A brief review: effects of resonant magnetic perturbation on classical and neoclassical tearing modes in tokamaks

This paper reviews the effects of resonant magnetic perturbation (RMP) on classical tearing modes (TMs) and neoclassical tearing modes (NTMs) from the theory, experimental discovery and numerical results with a focus on four major aspects: (i) mode mitigation, where the TM/NTM is totally suppressed or partly mitigated by the use of RMP; (ii) mode penetration, which means a linearly stable TM/NTM triggered by the externally applied RMP; (iii) mode locking, namely an existing rotating magnetic island braked and finally stopped by the RMP; (iv) mode unlocking, as the name suggests, it is the reverse of the mode locking process. The key mechanism and physical picture of above phenomena are revealed and summarized.     

Microchannel cooling technique for dissipating high heat flux on W/Cu flat-type mock-up for EAST divertor

As an important component of tokamaks, the divertor is mainly responsible for extracting heat and helium ash, and the targets of the divertor need to withstand high heat flux of 10 MW m⁻² for steady-state operation. In this study, we proposed a new strategy, using microchannel cooling technology to remove high heat load on the targets of the divertor. The results demonstrated that the microchannel-based W/Cu flat-type mock-up successfully withstood the thermal fatigue test of 1000 cycles at 10 MW m⁻² with cooling water of 26 l min⁻¹, 30 °C (inlet), 0.8 MPa (inlet), 15 s power on and 15 s dwell time; the maximum temperature on the heat-loaded surface (W surface) of the mock-up was 493 °C, which is much lower than the recrystallization temperature of W (1200 °C). Moreover, no occurrence of macrocrack and 'hot spot' at the W surface, as well as no detachment of W/Cu tiles were observed during the thermal fatigue testing. These results indicate that microchannel cooling technology is an efficient method for removing the heat load of the divertor at a low flow rate. The present study offers a promising solution to replace the monoblock design for the EAST divertor     

Experimental investigation of scrape-off layer blob high density transition in L-mode plasmas on EAST

Lithium Beam Emission Spectroscopy systems in the outer midplane and divertor Langmuir probe arrays embedded in the divertor target plates, are utilized to investigate the scrape-off layer (SOL) blob transition and its relation with divertor detachment on EAST. The blob transition in the near SOL is observed during the density ramp-up phase. When the plasma density, normalized to the Greenwald density limit, exceeds a threshold of fGW ∼ 0.5, the blob size and lifetime increases by 2 – 3 times, while the blob detection rate decreases by about 2 times. In addition, a weak density shoulder is observed in the near SOL region at the same density threshold. Further analysis indicates that the divertor detachment is highly correlated with the blob transition, and the density threshold of blob transition is consistent with that of the access to the outer divertor detachment. The potential physical mechanisms are discussed. These results could be useful for the understanding of plasma-wall interaction issues in future devices that will operate under a detached divertor and high density conditions (over the blob transition threshold).     

Design of improved compact decoupler based on adjustable capacitor for EAST-ICRF antenna

The Ion Cyclotron Radio Frequency (ICRF) heating antenna on EAST adopts a decoupling device to constrain power coupling among the radiation straps, which was discovered shortcomings such as long size, poor contact, and etc. In order to improve these weak points, a new type decoupler with terminal-loaded tunable capacitor is designed to replace the previous design. Besides the capability of the tunable admittance parameters of decoupler, the withstand voltage of the capacitor is the most significant consideration for working under high power. Therefore, the theoretical analysis carefully elaborates the capacitor withstand voltage, and the detailed analytical equations and criteria for design are given. After the comparative analysis of theoretical calculation and 3D simulation results, the decoupler design scheme is finalized. The capacitor-loaded decoupler has been successfully adopted for ICRF antenna at port N on EAST, and achieved the optimization of adjacent port isolation from −22 to −58 dB at 37 MHz without plasma to restrict mutual coupling. The new design of the decoupler has greatly improved its compactness and automatic adjustment performance, and could be good solution for the decoupling network of ICRF antennas.     

Effects of m=0 Harmonics on Quasi-Linear Stage of m=1 Double Tearing Mode

Effects of tearing modes are inves shows that the effect of the m = 0 harmonics on the early quasi-linear stage of m = 1 double tigated. The numerical calculation with the harmonics m = 0 included the m = 0 harmonics on the mode is negligible in the linear stage. As the mode begins to grow nonlinearly, both the current and flow profiles are pinched due to the m = 0 harmonics. To make a comparison we also carry out the calculation with m = 0 harmonics turned off. The profiles of the total current, poloidal magnetic field, and poloidal shear velocity in the cases with or without the m = 0 harmonics are compared and discussed. In addition, the formation of a poloidal velocity shear is found and its mechanism is investigated.     

