Study Of Properties Of Tearing Mode And Its Control In Advanced Tokamak Plasmas

Tearing mode(TM)instability is one kind of macroscopic magnetohydrodynamic(MHD)instabilities in tokamak plasmas,which is extremely dangerous.TM can break toroidally wellnested magnetic flux surfaces into helical magnetic islands,limit the particle and energy confinement,and even lead to major disruption during experimental discharges,which can badly damage the experimental devices and consequently cause great economic loss.Lack of bootstrap current due to the flattening effect of pressure profile inside the magnetic islands will lead to another more dangerous MHD instability called neo-classical tearing mode(NTM),which can further impair confinement and thus cause detrimental consequences.Reversed magnetic shear(RMS)configuration is believed to be one of the candidates for achieving steady-state operating scenario in advanced tokamaks.During RMS discharges,the confinement has been largely improved due to the formation of the internal transport barrier(ITB)so that the high core plasma pressure can be maintained.Despite all the advantages,RMS still has its own drawback.There exist two rational surfaces with the same safety factor in RMS configuration,which allows perturbations on both surfaces coupling to each other forming double tearing mode(DTM).The off-axis sawtooth triggered by fast magnetic reconnection as a result of DTM will lead to rapid collapse of temperature in the core region of tokamaks.Aiming to achieve steady-state confinement in tokamak devices,TMs/NTMs must be completely controlled.In experiments,electron cyclotron current drive(ECCD)has been adopted to successfully suppress TM/NTM magnetic islands.Nevertheless,no one can guarantee ECCD never fails.Once ECCD fails,the consequence is unbearable.Therefore,the ideal way to deal with instability problems is to predict the evolution tendency of plasma stability beforehand,rather than try to suppress instability after the mode is already unstable.For this purpose,the three-dimensional(3D)MHD spectroscopy has been developed to real time monitor multi-mode MHD stability through active detection.The feasibility and reliability of 3D MHD spectroscopy has been successfully validated in both DⅢ-D and KSTAR tokamak experiments.The research contents in this thesis include four main aspects,and they are properties of TM in advanced configuration,control of TM,control of multi-helicity TM and real time detection of TM.At first,one need to study properties of TM first before trying to control it.Then,the effectiveness and efficiency of basic control method should be studied to ensure the fundamental requirement.At last,investigation of real time detection of stability should be made in order to predict the stability and try to fundamentally solve the problem.The structure of this thesis and the main research contents are as follows.In Chapter 1,while looking back upon the relation between the total amount of energy consumption all over the world and the development of human society,the research background and meanings of magnetically confined nuclear fusion are briefly introduced.Next,the basic research method and theory have been introduced as well as research object.In Chapter 2,the MHD eigenvalue solver called MARS-F is introduced at first.Then,the influence of high plasma beta on TM instability in RMS configuration is systematically investigated using MARS-F.It is found that high plasma beta leads to multiple solutions and finite mode real frequency of TM eigenmode,and can strongly stabilize TM.In the large separation case,high plasma beta can transform TM into external kink mode,which can be stabilized by externally applied resistive wall.In Chapter 3,the MHD initial value code called MHD@Dalian Code is introduced at first.Then,the control of NTM in RMS configuration by ECCD is investigated using this code.It is found that the NTM and NTM triggered explosive burst can be effectively suppressed by ECCD under appropriate control conditions.In Chapter 4,the MHD@Dalian Code is also adopted to investigate the control of multihelicity NTMs by ECCD.It is found that the control effectiveness and efficiency of certain helicity NTM can be badly influenced by other helicity NTMs.Therefore,it is necessary to well design the control strategy based on real experimental environments.In Chapter 5,the 3D MHD spectroscopy is introduced at first.This method can be used for detecting low toroidal number multi-mode MHD stability(e.g.TM and kink instabilities)in experiments.Then,time domain method(TDM)of 3D MHD spectroscopy has been successfully applied into both DⅢ-D and KSTAR tokamaks to analyze plasma stability.TDM gives the temporal evolution of multiple MHD eigenmodes’ growth rates.It is found that the efficiency of TDM is sufficient for detecting MHD eigenmode in real time during experimental discharges.At last,the reliability of TDM is validated by MARS-F numerical simulation with experimental equilibrium.Moreover,the eigenmode structures of each mode are obtained.Last but not least,a summary of the research results in this thesis is given at first.Then,the innovation points of the research are listed.Furthermore,the prospects of future research have been made.