Observation of Doppler shift f_(D) modulated by the internal kink mode using conventional reflectometry in the EAST tokamak

In this paper we present a new experimental observation using a conventional reflectometry technique,poloidal correlation reflectometry(PCR),in the Experimental Advanced Superconducting Tokamak(EAST).The turbulence spectrum detected by the PCR system exhibits an asymmetry and induced Doppler shift f_(D)during the internal kink mode(IKM)rotation phase.This Doppler shift f_(D)is the target measurement of Doppler reflectometry,but captured by conventional reflectometry.Results show that the Doppler shift f_(D)is modulated by the periodic changes in the effective angle between the probing wave and cutoff layer normal,but not by plasma turbulence.The fishbone mode and saturated long-lived mode are typical IKMs,and this modulation phenomenon is observed in both cases.Moreover,the value of the Doppler shift f_(D)is positively correlated with the amplitude of the IKM,even when the latter is small.However,the positive and negative frequency components of the Doppler shift f_(D)can be asymmetric,which is related to the plasma configuration.A simulated analysis is performed by ray tracing to verify these observations.These results establish a clear link between f_(D)and IKM rotation,and are helpful for studying the characteristics of IKM and related physical phenomena.

Energetic-ion excited internal kink modes with weak magnetic shear in q0> 1 tokamak plasmas

The energetic particle driven internal kink mode is investigated in this paper for q0〉 1 tokamak plasma with weak magnetic shear. With the effect of energetic particles, the m/n = 1/1 internal mode structure in tokamak plasma does not appear as a rigid step-function when safety factor passes through q = 1 rational surface. It is found that even when the rational surface is removed, the mode may be still unstable under the low magnetic shear condition if the energetic particle drive is strong enough; with the low shear region of safety factor profile widening, the mode becomes more unstable with its growth-rate increasing. Furthermore, we find that the existence of the q = 1 rational surface does not have a significant effect on the stability of the plasma if energetic particles are present, which is very different from the scenarios of the ideal-MHD modes.

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 q_(0)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.

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 272455).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,respectively,in this work.The n=1 internal kink mode can cause NBI fast ions transporting in real space from regions of 0<s≤0.32 to 0.32<s≤0.53,where s labels the normalized plasma radial coordinate.The transport of fast ions is greater as the perturbation amplitude increases.The maximum relative change of the number of fast ions approaches 5%when the perturbation amplitude rises to 500 G.A strong transport is generated between the regions of 0<s≤0.05 and 0.05<s≤0.12 in the presence of the fishbone instability.Higher frequency results in greater transport,and the number of fast ions in 0<s≤0.05 is reduced by 30%at the fishbone frequency of 100 k Hz.Perturbations with higher n will lead to the excursion of fast ion transport regions outward along the radial direction.The loss of fast ions,however,is not affected by the internal MHD perturbation.Strong transport from 0<s≤0.05 to 0.05<s≤0.12 does not influence the plasma heating power of ITER,since the NBI fast ions are still located in the plasma core.On the other hand,the influence of fast ion transport from 0<s≤0.32 to 0.32<s≤0.53 needs further study.

Transition from fishbone mode toβ-induced Alfvén eigenmode on HL-2A tokamak

In the presence of energetic particles(EPs)from auxiliary heating and burning plasmas,fishbone instability and Alfvén modes can be excited and their transition can take place in certain overlapping regimes.Using the hybrid kinetic-magnetohydrodynamic model in the NIMROD code,we have identified such a transition between the fishbone instability and theβ-induced Alfvén eigenmode(BAE)for the NBI heated plasmas on HL-2 A.When the safety factor at magnetic axis is well below one,typical kink-fishbone transition occurs as the EP fraction increases.When q0 is raised to approaching one,the fishbone mode is replaced with BAE for sufficient amount of EPs.When q0 is slightly above one,the toroidicity-induced Alfvén eigenmode dominates at lower EP pressure,whereas BAE dominates at higher EP pressure.

Multichannel Microwave Interferometer for Simultaneous Measurement of Electron Density and its Fluctuation on HL-2A Tokamak

A nmltichannel microwave interferometer system has been developed on the HL- 2A tokomak. Its working frequency is well designed to avoid the fringe jump effect. Taking the structure of HL-2A into account, its antennas are installed in the horizontal direction, i.e. one launcher in high field side （HFS） and four receivers in low field side （LFS）. The fan-shaped measurement area covers those regions where the magnetohydrodynamics （MHD） instabilities are active. The heterodyne technique contributes to its high temporal resolution （1 μs）. It is possible for the multichannel system to realize simultaneous measurements of density and its fluctuation. The quadrature phase detection based on the zero-crossing method is introduced to density measureinent. With this system, reliable line-averaged densities and density profiles are obtained. The location of the saturated internal kink mode can be figured out from the mode showing different intensities on four channels, and the result agrees well with that measured by electron cyclotron emission imaging （ECEI）.

Global and Kinetic MHD Simulation by the Gpic-MHD Code

In order to implement large-scale and high-beta tokamak simulation, a new algorithm of the electromagnetic gyrokinetic PIC （particle-in-cell） code was proposed and installed on the Gpic-MHD code [Gyrokinetic PIC code for magnetohydrodynamic （MHD） simulation]. In the new algorithm, the vorticity equation and the generalized Ohm＇s law along the magnetic field are derived from the basic equations of the gyrokinetic Vlasov, Poisson, and Ampere system and are used to describe the spatio-temporal evolution of the field quantities of the electrostatic potential φ and the longitudinal component of the vector potential Az. The basic algorithm is equivalent to solving the reduced-MHD-type equations with kinetic corrections, in which MHD physics related to Alfven modes are well described. The estimation of perturbed electron pressure from particle dynamics is dominant, while the effects of other moments are negligible. Another advantage of the algorithm is that the longitudinal induced electric field, ETz = -δAz/δt, is explicitly estimated by the generalized Ohm＇s law and used in the equations of motion. Furthermore, the particle velocities along the magnetic field are used （vz-formulation） instead of generalized momentums （pz-formulation）, hence there is no problem of ＇cancellation＇, which would otherwise appear when Az is estimated from the Ampere＇s law in the pz-formulation. The successful simulation of the collisionless internal kink mode by the new Gpic-MHD with realistic values of the large-scale and high-beta tokamaks revealed the usefulness of the new algorithm.

The theory of early nonlinear stage of m=1 instability with locally flattened q-profile

In this paper,we analyze the modification of fast particles on the nonlinear radial displacement of m=1 internal kink mode with a shoulderlike equilibrium current theoretically.Using the matching method on the solutions of the outer and inner regions,we derive the analytical form of nonlinear radial displacement in the limit of q'=q"=0,which is valid to the cases of weak shear due to a slight flattening of the q(r)profile around q=1.We have taken into consideration the effects of the circulating and trapped fast particles on the nonlinear state of the mode.It is found that a fast particle can modify the nonlinear saturation level by the change of potential energy,depending on the fast particle properties.By the matching of linear dispersion relation to early nonlinear result,we also obtain the relations of radial displacement to the mode frequency and linear growth rate,and discuss the scaling for different stabilities of the MHD modes.