Shafranov Shift in Low-Aspect-Ratio Heliotron / Torsatron CHS

The MHD equilibrium properties of neutral-beam-heated plasmas have been experimentally investigated in the Compact Helical System (CHS), a low-aspect-ratio (A_p ~ 5) heliotron/torsatron. This configuration is characterized by a strong breaking of helical symmetry. The radial profiles measured by various diagnostics have shown significant Shafranov shift due to plasma pressure. The deviation of the magnetic axis from its vacuum position has reached 50% of the minor radius. When the three-dimensional equilibrium code VMEC is used to reconstruct the equilibrium from the experimental data, the result is in good agreement with the experimentally observed Shafranov shift as well as the diamagnetic pressure in plasmas with leq I .2% and beta_0 leq 3.3%. This beta value corresponds to half of the conventional equilibrium beta Iimit defined by the Shafranov shift reaching a value of half the minor radius. Although tangential neutral beam injection has caused pressure anisotropies p_parall/p_bot leq 3, the description of the equilibrium assuming isotropic pressure is consistent with the experiment