GTC Simulation of Ideal Ballooning Mode in Tokamak Plasmas

In the present paper,we first derive the eigenmode equation of the ideal ballooning mode in tokamak plasmas using a gyrokinetic equation.It is shown that the gyrokinetic eigenmode equation can be reduced to the magnetohydrodynamic(MHD) form in the long wavelength limit when kinetic effects are ignored.Then,the global gyrokinetic toroidal code(GTC) is applied for simulations of the edge-localized ideal ballooning modes.The obtained mode structures are compared with the results of ideal MHD simulations.The observed scaling of the linear growth rate with the toroidal mode number is consistent with the ideal MHD theory.The simulation results verify the GTC capability of simulating MHD processes in toroidal plasmas.     

Stability analysis of Alfvén eigenmodes in China Fusion Engineering Test Reactor fully non-inductive and hybrid mode scenarios

In this paper, NOVA/NOVA-K codes are used to investigate the stability of Alfvén eigenmodes(AEs) in the China Fusion Engineering Test Reactor(CFETR). Firstly, the stability of AEs excited by energetic alpha particles is investigated. For the fully non-inductive scenario, it is found that all AEs are stable, and the least stable toroidal mode number is n= 8. However, for the hybrid mode scenario, it is found that many AEs are unstable, and the least stable toroidal mode numbers are n= 7, 8. Secondly, the effect of energetic alpha-particle parameters and beam ions on AE stability is also presented. The threshold of the least stable AE is about β_(crit,α) = 1.12%,crit,less than the value of alpha-particle beta(β_α=1.34%). The result demonstrates that the AEs excited by alpha particles are weakly unstable. The effect of the beam ions on AE stability is found to be very weak in CFETR.     

Locked Mode Instability in HL-1M Tokamak

Locked modes are observed in many Tokamaks. The locked mode is typically a purely growing, non-rotating, non-axisymmetric magnetic field perturbation with poloidal mode number m=2 or 3 and toroidal mode number n=1. Major disruption often appears after mode locking. Locked mode is often triggered by error field, which is resulting from slight irregularities and shape variation of poloidal or toroidal field coils, magnetic material or eddy current in the vacuum vessel. It may be produced due to the change of     

Vertical Instability in EAST： Comparison of Model Predictions with Experimental Results

Growth rates of the axisymmetric mode in elongated plasmas in the experimental advanced superconducting tokamak （EAST） are measured with zero feedback gains and then compared with numerically calculated growth rates for the reconstructed shapes. The comparison is made after loss of vertical position control. The open-loop growth rates were scanned with the number of vessel eigenmodes, which up to 20 is enough to make the growth rates settled. The agreement between the growth rates measured experimentally and the growth rates determined numerically is good. The results show that a linear RZIP model is essentially good enough for the vertical position feedback control.     

Electron population properties with different energies in a helicon plasma source

The characteristics of electrons play a dominant role in determining the ionization and acceleration processes of plasmas. Compared with electrostatic diagnostics, the optical method is independent of the radio frequency(RF) noise, magnetic field, and electric field. In this paper, an optical emission spectroscope was used to determine the plasma emission spectra, electron excitation energy population distributions(EEEPDs), growth rates of low-energy and highenergy electrons, and their intensity jumps with input powers. The 56 emission lines with the highest signal-to-noise ratio and their corresponding electron excitation energy were used for the translation of the spectrum into EEEPD. One discrete EEEPD has two clear different regions,namely the low-energy electron excitation region(neutral lines with threshold energy of13–15 eV) and the high-energy electron excitation region(ionic lines with threshold energy?19 e V). The EEEPD variations with different diameters of discharge tubes(20 mm, 40 mm,and 60 mm) and different input RF powers(200–1800 W) were investigated. By normalized intensity comparison of the ionic and neutral lines, the growth rate of the ionic population was higher than the neutral one, especially when the tube diameter was less than 40 mm and the input power was higher than 1000 W. Moreover, we found that the intensities of low-energy electrons and high-energy electrons jump at different input powers from inductively coupled(H) mode to helicon(W) mode; therefore, the determination of W mode needs to be carefully considered.     

