of the Structure of Plasma Shock Waves in a Fast θ Pinch

The density, temperature, and magnetic field structures of plasma shock fronts have been observed by means of scattered light, a magnetic probe, and emission spectroscopy. The shocks and initial hydrogen plasma were generated by capacitor discharges through a single‐turn coil placed around a cylindrical quartz tube containing hydrogen. Electron velocity distributions (density and temperature) were obtained by analyzing the light scattered at 90° to the incident beam from a Q‐switched ruby laser. The observed temperature structure was compared with that calculated from observed particle and current density by assuming local resistive dissipation due to binary electron‐ion collisions. Agreement was good in two cases, but, in a case characterized by lower initial ωceτ(0.6), the velocity distribution developed enhanced wings and enhanced heating occured. These effects indicate a diffusion of high‐energy electrons from the hot, downstream region. Ion temperatures, obtained from Doppler broadening of a carbon l...

Emission Spectra from Pulsed Discharges in Liquid Helium

The time-resolved emission spectra from high-voltage pulsed discharges in liquid helium below 4.2 K, 1 atm, and cryogenic helium gas near 4.2 K, 1 atm, have been measured in the visible spectral range. In addition to the well known atomic lines, which exhibit strong and broadened profiles with continua at an early stage, some strong spectral features are observed near 23300 cm-1 ( ~430 nm) and 25300 cm-1 ( ~395 nm). The former can be assigned to He 2 transition ( g3Σg +– a3Σu +), and the latter is not observed in previously reported works. These features disappear in 77 K and 300 K, 1 atm, He gas under the same experimental conditions.

Fast pyroelectric detector for broadband radiated power measurements

Basic design requirements are presented for pyroelectric detectors to be used effectively when the radiation source emits part or all of its power in the vacuum ultraviolet range, as is the case for a typical plasma in magnetic confinement fusion research. The difficulty associated with the VUV wavelength range lies in the high photoelectric yield of the associated energetic photons. A relatively inexpensive, commercially available detector was modified to eliminate the spurious currents resulting from photoelectrons. The modifications have little effect on the basic characteristics of the commercial detector (time response, sensitivity, etc).

Experimental spheromak MHD stability studies

Abstract The n = 1 tilt and radial shift instability of spheromaks is shown to be stabilized by the use of conducting wall ( r w r s ≅1.2) and stabilization coils.