E. M. Fearon

Fabrication of polymer shells using a depolymerizable mandrel

A new technique for producing hollow shell laser fusion fuel capsules has-been developed that starts with a depolymerizable mandrel. In this technique we use poly({alpha}-methylstyrene) (PAMS) beads or shells as mandrels which are overcoated with plasma polymer. The PAMS mandrel is thermally depolymerized to gas phase monomer. which diffuses through the permeable and thermally more stable plasma polymer coating, leaving a hollow shell. Using this technique we made shells from 200 {mu}m to 4 mm diameter with 15 to 100 {mu}m wall thickness having sphericity better than 0.5 {mu}m and surface finish better than 10 nm RMS. 13 refs., 5 figs., 1 tab.

Fabrication of special inertial confinement fusion targets using a depolymerizable mandrel technique

A technique was developed for fabricating spherical shell targets for implosion physics experiments with diameters up to several millimeters and with unique structural features such as thin metal layers or texture on the inside surface. We start with a spherical bead or thin shell of poly(alpha‐methylstyrene) (PAMS) of the desired size, which can be textured by laser photoablation or overcoated with a thin layer of diagnostic material. The mandrel is next overcoated with plasma polymer (CH) 2–50 μm thick. Upon heating, the PAMS depolymerizes to a gaseous monomer which diffuses through the thermally stable plasma polymer coating leaving a hollow shell. Shells produced by this technique are uniform in wall thickness, and highly spherical. If the PAMS mandrel is textured, the mandrel topology is transferred to the inner wall of the plasma polymer shell. Likewise, thermally stable coatings on the mandrel are transferred to the inner shell wall.A technique was developed for fabricating spherical shell targets for implosion physics experiments with diameters up to several millimeters and with unique structural features such as thin metal layers or texture on the inside surface. We start with a spherical bead or thin shell of poly(alpha‐methylstyrene) (PAMS) of the desired size, which can be textured by laser photoablation or overcoated with a thin layer of diagnostic material. The mandrel is next overcoated with plasma polymer (CH) 2–50 μm thick. Upon heating, the PAMS depolymerizes to a gaseous monomer which diffuses through the thermally stable plasma polymer coating leaving a hollow shell. Shells produced by this technique are uniform in wall thickness, and highly spherical. If the PAMS mandrel is textured, the mandrel topology is transferred to the inner wall of the plasma polymer shell. Likewise, thermally stable coatings on the mandrel are transferred to the inner shell wall.

Development of Polyimide Ablators for NIF: Analysis of Defects on Shells, a Novel Smoothing Technique and Upilex Coatings

Abstract Over the last three years, LLNL has developed polyimide vapor deposition technology suitable for mandrel overcoating and fabrication of polyimide capsules. Agitated mandrels were overcoated with 4,4’-oxydianiline and pyromellitic dianhydride, and the PMDA/ODA coating was thermally converted to polyimide by heating to 300°C. Shells from this process did not meet smoothness requirements specified by the target designs for the National Ignition Facility (NIF). The defects and the possible mechanism(s) for defect generation were analyzed, and it was determined that surface roughness was the result of shell-pan interaction(s). A post-processing, shell smoothing technique was also developed which simultaneously levitates the shell while exposing it to solvent vapor. Efforts to form Upilex™, a high strength polyimide, using vapor deposition will also be discussed.

Infrared spectra of liquid and solid HT and HD in mixtures with T2

The collision‐induced fundamental vibration–rotation spectra of liquid and solid HT and T2 in a mixture of 50% H and 50% T have been recorded. The spectra of liquid and solid HD in a 90% HD plus 10% T2 mixture have also been observed. The frequencies of the numerous single and double transitions have been compared with those calculated from the known molecular constants. Deep holes have been observed in the Q (O) phonon band of solid HD, HT, and DT. The position and extent of these holes have been related to a theoretical treatment which attributes the holes to a coupling of the rotational and translational motion of the molecules through the anisotropic part of the intermolecular interaction. The effect of J=1 impurity molecules on the intensity of the sharp Q1(O) line of liquid HD is discussed and the effect of the radioactive decay heat on the temperature of the liquid and solid samples is evaluated.The collision‐induced fundamental vibration–rotation spectra of liquid and solid HT and T2 in a mixture of 50% H and 50% T have been recorded. The spectra of liquid and solid HD in a 90% HD plus 10% T2 mixture have also been observed. The frequencies of the numerous single and double transitions have been compared with those calculated from the known molecular constants. Deep holes have been observed in the Q (O) phonon band of solid HD, HT, and DT. The position and extent of these holes have been related to a theoretical treatment which attributes the holes to a coupling of the rotational and translational motion of the molecules through the anisotropic part of the intermolecular interaction. The effect of J=1 impurity molecules on the intensity of the sharp Q1(O) line of liquid HD is discussed and the effect of the radioactive decay heat on the temperature of the liquid and solid samples is evaluated.

