Spherical torus

Plasma science (the study of ionized gases) is critical to the development of fusion energy (involving the fusion of nuclei), which could be an abundant and attractive energy source in the future. The spherical torus concept creates a unique magnetic field structure to improve plasma stability and confinement at high pressure. This plasma configuration, shaped like a sphere with a narrow hole through the center, can confine a higher plasma pressure for a given magnetic field strength than a conventional tokamak, in which plasma is shaped like a donut. Conceived in 1985 by researchers at Oak Ridge National Laboratory, the spherical torus concept is now being tested at Princeton Plasma Physics Laboratory. Preliminary tests on a small device in the United Kingdom reached a high ratio of plasma pressure to applied magnetic field pressure (called toroidal beta) about 40 percent, that is roughly three times greater than the level reached in a tokamak. Initial results at Princeton are encouraging. The toroidal beta has reached approximately 22.5 percent; and the energy containment efficiency improved by as much as a factor of two when an additional heating method was applied. The goal is to produce and maintain high-performance spherical torus plasmas for pulse lengths of several seconds.

Scientific Impact: Initial results at Princeton suggest that the spherical torus experiment will have adequate heating power to test the theoretically predicted toroidal beta values in the range of 40 percent. This configuration also offers new opportunities to advance the science of high-temperature magnetized plasmas of interest to the astronomical plasma physics community.

Social Impact: Because the amount of fusion power produced is roughly proportional to the square of the plasma pressure, the spherical torus configuration could lead to smaller, less costly development steps to economical fusion power plants. This advance could help promote the availability of fusion as an attractive energy source.

Reference: "Features of Spherical Torus Plasmas," Y.-K. M. Peng and D. J. Strickler, Nucl. Fusion Vol. 26 (1986) 576.

"High-harmonic fast magnetosonic wave coupling, propagation, and heating in a spherical torus plasma," J. Menard, R. Majeski, R. Kaita, M. Ono, T. Munsat, et al., Phys. Plasmas 6, 2002 (1999).

"RF Experiments on Spherical torus Plasmas," R. Majeski, J. Menard, D. Batchelor, T. Bigelow, et al., AIP Conf. Proc. 485, 296 (1999).

URL: http://www.pppl.gov/projects/pages/nstx.html

Technical Contact: Don Priester, Research Division, 301-903-3752

Press Contact: Jeff Sherwood, DOE Office of Public Affairs, 202-586-5806

SC-Funding Office: Office of Fusion Energy Sciences