Suppression and mitigation of inter-ELM high-frequency Alfvén-like mode by resonant magnetic perturbation in EAST

Suppression and mitigation of a high-frequency Alfvén-like mode(HFAM) between type-I edge localized modes(ELMs) during ELM mitigation by resonant magnetic perturbation(RMP) is observed for the first time in the EAST tokamak. This mode is located near the edge pedestal region. The modeling result of the Alfvén continuum shows that the HFAM is located near the elipical Alfvén eigenmode(EAE) gap. During the application of n=1 RMP for ELM mitigation, the HFAM can be fully suppressed when the RMP amplitude exceeds a threshold,below which the HFAM is mitigated. The suppression is caused by a reduction of pedestal height induced by RMP. In the case using n=3 RMP, the mode is localized toroidally at specific phase depending on the phase of applied RMP, i.e. locked in the three-dimensional equilibrium formed by RMP. The dominant toroidal mode number of HFAM is around n=-6 and it reduces to -3 during the application of n=3 RMP, which indicates the existence of possible nonlinear coupling between the HFAM and RMP. Here the negative mode number denotes that the mode rotates in electron diamagnetic drift direction. The observation reported here improves the understanding of pedestal dynamics and its stability in RMP ELM control.     

Modulational instability of the coupled waves between fast magnetosonic wave and slow Alfvén wave in the laser–plasma interaction

By performing modulational instability analysis of the the nonlinear coupled dimensionless equations between a fast magnetosonic wave(FMSW) propagating obliquely with the magneticfield and a low-frequency slow Alfvén wave(SAW), we obtain the dispersion relation of the perturbation wave. The growth rate of the perturbation wave is obtained. It is found that the growth rate increases as background magnetic field increases, which is in agreement with that reported by Tiwary et al(2016 Phys. Plasmas 23 122307). A critical perturbation wave number is found. When the perturbation wave number is greater than or equal to the critical value, the growth rate is positive and it increases as the perturbation wave number increases, while the wave is stable. The maximum growth rate is reached when the frequency of the FMSW is half of the ion cyclotron frequency. The minimum growth rate is reached when the propagation direction of the perturbation wave is the same as that of the FMSW.     

Kinetic micro-instabilities in the presence of impurities in toroidal magnetized plasmas

The theoretical and numerical studies on kinetic micro-instabilities,including ion temperature gradient(ITG) driven modes,trapped electron modes(TEMs) in the presence of impurity ions as well as impurity modes(IMs),induced by impurity density gradient alone,in toroidal magnetized plasmas,such as tokamak and reversed-field pinch(RFP) are reviewed briefly.The basic theory for IMs,the electrostatic instabilities in tokamak and RFP plasmas are discussed.The observations of hybrid and coexistence of the instabilities are categorized systematically.The effects of impurity ions on electromagnetic instabilities such as ITG modes,the kinetic ballooning modes(KBMs) and kinetic shear Alfvén modes induced by impurity ions in tokamak plasmas of finite β(=plasma pressure/magnetic pressure) are analyzed.The interesting topics for future investigation are suggested.     

Effect of ideal internal MHD instabilities on NBI fast ion redistribution in ITER 15 MA scenario

Transport of fast ions is a crucial issue during the operation of ITER. Redistribution of neutral beam injection(NBI) fast ions by the ideal internal magnetohydrodynamic(MHD) instabilities in ITER is studied utilizing the guiding-center code ORBIT(White R B and Chance M S 1984Phys. Fluids 27 2455). Effects of the perturbation amplitude A of the internal kink, the perturbation frequency f of the fishbone instability, and the toroidal mode number n of the internal kink are investigated, respective...     

