Radially scanning magnetic probes to study local helicity injection dynamics

Rev Sci Instrum. 2018 Oct;89(10):10J103. doi: 10.1063/1.5036527.

Abstract

Two new magnetic probes have been deployed on the Pegasus spherical tokamak to study the dynamics of local helicity injection non-solenoidal plasma start-up and current drive. The magnetic radial array probe consists of 15 pickup coils (∼5 × 8 mm each) that measure B ̇ z ( R ) over a 15 cm linear extent. The coils consist of traces embedded in a printed circuit board. Three coil designs are utilized to balance frequency response and coil sensitivity. Helmholtz coil measurements are used to measure coil and full assembly bandwidths (∼2 MHz and ∼200 kHz, respectively) and sensitivities (0.18/0.35/0.96 mV T-1 s). The magnetic radial scanning probe is an array of Hall effect sensors that measure field strength ( | B | 177 mT) and direction at 8 spatial points (ΔR = 1.5 cm), supporting the studies of equilibrium field structure and low-frequency (≤5 kHz) current dynamics. It uses commercial surface-mount Hall effect sensors with chip-integrated amplifiers and compensators that are mounted in a 3-D printed frame. Helmholtz coil measurements indicate negligible cross-field gain nonlinearity and provide absolute calibration of the diagnostic. Both probes are constructed as an electrostatically shielded insertable air-side assembly that mounts within a radially translatable ultrahigh vacuum assembly from an existing probe.