The Studies Of The Coupling Between The ICRF Antenna And The Plasma On EAST

Heating in the ion cyclotron range of frequencies(ICRF)is one of the important auxiliary heating methods in the magnetic confinement fusion devices.Using ICRF wave to heat ions and electrons in the plasma core efficiently has been demonstrated in EAST and other tokamaks.The ICRF antenna can operate in various heating and current drive phasing cases in 25-70MHz.A key issue which is focused on in this paper is the improvement of the coupling between the ICRF antenna and the plasma.A variational principle code which can calculate self-consistently currents on the antenna conductors has been developed to include the model of the ICRF antenna in EAST.Taking into account two layers of antenna conductors and the tilting angle of the Faraday rods refered to the toroidal direction,the wave equations in the vacuum region are derived in slab geometry.Numerical results involving the coupling resistance and power spectra have been obtained,for the parameters of the EAST machine and a deuterium-hydrogen plasma.The dependencies of the coupling resistance on various antenna and plasma parameters in different phasing configurations are analyzed and discussed combined with the power spectra of the antenna.The asymmetry in the toroidal power spectra with the heating phasing(0,?,0,?),(0,?,?,0)and(0,0,?,?)occurs due to the nonzero tilting angle of the Faraday rods.The coupling efficiency could be improved significantly by applying the power spectra with the lower dominant parallel wave number and minimizing the antenna-plasma gap.Increasing the plasma density as well as reducing the density gradient and the toroidal magnetic field |B| in the plasma edge is also efficient to enhance antenna coupling resistance.Besides,the antenna parameters including the strap length,strap width and gaps among the straps have influence on the coupling resistance.The numerical results involving the antenna parameters are beneficial for guiding the ICRF antenna design in the future.The coupling characteristics of the ICRF antenna in B port are compared with the antenna in I port.The toroidal power spectra of the 2-strap ICRF antenna in B port are wider than 4-strap antenna in I port;while the coupling resistance in B port is larger than I port in dipole phasing.The poloidal phase difference in B port is an extra parameter to optimize the poloidal power spectra and the coupling efficiency.The near field structures with different antenna phasing cases are investigated;the fields excited inside the plasma are found to disperse far more in the parallel than in the perpendicular direction with respect to B.The rf sheath generation mechanism connected to the parallel electric fields and the interaction between the plasma and the first wall enhanced by the rf sheath are reviewed.The experiments were carried out to study the coupling characteristics of the ICRF antenna in EAST.The dependencies of the coupling resistance on various parameters including the antenna position,the central electron density,gas injection and the antenna phasing have been studied.The results obtained show that the experimental data are consistent with theoretical simulations.We find that the antenna loading resistance decreases sharply at the L-H transition due to the change of the plasma density profiles in the scrape-off layer(SOL).The effect of the low hybrid wave(LHW)on the ICRF coupling during the H-mode is observed.The theoretical interpretation of the results is discussed,together with the efficient methods to optimize the coupling efficiency.