Analytical compensation of axisymmetric equilibrium fluxes picked up by locked mode detectors in tokamaks

The influence of resonant magnetic perturbations (RMPs) with a large m/n = 3/1 component on electron density has been studied on J-TEXT tokamak by using externally applied static and rotating RMPs, where m and n are the poloidal and toroidal mode number, respectively. The detailed time evolution of electron density profile, measured by the polarimeter-interferometer, shows that the electron density ne first increases (decreases) inside (around/outside) of the 3/1 rational surface (RS), and it is increased globally later together with enhanced edge recycling. Associated with field penetration, the toroidal rotation around the 3/1 RS is accelerated in co-Ip direction and the poloidal rotation is changed from electron to ion diamagnetic drift direction. Spontaneous unlocking-penetration circles occur after field penetration if the RMPs amplitude is not strong enough. While for sufficiently strong RMPs, the 2/1 locked mode is also triggered due to mode coupling, and the global density is increased. The field penetration threshold is found to be linearly proportional to the neL (line-integrated density) at the 3/1 RS but to (neL) 0.73 for the ne at plasma core. In addition, for rotating RMPs with a large 3/1 component, field penetration causes global increase in electron density.

Section: Methodsmentioning

confidence: 99%

Plasma response to m/n = 3/1 resonant magnetic perturbation at J-TEXT Tokamak

Wang

et al. 2016

“…The rotating m/n = 2/1 TMs are monitored by the routine poloidal and toroidal Mirnov arrays. The phase and amplitude of locked mode are measured by two sets of the locked mode detectors [44] . In which, each set contains two saddle loops installed at two opposite toroidal locations outside of the vacuum vessel.…”

Section: Methodsmentioning

confidence: 99%

First observation of unlocking the locked mode by electrode biasing on J-TEXT tokamak

Chen¹,

Wang²,

et al. 2020

Major disruptions, often caused by locked m/n = 2/1 modes (m/n is the poloidal/toroidal mode number), are great threat to a tokamak fusion reactor and should be mitigated or avoided. The locked 2/1 modes have been unlocked by electrode biasing (EB) for the first time in J-TEXT tokamak experiments. The application of a sufficiently negative EB voltage in plasma edge region quickly changes the phase and amplitude of the locked 2/1 mode and drives the mode to rotate in about 10–30 ms. For a larger EB current or a smaller locked mode amplitude, the mode is more easily to be unlocked, revealing a new method for mode unlocking and avoiding plasma disruptions.

“…Three sets of diagnostics are used to measure the TM and the plasma responses to RRMP fields, as shown in figure 1. Two pairs of locked mode detectors (LMDs) [44], measuring the radial magnetic field, b r , were installed at the low field side mid-plane (θ LMD = 0°) and outside the stainless-steel vacuum vessel with 16 mm thickness. Since the high frequency (>500 Hz) component of b r is completely screened, only the static or quasi-static component of b r from the plasma can be measured.…”

Section: Experimental Set-upmentioning

confidence: 99%

Plasma response to rotating resonant magnetic perturbations with a locked mode in the J-TEXT tokamak

Wang¹,

Ren²,

et al. 2019

Self Cite

Disruption caused by locked modes (LMs) is one of the most critical issues to be solved for tokamak fusion reactors. This paper aimed to understand the plasma response to external rotating resonant magnetic perturbations (RMPs), which are applied at a few kilohertz for controlling the pre-existing LMs. In the J-TEXT tokamak, the plasma response to a rotating RMP showed a feature of travelling wave, if no tearing mode (TM) existed. With a LM, the application of a rotating RMP led to unique features of plasma responses, which could be roughly described by the formula δb θ (θ, t) = 2Asin(mθmθ O)cos(2πf RRMP t), where δb θ , A, m, θ O and f RRMP are the perturbed poloidal magnetic field, the amplitude, the poloidal mode number, the poloidal location of the TM's O-point and the frequency of the rotating RMP. These features are analogous to that of a standing wave in the TM rest frame, with the nodes locating at the O-/X-points. The perturbations of electron temperature, T e , due to plasma responses were zero or minimal inside the magnetic island or outside the magnetic island but at the same poloidal positions as the O-/X-points. Both δb θ and the T e perturbations increased linearly with the rotating RMP. A phenomenological model is proposed that the rotating RMP drives the phase oscillation of the TM by applying a periodical electromagnetic torque. Then the phase oscillation of TM, combined with the poloidal gradient of the plasma parameters, induces the plasma response measured experimentally. The nonlinear numerically modelling confirmed the forced phase oscillation of the TM, and qualitatively verified the amplitude and phase dependences among the plasma response, the phase oscillation of the TM and the rotating RMP, which have been predicted by the model.