Study On Current Driven By Ohkawa Mechanism Of Electron Cyclotron Waves

Electron cyclotron current drive(ECCD) plays an important role in controlling plasma current profile and magnetohydrodynamic instabilities in tokamaks.The effect of electron trapping has an important influence on the Fisch-Boozer mechanism dominated ECCD.The normalized current drive efficiency of ECCD decreases rapidly with the increase of the radial normalized radius of tokamak plasma,which makes the current drive efficiency of ECCD very low in far off-axis radial region(?>0.6) of tokamak plasmas.Since the current drive mechanisms of electron cyclotron waves have been proposed till now,both theoretical and experimental studies believe that the Fisch-Boozer mechanism is the dominant mechanism,regarding the Ohkawa mechanism as the disadvantageous mechanism of ECCD,and the current driven by the Ohkawa mechanism contributes little to the total driven current of electron cyclotron waves.Therefore,there is a lack of comprehensive and in-depth study on noninductive current drive by the Ohkawa mechanism of electron cyclotron waves.On the basis of in-depth analysis of current drive via the Ohkawa mechanism of electron cyclotron(EC) waves,this thesis focuses on the Ohkawa mechanism dominated current drive(OKCD) in the far off-axis region of tokamak plasma by using coupled ray-tracing equations and bounce-averaged kinetic equation.The problem of effective local non-inductive OKCD has been solved theoretically.The current drive capability of two mechanisms of EC waves in the far off-axis region of tokamak with different inverse aspect ratios has been studied comparatively.A new synergy effect of the EC waves and low hybrid wave current drive has been discovered.Firstly,the research background and significance of this thesis are briefly introduced,the research progress of current drive mechanisms of EC waves and the current driven by combining two kinds of RF wave are summarized.In Chapter 2,the complex physical problems in the interaction of RF waves with plasmas are briefly introduced,and the coupled bounce-averaged kinetic equation and ray tracing equations are obtained by simplifing the problem of RF current drive.After this,the basic physics of EC waves and low hybrid wave are briefly introduced.Secondly,a localized and efficient current drive method in the outer half region of the tokamak with large inverse aspect ratio is proposed via the Ohkawa mechanism of EC waves.Far off-axis OKCD via EC waves was investigated in high electron beta ?_e HL-2M-like tokamaks with large inverse aspect ratio and in EAST-like tokamaks with low inverse aspect ratio.OKCD can be driven efficiently,and the driven current profile is localized spatially in the radial region,ranging from 0.62 to 0.85,where the large fraction of trapped electrons provides excellent advantage for OKCD,and the current drive efficiency increases with an increase in minor radius then drops beyond a certain point.The highest current drive efficiency can reach 0.18,the total driven current by OKCD can reach 20~32kA/MW in HL-2M-like tokamaks.The current drive is less efficient for the EAST-like scenario due to lower inverse aspect ratio.The results show that OKCD may be a valuable alternative current drive method in large inverse aspect ratio tokamaks and the potential capabilities of OKCD can be used to suppress some important MHD instabilities in the region far off-axis.Thirdly,a systematic comparative study between OKCD and ECCD is performed.The results show that OKCD has more advantages than ECCD for far off-axis localized current drive in tokamaks with large inverse aspect ratios,while ECCD is more efficient than OKCD in tokamaks with low inverse aspect ratios.The results of local current driving on the q=2 rational surface show that both OKCD and ECCD are effective for control of the m=2/n=1 tearing mode(TM) or neoclassical tearing mode(NTM).The results further confirm that OKCD can be a valuable alternative far off-axis localized current drive method to replace ECCD in large inverse aspect ratio tokamaks or in the radial position where the local inverse aspect ratio is large enough.Fourthly,a new synergy mechanism between OKCD of EC waves and lower hybrid current drive(LHCD)is discovered and discussed.And the methodology to achieve this synergy effect is also introduced.Improvement of OKCD efficiency can be achieved up to a factor of~2.5 in far off-axis radial region(?>0.6)of tokamak plasmas.Making EC wave heating the electrons with parallel velocity in the co-I_p direction and LH wave heating the electrons with parallel velocity in the counter-I_p direction,the mechanism of this new synergy effect comes from the results of electron trapping and detrapping processes.The OKCD makes the low speed barely passing electrons to be trapped(trapping process),the LHCD pulls some of the high speed barely trapped electrons out of the trapped region in velocity space(detrapping process)and accelerates the detrapped electrons to a higher speed.Fifthly,far off-axis OKCD simulation is performed by using TORAY-GA linear current drive code,and the results are compared with those calculated by CQL3D quasi-linear code.It is found that the radial location of OKCD profile is almost identical between the linear and the quasi-linear calculations.However,there are significant differences in the calculation of the total driven current Iec and the peak value of the driven current profile jecpeak between the two models.The simulation results shown that the TORAY-GA code can be used to calculate OKCD quickly and accurately by multiplying some appropriate scaling factors in practical engineering.Finally,we summarize the thesis and put forward the prospects of the work.