发酵优化制备L-赖氨酸盐酸盐-13C6、15N2

采用北京棒杆菌（Corynebacterium pekinense）ZLD118（HS^-、AEC^r）发酵制备稳定同位素双标记的L-赖氨酸盐酸盐-¹³C₆、¹⁵N₂。对菌种的发酵培养条件和发酵培养基配方进行优化,采用优化后的发酵工艺制备高丰度稳定同位素¹³C、¹⁵N双标记L-赖氨酸,经离子交换树脂提纯、活性炭脱色精制后,产品的¹³C丰度达98.61%,¹⁵N丰度为98.15%,纯度为98.47%,产率为88.8%。结果表明,该工艺可进一步用于放大制备。     

Space-resolved vacuum-ultraviolet spectroscopy for measuring impurity emission from divertor region of EAST tokamak

The measurement of impurity distribution in the divertor region of tokamaks is key to studying edge impurity transport. Therefore, a space-resolved vacuum-ultraviolet (VUV) spectrometer is designed to measure impurity emission in the divertor region on EAST. For good spectral resolution, an eagle-type VUV spectrometer with 1 m long focal length with spherical holograph grating is used in the system. For light collection, a collimating mirror is installed between the EAST plasma and the VUV spectrometer to extend the observing range to cover the upper divertor region. Two types of detectors, i.e. a back-illuminated charge-coupled device detector and a photomultiplier-tube detector, are adopted for the spectral measurement and high-frequency intensity measurement for feedback control, respectively. The angle between the entrance and exit optical axis is fixed at 15°. The detector can be moved along the exit axis to maintain a good focusing position when the wavelength is scanned by rotating the grating. The profile of impurity emissions is projected through the space-resolved slit, which is set horizontally. The spectrometer is equipped with two gratings with 2400 grooves/mm and 2160 grooves/mm, respectively. The overall aberration of the system is reduced by accurate detector positioning. As a result, the total spectral broadening can be reduced to about 0.013 nm. The simulated performance of the system is found to satisfy the requirement of measurement of impurity emissions from the divertor area of the EAST tokamak.     

Nonlinear evolution and secondary island formation of the double tearing mode in a hybrid simulation

Double tearing modes(DTMs), induced by double current sheet configurations or two neighboring rational surfaces with the same safety factor in tokamaks, are widely observed in solar, space, and fusion plasmas. In this paper, the evolution of DTMs without a guide field is investigated numerically using a hybrid model(electron fluid+ion PIC). The overall evolution processes of DTMs are qualitatively consistent with previous works using other models. The particle dynamics during the evolution of DTMs is analyzed in detail. Behaviors of ions and electrons present different characteristics around the reconnection region which gives rise to Hall effects producing the out-of-plane quadrupole magnetic field. In the explosive reconnection process with interactions between two DTMs islands, the asymmetric drive and the thin current layer feature lead to the emergence of secondary magnetic islands which develop with the late evolution of the DTMs.     

Suppression of the m/n=2/1 tearing mode by electron cyclotron resonance heating on J-TEXT

Stabilization of tearing modes and neoclassical tearing modes is of great importance for tokamak operation.Electron cyclotron waves(ECWs)have been extensively used to stabilize the tearing modes with the virtue of highly localized power deposition.Complete suppression of the m/n=2/1 tearing mode(TM)by electron cyclotron resonance heating(ECRH)has been achieved successfully on the J-TEXT tokamak.The effects of ECW deposition location and power amplitude on the 2/1 TM suppression have been investigated.It is found that the suppression is more effective when the ECW power is deposited closer to the rational surface.As the ECW power increases to approximately 230 kW,the 2/1 TM can be completely suppressed.The island rotation frequency is increased when the island width is reduced.The experimental results show that the local heating inside the magnetic island and the resulting temperature perturbation increase at the O-point of the island play dominant roles in TM suppression.As the ECW power increases,the 2/1 island is suppressed to smaller island width,and the flow shear also plays a stabilizing effect on small magnetic islands.With the stabilizing contribution of heating and flow shear,the 2/1 TM can be completely suppressed.     

MHD Flow Layer Formation at Boundaries of Magnetic Islands in Tokamak Plasmas

Non-linear development of double tearing modes induced by electron viscosity is numerically simulated. MHD flow layers are demonstrated to merge in the development of the modes. The sheared flows are shown to lie just at the boundaries of the magnetic islands, and to have suffcient levels required for internal transport barrier （ITB） formation. Possible correlation between the layer formation and triggering of experimentally observed ITBs, preferentially formed in proximities of rational flux surfaces of low safety factors, is discussed.     

