Research And Design Of Feedback Control On MHD Instabilities In KTX Reversed Field Pinch

The thesis mainly describes the physical design of feedback control system on magnetohydrodynamics (MHD) instabilities for Keda Torus experiment (KTX) reversed field pinch (RFP) and the research on optimal feedback control of m=l tearing modes (TMs) and resistive wall modes (RWMs).KTX is the first large RFP machine in China, which is under construction in USTC, shouldering the mission of exploring the advanced RFP scenario. RFP configuration is characterized by multiple MHD modes existing simultaneously. RWM limits the increase of discharge duration and plasma current. TM may develop leading to wall-locking phenomenon, producing severe plasma-wall interaction, which may lead to premature termination of the discharges. For the sake of enlonging the discharge duration and improving the confinement performance, RWMs and TMs should be feedback controlled at the same time in KTX.First, the design of feedback control system is accomplished, which consists of the sensor array, the actuator array and digital PID controller. The sensor array is4×48saddle sensors and magnetic probes placing at the center of saddle sensors. The actuator array is4^24saddle coils, each fed by an individual power amplifier. The digital PID controller is the brain of the system with the control loop of200microseconds. In pactice, KTX feedback system can routinely run in the intelligent shell and model control schems, and the clean mode control technique will also be utilized, the most advanced control technique ever in RFP. Taking advantage of abundant mangetic signal, the feedback control system can work using the polodial or toroidal perturbed field, exploring more efficient control methods experimentally.Second, an MHD model based on the geomertry and parameters of KTX is built. In the model, the eigenfunction of TM perturbation is taken in terms of the basis solutions of Newcomb’s equation. Combining the fluid equations considering the balance between the viscous torque and the electromagnetic (EM) torque and the imposed feedback law, we obtain a full dynamic model for TM. The impact of sidebands is considered in raw mode control (RMC) schem. This model can evolve the phase and rotation of TM by imposing its amplitude. Moreover, the profile of perturbed field and feedback current, power can also be evolved. Similarly, the RWM’s feedback control can be included into the model. The main different lies on the fact that RWMs in RFP are non resonant modes with no rational surface and the assocated EM torque, which is zero except the vicinity of the rational surface. Based on the model, a code named KTX-FC is developed by fortan90languages, which can be applied to various RFPs only by adjusting the geometry and related paramenters.Third, simulations of feedback control on TM and RWM in KTX are carried out. The locking threshold of the main TMs in KTX is expected at the level of1mT, which means their feedback control is indispensable. The choice of1.5mm thick copper shell is found in the optimal range. It is first discovered in simulations that the different tendency using radial sensor from the other ones when placed outside the vessel. In the RMC sheme, the feedback with radial sensor needs more sensors and the in-vessel location. The spectrum of instable RWM depands sensitively on the equilibrium; feedback control can not only stabilize RWM while the complex gain can also bring the mode rotation. The latency affects the feedback performance of RWM and the smaller the latency the better the achived feedback stabilization. The feedback frequency is required to be at least5kHz accoding to the disrete time control reaseach. The CMC is more efficient than RMC for both TMs and RWMs.