EPICS data acquisition system for FPGA-based plasma electron density measurement on KSTAR

The Korea Superconducting Tokamak Advanced Research (KSTAR) has several diagnostics devices, including a millimeter-wave interferometer (MMWI), far-infrared interferometer (FIR), two-color interferometer, etc., for measuring plasma electron densities. The MMWI diagnostic device measures the electron density of the plasma from the phase difference generated by transmitting a 1 mm (280 GHz) electromagnetic wave into it. The FIR device is similar to the MMWI, but uses a far-infrared (119 um) laser as its electromagnetic wave source. The MMWI is the primary plasma density measurement system in KSTAR. However, its signal contains significant noise and hundreds of fringe-jump events from the phase comparator circuit. The original analogue circuit of the phase comparator is limited in noise handling since it needs to average the raw IF signal to be treated in analogue circuit with limited bandwidth. To handle noise with more flexibility and to reduce occurrence of fringe-jump errors, we developed a new data acquisition (DAQ) system using a high-speed FPGA-based digitizer. The MMWI signal resembles a 10 MHz sine wave. We acquired data at 100 MSPS using a new digitizer. The phase difference per sample was 36°, which lead to inaccurate results due to noise. To increase the accuracy, the new DAQ system has interpolation logic within the FPGA that increases the phase resolution. In addition, the phase comparative value is transmitted to two channels by inverting the signal. The signal that does not travel through the plasma has a phase difference of 180°. The FPGA-based system was constructed using the same algorithm as existing phase comparator circuit [2]. The detected phase was averaged to obtain a phase value, and the result was transmitted as 1280 phase values per msec. In the EPICS Input/Output Controller (IOC), the density is calculated using data received from the direct memory access (DMA) via peripheral component interconnect (PCI) Express. The above FPGA logic was successfully used for real-time feedback control in the ITER CODAC environment. This paper introduces a new, detailed MMWI DAQ system design concept using FPGA, and its operational results.