Numerical Study On The Stabilization Performance Of CFETR Blanket To The Plasma Vertical Instability

The target of CFETR(China Fusion Engineering Test Reactor)is to fulfill self-sufficient deuterium-tritium nuclear fusion,and the fusion power is up to 1 GW.Thus,the elongated plasma configuration is adopted to achieve high operating parameters,the blanket is used for tritium self-sufficiency,energy exchange and other functions.The elongated plasma is intrinsically vertically unstable,while the blanket structures occupy a lot of place inside the vacuum vessel,which increases the distance between the control coils and the plasma,thereby reducing the control coils'controllability of the plasma vertical instability.As a result,it is necessary to analyze the influences of blanket on the control of plasma vertical instability.The influences of CFETR blanket on the stabilization performance of plasma vertical instability are analyzed based on the rigid linear model in TokSys.In the analysis of breakdown,firstly,3 points at high field side,center of vacuum vessel and low field side are set as the breakdown centers,then the initial magnetization optimizations are performed.The optimization results show that when the current limit of the control coil is 55 kA per turn,the radius of the null-field(poloidal magnetic field less than 20 Gs)area is around 2 m,and the maximum unidirectional volt-seconds that the poloidal field coils of CFETR can provide is 231 Vs(for the whole discharge process,it is about 462 Vs).Taking the initial magnetizations as the initial conditions,the breakdown trajectories are optimized,and volt-seconds consumptions are calculated under the conditions that the slew rate limit of coil current is 20 kA/s per turn,the voltage limit of power supply is 10 kV and the equivalent resistivity of the blanket structures is 10res(10 times the resistivity calculated based on the material compositions of the blanket structures).The results show that the breakdown closer to the high field side consumes fewer volt-seconds.Then,setting the center of vacuum vessel as the breakdown center,the influences of different equivalent resistivities of the blanket structures on breakdown are evaluated,which show that the higher the equivalent resistivity of the blanket structures,the easier the breakdown to accomplish.Then,the stability parameter,the stability margin and the growth rate of plasma vertical displacement are used to assess the influences of different equivalent resistivities of the blanket structures on the passive controllability of CFETR plasma vertical instability.For all the cases,the calculated values of stability parameter are larger than 1,of stability margin are larger than 0.These results imply that with the help of passive control from those passive conductors,the characteristic time of growth of CFETR plasma vertical instability is larger than the response time of the feedback control system,thus feedback control can be used to control the plasma vertical instability.On the other hand,the calculated value of the growth rate of plasma vertical displacement increases as the equivalent resistivity of the blanket structures increases,while reducing the equivalent resistivities of blanket modules No.2 and No.5 can distinctly reduce the growth rate.At last,the feedback controllability of CFETR plasma vertical instability is evaluated.Firstly,the impacts of different equivalent resistivities of the blanket structures on the feedback controllability are estimated based on the feedback capability parameter.The computed value of the feedback capability parameter increases as the equivalent resistivity increases.When reducing the equivalent resistivities of blanket modules No.2 and No.5,the feedback capability parameter becomes smaller.These mean that the feedback controllability weakens as the equivalent resistivity increases,while the feedback controllability of CFETR can be improved by reducing the equivalent resistivities of blanket modules No.2 and No.5.These suggest that for the designs of CFETR,for the circuits formed in the toroidal direction through the "blanket-supporting structure-vacuum vessel" method,there should be appropriately more contact positions to reduce the overall equivalent resistivity of the blanket structures,and the contact positions should be distributed as much as possible in the positions of the blanket modules No.2 and No.5 so that the equivalent resistivities of these two positions can be reduced.Then,the calculation formula of the maximum controllable displacement is derived based on the TokSys rigid linear model and the analysis method used by D.A.Humphreys.In the cases of that the equivalent resistivities of the blanket modules No.2 and No.5 are 5res,the other modules are 1 Ores,the limit of control coil's power supply voltage is 10 kV,and the postpone time of the feedback control system is 1 ms,the maximum controllable displacements based on the TokSys model and D.A.Humphreys's analysis result are calculated and compared.Since the maximum controllable displacement based on the TokSys model takes into account the influences of the damped currents in the resistive conductors,its calculated value is around twice of that based on D.A.Humphreys's analysis result.Besides,the calculated value of the maximum controllable displacement also shows that the plasma vertical instability of CFETR can not be controlled only by the poloidal field coils outside the vacuum vessel,the in-vessel coils are needed for the feedback control.Then,to maximize the feedback controllability,the locations of the in-vessel coils are optimized by setting the mutual inductances as the optimization targets.The optimization result shows that the best locations for the in-vessel coils are the innermost and topmost(or lowermost)points(i.e.Rici=11.38 m?ZIC1=4.1 m,RIC2=11.38 m?ZIC2=-4.1 m)of the region that the in-vessel coils can be placed.After the optimization,validation is conducted by placing the in-vessel coils at 8 different locations,which shows the correctness of the optimization result.