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Tendency of MHD forces to create localized, collimated plasma-filled flux tubes and ion orbit instability in a flux tube

Author: Paul M Bellan
Requested Type: Consider for Invited
Submitted: 2006-12-18 14:04:37

Co-authors: S. You, G. S. Yun, D. Kumar, E. Stenson, and S. K. Tripathi

Contact Info:
Caltech
1220 E. California Blvd
Pasadena, CA   91125
USA

Abstract Text:
Although spheromak formation is traditionally explained by the Taylor relaxation process, a zero-beta model, we have found that the effect of finite beta can actually be extremely important even if beta is small. This insight has resulted from experiments using two types of magnetized plasma guns, namely (i) a gun with planar electrode geometry [1] and (ii) a gun with two electrodes that form a horse-shoe shape magnetic field analogous to a solar coronal loop. Various diagnostics are used, including high-speed photography at rates up to 10 million frames/second, spectroscopy to measure velocities via Doppler shift or densities via Stark broadening [2], He-Ne laser interferometry [3], and magnetic probes [4]. Measurements indicate that instead of the pressure being spatially uniform (i.e., zero beta), highly localized structures develop with the morphology of a collimated plasma-filled magnetic flux tube surrounded by near-vacuum. The dynamics can be explained in terms of first the creation of these plasma-filled collimated flux tubes and then their deformation via stretching, bending, kinking, and/or mutual interaction. This is quite different from the Taylor relaxation model where uniform plasma pressure is assumed. The flux tubes become plasma-filled because MHD JxB forces ingest plasma from wall sources into the flux tube [5, 6]. The ingested plasma carries frozen-in magnetic flux and the pile-up of this ingested flux collimates the flux tube. Ingestion can occur into one or both ends of a flux tube, occurs at an Alfvenic velocity, and can result in the formation of dramatic magnetized plasma jets analogous to astrophysical jets. Ingestion has been imaged in the type (ii) gun using a two-color scheme whereby different plasma species are used at the two ends of a flux tube. Imaging with atomic line filters clearly distinguishes the inflows from the respective ends. It has also been found that if the MHD driven inflow from the cathode is sufficiently fast, these ions with negative vz develop an orbit instability [7] whereby a large radially directed vzBtheta force causes ions to be radially expelled from a flux tube with helical magnetic field rather than being magnetically confined. This kinetic instability has been experimentally observed and agrees with a Hamiltonian-based model and also with direct numerical calculations of trajectories in a helical magnetic field.

[1] S. C. Hsu and P. M. Bellan, Phys. Plasmas 12, art. 032103 (2005)

[2] G. S. Yun, S. You, and P. M. Bellan, submitted for publication.

[3] D. Kumar and P. M. Bellan, Rev. Sci. Instrum. 77, Art. No. 083503 (2006).

[4] C.A. Romero-Talamas, P. M. Bellan, and S. C. Hsu, Rev. Sci. Instrum. 75, 2664 (2004)

[5] P. M. Bellan, Phys. Plasmas 10 Pt 2, 1999 (2003)

[6] S. You, G. S. Yun, and P. M. Bellan, Phys. Rev. Lett. 95, art. 045002 (2005)

[7] S. K. Tripathi, G. S. Yun, and P. M. Bellan, submitted for publication.

Characterization: E2,E5

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