Bootstrap current fraction scaling for spherical torus plasmas

Dependence of the bootstrap current fraction of spherical torus plasma on the aspect ratio and the triangular deformation is studied by using a simple equilibrium model, the Solov'ev-type equilibrium. Two bootstrap current models are used: one is the single-ion collisionless regime model, the other is the recently developed arbitrary collisional regime model. It is found that besides the well known favourable effect of the small aspect ratio on the enhancement of the bootstrap current fraction, the triangularity is beneficial for raising the total bootstrap current though it does not change its fraction much.