HTPD 2018

To study Local Helicity Injection (LHI) dynamics and current drive, a new insertable B ̇ magnetic probe was deployed on the Pegasus spherical tokamak. The Magnetic Radial Array (MrA) probe consists of an array 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 (PCB), with twisted-pair wires bringing the signal off the PCB to reduce noise. Three different coil designs are utilized to balance frequency response and coil sensitivity. Helmholtz coil measurements confirm bandwidth of ≲3.5 MHz and sensitives of 0.18/0.35/0.96 mV T^(-1) s. The probe uses the carbon armor and vacuum assembly from an existing probe. MrA probe measurements during LHI show significant magnetic activity at ~600 kHz that is localized to the plasma edge. To complement this high-speed B ̇ array, a lower-bandwidth (≤40 kHz) B(R) probe array is being developed. It utilizes ratiometric Hall effect sensors (with built-in amplifiers and compensators) that are mounted in a 3D printed form. This probe will provide measurements of field strength (|B|≤120 mT) and direction at 10 spatial points (ΔR=1.5 cm), to support studies of equilibrium field structure and current dynamics. Work supported by US DOE grant DE-FG02-96ER54375