No abstract

The effect of resonant magnetic perturbations (RMPs) on particle confinement is studied in J-TEXT tokamak by using externally applied rotating RMPs. It is found that RMPs cause improved (degraded) particle confinement when its frequency is higher (lower) than the natural m/n = 2/1 tearing mode frequency, and the amount of change in electron density is proportional to the difference between these two frequencies, where m and n are the poloidal and toroidal mode number, respectively. These results reveal the important role of the relative rotation between RMPs and the electron fluid in affecting the particle confinement. The experimental results are compared to numerical ones based on nonlinear two-fluid equations, and quantitative agreement is found. Resonant magnetic perturbations (RMPs) often exist, e.g., in solar flares, magnetotail and fusion devices, due to intrinsic plasma instabilities and have attracted much research efforts, since they are associated with magnetic reconnection. In fusion devices RMPs can also be generated by external coil current, which have important applications in fusion plasmas. Static RMPs are able to suppress or mitigate edge localized modes (ELMs) [1-5] and to affect other instabilities [6-12]. In addition, rotating RMPs were used to study the field penetration [13], their effect on plasma rotation [14] and plasma response on TEXTOR [15]. On DIII-D, rotating RMPs are utilized to control NTMs rotation [16] and detect the intrinsic error field [17].

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.

This study aimed to unlock the m/n = 2/1 locked mode (LM) performed in J-TEXT tokamak by using rotating resonant magnetic perturbations (RMPs), where m and n are the poloidal and toroidal mode numbers, respectively. In the experiments, to maintain the LM, mode locking occurs by using static RMPs generated by a set of saddle coils. After mode locking, another rotating RMP with frequency of several kilo-Hz is applied to drive the static LM to rotate. The unlocking of LM is realized by using rotating RMP with different frequency and amplitude. It is found that the unlocking process contains two stages, i.e. the oscillating stage and the unlocking stage. In the oscillating stage, the rotating RMP with amplitude that is not strong enough causes the LM to oscillate around its locked phase and produces magnetic fluctuation to behave as a standing wave-like structure in poloidal direction. When the amplitude of the rotating RMP is strong enough, it first causes the LM to oscillate and then transforms to mode unlocking quickly in less than 1 ms, namely the unlocking stage. Further analysis shows that the unlocking of LM is determined by the torque balance between the viscous torque and the electromagnetic torques exerted by both the static and the rotating RMP. In addition, the unlocking process is sensitive to both the amplitude and the frequency of the rotating RMP as well as the amplitude of static RMP. Nonlinear numerical modeling based on reduced MHD equations is also performed to understand the unlocking process, and numerical results qualitatively agree with the experimental ones.

The effect of resonant magnetic perturbations (RMPs) on high density limit and low-q limit discharges is studied in J-TEXT tokamak. It is found that a moderate amplitude of applied RMPs increases the density limit and delays the disruption, and the disruption precursor is suppressed with a slight reduction in toroidal plasma rotation. A too large amplitude of RMPs, however, leads to a lower density limit and an earlier disruption. For the optimal RMPs amplitude, the disruption is delayed by about 150 ms. For low-q discharges, the applied static RMP can either lower the limit of the edge safety factor q a from 2.15 to nearly 2.0 or sustain stable plasma discharge with q a = 2.1, and the disruptive precursor is also suppressed.