K. Shinohara

M. Ishikawa

M. Takechi

Y. Todo

Todo,  Y.

Shinohara,  K.

Takechi,  M.

Ishikawa,  M.

NIFS-Repository (National Institute for Fusion Science)

References[1] K. Shinohara et aI., NucI. Fusion 41,603 (2001).[2] Y. Todo and T. Sato, Phys. Plasmas 5, 1321 (1998).Fig. 1. Frequency and peak location of the EPM. Solidcurves are the n=1 local shear Alfven frequency and dashedcurve is the q-profile.o423 ..c560.60.50.48<{0.3'80.20.100 0.2 0.4 0.6 0.8ria·500 1000 1500 2000 2500wAtFig. 2. Time evolution of the frequency spectrum.Contour levels are 0.19, 0.39, and 0.59 of the maximumvalue.frequency. Frequency of the EPM is close to the local shearAlfven frequency.Frequency of the toroidal electric field is analyzed.Time evolution of the frequency spectrum is shown in Fig. 2.The contour levels are 0.19,039, and 0.59 of the maximumvalue. It is found that the frequency changes upwards by 8%(-4kHz) and downwards by 17% (-9kHz) of the frequencyin the linear growth phase in 3000 Alfven time (-2Ams).This frequency change is close to the experimental results.§ 2. Simulation of Fast Frequency SweepingMode in a JT-60U ExperimentInstabilities with frequency chirping in the frequencyrange of Alfven eigenmodes have been found with energeticneutral beam injection in JT-60U [1]. One type of instabilityappears with frequency inside the toroidal Alfven eigenmode(TAE) gap and the frequency changes rapidly by 10-20 kHzin 1-5 ms. This type of instability is called the fast frequencysweeping (FFS) mode. Since the plasma profile changes in amuch longer time scale, some nonlinear effects play animportant role in the frequency change of the FFS modeduring the interplay between the mode and the energetic ionscreated by the neutral beam injection.Using the experimental q-profile, bulk pressure profile,and density profile we have carried out a particle-magnetohydrodynamic (MHD) hybrid simulation [2]. In thissimulation the plasma is divided into to two parts, namely,energetic ions and bulk plasma. The bulk plasma and theelectromagnetic field are described by the MHD equations.This approximation is reasonable under the condition thatthe energetic ion density is much less than the bulk plasma.The drift-kinetic description is employed for the energeticions. To complete the equation system in a self-consistentway, we take account of the energetic ion effects on the bulkplasma in the MHD momentum equation. The (jj particlesimulation method is applied to the energetic ions. Themajor and minor radii are Ro=3.4m a=l.Om, respectively.The magnetic field at the magnetic axis is 1.2T. The bulkplasma and the beam ions are deuterium. Taking intoaccount the FFS burst interval is less than 10ms whereas theslowing down time is of the order of magnitude lOOms,energetic ion pressure is assumed to be 20% of the classicaldistribution. Energetic ion distribution in the velocity spaceis a slowing down distribution with maximum velocity thatis 80% of the injection velocity. This maximum velocitycorresponds to the Alfven velocity at the magnetic axis.Perpendicular velocity of energetic ions is neglected forsimplicity.Nonlinear evolution of the energetic ions and theMHD equation is followed until WAt = 3000 where WA isthe Alfven frequency. An energetic particle mode (EPM)with the primary poloidalltoroidal mode number m/n=2/l isdestabilized. Figure I shows frequency and peak location ofthe EPM with the q-profile and n=1 local shear AlfvenTodo, Y.Shinohara, K., Takechi, M., Ishikawa, M. (JAERI)323

Simulation of Fast Frequency Sweeping Mode in a JT-60U Experiment

Simulation of Fast Frequency Sweeping Mode in a JT-60U Experiment

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