Measurement of the Pfirsch-Schlueter and bootstrap currents in HSX

The Helically Symmetric Experiment is a quasi-helically symmetric axis stellarator that, to a good approximation, has no toroidal curvature. The helical axis of symmetry in |B| and lack of toroidal curvature results in a Pfirsch-Schlueter current that rotates as a function of toroidal angle. Also, the magnitude of this current is reduced by a factor of 1/|n-mι| approx. = 1/3 in HSX. Another consequence of the helical axis is that the bootstrap current flows in the direction opposite to that in a tokamak and acts to reduce the rotational transform. These currents are measured in HSX by an external Rogowski coil and an external dB/dt pickup coil array. A 10-chord Thomson scattering system measures the radial profiles of the electron temperature and density. Ion profiles are assumed. Two numerical codes, VMEC and BOOTSJ, calculate the equilibrium MHD current densities and bootstrap current densities in HSX. An additional code, V3FIT, calculates the magnetic field due to the main magnetic field and plasma currents. Comparisons of numerical estimates and recent measurements are presented for 1 Tesla QHS plasmas heated by 50 kW ECRH. The helical nature of the Pfirsch-Schlueter current is confirmed, and the bootstrap current, as calculated by BOOTSJ provides a good estimate of the toroidal current measured in HSX. (author)