A Hall sensor array for internal current profile constraint

Measurements of the internal distribution of B in magnetically confined plasmas are required to obtain current profiles via equilibrium reconstruction with sufficient accuracy to challenge stability theory. A 16-channel linear array of InSb Hall effect sensors with 7.5 mm spatial resolution has been constructed to directly measure internal B(z)(R, t) for determination of J (psi, t) associated with edge-localized peeling mode instabilities in the Pegasus Toroidal Experiment. The diagnostic is mounted in an electrically isolated vacuum assembly which presents a slim, cylindrical profile (similar to 1 cm outside diameter) to the plasma using graphite as a low-Z plasma facing component. Absolute calibration of the sensors is determined via in situ cross-calibration against existing magnetic pickup coils. Present channel sensitivities are of order of 0.25 mT. Internal measurements with bandwidth of <= 25 kHz have been obtained without measurable plasma perturbation. They resolve n=1 internal magnetohydrodynamics and indicate systematic variation in J(psi) under different stability conditions. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3475539]