Magnetic-island-induced ion temperature gradient mode: Landau damping,equilibrium magnetic shear and pressure flattening effects

Characteristics of the magnetic-island-induced ion temperature gradient (MITG) mode are studied through gyrofluid simulations in the slab geometry,focusing on the effects of Landau damping,equilibrium magnetic shear (EMS),and pressure flattening.It is shown that the magnetic island may enhance the Landau damping of the system by inducing the radial magnetic field.Moreover,the radial eigenmode numbers of most MITG poloidal harmonics are increased by the magnetic island so that the MITG mode is destabilized in the low EMS regime.In addition,the pressure profile flattening effect inside a magnetic island hardly affects the growth of the whole MITG mode,while it has different local effects near the O-point and the X-point regions.In comparison with the non-zero-order perturbations,only the quasi-linear flattening effect due to the zonal pressure is the effective component to impact the growth rate of the mode.     

The Effect of High-Pressure Arc Discharge Plasma on the Degradation of Chlorpyrifos

A study is conducted to determine the effect of a kind of high-pressure arc discharge plasma on the degradation rate and kinetic equations of chlorpyrifos in different solvents with the treated times and concentrations as variables. The degradation rate was sorted in different solvents as water, methanol, acetone and then acetoacetate. The tendencies of the degradation rates with treated time in water and methanol were optimally fitted with first-order kinetics equations while those in acetone and acetoacetate were fitted with zeroth-order kinetics equations. The difference was attributed to the stronger polarity of water and methanol. The weak correlation of the degradation rates with time was mainly because the high-temperature of the arc discharge tube and the chemically-active species generated by the discharge. The degradation half-life was extended with increase of chlorpyrifos concentration. A degradation half-life less than 3 min was achieved for chlorpyrifos in water and methanol when the initial concentration was less than 300 μg/ml.     

Influence of toroidal rotation on the tearing mode in tokamak plasmas

The stabilizing mechanism of toroidal rotation on the tearing mode is studied using the 3 D toroidal resistive magnetohydrodynamic code M3 D. It is found that the dominating mechanism,either the centrifugal effect or the Coriolis effect, depends on the specific pressure β and rotation frequency Ω. On the premise that Ω is sufficiently large, when β is greater than a critical value,the effect of the centrifugal force is dominant, and the stabilizing effect mainly comes from the modification of equilibrium induced by the centrifugal force; when β is less than a critical value,the stabilizing effect from the Coriolis force overcomes that from the centrifugal force. However,if Ω is small, then the effect of equilibrium modification due to the centrifugal force is not significant even if β is large. Finally, the results showed that toroidal rotation shear enhances the stabilizing effect.     

Study of Kinetic Shear Alfvén Modes Driven by Ion Temperature Gradient in Tokamak Plasmas

Discrete kinetic shear Alfv(?)n modes driven by ion temperature gradient (AITG) is investigated in the full gyrokinetic limit. It is shown that AITG instability may occur when the plasma pressure gradient is well below the threshold value for ideal magnetohydrodynamics (MHD) ballooning instabilities, and that the critical magnetic shear required to completely stabilize the former is significantly higher than that for the latter. In addition, finite-β(=plasma pressure/magnetic pressure) effects on toroidal electrostaticion temperature gradient (ITG) modes are     

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.     

Implementation of field-aligned coordinates in a semi-Lagrangian gyrokinetic code for tokamak turbulence simulation

Field-aligned coordinates have been implemented in the gyrokinetic semi-Lagrangian code NLT,Ye et al (2016 J.Comput.Phys.316 180),to improve the computational efficiency for the numerical simulations of tokamak turbulence and transport.4D B-spline interpolation in fieldaligned coordinates is applied to solve the gyrokinetic Vlasov equation.A fast iterative algorithm is proposed for efficiently solving the quasi-neutrality equation.A pseudo transform method is used for the numerical integration of the gyro-average operator for perturbations with a high toroidal mode number.The new method is shown to result in an improved code performance for reaching a given accuracy.Some numerical tests are presented to illustrate the new methods.     

