Disruption control on FTU and ASDEX upgrade with ECRH

Abstract

The use of ECRH has been investigated as a promising technique to avoid or postpone disruptions in dedicated experiments in FTU and ASDEX Upgrade. Disruptions have been produced by injecting Mo through laser blow-off (FTU) or by puffing deuterium gas above the Greenwald limit (FTU and ASDEX Upgrade). The toroidal magnetic field is kept fixed and the ECRH launching mirrors are steered before every discharge in order to change the deposition radius. The loop voltage signal is used as disruption precursor to trigger the ECRH power before the plasma current quench. In the FTU experiments (I p =0.35-0.5 MA, B t =5.3 T, P ECRH =0.4-1.2 MW) it is found that the application of ECRH modifies the current quench starting time depending on the power deposition location. A scan in deposition location has shown that the direct heating of one of the magnetic islands produced by magnetohydrodynamic (MHD) modes (either m/n=3/2, 2/1 or 3/1) prevents its further growth and also produces the stabilization of the other coupled modes and current quench delay or avoidance. Disruption avoidance and complete discharge recovery is obtained when the ECRH power is applied on rational surfaces. The modes involved in the disruption are found to be tearing modes stabilized by a strong local ECRH heating. The Rutherford equation has been used to reproduce the evolution of the MHD modes. The minimum absorbed power value found for disruption avoidance is 0.4 MW at 0.5 MA with deposition on the q=2 surface. In the similar set of experiments carried out in ASDEX Upgrade L-mode plasmas (I p =0.6 MA, B t =2.5 T, P ECRH = 0.6 MW ~ P OHM) the injection of ECRH close to q=2 significantly delays the 2/1 onset and prolongs the duration of the discharge: during this phase the density continues to increase. No 2/1 onset delay is observed when the injected power is reduced to 0.35 MW.

Simulation of feedback control system for NTM stabilisation in ASDEX Upgrade

“…In the future, the simulation will be used to prepare experiments where the gyrotron power is also controlled from the central controller, and others on pre-emptive control [2]. It is intended that more intelligence will be integrated into the controller, allowing simpler usage by an experiment leader.…”

“…Worse still, NTMs can lead to disruptions which compromise the lifetime of structural components [2]. Several methods to mitigate NTMs have been investigated [3], of which localised Electron Cyclotron Resonance Heating (ECRH) is the preferred method to stabilise an NTM.…”

Detection of Disruptions in the High-β Spherical Torus NSTX

“…Control of the m/n=2/1 neoclassical tearing mode [61,62] has been demonstrated using gyrotrons to drive currents inside the magnetic island [62][63][64][65][66][67]. Furthermore, even when instabilities grow large and result in significant modifications of the plasma state, active control "recovery techniques" can be envisioned, for instance, using ECCD on locked modes in DIII-D [68], or ECH as demonstrated on AUG [69,70] and FTU [69] to recover from locked mode and density limit disruptions.…”

Study of the change of electron temperature inside magnetic island caused by localized radio frequency heating

“…Magnetic islands due to neoclassical tearing modes (NTMs) have been successfully suppressed by electron cyclotron current drive (ECCD) on ASDEX Upgrade [11], JT-60 [12], and DIII-D [13]. Alternatively, Electron cyclotron resonance heating (ECRH) is also able to reduce the island size or to slow down the island growth on TEXTOR [14], FTU [15], ASDEX Upgrade [15], and T10 [16]. To stabilize magnetic islands more efficiently, the modulated technique to deposit the rf power around the island's O-point has been utilized in experiments [11][12][13][14][15].…”

“…Alternatively, Electron cyclotron resonance heating (ECRH) is also able to reduce the island size or to slow down the island growth on TEXTOR [14], FTU [15], ASDEX Upgrade [15], and T10 [16]. To stabilize magnetic islands more efficiently, the modulated technique to deposit the rf power around the island's O-point has been utilized in experiments [11][12][13][14][15]. Theoretical study indicates that the stabilization by ECRH is more effective than that by ECCD for a sufficiently large island [17].…”