G. A. Sawyer

Plasma experiments on the linear scyllac theta pinch

Experiments on a 5‐m‐long theta pinch both with and without strong magnetic mirrors (R ≈ 2.5) are reported. When the experiment is operated without magnetic mirrors, the particle loss rate is found to scale from shorter theta‐pinch experiments directly as the length and inversely as the ion thermal velocity. The electron temperature scales with length (l2/5) as predicted by a theory which accounts for thermal conduction to the ends. Addition of the magnetic mirrors increases plasma confinement time from 11.5 to 18.9 μsec. a smaller increase than predicted by available theories, and also produces an m = 1, k ≈ 0 instability which is moderated under certain conditions by “line tying” effects.

Plasma End Losses and Heating in the ``Low‐Pressure'' Regime of a Theta Pinch

A neutron‐producing plasma with ion energy ∼3–4 keV has been produced at filling densities 10–50 μHg without negative bias magnetic fields in a 570‐kJ theta pinch. Axial interferograms, taken with a ruby‐laser‐illuminated Mach—Zehnder interferometer show that a stable compressed plasma core exists throughout the magnetic half cycle with no ionized impurities outside the core, and no drift toward the wall. The interferograms give peak plasma densities of 2 to 5 × 1016 cm‐3, and also indicate a loss of particles as a function of time. Plasma containment times (e‐folding times of N) before peak compression are 6 to 30 μsec. The observed loss rates are approximately in agreement with predictions of free flow through an orifice whose radius is equal to an ion Larmor radius. Soft x‐ray measurements yield ∼300 eV electron temperature for all filling pressures. Absolute intensities of the soft x‐ray emissions show the impurity level to be <0.1%. The ion energy for the low‐pressure regime deduced from pressure bal...

Plasma equilibrium and stability in the Scyllac toroidal sector experiments

Experiments are reported on helical plasma equilibrium and stability in the Scyllac toroidal θ-pinch sectors (120°) which have major radii of 2.375 and 4.0 m with coil arc lengths of 5.0 and 8.4 m, respectively. In these experiments the outward toroidal drift force was compensated by a combination of l = 1 helical and l = 0 bumpy fields which are generated by shaping the inner surface of the compression coil or by driven l = 1 windings. Time-resolved measurements were made of the gross plasma-column motion, the plasma radius, the magnetic flux excluded by the plasma, the external magnetic field, the plasma density, the electron and ion temperatures, and the plasma β at axial locations of minimum and maximum plasma radius. These data are used to study the approach to the theoretically predicted toroidal equilibrium (including axial pressure equilibrium). The plasma column remained in stable equilibrium for 7 – 10 μs in the 8-m sector compared with 4 – 7 μs in the 5-m experiment, at which times the onset of a terminating m = 1, k ≈ 0 sideways motion occurred. The results show that the plasma achieved axial pressure equilibrium (nkT = const) in 4 – 6 μs, while maintaining equilibrium in the toroidal plane for 10 μs or longer. The measurements of the plasma radius, β and magnetic field in the various experiments have confirmed in detail the stable toroidal equilibrium observed in the streak photographs during the first 4-10 μs of the discharge. The observed toroidal equilibria of the high-β, θ-pinch plasma are in quantitative agreement with MHD sharp-boundary theory and confirm the theoretical scaling of the equilibrium field between the 5-m and the 8-m sector experiments.

X-RAY SPECTROSCOPIC MEASUREMENTS OF A HIGH-TEMPERATURE PLASMA

Abstract The X-ray spectrum emitted from the deuterium plasma in the Scylla fast magnetic compression experiment has been studied with a beryl, single-crystal, X-ray diffraction spectrometer. Prominent lines are identified from the HeI-like spectra of Na X, Mg XI, A1 XII and Si XIII wall impurity ions as well as the H-like Lyman series of O VIII. The shape and absolute magnitude of the continuum spectrum indicate that the plasma electron temperature is 345±40 eV and that the soft x-radiation arises primarily from electron recombination with small amounts of highly stripped impurity ions. Broadening measurements of the spectral lines in the X-ray region show that the Doppler temperature of impurity ions is considerably greater than the corresponding deuterium ion temperature.

Measurement of β, n, and Ti in High‐ and Low‐Pressure Theta‐Pinch Operation

Three coordinated measurements on the Scylla III theta‐pinch yield: β, which is the ratio of plasma pressure to external magnetic field pressure; n, which is the plasma density as a function of radius; and Ti, which is the ion temperature. The three measurements are (1) a differential magnetic loop‐probe measurement that determines total flux excluded by the plasma, (2) side‐on‐measurement of the profile of light intensity emitted by the plasma that yields the relative density profile, and (3) a side‐on gas‐laser interferometer measurement that gives the absolute density integrated through the plasma. Electron temperature is measured separately by x‐ray absorption and it is assumed that electron and ion temperatures are not a function of radius. In the low‐pressure regime, (P initial = 20 mTorr) without bias magnetic field, typical measured values are β = 0.70 ± 0.15 and ne = ni = 3.0 × 1016/cm3 on axis, Ti = 2.2 keV, and the plasma density decreases monotonically with increasing radius from the axis, rea...

Coordinated Measurements of Plasma Density and Cooperative Light Scattering in a Theta‐Pinch Plasma

Measurements have been made on the spectral broadening of ruby laser light scattered at a small forward angle by density fluctuations in a theta‐pinch plasma, in order to systematically investigate the variations in scattering cross section with reverse bias magnetic field and with time during the first half‐cycle of the compression field. Measurements of density under the same plasma conditions were made so that the effects of cross section variations could be separated from density variations. The principal results are: (1) The observed cross section is approximately six times the theoretical cross section for a plasma with the Scylla III temperature and density (no bias magnetic field). The large cross section is attributed to superthermal density fluctuations (turbulence) in the plasma. (2) The scattering cross section for a plasma with a bias field of ‐800 G is three times larger than for the plasma with no bias field. (3) There is little difference in the scattering cross sections measured at two ti...