Triple Langmuir Probe Circuit Response to Dynamic Loading

Recently, an array of Langmuir probes was installed in the divertor region of the National Spherical Tokomak eXperiment (NSTX) and has been successfully tested [1]. The array is backed by a custom designed electronics system that allows biasing the probes, collecting the signals, reducing noise and amplifying circuitry and is suited to operate both as a single Langmuir probe and as a triple Langmuir probe (TLP). While the probe data has been useful in understanding the plasma characteristics during steady plasma discharges in NSTX, certain modifications aid interpretation of the transient events ($\sim \mu $s scale) such as during Edge Localized Modes (ELMs). During high-flux transients, the bias circuit may drift from the nominal values before on-board control circuitry can respond. The details of the circuit, its response to dynamic loading and the resulting impact on signal interpretation is presented. [1] M.A. Jaworski, J. Kallman, R. Kaita, H. Kugel, B. LeBlanc, R. Marsala, and D.N. Ruzic, ``Biasing, acquisition and interpretation of a dense Langmuir probe array in NSTX,'' 18th Topical Conference on High Temperature Plasma Diagnsotics, 2010.