Spherical Torus

The spherical torus is a tokamak with the central hole reduced to the minimum technically feasible size. In general, the central hole of the torus must be sufficiently large to fit at least the internal legs of the toroidal field coil. In present designs it must also be sufficiently large to accommodate the solenoid of the ohmic heating system. Actually, one of the most important topics in modern tokamak research is to study and develop efficient methods of driving the plasma current in steady state without using the intrinsically pulsed transformer action, and therefore completely eliminating the ohmic solenoid. This will be possible, in great part, by careful and precise control of the self-sustaining thermoelectric current ("bootstrap" current) inside the plasma. The development of appropriate means of auxiliary heating and current profile control is even more important in the case of spherical tokamaks.

Aspect ratio - The single most important geometric characteristic of a tokamak is the aspect ratio, a number larger than unity corresponding to the ratio between the major and the minor radii of the plasma torus.

The figure shows on the left side the geometric difference between low (top) and high (bottom) aspect ratio tori. The magnetic topology of a spherical, or low aspect ratio torus is illustrated on the right side, which shows how a magnetic field line on the outer edge of the torus becomes highly twisted near the central hole.

High and low aspect ratio tokamaks - The operation of the presently large tokamaks depends on high values of the toroidal magnetic field and relatively large sizes. Typically, the toroidal magnetic induction is B ~ 4 T at the plasma center, and the major radius of the torus is R ~ 3 m. These conventional tokamaks have aspect ratios A = R/a ~ 3, considered high in modern magnetic confinement fusion research (a ~ 1 m designates the minor radius of the plasma torus). The low aspect ratio tokamaks, or spherical tori, have the advantage of smaller overall size, for a given plasma size, and low magnetic field. These characteristics lead naturally to lower cost of the device. Due to the increase in plasma stability, a consequence of the large twisting of the magnetic field lines in the central region, the spherical torus can operate with a high ratio of plasma pressure to magnetic field pressure, which is an advantageous feature for reactors. However, these advantages only become apparent for aspect ratios lower than A ~ 2.

The plasma in the TCABR tokamak has a circular cross-section with aspect ratio A = 3.4, a major radius R = 0.61 m, a magnetic field B = 1.1 T, and a total plasma current I = 0.1 MA.

Illustration of the main components of the TCABR high aspect ratio tokamak presently installed in the plasma laboratory of the Applied Physics Department of the University of São Paulo (http://www.if.usp.br).