| Ion cyclotron range of frequencies(ICRF)heating and current drive is one of the most important auxiliary heating methods to obtain stable and high parameter plasma in tokamaks.At present,ICRF antennas on tokamaks at home and abroad,including the EAST,are faced with the common problems of low coupling and high field-induced sheath during operation.With the upgrade of RF heating system and plasma parameter improvement,as well as the tokamaks like ITER and CFETR for a higher operating level in the future,the simple structural optimization for classical ICRF antennas can no longer meet the heating requirements of ICRF.TWA(Traveling Wave Array)antenna is regarded as the most promising solution to work out the problems.However,TWA antennas operating at tens of MHz ion cyclotron range of frequencies are rarely studied.In this paper,the design and experimental study of TWA antenna for tokamaks are carried out based on this frequency band.Firstly,the circuit and key structures of a TWA section are designed and optimized based on EAST operation conditions.Through the analysis and solution of antenna impedance circuit and the optimization of antenna design,a six-current strap antenna model with 35 MHz as the central frequency is obtained.Within the bandwidth of more than 3 MHz,the S11 of the input port is lower than-30 dB and the voltage standing wave ratio is lower than 1.2 in vacuum.Based on the calculation and analysis of the excitation power spectrum,the principle and method of adjusting current strap spacing and phase to achieve accurate selection of power spectrum peak are expounded.The optimal power feed and current strap spacing collocation can be obtained through the simulation.Low port reflection and high power injection in the designed model are the basis for avoiding common problems of low coupling and high field-induced sheath.Based on the preliminary design of the antenna model,the modeling and analysis of power coupling and wave fields of the antenna are carried out.By using the RAPLICASOL method,the load resilience of TWA antenna is preliminarily evaluated.It shows that the antenna can still maintain good power injection within a certain bandwidth with a cold plasma load.Then the influence of plasma density profile,Bo and tilted angle on power coupling and fields for TWA antenna is investigated.And the mechanism of wave propagation and resonance for slow wave is studied by solving the dispersion relation.The power dissipation is added to improve the model.Furthermore,through the simulation and analysis of wave field characteristics,the results of power coupling are better understood.The higher SOL density is more favorable to the RF performance and power coupling of the antenna.In the low density range,high magnetic field intensity and small tilted angle will cause the increase of FW cut-off density,which is not conducive to the coupling and power flow.While in the high density range,the change of magnetic field intensity and the tilted angle will cause the change of dielectric tensor elements,and affect the energy storage of wave power in the plasma region.In addition,the long-field line-sheath of the ICRF antenna for EAST is calculated and the structures are optimized in this paper.The electric field distribution on TWA antenna surface is evaluated by simulation,and the characteristics of electric field for antenna structures are described.The simulation results of coupling and wave fields also show the potential of TWA antenna to solve the problem of low coupling and high field-induced sheath.In addition to simulation and analysis,this paper also carried out the design and experimental study of TWA antenna prototype.In order to match the test conditions of the TITAN platform,the designed antenna is centered around 53 MHz,49-59 MHz is the optimized low reflection band and S11 is lower than-25 dB in this frequency range.The voltage and current at the feeding connection tap are relatively stable,and the power reflection is low.The evaluation of structure and assembly tolerance has been conducted before the experiment,and the results meet the design requirements.In the experiment,the full frequency scanning of the generator is carried out under the power of 200 kW,and the experimental results are consistent with the simulation.Considering the influence of high temperature deformation on antenna performance,the port response of the antenna after baking is tested,which is consistent with expectation.During the high power test of 2 MW/3 s,the forward injected power of the transmitter was good and no obvious reflection or high voltage arcing occurred.The whole experiment results not only show the excellent properties of high coupling and low field-induced sheath for TWA antenna and the great potential of future application,but also further verify the simulation method used in the design and analysis. |
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