Research On The RF System Of The High-power RF Based Negative Ion Source At HUST

High-power and high-current ion source is a core component of the neutral beam injector system for fusion, and RF driven negative ion source is already the ITER reference design. Research on key technologies of RF negative hydrogen ion sources has been carried out at HUST since 2011. At present, a driver test facility for a RF negative hydrogen ion source(“the HUST driver test facility”, for short) was developed at HUST. The driver, where plasma is produced, is the fountainhead of RF ion sources.The RF system is the heart of the HUST driver test facility since it supplies RF power for plasma. It consists of a RF generator, a transmission line, a matching unit and a RF coil. The output power of the RF generator is transmitted by the transmission line and the matching unit to the RF coil, and then it is coupled into plasma by the RF coil which is wound outside cylindrical driver.This thesis centers around researching on the RF system. The work could be divided into three parts: 1, the accomplishment of transmitting RF power to the RF coil; 2, the investigation of how RF power influences on plasma ignition and extinction; 3, the discussion on the method of matching impedance online.When plasma is equivalent to a homogeneous medium, the plasma effective plasma conductivity is deduced. By means of finite element method, equivalent impedance of the driver is calculated with a reasonable accuracy by a simplified 2D model, and the calculated equivalent impedance range is significant to design the matching unit. A common L-type matching unit and an improved matching unit based on a RF transformer is illustrated, and their electrical parameters is deduced. In order to reduce cost and benefit for engineering, the improved matching unit is finally chosen, which is composed of an RF transformer with the turn ratio of 3:1, a series variable capacitance and a parallel variable capacitance. The design and implementation of the RF transformer is key, and the rationale of a RF transformer with new structure is presented, which has the advantages of high power capability, adjusting turn ratio easily and so on. According to the calculated equivalent impedance range, the accomplished matching unit accomplishes to transmit 20 kW RF power to the RF coil at 1MHz, and then plasma is successfully ignited.The experiments on hydrogen plasma ignitions and extinctions is carried out to investigate systematically the RF power necessary to ignite hydrogen plasma with a 1kW RF generator. The way that matching unit is tuned before plasma ignition is brought up and put into use. With regard to some operation parameters, heating current of the starter filament is required for initial plasma ignitions; the decrease of pressure from 10 to 4Pa leads to rapid increase of the RF power necessary to ignite plasma; the increase of the turn number from 4 to 10 makes for decreasing the RF power necessary to ignite plasma. According to the influence of RF power on plasma extinctions, a method that plasma need be ignited at a higher pressure(e.g. 2.5Pa) then can operate at 0.3Pa is put forward to assure the good running at a high RF power, and the method is already verified. Typical spectrum is presented since it is a main criterion of plasma ignition. These experiments help to deeply understand RF ion source and well prepare for the running of high-power.The developed matching unit for high-power is introduced, what's more is that the curve of loss of matching condition in experiments is also presented. As for the five parameters, the discussion shows that the operation frequency, the capacity of CS and the equivalent inductance are key factors of the loss of matching condition. Based on the above experimental results and discussion, a method to tune online is discussed, which need adjust the operation frequency as well as the capacity of the parallel capacitance togetherIn addition, an optimization of the number of RF coil turns is come up with. A new Power Coupling Efficiency Factor is defined to represent coupling ability of RF coil. The optimization need find out the turn number with higher Coupling Efficiency firstly, and ensure that their corresponding electrical requirements of the matching unit is appropriate, then take account of the both apects to choose a optimal turn number. The experiments on the influence of turn number on the RF power necessary to ignite plasma shows that this optimization is efficient.