Abstract Details

bellantalk_icc_2006_austin4b.pdf2006-03-09 00:35:12Paul 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

Contact Info:
1200 E. California Blvd
Pasadena, CA   91125-9

Abstract Text:
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 [1] 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 [2], (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 [2], (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 [4]. 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.

[1] S. C. Hsu and P. M. Bellan, “On the jets, kinks, and spheromaks formed by a planar magnetized coaxial gun�, Phys. Plasmas 12, art. 032103 (2005)
[2] P. M. Bellan, “Why current-carrying magnetic flux tubes gobble up plasma and become thin as a result�, Phys. Plasmas 10 Pt 2, 1999 (2003)
[3] S. C. Hsu and P. M. Bellan, “Experimental identification of the kink instability as a poloidal flux amplification mechanism for coaxial gun spheromak formation�, Phys. Rev. Letters 90 (2003) art. 215002
[4] S. You, G. S. Yun, and P. M. Bellan, “Dynamic and stagnating plasma flow leading to magnetic-flux-tube collimation�, Phys. Rev. Lett. 95, art. 045002 (2005)

Characterization: A1

Group with Sett You's abstract, put this one before his.

The University of Texas at Austin

Innovative Confinement Concepts Workshop
February 13-16, 2006
Austin, Texas

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