E. M. Little

Effects of Ionization and Magnetic Initial Conditions on a Magnetically Compressed Plasma (Scylla)

The effects of strong preionization and the application at steady bias magnetic fields on the operation of the Magnetic compression device Scylla are studied. It is shown that both strong preionization and a bias field B/sub o/ antiparallel to the main compression field B/sub z/ are necessary to produce d-d neutrons during the first half-cycle of B/sub z/. Other aspects of the plasma activity are also shown to depend strongly upon the sign of B/sub o/. Application of bias fields with weak preionization leads to production of hard x rays, which occur on the half-cycle of the discharge preceeding that of neutron emission. When hard x rays are produced the plasma is not hydromagnetic. The hard x rays are extinguished when there is strong preionization, leading to a hydromagnetic plasma. In the case of the hydromagnetic plasma it is concluded that the antiparallel B/sub o/ is most effective early in a given half-cycle and affects the plasma primarily during its preheating, ionization phase, rather than during the later adiabatic-compression phase. An interpretation is given in terms of a plasma sheath that has special properties when it separates magnetic fields of opposite signs. (auth)

RFP energy evolution and containment in the presence of giant sawteeth

Experimental measurements of internal energy in the ZT‐40M Reversed Field Pinch are presented that reinforce the interpretation of the soft x‐ray sawtooth oscillations seen in that device as indicating an internal energy loss. Soft x‐ray spectra are presented that support the contention that hot electron confinement, as well as energy confinement, is severely degraded by the presence of large scale sawteeth, presumably during the fall time of the soft x‐ray flux. The behavior of the equilibrium fields may be explained as the result of a combination of current redistribution and a decrease and recovery in internal βp during a sawtooth.

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...

Faraday Rotation Measurements on the Scylla IV Theta Pinch

Faraday rotation measurements have been combined with Mach‐Zehnder interferometer measurements to yield the value of the internal magnetic field Bi in the Scylla IV plasma as a function of time and radius. Since the Faraday rotation is proportional to ∫ Bi(l)ne(l) dl and the Mach‐Zehnder interferometer provides ∫ ne(l) dl, the data give the average value of the internal magnetic field Bi(r) which exists over the plasma length l at the radius r. The measurements of Bi(r) are combined with those of the external magnetic field Be to determine the average plasma β as a function of radius, β = 1 − Bi2(r)/Be2. The Faraday rotation measurement employs a 1.5‐mm diam beam of visible radiation (6328 A) from a He‐Ne gas laser as a plasma magnetic‐field probe and a calcite Wollaston prism as the polarization analyzer with silicon diode detectors. In the low‐pressure plasma regime (10‐25 mTorr) the β values in the central plasma‐core (∼5‐mm diameter) range in value between 0.9 and 1.0 with initial reversed bias magnet...

Injected Magnetic Compression Experiment

The injection of the plasma, produced by the collision of two D plasmas from coaxial guns, into the Scylla magnetic compression device is studied. The plasma formed by the two guns generally has fast and slow components. The neutron production obtalned by trapping each of these components is measured for the cases in whlch the guidlng magnetic field is parallel or antiparallel to the compression field. These measurements are also made using only one of the plasma guns. The use of this plasma injection device is compared with the older Scylla method. (T.F.H.)