- The results of an experimental study of the transient loss of plasma from a 25-cm-long theta pinch initially containing a reversed trapped magnetic field are presented. The plasma, amenable to MHD analyses, was a doubly ionized helium plasma characterized by an ion density N(,i) = 2 x 10('16) cm('-3) and an ion temperature T(,i) = 15 eV at midcoil and by N(,i) = 0.5 x 10('16) cm('-3) and T(,i) = 6 eV at a position 2.5 cm beyond the end of the theta coil.
Local diagnostics with a temporal resolution of 0.1 (mu)s allow the physics affecting the plasma loss to be identified with a spatial resolution of less than 1 cm. Direct, independent measurements were made of plasma diamagnetism, luminosity, magnetic fields, electron densities, electron temperatures, and impact pressure. Impact pressure data are combined with local density data to yield flow velocities.
Values of N(,e) and T(,e) are obtained from spectroscopic measurements: N(,e) is obtained from techniques that utilized Stark broadening of the HeII 4686-(ANGSTROM) line, and T(,e) is obtained from the ratio of the intensity of the HeII 4686-(ANGSTROM) line to that of the underlying continuum emission and also from the absolute intensity of the HeII 4686-(ANGSTROM) line. The conditions required to perform these measurements in the time-dependent, inhomogeneous, flowing plasma produced in this experiment are identified and are shown to be fulfilled. Evaluation of the accuracy of the spectroscopic techniques is accomplished by comparing the N(,e) and T(,e) data obtained from spectroscopic measurements performed in the end region with data obtained from Thomson scattering.
Various plasma phenomena observed inside the theta coil are studied. A closed magnetic field structure forms during radial collapse and is subsequently annihilated by classical field diffusion. Stability of the plasma column is studied: the resistive tearing instability is shown to be resistively damped; Rayleigh-Taylor flutes are observed to occur after radial collapse and are shown to be driven by the radial oscillation of the plasma column; and rotation is identified as the mechanism driving an m = 2 instability. The amount of rotational energy possessed by the plasma column is obtained using radial momentum balance; the column is shown to possess rotational energy throughout the end-loss event. End-shorting of the radial electric field within the current sheath and the subsequent propagation of a torsional Alfven wave from each end of the coil toward midcoil is shown to have been the most probable cause of the observed rotation.
Plasma flow from the coil did not begin until completion of radial collapse. Axially directed electromagnetic body forces are shown to have had a minimal effect on the rate of plasma loss; the primary driving mechanism for plasma loss is identified as an axial pressure gradient. At the ends of the coil, the exhausting plasma is preceded by an outward propagating shock wave, which forms as the result of the interaction of end-loss plasma with the ambient background gas. Evidence is given indicating both formation of effective ends within the coil upon completion of radial collapse and propagation of a rarefaction wave moving from each effective end toward midcoil at the cusp speed. Efflux of plasma is obtained by two virtually independent methods and found to agree with the predictions of the transient, analytic, MHD theories of Wesson and Freidberg and Weitzner. Finally, the normalized particle loss time for loss of plasma from within the identified effective coil length is found to agree well with that predicted by the numerical results of Brackbill, Menzel, and Barns.
- Dissertation Note:
- Ph.D. The Pennsylvania State University 1980.
- Source: Dissertation Abstracts International, Volume: 41-12, Section: B, page: 4559.
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