Application of local Monte Carlo method in neutronics calculation of EAST radial neutron camera

The Local Monte Carlo (LMC) method is used to solve the problems of deep penetration and long time in the neutronics calculation of the radial neutron camera (RNC) diagnostic system on the experimental advanced superconducting tokamak (EAST), and the radiation distribution of the RNC and the neutron flux at the detector positions of each channel are obtained. Compared with the results calculated by the global variance reduction method, it is shown that the LMC calculation is reliable within the reasonable error range. The calculation process of LMC is analyzed in detail, and the transport process of radiation particles is simulated in two steps. In the first step, an integrated neutronics model considering the complex window environment and a neutron source model based on EAST plasma shape are used to support the calculation. The particle information on the equivalent surface is analyzed to evaluate the rationality of settings of equivalent surface source and boundary. Based on the characteristic that only a local geometric model is needed in the second step, it is shown that the LMC is an advantageous calculation method for the nuclear shielding design of tokamak diagnostic systems.     

射频波电流驱动抑制双撕裂模不稳定性研究

采用不可压缩磁流体模型,在圆柱位形下研究了射频波电流驱动对双撕裂模不稳定性的影响。结果表明,托卡马克装置中沉积在有理面上的同向驱动电流能有效减缓3/1双撕裂模的发展。电流驱动幅值约占等离子体电流初始值的4%、驱动电流沉积宽度约为小环半径的7%时,抑制效果较好。此外,研究表明,在双撕裂模进入快速增长阶段前加入外部驱动电流,才能有效抑制撕裂模的磁爆发。     

Effects of resonant magnetic perturbation on locked mode of neoclassical tearing modes

The effects of externally applied resonant magnetic perturbation (RMP) on the locked mode of the neoclassical tearing mode (NTM) are numerically investigated by means of a set of reduced magnetohydrodynamic equations. It is found that, for a small bootstrap current fraction, three regimes, namely the slight suppression regime, the small locked island (SLI) regime and the big locked island (BLI) regime, are discovered with the increase of RMP strength. For a much higher bootstrap current fraction, however, a new oscillation regime appears instead of the SLI regime, although the other regimes still remain. The physical process in each regime is analyzed in detail based on the phase difference between the NTM and the RMP. Moreover, the critical values of the RMP in both SLI and BLI regimes are obtained, and their dependence on key plasma parameters is discussed as well.     

Effects of plasma radiation on the nonlinear evolution of neo-classical tearing modes in tokamak plasmas

The effects of plasma radiation on the nonlinear evolution of neo-classical tearing modes are investigated based on a set of reduced magnetohydrodynamic equations. It is found that the radiation can reduce the pressure near the rational surface. During the nonlinear evolution, the magnitude of perturbed bootstrap current is drastically enhanced in the presence of the radiation. Besides, the radiation can increase the growth rate of the magnetic islands by diminishing the pressure, such that the magnetic islands do not saturate compared with that without radiation. On the other hand, with the increase of the ratio of parallel to perpendicular transport coefficient ${chi }_{parallel }/{chi }_{perp },$ the reduction of pressure can further increase the growth rate of magnetic islands in the presence of plasma radiation. Finally, the mechanisms of the destabilizing effects driven by the radiation are discussed in detail as well.     

On theoretical research for nonlinear tearing mode

The analytical approaches for nonlinear tearing mode have been reviewed.It is shown that Rutherford's model has triggered numerous studies on the nonlinear tearing mode.Its physical picture is clear meanwhile its mathematical method is ingenious but still puzzling to understand.It is trying to find how the ‘nonlinear behavior' resulted from the linear equation by a nonlinear transform.It is indicated that Li's model for the tearing mode includes the linear growth,Rutherford's behavior and the new behavior.It was found that the quasilinear modification of magnetic field provided a new damping mechanism for nonlinear growth.The new behavior w~ t1/2 becomes dominant if the mode is weakly unstable.It is shown that many analytical methods have been developed to calculate the criterion parameter D' of the tearing mode.Li's instability criterion can cover the previous results in the limit cases.     

Suppression of Neoclassical Tearing Modes Towards Stationary High-Beta Plasmas in JT-60U

Results from stabilization of neoclassical tearing modes （NTMs） in JT-60U are described. NTM stabilization and confinement improvement have been demonstrated by employing a real-time NTM stabilization system, where the identification of the location of an NTM and the optimization of the injection angle of the electron cyclotron wave are performed in real time. Also, a high-beta plasma with the normalized beta of 3 has been sustained by suppressing NTM by applying the electron cyclotron current drive （ECCD） before the onset （＇preemptive ECCD＇）. In addition, a simulation code for analysis of the NTM evolution has been developed by combining the modified Rutherford equation with the transport code TOPICS. It is found that the simulation well reproduces the NTM behavior in JT-60U. The simulation also shows that the ECCD width is also important for NTM stabilization, and that the EC wave power for complete stabilization can be reduced by narrowing the ECCD profile.