Collision induced infrared lines in solid hydrogens caused by tritium radioactivity

Abstract We report the first observation of infrared absorption lines due to the presence of radioactive tritium in crystals of the solid hydrogens. Two prominent lines appear at the low-frequency side of the collision-induced spectrum, and are interpreted as due to the presence of positive and negative ions or electrons created during the radioactive process.

Preparation of hollow shell ICF targets using a depolymerizing model

A new technique for producing hollow shell laser fusion capsules was developed that starts with a depolymerizable mandrel. In this technique we use poly(alpha-methylstyrene) (PAMS) beads or shells as mandrels which are overcoated with plasma polymer. The PAMS mandrel is thermally depolymerized to gas phase monomer, which diffuses through the permeable and thermally more stable plasma polymer coating, leaving a hollow shell. We have developed methods for controlling the size of the PAMS mandrel by either grinding to make smaller sizes or melt sintering to form larger mandrels. Sphericity and surface finish are improved by heating the PAMS mandrels in hot water using a surfactant to prevent aggregation. Using this technique we have made shells from 200 {mu}m to 5 mm diameter with 15 to 100 {mu}m wall thickness having sphericity better than 2 {mu}m and surface finish better than 10 nm RMS.

Progress Toward Meeting NIF Specifications for Vapor Deposited Polyimide Ablator Coatings

Abstract We are developing an evaporative coating technique for deposition of thick polyimide (PI) ablator layers on ICF targets. The PI coating technique utilizes stoichiometrically controlled fluxes from two Knudsen cell evaporators containing a dianhydride and a diamine to deposit a polyamic acid (PAA) coating. Heating the PAA coating to 300°C converts the PAA coating to a polyimide. Coated shells are rough due to particles on the substrate mandrels and from damage to the coating caused by the agitation used to achieve a uniform coating. We have developed a smoothing process that exposes an initially rough PAA coated shell to solvent vapor using gas levitation. We found that after smoothing the coatings developed a number of wide (low-mode) defects. We have identified two major contributors to low-mode roughness: surface hydrolysis, and deformation during drying/curing. By minimizing air exposure prior to vapor smoothing, avoiding excess solvent sorption during vapor smoothing, and using slow drying we are able to deposit and vapor smooth coatings 160 μm thick with a surface roughness less than 20 nm RMS.

Vapor-Deposited Polyimide Ablators for NIF: Effects of Deposition Process Parameters and Solvent Vapor Smoothing on Capsule Surface Finish

Abstract Recent progress made at LLNL on fabricating NIF scale polyimide capsules using vapor deposition techniques is detailed. Our major focus has been on improving the capsule surf ace finish through better understanding of the origin of surface roughness created during the deposition process and implementation of a post-deposition vapor smoothing procedure prior to imidization. We have determined that the most important factors during the deposition process that impact surface finish include mandrel quality, monomer mixing, selfshadowing, and abrasion. We have shown that high rate deposition (above 10 μm/h) is effective at reducing roughness, which we believe is due to the shorter total time of shell agitation in the bouncer pan. By adjusting the coating conditions, coatings up to 160 μm thick have been reproduc-My fabricated with 300 nm RMS roughness. Solvent vapor smoothing, a new technique also developed at LLNL, further improves the surface to 30 nm RMS.

Pyrolytic Removal Of The Plastic Mandrel From Sputtered Beryllium Shells

Abstract An engineering model is presented for the removal of the plastic mandrel from the inside of a sputtered Be shell. The removal is accomplished by forcing heated air in and out of the 4 to 5 μm laser drilled fill hole in the capsule wall by cycling the external pressure between 2 and 5 atm. The plastic is combusted to CO2 and H2O by this exposure, thus removing the mandrel. Calculations are presented to evaluate the various parameters in the approach. Experimental confirmation of the effectiveness of the removal is shown.

Separation of hydrogen isotopes by selective adsorption with production of high‐purity D–T and T2

Selective adsorption on a solid at 20 K provides the means to separate hydrogen isotopes, and particularly to purify small quantities of molecular D–T or produce pure J=1 T2 for nuclear spin polarization. We develop adsorption isotherms and separation factors for several solids from literature data. Our computational model of adsorption column operation, developed to determine column sensitivity to key parameters, is described. Model results show that column diameter and length, particle diameter, choice of solid, and the fractional void space of the packed solid are all key parameters in determination of total purification capacity of a column. Zeolite 13X is shown to be the optimal adsorbent. Column design and operation is discussed in context of the calculational results. We present experimental data that demonstrate successful operation of an adsorption column in purification of D–T gas by removal of T2, and discuss advantages of this technique over conventional separation means.