Plasmoid propagation in a transverse magnetic field and in a magnetized plasma

Abstract

The propagation of plasmoids (neutralized ion beams) in a vacuum transverse-magnetic field has been studied in the University of California, Irvine laboratory for several years [Phys. Fluids 24, 739 (1981); 25, 730, 2353 (1982); 26, 2276 (1983); J. Appl. Phys. 64, 73 (1988)]. These experiments have confirmed that the plasmoid propagates by the E×B drift in a low beta and high beta plasmoid beam (0.01<β<300), where β is the ratio of beam kinetic energy to magnetic field energy. The polarization electric field E arises from the opposite deflection of the plasmoid ions and electrons, because of the Lorentz force, and allows the plasmoid to propagate undeflected at essentially the initial plasmoid velocity. In these experiments, plasmoids (150 keV, 5 kA, 50–100 A/cm2, 1 μ sec) were injected into transverse fields of Bt=0–400 G. Anomalously fast penetration of the transverse magnetic field has been observed as in the ‘‘Porcupine’’ experiments [J. Geophys. Res. 91, 10,183 (1987)]. The most recent experiments are aimed at studying the plasmoid propagation dynamics and losses in the presence of a background, magnetized plasma which is intended to short the induced polarization electric field and stop the beam. Background plasma was generated by TiH4 plasma guns fired along Bt to produce a plasma density, np =1012 −1014 cm−3 . Preliminary results indicate that the beam propagation losses increase with the background plasma density; compared to vacuum propagation, roughly a 50% reduction in ion current density was noted 70 cm downstream from the anode for np∼1013 cm−3 . Principal diagnostics include magnetically insulated Faraday cups, floating potential probes, calorimeters, microwave interferometer, and thermal-witness paper.

Plasma penetration of the dayside magnetopause

“…The cross-field width of the plasmoids is much greater than 140 km, and the crossfield distance between spacecraft 1 and 3 of 1400 km will serve as an order of magnitude estimate. These observations are consistent with plasma entry through impulsive penetration 9 , where the plasmoids enter by both expulsion of the local magnetic field and by polarization 14 or a combination of the two 45 .…”

“…Since our estimate is that the plasmoids are much larger than 140 km, their theory predicts that penetration should occur through expulsion in the present case. Plasmoids could penetrate through an expulsion of the ambient magnetic field, which is followed by a magnetic diffusion and a convection phase, as outlined by Wessel et al 14 . A combination of polarization and expulsion is also possible 45 .…”

“…A refined requirement, 10 m i /m e , was later found experimentally 15 . Experimental studies showed that plasmoids can penetrate magnetic fields, while the relative change of the magnetic field, caused by the plasmoid, remains small even for large values of the kinetic beta β k = W K /(0.5µ 0 B 2 ) 14 . Experiments with laser produced plasmas have shown that wide plasmoids can break up into smaller plasmoids as they move across the field 16 , and this has been attributed to the Rayleigh-Taylor instability 17 .…”

“…This polarization allows the charged particles to continue moving with their original velocity through the means of an E×B-drift 12,13 . Plasmoids with high kinetic energy density can penetrate ballistically by expelling the ambient magnetic field from the interior of the plasmoid 14 . This phase is then followed by a magnetic diffusion phase, in which the plasmoid is magnetized by the ambient field, and finally the plasmoid can E × B-drift as described above.…”

Propagation of a narrow plasma beam in an oblique magnetic field

“…The beam quickly magnetizes when entering the field. 3 In previous work, the propagation of intense, narrow ion beams was studied in an axial field 4 and in a transverse magnetic field. 2 ' 3 ' 5 Our results extend these studies to the general case of narrow beam propagation at an arbitrary angle with respect to the magnetic field.…”