Novel approaches for mitigating runaway electrons and plasma disruptions in ADITYA tokamak

Abstract

This paper summarizes the results of recent dedicated experiments on disruption control and runaway mitigation carried out in ADITYA, which are of the utmost importance for the successful operation of large size tokamaks, such as ITER. It is quite a well-known fact that disruptions in tokamaks must be avoided. Disruptions, induced by hydrogen gas puffing, are successfully avoided by two innovative techniques in ADITYA using a bias electrode placed inside the last closed flux surface and applying an ion cyclotron resonance pulse with a power of ∼50 to 70 kW. These experiments led to better understanding of the disruption avoidance mechanisms and also can be thought of as one of the options for disruption avoidance in ITER. In both cases, the physical mechanism seems to be the control of magnetohydrodynamic modes due to increased poloidal rotation of edge plasma generated by induced radial electric fields. Real time avoidance of disruption with identifying proper precursors in both the mechanisms is successfully attempted. Further, analysing thoroughly the huge database of different types of spontaneous and deliberately-triggered disruptions from ADITYA, a significant contribution has been made to the international disruption database (ITPA). Furthermore, the mitigation of the runaway electron generated mainly during disruptions remains a challenging topic in present tokamak research as these high-energy electrons can cause severe damage to in-vessel components and the vacuum vessel. A simple technique has been implemented in ADITYA to mitigate the runaway electrons before they can gain high energies using a localized vertical magnetic field perturbation applied at one toroidal location to extract runaway electrons.

Simulation of runaway electrons, transport affected by J-TEXT resonant magnetic perturbation

“…As a kind of mitigation method, the resonant magnetic perturbation (RMP) is applied to increase the runaway electrons, losses to depress the knock-on avalanche mechanism of runaway electrons. The method has been proved effective in various experiments: in Tore Supra, the applying of an ergodic divertor leads to enhancement of runaway electron losses [4]; runaway electrons are absent if applying an extremely high magnetic perturbation field in JT-60U [5,6]; in TEXTOR, the runaway losses are enhanced by RMP and the runaway electrons can be effectively suppressed [7][8][9][10]; and DIII-D [11], HT-7 [12], ADITYA [13], also have the same results. In this work, two sets of coils, SRMP and DRMP, are used on the J-TEXT tokamak [14,15] as the RMP field to study their possible influence on the magnetic topology as well as the orbits and the loss rate of runaway electrons.…”

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

“…To start with, the initiation of the ICR pulse is pre-fixed time prior to the gas injection disrupting the plasma. After successfully avoiding the disruptions with the pre-fixed pulse, real time application of ICR pulse has been carried out by detecting the increase in H␣ emission signal due to gas puff and used it as precursor for triggering the ICR pulse to avoid disruptions [3]. The developed circuit detects and compares the increase in H␣ emission due to gas puffing (Fig.…”

“…Many innovative experiments related to immediate ITER requirements such as disruption avoidance and runaway mitigation are carried out in this device. Disruptions, induced by hydrogen gas puffing, are successfully avoided by two innovative techniques in ADITYA using a bias electrode placed inside the Last Closed Flux Surface (LCFS) and applying an Ion Cyclotron Resonance (ICR) pulse with power of ∼50-70 kW [3]. A simple technique has been implemented in ADITYA to mitigate the runaway electrons before they can gain high energies using a localized vertical magnetic field perturbation applied at one toroidal location to extract the runaway electrons [3].…”

25th IAEA Fusion Energy Conference: summary of sessions EX/S, EX/W and ICC

“…A number of papers addressed the question of disruption avoidance and disruption control using a variety of methods [5][6][7][8][9][10]. A feedback-based mode rotation control method to exert an electromagnetic torque on the plasma was used in DIII-D as well as RFX-mod to prevent mode locking of a (2, 1) tearing mode (TM) and thereby prevent the onset of a disruption [5].…”

“…This has been helpful in considerably expanding the disruption free operating regime of the device (see figure 2). Suppression of unstable MHD activity and concomitant avoidance of disruption was demonstrated on tokamak ADITYA using biased electrodes inside the last closed flux surface [8,10]. In these experiments disruptions were artificially induced by the introduction of a gas puff that resulted in an abrupt termination of the current.…”