Experimental Studies Of Helicon Plasma Discharge And Fuel Retention

Plasma facing components(PFC)is essential to ensure a stable sustainment of tokamak plasma confinement and burning in present and future fusion devices because enormous amounts of energetic particles will impinge on the PFC during the plasma operation which can result in a series of issues,such as fuel retention,impurities formation,blistering,co-deposition and so on.PFC development for operation in a large-scale fusion device requires thorough testing in the intended operational conditions which are comparable to the real operation conditions,even beyond design limits.Two specially designed linear helicon devices(High Magnetic field Helicon eXperiment,HMHX and Permanent-Magnet Helicon source for plasma-material Interaction eXperiments,PMHIX)are constructed in Soochow University for the studies of PWI.Compared with tokamaks,HMHX and PMHIX have the advantages that the plasma conditions can be accurately controlled and monitored for long periods of time,also the properties of the plasma source can be tailored to study the phenomena of interest.These two linear devices are used to simulate the conditions specific to a fusion plasma both at the first wall and in the divertor of fusion devices.The design and construction of HMHX and PMHIX,including their magnetic system,vacuum vessel,RF system,and diagnostic tools are described and initial measurements of electron density ne,electron temperature Te,electron energy distribution function EEDFs,ion energy distribution function IEDF,ion flux ? and heat flux q in argon plasmas are presented in chapter 3.With controllable magnetic field and substrate location,the devices can be operated at a power of 2 kW to obtain Ar+ ion beam with advanced parameter(ion flux of 1020-1022m-2s-1 and electron density of 1018-1020m-3).The heat flux can effectively be controlled by the flow rate of gas,position and negative bais of sbustrate.Ion beam with high heat flux up to 1MWm-2 can be achieved which reaches one tenth of that of divertor region.Then plasmas with high parameters are used to investigate the PWI issues,for example,impurity removal,hydrogen recycling and retention in plasma facing materials.1.Impurity removal:The ion flux and energy of atomic N ion(N+)and molecular N2 ion(N2+)are controlled by varying the flow-rate ratio of N2/(N2+Ar)(a)in helicon-wave excited plasma(HWP)with Ar/N2 gas mixtures.Due to electron-impact ionization,Arm*are involved,which could remarkably enhance the atomic N+and molecular N2+ ion creation,especially at the a of 0.5.The ion density ratio[N+]/[N2+]is the function of ne rather than Te.The maximum density and flux of atomic N+are obtained at the a of 0.5,which are 2.5 x 1018 m-3 and 8.6 x 1021 m-2s-1 respectively.The N+ion-beams are formed with a speed near to Mach 3 and the energy of the ion-beam increases from 30 eV to 50 eV with increasing the a.As the a is increased,there is an increase in the Te and potential gradients,which result in larger ion-beam energies.High removal rate of N2,on average about 1.1×1024 N2/m2h,is obtained during the 40 min Ar HWP discharge cleaning(sample is pre-exposed by Ar/N2 HWP).Our present experimental results indicate that HWP has potential applications in wall conditioning in steady-state superconducting fusion reactors such as ITER and DEMO.2.Hydrogen recycling and retention in plasma facing materials(tungsten):Continuous and steady deuterium HWP beam is realized by the PMHIX device.The effect of mixed D-He plasma irradiation at different He concentrations on D retention in tungsten samples is investigated by TDS.Results indicate that total deuterium dropped up to 37%,when 5%of He+ was added to pure D plasma.We also find that pre-exposing tungsten samples to He+HWP ions to a fluence of 1.0×1024m-2 can drastically reduce total deuterium retention from 7.30×1020 Dm-2 down to 2.44×1020Dm-2.By tuning the DC bias voltage and coils current,the ion energy(driven by the bias voltage)and the ion flux(related to the plasma density)could be separately controlled.And the effect of negative bias voltage and magnetic field on D retention in tungsten samples is also investigated.Based on the research on HMHX and PMHIX,the HWP wall conditioning scheme on EAST has been proposed.In chapter 6,HWP discharge cleaning experiments under steady magnetic field were carried out on EAST for the first time by using 13.56MHz/5kW and 27MHz/50kW radio frequency power source.Toroidally homogeneous but poloidally inhomogeneous HWP plasmas were obtained.Emphases were taken on the influence of cleaning parameters on the cleaning effects,including toroidal field,RF power and pulse width.It is found that localized power deposition of He HWP lead obvious hydrogen release(D is mostly released in the form of HD).Outgassing rate of the hydrogen increase with RF power reaching its maximum value at?15kW,after that,outgassing rate drift down slowly.For the 13.56MHz/5kW radio frequency power source,the impurities outgassing rate increase with pulse width reaching its maximum value at 2s,then the outgassing rate keep relatively high and constant.Also,CCD camera,OES and electrostatic probe are used to determine plasma parameters.