Fast scanning probe for the NSTX spherical tokamak

We describe a fast reciprocating Langmuir probe and drive system, which has four main new features: (1) use of high-temperature, vacuum, circuit boards instead of cables to reduce weight and increase to 21 the number of possible connections, (2) rotatable and removable shaft, (3) 10 tip construction with designed hardware bandwidth up to 10 MHz, and (4) a detachable and modular tip assembly for easy maintenance. The probe is mounted in a fast pneumatic drive capable of speeds ∼7 m/s and ∼20g's acceleration in order to reach the scrape-off layer (SOL) and pedestal regions and remain inserted long enough to obtain good statistics while minimizing the heat deposition to the tips and head in a power density environment of 1-10 MW/m². The National Spherical Torus Experiment SOL features electron temperature, T_e∼10-30 eV, and electron density, n_e∼0.1-5×10¹² cm^-3 while the pedestal features n_e∼0.5-1.5×10¹³ cm^-3 and T_e∼30-150 eV. The probe described here has ten tips which obtain a wide spectrum of plasma parameters: electron temperature profile T_e(r), electron density profile n_e(r) and Mach number profile M(r ), floating potential V_f(r), poloidal and radial electric field profiles E_θ(r) and E_ρ(r), saturation current profile I_sat(r), and their fluctuations up to 3 MHz. We describe the probe and show representative radial profiles of various parameters.