Investigation of E–H mode transition in magnetic-pole-enhanced inductively coupled neon–argon mixture plasma

In this paper, E–H mode transition in magnetic-pole-enhanced inductively coupled neon–argon mixture plasma is investigated in terms of fundamental plasma parameters as a function of argon fraction(0%–100%), operating pressure(1 Pa, 5 Pa, 10 Pa and 50 Pa), and radio frequency(RF) power(5–100 W). An RF compensated Langmuir probe and optical emission spectroscopy are used for the diagnostics of the plasma under study. Owing to the lower ionization potential and higher collision cross-section of argon, when its fraction in the discharge is increased, the mode transition occurs at lower RF power; i.e. for 0% argon and1 Pa pressure, the threshold power of the E–H mode transition is 65 W, which reduces to 20 W when the argon fraction is increased. The electron density increases with the argon fraction at afixed pressure, whereas the temperature decreases with the argon fraction. The relaxation length of the low-energy electrons increases, and decreases for high-energy electrons with argon fraction, due to the Ramseur effect. However, the relaxation length of both groups of electrons decreases with pressure due to reduction in the mean free path. The electron energy probability function(EEPF) profiles are non-Maxwellian in E-mode, attributable to the nonlocal electron kinetics in this mode; however, they evolve to Maxwellian distribution when the discharge transforms to H-mode due to lower electron temperature and higher electron density in H-mode. The tail of the measured EEPFs is found to deplete in both E-and H-modes when the argon fraction in the discharge is increased, because argon has a much lower excitation potential(11.5 eV) than neon(16.6 eV).     

Observations of mode frequency increase and the appearance of ITB during the m/n = 1/1 kink mode in EAST high electron temperature long pulse operation

A new long-pulse high electron temperature(T_e) regime has been achieved on experimental advanced superconducting tokamak by pure radio frequency heating. In this new scenario, there are mainly two confinement states involving two magneto-hydrodynamic(MHD) modes, one of which is identified as m/n = 1/1 kink mode(where m and n are the poloidal and toroidal mode numbers, respectively). The frequency evolution of the kink mode is investigated through the three-dimensional, toroidal, and n...     

Investigation of the Parallel Flow in the Edge Plasma of EAST

The effects of the E × B drift and ballooning-like transport on the edge plasma in EAST tokamak are investigated with a simplified fluid model. The simulation results show that the E × B drift and ballooning-like transport affect the plasma behavior. When the toroidal field is reversed, at the low field side the density is much larger and temperatures （both electron and ion） are lower, and the profiles of the density and temperatures become more symmetric. With the ballooning-like transport considered, the spatial ballooning-like distribution at the low field side is not very important, but the magnitude affects the ratios of the inner/outer particle flux and energy as well as the E × B drift. At the top of the scrape-off layer, the Mach number exceeding 0.3 for the normal toroidal field and approaching 0.2 for the reversed toroidal field in the simulation are obtained when the drift and ballooning-like transport are included in the model.     

Study of Plasma Instabilities and Effects of Velocity Profile in Internal Transport Barrier

The effects of plasma velocity profiles, that are not monotonous but with maximum/minimum, on plasma instability and transport in an internal transport barrier (ITB) are studied. Ion temperature gradient (ITG) instability is investigated. Results from the gyro-kinetic theory are presented. It is found that the mode structures and stability prop     

Flow Shear Stabilization of Ion Temperature Gradient Modes in an Internal Transport Barrier

Ion temperature gradient(ITG) driven instability is studied with gyro-kinetic theory in an internal transport barrier(ITB) of tokamak plasmas.The stabilization effects of a parallel velocity shear on the modes are investigated.It is found that the mode structures and stability properties,as well as the effects of a velocity shear,in an ITB are significantly different from that in off-ITB regions.