Analysing the impact of anisotropy pressure on tokamak equilibria

Abstract

Neutral beam injection and ion cyclotron resonance heating induce pressure anisotropy. The axisymmetric plasma equilibrium code HELENA has been upgraded to include anisotropy and toroidal flow. We have studied, using both analytical and numerical methods, the determining factors for anisotropic equilibria and their impact on flux surfaces, the magnetic axis shift, and the displacement of pressure and density contours from the flux surface. With p∥/ p⊥≈ 1.5, p⊥ can vary by 20% on the s=0.5 flux surface, in a MAST-like (MAST: Mega Amp Spherical Tokamak) equilibrium. We have also re-evaluated the widely applied approximation to the anisotropy in which p*= (p∥+p⊥)/2, the average of the parallel and perpendicular pressures, is taken as the approximate isotropic pressure. We show that an isotropic reconstruction can yield a correct p* only by giving an incorrect RBφ. We find the reconstructions of the same MAST discharge with p∥/p⊥≈1.25 using isotropic and anisotropic models to have a 3% difference in a toroidal field and a 66% difference for a poloidal current.