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Motion and equilibrium of a spheromak in a toroidal flux conserver

Brown, M. R. and Cutrer, D. M. and Bellan, P. M. (1991) Motion and equilibrium of a spheromak in a toroidal flux conserver. Physics of Fluids B, 3 (5). pp. 1198-1213. ISSN 0899-8221. http://resolver.caltech.edu/CaltechAUTHORS:BROpofb91

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Abstract

A number of experiments have been performed on spheromaks injected into the empty vacuum vessel of the Caltech ENCORE tokamak (i.e., without tokamak plasma) [Phys. Rev. Lett. 64, 2144 (1990); Phys. Fluids B 2, 1306 (1990)]. Magnetic probe arrays (in a number of configurations) have been used to make single shot, unaveraged, in situ measurements of the spheromak equilibrium. These measurements are important because (i) they reveal for the first time the equilibrium structure of spheromaks in a toroidal geometry, (ii) they provide a reliable estimate of magnetic helicity and energy of spheromak plasmas used in injection experiments [Phys. Rev. Lett. 64, 2144 (1990)], and (iii) they constitute the first measurements of spheromak motion across and interaction with static magnetic fields (which are useful in corroborating recent theories). Probe measurements in the tokamak dc toroidal field show for the first time that the spheromak exhibits a ``double tilt.''The spheromak first tilts while in the cylindrical entrance region, emerging into the tokamak vessel antialigned to the dc toroidal field, then expands into the tokamak vacuum vessel, and finally tilts again to form an oblate (nonaxisymmetric, m=1) configuration. In addition, the spheromak drifts vertically in the direction given by Jcenter×Btok, where Jcenter is the unbalanced poloidal current that threads the center of the spheromak torus. Probe arrays at different toroidal locations show that the spheromak shifts toroidally (horizontally left or right) in the direction opposite that of the static toroidal field. In the absence of toroidal flux, the m=1 object develops a helical pitch, the sense of the pitch depending on the sign of the spheromak helicity. The spheromak equilibrium in the toroidal vessel is well fit by a pressureless infinite cylindrical model; however, there is evidence of deviation from m=1 symmetry because of toroidal effects, nonuniform J/B profile, and finite beta. Experiments performed in a test facility consisting of the spheromak gun and a replica of the entrance region (with a closed end) show that the spheromak is generated with its axis coaxial with that of the gun. Coherent, m=2 magnetic modes are observed during the formation stage rotating in the E×B direction at about 125 kHz (rotation velocity corresponding to 40% of the Alfvén speed).


Item Type:Article
Additional Information:Copyright © 1991 American Institute of Physics. Received 27 August 1990; accepted 7 January 1991. It is a pleasure to acknowledge the technical assistance of Frank Cosso and Larry Begay, as well as numerous enlightening discussions with Dr. Cris Barnes, Dr. Tom Jarboe, Dr. Juan Fernández, Dr. Fred Wysocki, and Dr. George Marklin at Los Alamos National Laboratory and Dr. Chales Hartman and Dr. Jim Hammer at Lawrence Livermore National Laboratory. This work was supported under U.S. Department of Energy Grant NO. DE-FG03-86ER53232. [M.R.B. was an] U.S. Department of Energy, Fusion Energy Postdoctoral Research Fellow.
Subject Keywords:SPHEROMAK DEVICES, EQUILIBRIUM, TOROIDAL CONFIGURATION, MAGNETIC FLUX, TOKAMAK DEVICES, MAGNETIC PROBES, PLASMA CONFINEMENT
Record Number:CaltechAUTHORS:BROpofb91
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:BROpofb91
Alternative URL:http://dx.doi.org/10.1063/1.859813
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:9395
Collection:CaltechAUTHORS
Deposited By: Archive Administrator
Deposited On:18 Dec 2007
Last Modified:26 Dec 2012 09:48

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