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.     

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.     

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).     

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.     

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.     

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.     

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 study of ELM-induced filament structures using the VUV imaging system on EAST

A high-speed vacuum ultraviolet(VUV) imaging system has been developed on the Experimental Advanced Superconducting Tokamak(EAST), which selectively measures line emission with a central wavelength of 13.5 nm(CVI, n=4–2). It has been employed to study edge/pedestal plasma behavior in EAST. Edge localized mode(ELM)-induced filament structures have been captured by the VUV imaging system during the ELMy high confinement mode discharge with both high temporal and spatial resolutions. The typical features(i.e.poloidal width and pitch angle) of the observed filaments are quantitatively characterized based on the VUV imaging data, and the dependence of these features on basic plasma parameters is analyzed. It is found that the poloidal width is proportional to the heating power, and the pitch angle is inversely proportional to the edge safety factor q.95 The scatterplot shows a positive trend between the poloidal width and the ELM amplitude defined by the relative change in stored energy. These results are based on the condition that the perturbation induced by ELMs is confined to a narrow layer in the plasma.     

Frequency multiplication with toroidal mode number of kink/fishbone modes on a static HL-2A-like tokamak

In the presence of energetic particles (EPs), the long-lived mode (LLM) frequency multiplication with n = 1, 2, 3, or higher is often observed on HL-2A, where n is the toroidal mode number. Hybrid kinetic-MHD model simulations of the energetic particle (EP) driven kink/fishbone modes on a static HL-2A-like tokamak using NIMROD code find that when the background plasma pressure is relatively high, and the EP pressure and the beam energy are relatively low, the mode frequency increases almost linearly with EP pressure, and the frequency is proportional to n ('frequency multiplication'), even in the absence of any equilibrium plasma rotation. In addition, the frequency multiplication persists as the safety factor at the magnetic axis q0 varies. In the absence of EPs, the growth rate of the 1/1 mode is the largest; however, as the EP pressure increases, the growth rate of 2/2 modes or 3/3 modes becomes dominant, suggesting that higher-n modes are more vulnerable to EPs. These results may shed light on the understanding of the toroidal mode number dependence of kink/fishbone modes in the advanced scenarios of tokamaks with weak or reversed central magnetic shear.     

RF calculation and thermal-stress analysis of the ECRH launcher on EAST tokamak

EAST is a medium sized superconducting tokamak with major radius R = 1.8 m, minor radius a = 0.45 m, plasma current I_p ≤ 1 MA, toroidal field B_T ≤ 3.5 T and expected plasma pulse length up to 1000 s. An electron cyclotron resonance heating (ECRH) launcher for four-beam injection is being installed on EAST tokamak. Four electron cyclotron wave beams which are generated from four sets of 140 GHz/1 MW/1000 s gyrotrons will be injected into the plasma by the spherical focusing mirrors and plane mobile mirrors. The focusing mirrors are spherical to focus Gaussian beams after reflection. Four plane mobile mirrors independently steer continuously in the poloidal and toroidal direction controlled by motors. With the suitable distance between mirrors and appropriate focal length of focusing mirror, the beam radius in the resonance layer of plasma is 31.145 mm. The heat from plasma radiation and metal losses is loaded on the mobile mirror. In order to decrease the temperature and thermal stress, the inner equivalent diameter of water channels is 8 mm and the suggested water velocity is 4 m/s.     

Experimental study of core and edge fluctuations by reflectometry on EAST tokamak

An eight-channel poloidal correlation reflectometer(PCR) with O-mode polarization has been installed in the EAST tokamak to measure the fluctuations from core to edge. The PCR launches eight different frequency microwaves(20.4, 24.8, 33, 40, 42.4, 48, 52.6, 57.2 GHz) into the plasma from the low field side and two poloidally separated antennae are used to receive the reflected waves. As a result, the diagnostic can measure fluctuations in eight(radial)× two(poloidal) spatial positions. The diagnostic has been applied to study the core and edge pedestal fluctuations during an inter-ELM phase in H-mode plasma. This inter-ELM phase can be divided into two stages. In the first stage, a low frequency(50 k Hz) broadband fluctuation dominates in the pedestal gradient region. In the second stage, this fluctuation is strongly suppressed and quasi-coherent fluctuations(QCFs) appear. The QCF's amplitude increases with the pedestal density gradient, implying density gradient driven instabilities. But the core fluctuations inside the pedestal show no evident changes during the inter-ELM phase.     

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.     

Influence of stationary driven helical current on the m=2/n=1 resistive tearing mode

The influence of stationary driven helical current on tearing mode instability in the m=2/n=1 rational surface is explored numerically using resistive magnetohydrodynamic simulation in cylindrical geometry. The results indicate that the flip instabilities result from the sustained injection of the sufficiently strong helical current driven in the island O-point. The driven helical current induces high order harmonics of instabilities due to the delay of suppressing timing and the increase of its current intensity. With the appropriate current density values, the development of the perturbed kinetic energy can be limited and the occurrence of the flip instabilities can be delayed for a long time. The radial deviation of the current deposition can lead to poor inhibition effect, and the effect of current bias on the boundary is greater than that on the axis.     

