| Due to high neutralization efficiency of negative hydrogen ions at high beam energy,a negative hydrogen ion source(NHIS)based neutral beam injection(NBI)system will play an important role in the future magnetic confinement fusion device.Compared with traditional hot cathode arc sources,radio-frequency(RF)inductively coupled NHIS operating at a high power and a low pressure has a series of advantages,such as long lifetime,simple structure of the chamber,no filament material contamination,basically free maintenance in operation,highly uniform plasma density and easily controlled beam current,which have been selected as the reference source for ITER.In order to optimize the NHIS and produce high density of negative hydrogen ions,it is well worth to systematically examine the complex physics related to production,transport processes,electron kinetics and especially the power absorbtion mechanism of this source.In this thesis,the main contents are given as follows.In order to give a comprehensive knowledge of this two-chamber ICP source,the axially and radially resolved measurements of the electron density,effective electron temperature,and electron energy probability function(EEPF)in an argon discharge are conducted by means of a Langmuir probe for various powers and gas pressures.A hybrid model within COMSOL Multiphysics is employed to give a deep understanding of the plasma production and transport behavior and to reproduce the experimental results.It is found that the diffusion combined with the nonlocal electron kinetics plays a predominant role in two-chamber ICPs at low pressures.Along the axial direction,both the electron density and the electron temperature peak at the center of the driver region.The depletion of high-energy tails of EEPFs with increasing the axial position demonstrates the cooling mechanism of energetic electrons in the expansion region.Along the radial direction,the spatial distribution of the electron density exhibits a "bell shape" for various powers and pressures.However,the radial distribution of-the effective electron temperature evolves gradually from a `convex shape" to a "concave shape" with increasing gas pressure,indicating a transition from nonlocal to local electron kinetics.Furthermore,the RF power transfer efficiency is experimentally and numerically investigated in this plasma source operating in hydrogen.The discharge is operated in a low pressure range of 0.1-3 Pa at a driving frequency of 2 MHz and an applied power of up to 6 kW.In the experiment,the power transfer efficiency is determined by measuring the applied power and the current through the antenna coil both with and without discharge operation.Fundamental properties,such as electron density and effective electron temperature,are obtained by means of a Langmuir probe.The effect of the antenna coil turns,N,is also studied in a range of 5-p turns.It is found that more coil turns can significantly enhance the power transfer efficiency due to a remarkably increasing quality factor of the system.Moreover,the experimental results show that the power transfer efficiency first increases and then reaches the maximum with increasing the applied power,while it first increases quickly and then rises at a slower rate with increasing gas pressure.In order to give a comprehensive knowledge of the power absorption mechanism,a self-consistent hybrid model is developed.It is found that the numerical results are in reasonable agreement with that measured in the experiment.The numerical results and the analytic solutions in the limit cases of low and high pressures can well explain the various trends of the power transfer efficiency obtained in the experiment.These trends mainly depend on the quality factor Q,the electron density,and the effective electron collision frequency.Finally,the electron characteristics and spatial distributions of plasma parameters are investigated by means of a Langmuir probe in a typical two-chamber ICP source with hydrogen discharge.Moreover,influences of the gas pressure,RF power and frequency on the EEPFs,electron density and electron temperature are also presented.The results indicate that the measured EEPFs evolve from a three-temperature structure distribution to a Maxwellian distribution as the pressure or the axial distance increases.Different characteristic frequencies calculated based on the measured plasma parameters show that stochastic heating of electrons dominates at pressures lower than 0.3 Pa and has to be considered for pressures lower than 1.0 Pa,while Ohmic heating dominates at higher pressures.Furthermore,the EEPFs as a function of the total electron energy evolve from an identical shape to different shapes with increasing the axial position and pressure,indicating a transition of electron kinetics from nonlocal to local regime.This can be explained by the calculated electron energy relaxation length.Besides,the electron density exhibits a "bell-shaped" profile in the driver region,while the electron temperature shows a relatively uniform distribution there,and they decrease to low values in the expansion region.In order to validate the experimental results,a COMSOL Multiphysics soft is used to calculate the electron density and electron temperature under different discharge conditions.The simulated axial distributions of the plasma parameters agree well with the measured results at 5.0 Pa,while the calculated electron density is lower and the calculated electron temperature is higher at lower pressures,i.e.,l Pa and 2 Pa.In addition,there is no frequency dependence of axially resolved EEPFs,electron density,and electron temperature in high power deposition discharges(1.5 kW). |
