Filterscopes: Spectral line monitors for long-pulse plasma devices^a)

A photomultiplier (PMT)-based diagnostic system for monitoring spectral lines along multiple viewchords, named the “Filterscope” [R. J. Colchin et al, Rev. Sci. Instrum. 74, 2068 (2003)], is currently in use at the DIII-D, NSTX, and CDX-U fusion plasma devices in the US, and has been installed at the KSTAR device in Korea. This diagnostic has recently been upgraded for application to long-pulse devices, such as KSTAR, EAST in China, and the future ITER in France. A new data acquisition system, employing the PXI instrumentation platform with an embedded Windows microprocessor controller, can simultaneously record up to 72 channels at $100kHz$ sampling rates for plasma periods lasting up to $20min$⁠. Based on the average signal level during an adjustable time interval (⁠$100ms$ in the present DIII-D implementation), the controller digitally adjusts PMT dynode voltage throughout the course of a discharge, thereby maintaining the output signals at a level where they are neither saturated nor dominated by digitizer noise. The new system’s ability to accommodate large variations in source strength, discharge to discharge and within a single discharge, has proved particularly valuable during DIII-D operations, since changes between top, bottom, and double-null divertor magnetic configurations lead to large temporal variations in signal brightness.