Demonstration of Prototype Mirror Langmuir Probe Control System Using a Red Pitaya Field Programmable Gate Array Board

High bandwidth, high spatial resolution measurements of electron temperature, density and plasma potential are valuable for resolving turbulence in the boundary plasma of tokamaks. While Langmuir probes can provide such measurements their temporal and spatial resolution is limited by the sweep rate for obtaining I-V characteristics or by the need to use multiple electrodes, each sampling a single plasma quantity at high bandwidth. The Mirror Langmuir Probe (MLP) bias technique overcomes these limitations by rapidly switching the voltage on a single electrode among three bias states, each dynamically optimized for local plasma conditions. The MLP system on Alcator C-Mod used analog circuitry to perform this function, measuring Te, Vf and Isat at 1.1 MHz. Recently, a new prototype digital MLP controller has been implemented on a Red Pitaya (RP) FPGA board, which reproduces the functionality of the original controller, performs all data acquisition, and is readily customizable at a fraction of the development time and implementation cost. A second RP was used to test the MLP by simulating the current response of a physical probe using C-Mod experimental measurements.