|bellantalk_icc_2006_austin4b.pdf||2006-03-09 00:35:12||Paul Bellan|
Measurements of plasma jets and collimated flux tubes that are the precursors of spheromak self-organization
Author: Paul M Bellan
Submitted: 2005-12-20 14:20:21
Co-authors: S. You, G. S. Yun, D. Kumar
1200 E. California Blvd
Pasadena, CA 91125-9
Spheromak formation is traditionally explained in terms of the Woltjer-Taylor theory of plasma relaxation whereby a zero-beta plasma spontaneously self-organizes via MHD instabilities to a minimum energy state consistent with conservation of magnetic helicity. The zero-beta assumption implies that plasma pressure is either zero or uniform. It is understood that pressure in a real plasma is in fact non-uniform (otherwise the plasma is not confined), but nevertheless, the Woltjer-Taylor relaxation is normally envisioned as taking place in a plasma with a nearly uniform density.
The Caltech experimental program uses a planar  electrode geometry to elucidate the details of this relaxation process. Spheromak-precursor plasmas are created where the steps in relaxation are very obvious and clear-cut. A variety of diagnostics are used, including high-speed photography at rates up to 10 million frames/second, spectroscopy to measure velocities via Doppler shift and densities via Stark broadening, He-Ne laser interferometry, and magnetic probes. Measurements indicate that the configuration is far from being spatially uniform and instead has the morphology of a collimated magnetic tube filled with dense plasma and surrounded by near-vacuum. The steps towards spheromak formation involve (i) filling of the flux tube by an MHD-driven jet process , (ii) collimation of this flux tube via forces resulting from the accumulation of frozen-in magnetic flux convected by the plasma filling the flux tube , (iii) kink instability of the collimated flux tube [1, 3], and as discussed in our adjacent presentation, (iv) sausage instability.
In order to address any doubts regarding whether plasma is really convected into the flux tube by the MHD-jet process, the neutral density was measured immediately before breakdown using a fast ion gauge at a location occupied just after breakdown by the dense plasma jet . The neutral density before breakdown is several orders of magnitude smaller than the plasma density just after breakdown, thereby establishing that the dense plasma was convected in by the jet and was not the result of ionizing in-place neutral gas. These results show that the initial magnetic topology is a strong function of the gas injection arrangements.
 S. C. Hsu and P. M. Bellan, â€śOn the jets, kinks, and spheromaks formed by a planar magnetized coaxial gunâ€?, Phys. Plasmas 12, art. 032103 (2005)
 P. M. Bellan, â€śWhy current-carrying magnetic flux tubes gobble up plasma and become thin as a resultâ€?, Phys. Plasmas 10 Pt 2, 1999 (2003)
 S. C. Hsu and P. M. Bellan, â€śExperimental identification of the kink instability as a poloidal flux amplification mechanism for coaxial gun spheromak formationâ€?, Phys. Rev. Letters 90 (2003) art. 215002
 S. You, G. S. Yun, and P. M. Bellan, â€śDynamic and stagnating plasma flow leading to magnetic-flux-tube collimationâ€?, Phys. Rev. Lett. 95, art. 045002 (2005)
Group with Sett You's abstract, put this one before his.