High Field Utilization
All current-carrying toroidal plasmas require a toroidal magnetic field to stabilize the plasma against kinking. In tokamaks this field can be the dominant magnetic field in the system. One of the engineering advantages of the spherical torus is its ability to confine plasmas with relatively low values of toroidal magnetic field. Theoretically, the ratio of the plasma current to the required stabilizing toroidal field is predicted to increase as the aspect ratio is reduced. One of the principal experimental thrusts of the Pegasus experiment is to determine the boundary of this instability in terms of the plasma current normalized by the central rod current ITF. In addition, at large values of Ip/ITF, the normalized plasma pressure, known as β, can become quite large. Experimental campaigns to access values of high Ip/ITF include scenario development, plasma control techniques, tearing mode suppression, and current profile diagnosis and control.
Static Field Gun Injection for Non-Inductive Startup
Typical tokamaks rely upon large solenoids in the centerstack of the machine to inductively form and sustain a hot, high-current plasma. However, the ST design relies upon a very small centerstack, so it would be very advantageous to completely remove the solenoid to allow as low an aspect ratio as possible. To this end, PEGASUS is studying the use of plasma gun injectors for startup of an ST plasma without any induction from the central solenoid.
Plasma Guns and External Induction
Pegasus studies the effectiveness of coupling induction from the solenoid to a gun-produced plasma in order to expand the accessible parameter space beyond what induction alone can provide.