Research On Plasma Production And Detection Techniques For A High-power Rf Negative Ion Source

Neutral beam injection(NBI)is one of the main plasma heating methods in magnetic confinement fusion(MCF)devices.The negative ion source based NBI(N-NBI)is thought to be the inevitable choice for large-scale MCF devices due to its high neutralization efficiency even the beam energy reaches highly as 1 MeV.Due to the advantageous features of simple structure and principally maintenance-free in operation,the high-power RF negative ion source was chosen as the reference source of International Thermonuclear Experimental Reactor Project(ITER)in 2007 and has become one of the research focuses in fusion engineering field in recent years.Under the support of the Ministry of Science and Technology of China,Huazhong University of Science and Technology(HUST)has launched the project,“Key Technology and Engineering Research of Driver in the ITER High-frequency Negative Ion Source” since 2011,aiming to explore mechanisms and technologies of low RF frequency,low pressure and high power plasma production.Up to now,HUST has developed a test setup of RF ion source(HUST source)with successful plasma production.Plasma production at low RF frequency,low pressure and high power conditions faces much more difficulties than conventional inductively coupled discharges,of which utilization efficiency of the RF power is a key issue.This article takes the HUST source as the research object,focuses on the RF power utilization efficiency by studying the theory and engineering problems of coil-plasma RF power coupling mechanism,online impedance matching,online detection of RF power and impedance,plasma diagnostics and ion source control,etc.The main research contents are as follows:(1)Characterization of RF power coupling between the coil and plasmaAn analytical model and a finite element model are both employed to study systematically the characteristics of the RF power coupling between the coil and plasma in HUST source.Influences of plasma parameters and operating parameters on the driver impedance and power coupling efficiency are discussed,which could provide certain guidances for selection of the source operating frequency and pressure.A 3D finite element model of the driver containing Faraday screen(FS)is presented for electromagnetic(EM)simulation,which provides clear physical images of the EM fields in the driver and reveals the role FS plays in suppressing the capacitive coupling configuration in the source.(2)Online tuning of the matching networkFor the first time,online impedance matching strategies and an auto-matching algorithm combining both capacitance and frequency adjustments are presented for high-power RF plasma applications.The strategies aim to meet the requirements of matching performance,prioritize frequency adjustment and reduce capacitance adjustment,such that to provide faster and more efficient matching schemes according to the impedance variation in practical engineering applications.The auto-matching algorithm integrates and improves the typical cyclic variable method and gradient descent method,and embeds the function of selecting automatically the optimal matching strategy and optimizing the matching parameters based only on the the reflection coefficient when the impedance is not available.It improves effectively the auto-matching speed and stability compared to conventional algorithms.(3)Development of the online RF power and impedance detection systemAn online detection system for the driver impedance and RF power is developed in-house.A capacitive voltage divider and a PCB-type Rogowski coil current transformer(CT)are used to sample respectively the high-frequency,high coil voltage and large coil current.A purely analog signal processing method is used to process the sampling signals and obtain the RF power and impedance,which has fater response than conventional digital signal processing method.With low-power(< 1 kW)online RF power and impedance measurement experiments,this system is proved feasible with good measurment resolution.(4)Development of the RF Langmuir probe diagnostics systemA highly integrated and automatic RF Langmuir probe system for diagnostics of the plasma in HUST source are developed in-house.The addition of the compensation electrode to the conventional RF chokes reduces effectively the RF compensation difficulty at low RF frequencies.Through a plasma parameter interpretation program developed in-house,the probe data can be processed quickly and automatically to evaluate the plasma parameters.On the basis of that,different probe data interpretation algorithms based on multiple electron and ion collection theories are studied and compared.(5)Design and implementation of the ion source control systemThe ion source control system uses PAC(Programmable Automation Controller)as the main controller and adopts a distributed hardware architecture.A hybrid program pattern based on event structure,message queue state machine and producer-consumer pattern is presented to develop the control software.The system has a compact structure with good stability,reliability and strong expandability.Based on the control system,the plasma production at low RF frequency,low pressure,and high power conditions has been successfully achieved in HUST source.(6)Experimental and testing reasearch of HUST sourceThe experimental and testing campaigns performed on HUST source are introduced,including operationg parameters detection,RF plasma diagnostics and high-voltage power supply commissioning.In summary,this work provides a complete technical implementation scheme for RF plasma production and detection in HUST source.Meanwhile,lots of precious operating experiences for high power RF ion sources are obtained as well.Moreover,it lays the foundation for the follow-up research on the complete RF negative ion source.