Spherical tokamak with a plasma center column

Abstract

Low aspect ratio toroidal pinches such as the standard (q>1) and the ultralow q (q<1) spherical tori or tokamaks (ST), would have a far more robust reactor engineering design if a plasma center column (PCC) can be used in place of a material center post. Biased electrodes across the plasma center column would drive a plasma current to produce the toroidal magnetic field in lieu of the toroidal field (TF) coils. The operation of such a device is naturally divided into two distinct phases: formation by driven relaxation under magnetic helicity injection and sustainment by auxiliary current drive and heating such as rf and neutral beam injection (NBI). The initial design constraints of a ST-PCC experiment are primarily motivated by the formation rather than the sustainment physics. With a Taylor-relaxed plasma as the baseline case, it is shown that three essential factors guide the design. First, the flux amplification factor determines the aspect ratio of the ST-PCC. Second, the plasma shaping in general and plasma elongation in particular gives the most freedom in shaping the q profile of the relaxed plasma. Two examples are the standard spherical tokamak with q>1 throughout the plasma and the ultralow q (ULQ) spherical tokamak with q much less than unity for the bulk of the plasma. Third, the vacuum bias magnetic flux plays the second most important role in modifying the q profile. As an example, it is shown how the bias flux can be designed to delineate a standard spheromak experiment from that of an ULQ ST-PCC. These physics understandings help define the design space of the ST-PCC experiments and directions for optimization.