Researches On The Phase Calculating And Readout System For LHCD Feedback Control

Lower Hybrid Current Drive (LHCD) is an effective method to sustain steady-state operation in tokamak. The energy of Lower Hybrid Waves (LHW) can be coupled into plasma to accelerate electrons by means of the Landau damping. The current drive efficiency depends on the plasma density, toroidal magnetic field and launched LHW power coupling spectrum. With various phase differences at each waveguide, the power coupling spectrums are diverse. If the phase differences are too small, some of the microwave beam cannot arrive at the center of plasma and the drive efficiency will be quite low. Otherwise, if the difference is too big, most of the LHW power will be absorbed by those outside electrons and the conversion efficiency will still be unsatisfactory. By feedback control of the phase differences at each waveguide, a specific power coupling spectrum can be maintained in purpose of increasing the drive efficiency and improving the plasma confinement. This is the prerequisite that the LHW system could achieve various goals in physics experiments.An LHW system with frequency of 2.45GHz, of which the microwave is derived from 20 KU-2.45 Klystron Amplifiers made in Russia, is applied on EAST (Experiment Advanced Super-conducting Tokamak). Since the phase of the klystron amplifier's output microwave is quite sensitive to the negative high voltage on its cathode, approximately a 10 degrees' phase movement can be brought about by a change of 1% on the negative high voltage, a feedback control, which should be completed much faster than the negative high voltage's fluctuations, must be achieved in order to maintain an experiment required power coupling spectrum.Considering about each part of the whole feedback control procedure, the software execution of feedback control will occupy most of the time cost, so a high-speed phase calculating and readout system are designed and implemented to satisfy the speed requirement of the entire feedback control.In the LHW system of EAST, the phase detector can only detect the magnitude of the phase difference in the range from -180°to +180°between the two input signals, but cannot recognize whether the current phase is positive or negative. This makes the signed phase calculation more difficult. Also, since the phase voltage output around 0°and±180°of the phase detector needs a high-order polynomial to describe, the high-order root operation will increase the time cost of phase calculation.According to the difficulties mentioned above, a phase calculating system, which is a hardware circuit, is designed and implemented as a solution. The main technical features of this system are as follows:1. A high-speed analog-digital converter converts the phase voltage into digital signals, which is a necessary preparation for the use of high-speed digital signal process technique.2. By analyzing the characteristic relationship between the phase voltage and the phase differences, a proper phase calculating algorithm for the specific phase detector is designed and the corresponding algorithm model is constructed in Matlab/Simulink environment.3. A large capacity and high performance FPGA (Field Programmable Gate Array) device, by taking the advantages of its embedded DSP cores and other abundant resources, plus some specific circuit modules for special calculations, is present to implement the phase algorithm model in hardware.4. Parallel process of phase calculating for multi-channels is implemented in a single FPGA device.As a result of the applications of these technologies, the phase calculating system has the ability to complete the calculation of all phase differences at 20 waveguides in the EAST LHW system in a considerable short time, and has better price/performance ratio than traditional DSP solutions.On the basis of the accomplishment of the phase calculating system, a data readout system is then designed and implemented based on PCI bus and fiber channel, with a RS-485 interface that has successful experience on the HT-7 Tokamak. When the connection between the readout system and the phase calculating system is constructed only through the fiber channel, it can not only provide a high speed data transmission path but also isolate the ground of the two systems. The readout system is running on QNX platform, a real-time operating system, so the software response is enhanced.