Double Shell Inertial Confinement Fusion Target Fabrication Technology Basic Research

Inertial confinement fusion(ICF)is an approach to fusion that relies on the inertia of the fuel mass to provide confinement.High power laser facility is the main ICF driver.There are two primary reasons for pursuing fusion research,an understanding the behavior of plasmas that make up most of the known universe and the creation of a new energy source.There are three ignition mode;central ignition,fast ignition and volume ignition.The related ignition target is cryotargets,cone-shell targets and double shell targets,repectively.Compared with the basic cryogenic targets,the ignition double-shell targets are more easily to prepared and handled at room temperature.The simplicity of using noncryogenic high-pressure gaseous fuel,simple laser pulse shapes,less stringent requirement on laser symmetry and vacuum hohlraums for indirect drive makes it worth while to reconsider this type of capsule. Therefore,it is necessary to make some research on preparing the double shell targets.Achieving double-shell ignition will involve its own set of demanding fabrication requirements,such as the smooth(seamless)mid- to high-Z inner shells and low-Z outer shell,high-pressure fuel fill,the assembly of highly concentric inner and outer shells.A set of double shell targets has been design and prepared by Los Alamos National Laboratory(LANL)for past 20 years.Highly concentric inner and outer shells assembly and seamless outer shell remain problemly.The research of double shell targets is just beginning in China.Beside the based fabrication technology,new method should be pursue to completely solve above challenges.Research of the inner shell fuel loading technique,single shell coating using heated drop-tower technique,and the decomposable poly(alpha-methylstyrene)(PAMS)mandrel technique are presented in this paper according to ICF target fabrication capability,level and demanding of the requirements.Technique of coating and PAMS pyrolysis is assistant to develop new way to fabricate high quality double shell targets.HGM has been one of targets widely used in ICF experiments,will be main inner shell in double shell target for a periodic time in China.Techniques of filling HGM with the fuel gas or the diagnosis gas are investigated.Content includes measurement of the HGM mechanism strength,the mixture of deuterium and neon gas fill process,comparision of argen fill method, the influence of gas permeability on HGM,pre-filling gas procedure to select better quanlity HGM used for targets,calculation of fuel density just before the target core being compressed, and analysis of the calculation uncertainness.Difficulty is discussed how to fill and measure the mixture gas ratio and total pressure while HGM is inner shell of the double shell target, solution methods are presented on the same time.Experiments and calculations show that neon permeability is 2.6×10^-18mol·m^-1·s^-1·Pa^-1at 350℃,while deuterium is 4.3×10^-18 mol·m^-1·s^-1·Pa^-1.It is concluded that D₂/Ne can be filled simultaneously.Filling time is based on the neon permeability because the neon permeability is more small.No Ar is found below 500℃by the method of gas diffusion.Under the condition of 600℃and 1.0 MPa Ar out of microspheres for 24 hours,there is as much as 0.003 MPa Ar inner the hollow glass shell. The surface finish decreases from no more than 20 nm to 50 nm-100 nm and the yield of microspheres which survived the fill procedure is no more than 50%with the diffusion fill temperature of 600℃.Equilibrium temperature and compositions significantly effecting gas permeability is well known,experiments indicate that wall thickness,fill process and surface erosion can also work in this paper.The deuterium permeation coefficient is 5.0×10^-22 mol·m^-1·s^-1·pa^-1with wall thickness above 2μm at room temperature,while the permeation coefficient is 1.56×10^-20mol·m^-1·s^-1·Pa^-1with wall thickness below 1μm.The permeation coefficient of thin wall HGM increased 30 times more than that of thick wall HGM.The main reason for effects of wall thickness on the HGM permeability is microcracks on the HGM surface.The permeation coefficient increases about 50%if HGM experienced one cycle of filling and deflating,and doubles after two cycles.Effects of HGM fabrication technology, surface microcrack and ion channels is evaluated.Coating in microsphere surface using heated drop-tower technique is a method to form an uniform layer aroud an existed shell.An layer can be prepared so far as its material dissolve in a solution with the help of technique.A detailed description of coating formation and a physical and chemical change in the process are presented.On the basis of this physical picture,and the heat,mass and momentum transfer of the transformation process,a physical model is established and the related mathematic model is developed.This paper put emphases on polyvinyl alcohol(PVA)coating aroud polystyrene(PS)shell,and then poly(alpha-methylstyrene)(PAMS)coating aroud HGM.With the help of model,the influences of processing parameters on the transformation are simulated.Then the key processing parameters that greatly affectes the coating quality and properties has been found to be initial solution concentration,shell diameter,furnace temperature and temperature profile and thermal conductivity of the furnace gas.High furnace temperature and high ratio in He/Ar mixture can shorten PVA coating dry time.Smaller initial shell diameter and dense solution can form thicker coating in the same furnace.Experiments show that the PVA coating can be prepared under the conditions of 5%PVA solution and 300μm PS diameter at 250℃, resulting 2.2μm PVA caoting.The PS with 250μm to 550μm in diameter have PVA coating 2.4μm to 1.0μm in thickness.The surface finish is 3 nm～10 nm.The decomposable PAMS mandrel technique,as one of the most important methods of capsule fabrication,is the key technology to prepare the glow discharge polymer(GDP)shells for cryotargets,large-diameter or thick wall HGM,polyimide shells and metal shells.The PAMS/GDP process is investigated in this paper.Inner PAMS shells are overcoated with a GDP layer by the and are then heated to high temperature,leaving the GDP shell.Thermal gravity(TG),weight before and after thermal process,and pyrolysis gas chromatograph(PGC) /mass specture(MS)are used to provide charaction of four PAMS sample with different molecular weight for themal degradation process and degradation products.The pore stucture of GDP coating is characterized by the gas adsorption-desorption method and scannning electron microscope.Molecular diameter of PAMS degradation products are determined from ab-initio of quantum chemistry calculation.Products diffusion process through GDP coating is conjectured.The weight-average molecular weight of 1#,6#,7# and 9# PAMS is 18 k,114 k,244 k and 76 k,respectively.TG analysis indicated that pyrolysis temperature of 1# PAMS covers from 270℃to 410℃,that of 6#,7# and 9# covers from 200℃to 340℃.6#,7# and 9# have 2%mass fraction tetrahydrofuran which is residual solvent.Degradation products indicated by MS can be divided into three groups;monomers,dimers and other small molecules such as benzene,styrene,ethylbenzene,isopropylbenzene and neobutylbenzene. The yield of monomers is 94%～100%.The PAMS degradation is depolymerized.The weight variation at the different equilibrium temperature shows that degradation rate is higher after 255℃.The activation energy for the rate of pyrolysis of whole polymer is 244 kJ/mol.The GDP coating is compact with a little holes and pores.The process of products diffusion through GDP coating is belong solution- diffusion model.PAMS/GDP experiments show that PAMS does't molten to flow liquid during degradation,but the degradation can reduce surface finish of GDP coatings.