Multi-channel control circuit for real-time control of events in Aditya tokamak

Tokamak plasma is prone to many random events having potential for causing severe damages to the machine, such as disruptions, production and elimination of high-energy runaway electrons etc. These events can be mitigated by obtaining pre-cursor signal leading to these events and then taking proper measures just before their onset to avoid their happenings, like disruptions can be mitigated by massive gas injection or putting a bias voltage on an electrode placed inside the plasma, the runaways can be mitigated by gas injection and by applying specific magnetic fields. Hence for real time control of these events, the pre-cursors should be electronically recorded and the mitigation techniques should be initiated by sending triggers to their individual operational systems. To implement these methodologies of real-time controlling of events in Aditya Tokamak, a low cost multi-channel Micro-Controller based timing circuit is designed and developed in-house. This circuit first compares the precursor signals fed into it with the pre-set values and gives a trigger output whenever the signals overshoot the pre-set values. The circuit readies itself for operation along with start of the tokamak discharge and waits up to an initial pre-determined delay and then initiates a trigger at the time of overshooting of precursor signal. The circuit is fully integrated and assembled in compact enclosure with local LCD for threshold and initial trigger-delay monitoring and indicators for full stand-alone operation. The system has been successfully tested in the disruption control experiment with two different techniques viz., electrode biasing and ICRH pulse techniques in Aditya tokamak. The increase in MHD oscillation amplitude and the gas puff induced Hα intensity increase are used as a precursor for triggering the electrode bias and ICR pulse respectively in real time disruption control application. The runaway (RE) mitigation during disruption phase with local vertical field perturbation (LVF) in real time feedback mode is performed using this circuit, where runaway flux amplitude beyond pre-set threshold is used as a precursor for triggering the LVF current pulse. The detailed design features and results will be described in this paper. In addition to that the circuit is also used for single loop online plasma electron density control as well as position control.