Numerical simulation of plasma response to externally applied resonant magnetic perturbation on the J-TEXT tokamak

Nonlinear magnetohydrodynamic (MHD) simulations of an equilibrium on the J-TEXT tokamak with applied resonant magnetic perturbations (RMPs) are performed with NIMROD (non-ideal MHD with rotation, open discussion). Numerical simulation of plasma response to RMPs has been developed to investigate magnetic topology, plasma density and rotation profile. The results indicate that the pure applied RMPs can stimulate 2/1 mode as well as 3/1 mode by the toroidal mode coupling, and finally change density profile by particle transport. At the same time, plasma rotation plays an important role during the entire evolution process.     

Experimental observation of reverse- sheared Alfvén eigenmodes (RSAEs) in ELMy H-mode plasma on the EAST tokamak

Reverse-sheared Alfvén eigenmodes (RSAEs) have been observed by using an interferometer and ECE diagnostics in NBI heated ELMy H-mode plasma on EAST tokamak. A typical feature of these modes is a fast frequency sweeping upward from ∼80 kHz to ∼110 kHz in hundred milliseconds during which the plasma temperature, density and rotation keeps no change. Only core channels of the interferometer can observe these modes, implying a core localized mode. The ECE measurement further showed that these modes located at about ρ=0.37–0.46, just around the position of q_min with ρ∼0.4. These core localized modes are very weak in the magnetic fluctuations measured by mirnov probes mounted at the machine vacuum vessel. A multiple frequency fluctuation component, seemingly the so-called ‘grand cascades’, was also clearly observed on the ECE signal at ρ=0.46. During the phase, a transient internal transport barrier (ITB) in ion temperature and toroidal rotation was observed and the ITB foot was just close to the position of q_min . A modulation of RSAE frequency by ELM event was observed and this modulation could be attributed to rotation decrease orq_min increase due to ELM. Further study of these modes in EAST can provide valuable constraints for the q profile measurement and will be important for the long pulse operation.     

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.     

Preliminary design and performance study of EAST liquid lithium limiter based on CPS

Lithium has the ability of H recycling suppression and impurities absorption and it can be used as plasma facing material (PFM) in tokamaks. Lithium conditioning experiments were launched on EAST, HT-7 and some other tokamaks for many years by using the methods of GDC, IRCF and evaporation. Liquid lithium has better performances in effective lifetime and heat removal aspects compared to non-liquid lithium. While, applying liquid lithium in the tokamak would cause the safety problem as the lithium can react with many substances violently and the magnetohydrodynamic behavior is difficult to be handled. EAST liquid lithium limiter (LLL) system is under developing and will be applied in EAST to study the main technologies of the liquid lithium application. The normal operation temperature of the limiter is expected as 230–550 °C under the active cooling of water. Capillary porous system (CPS) is used to prevent the lithium from splashing under large electromagnetic force by increasing the surface tension of the lithium. In order to investigate the cooling performance of the cooling design, the thermal-hydraulic analysis was done which shows that with 3 m/s flowing velocity, the water can keep the limiter under 550 °C all the time if the heat flux is lower than 0.7 MW/m². Under heat flux of 1 MW/m², the limiter should be retreated within 7 s to avoid erosion. The pressure drop of the coolant under 3 m/s is less than 40 kPa with temperature difference nearly 34 °C which meet the design requirements very well. The key manufacture process and technologies like vacuum bonding between the CuCrZr heat sink and 316L guide plate were well studied in the R&D process. The heating test on the test bench showed that the CPS can be heated efficiently by the heaters attached into the heat sink.     

The theoretical study on intermittency and propagation of geodesic acoustic mode in L-mode discharge near tokamak edge

Through a systematically developed theory, we demonstrate that the motion of Instanton identified in Zhang et al(2017 Phys. Plasmas 24 122304) is highly correlated to the intermittent excitation and propagation of geodesic acoustic mode(GAM) that is observed in tokamaks.While many numerical simulations have observed the phenomena, it is the first theory that reveals the physical mechanism behind GAM intermittent excitation and propagation. The preceding work is based on the micro-turbulence associated with toroidal ion temperature gradient mode, and slab-based phenomenological model of zonal flow. When full toroidal effect is introduced into the system, two branches of zonal flow emerge: the torus-modified low frequency zonal flow(TLFZF), and GAM, necessitating a unified exploration of GAM and TLFZF. Indeed, we observe that the transition from the Caviton to Instanton is triggered by a rapid zero-crossing of radial group velocity of drift wave and is found to be strongly correlated with the GAM onset. Many features peculiar to intermittent GAMs, observed in real machines,are thus identified in the numerical experiment. The results will be displayed in figures and in a movie; first for single central rational surface, and then with coupled multiple central rational surfaces. The periodic bursting first shown disappears as being replaced by irregular one, more similar to the intermittent characteristics observed in GAM experiments.     

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。