Bootstrap current in enhanced reversed shear tokamaks for Volume Neutron Source applications

The bootstrap current is evaluated for two reference tokamak designs for a Volume Neutron Source (VNS). One is a larger aspect ratio design using superconducting coils (VNS-SC) and the other is a small aspect ratio design using a solid core with normal conducting toroidal field coils (VNS-ST). The target plasma profiles are taken as representative of the recently observed enhanced reverse shear plasmas with hollow magnetic safety factor (q) profiles in the core and correspondingly peaked density profiles. The higher q in the plasma center in combination with peaked density is shown to move the peak in the bootstrap current toward the plasma center. This reduces the current drive requirements to a very small axial seed current and a source localized around the location of the desired minimum in the q profile. Very high bootstrap current fractions can be attained in the VNS-SC design with normalized betas (defined in terms of the vacuum toroidal magnetic field at the geometric center of the plasma) of β_N ⩽3.5. The bootstrap current is lower in the VNS-ST design because of its lower aspect ratio; the highest bootstrap fraction found in these limited cases is 50% at β_N=5