Design of FPGA-based triggering and synchronization system for laser photo detachment diagnostic in ROBIN

Abstract

ROBIN is a single RF driver based negative ion test bed currently in operation at IPR, Gandhinagar, India. To understand and have deeper insights of physical phenomena, several diagnostics have been interfaced with ROBIN system. To quantify the negative ion density, laser photo detachment (LPD) diagnostic is configured. LPD at ROBIN is based on single high power Nd:YAG laser which has a pulse width of 3 ns and repetition rate of 20 Hz. Successful integration of LPD needs control on laser energy, precise triggering and detection of weak signals. Precise triggering has two aspects. One is to control laser energy via a delay between the flash lamp and Q-switch pulses; the second is to trigger it in phase with the 1 MHz RF signal. This required a triggering system which can work with an accuracy of few nanoseconds. A triggering and synchronization system (TSS) based on a custom field programmable gate array(FPGA) is designed and integrated in order to assure reliable and synchronized operation of the laser. Precise delay between laser pulses is necessary to obtain appropriate laser energy and to prevent the active Nd:YAG rod medium from thermal damage. In order to synchronize the laser operation with 1 MHz RF, Rogowskii coil based peak detection is used and interfaced with TSS. The LPD setup is HV referenced; hence the interfaces were designed with optical isolation. Another challenge is to detect weak signals from the LPD probe. For this, a custom low-noise pre-amplifier circuit is designed to measure signal in the range of 1-2 mA in the presence of large RF noise. The engineering solution designed for LPD interface contains mix of analog and digital systems. The TSS is based on a low cost FPGA platform, with custom analog-based interfaces. The unit has been integrated without using any specific commercial system for the desired goal. The data from diagnostic is being currently analysed. This paper presents the details regarding the design of interfacing electronics of the LPD, along with some test results. In addition, the FPGA firmware, the control software and signal acquisition process are presented.