A field programmable gate array unit for the diagnosis and control of neoclassical tearing modes on MAST

Abstract

A real-time system has been developed to trigger both the MAST Thomson scattering (TS) system and the plasma control system on the phase and amplitude of neoclassical tearing modes (NTMs), extending the capabilities of the original system. This triggering system determines the phase and amplitude of a given NTM using magnetic coils at different toroidal locations. Real-time processing of the raw magnetic data occurs on a low cost field programmable gate array (FPGA) based unit which permits triggering of the TS lasers on specific amplitudes and phases of NTM evolution. The MAST plasma control system can receive a separate trigger from the FPGA unit that initiates a vertical shift of the MAST magnetic axis. Such shifts have fully removed m∕n = 2∕1 NTMs instabilities on a number of MAST discharges.

Neoclassical tearing mode control using vertical shifts on MAST

“…Although 2/1 NTMs occur frequently in other high performance scenarios on MAST and also result in locked mode disruptions, this particular scenario was chosen because its performance ensures the early appearance of a 2/1 NTM. A field programmable gate array system was developed to trigger the MAST vertical control system, upon detection of a 2/1 NTM by the MAST Mirnov coil array [10]. Vertical shifts of the order of 1-2 cm in these scenarios result in an H-L back transition on MAST.…”

“…Triggered H-L transitions are shown to stabilize m/n = 2/1 NTMs (where m is the poloidal mode number and n the toroidal mode number) and prevent locked mode disruptions for several double null discharges on MAST, typically doubling the H-mode duration [10], with no significant lasting decrease in electron temperature (T e ), density (n e ) or ion temperature (T i ) measured in the plasma core (figure 1). The H-mode phase is typically recovered, and the NTM removed, within 20 ms of onset using this method.…”

Overview of physics results from MAST towards ITER/DEMO and the MAST Upgrade

“…Examining the heat transport inside the island, however, is a more fundamental method to understand NTM stability. Using field-programmable gate array (FPGA) based event triggering of the Thomson scattering system [90,91] the T e and n e profiles at eight time slices through the rotating 2/1 island of an NTM have been measured for the first time in an ST [92]. A 2D simple heat transport model assuming divergence-free diffusive heat transport in the island with constant parallel and perpendicular heat diffusivity has been used to fit to the T e profile.…”

“…A key feature of this analysis is that no specific models for parallel or perpendicular heat transport are assumed, which has not been the case in many previous analyses of NTM growth. The same technique of intelligent event triggering can be used to mitigate the NTM in H-mode by shifting the plasma briefly of DND triggering a short L-mode transition [36] leading to a temporary reduction of β p [91]. Disruption avoidance and mitigation are key features for ITER due to the large forces and the potential for runaway electron generation.…”