| Keda Torus Experiment(KTX)is a ring magnetic confinement fusion device different from Tokamak and stellarator.As a revered-field pinch(RFP)device,the majority radius of KTX is 1.4 m and the small radius is 0.4 m.However,the current of KTX is too small to realize the highly constrained quasi-single helix state(QSH).In this paper,ideal MHD simulation and VMEC code are used to obtain three-dimensional helical equilibrium in the KTX configuration,which is used to study the MHD stability characteristics of QSH in RFPs,and theoretically simulated the possible factors affecting different state transitions.In chapter 1,the background of magnetic confinement fusion and its related devices are briefly introduced,and the equilibrium and magnetofluid instability characteristics under the reverse field pinch configuration are introduced as well as the related simulation results.In chapter 2,in the RFP,the magnetic field is generated mainly by internal currents passing through the self-organization mechanism.This property makes RFP plasmas prone to MHD instability,which destroys the toroidal symmetry of the magnetic field and generates an inverted toroidal magnetic field.In the RFP,there are three types of equilibrium states,namely single helicity(SH),multiple helicity(MH),and quasi single helicity(QSH).The appearance of QSH state reduces the magnetic field fluctuation and improves the constraint performance.In this chapter,the simulation method and input parameters are introduced,and the transformation between MH state and QSH state is realized by changing the safety factor profile,and the MHD instability in these two states is analyzed.In chapter 3,Mercier criterion is used to analyze the exchange mode stability under different safety factor profiles.It is concluded that in the theoretical simulation of KTX experiment may be easier to achieve the QSH state of NFP = 6,and the negative magnetic shear at the core is more conducive to the confinement of plasma.In this way,the possibility of different QSH states in KTX is proved theoretically,and then the characteristics of QSH under different q profiles are studied.In this chapter,we also study the influence of secondary mode on plasma confinement in the Boozer coordinate system.It is shown that increasing the secondary mode can reduce the confinement performance of plasma by increasing the polar drift.In the last chapter,different safety factor profiles in other RFP devices and the effect of current on the equilibrium transformation of the 3D helical structure are further studied.The results show that the increase of ohmic current can form the transformation of the ring structure to the helical structure,which is consistent with the experimental results.When the q profile is transformed from positive magnetic shear to negative magnetic shear,the magnetic structure is also transformed between the toroidal and helical structure,and there is a best condition of weak negative magnetic shear,which makes the three-dimensional equilibrium is the most stable under this condition.Therefore,the research of this part of work has a certain theoretical reference significance for the formation of better QSH state in the experiment.Finally,summarizing the full text and putting forward the work prospect. |
