Laser Fusion Targets Research Articles

Cryogenic laser fusion target material design considerations

Laser fusion target designs exhibit improved performance when the fusion fuel, a deuteriumtritium (DT) mixture, is frozen into a uniform, solid shell. The formation of'such a shell requires rapid isothermal cooling of the target to cryogenic temperatures. The cooling rate must be sufficiently fast to prevent significant, gravitaqtionally driven downward flow of the DT as it passes from the gaseous through the liquid state. Because it is not possible to measure the uniformity of a solid DT layer in opaque, multishell targets, we have modeled such targets to calculate the cooling rate and, hence, the expected thickness uniformity of the DT shell. The presented results provide target designers with practical guidelines for the selection of materials and configurations, which will assist in the fabrication of high-quality cryogenic targets.

Characteristics of thick non-planar SiO 2 coatings

We successfully developed a new process for making strong smooth thick SiO 2 films on hemispherical Kovar mandrels of various sizes designed for multishell laser fusion targets. The surface finish obtainable with a SiO 2 coating 13 μm thick on a mandrel 260 μm in diameter is approximately 30 nm peak to peak, with a few defects approximately 0.3 μm deep. The r.f. magnetron sputtered SiO 2 films were dense and crystalline.

Mass spectrometer determination of argon contents in laser fusion target pellets

A system for measuring argon contents in individual laser fusion targets using a getter pumped closed volume quadrupole mass spectrometer has been developed. Accuracy of 10% can be obtained, limited by the calibration standard and electron multiplier drift. Measurements are simple, unambiguous and can be performed at up to one per hour with a sensitivity equivalent to 1 × 103Pa (.01 atm) argon in a 140 pm internal diameter sphere.

Materials problems with inertial confinement fusion targets

The fabrication of advanced laser fusion targets requires careful attention to materials properties that are unimportant in target design. Specific problems encountered in the fabrication of high-Z pusher shells of a typical three-shell high-yield target are discussed. Neither electroplated gold nor chemical-vapor-deposited tungsten were usable, although both produced as-deposited coatings that met design criteria. We also observed that hydrogen permeability of chemical-vapor-deposited tungsten decreases drastically for coatings thicker than 25 pm.

Six-beam irradiation and implosion of laser fusion targets: Laser focus dependence

DT-filled 115-μm-diam. glass microballoons have been irradiated with the six-beam ZETA laser (Nd: phosphate glass) at 2 TW. The absorption uniformity and implosion symmetry have been studied with x-ray imaging for variations in the laser focal conditions. Symmetry, as indicated by the x-ray spatial emission of both the absorption region and the imploded core, has been found to be a strong function of illumination uniformity.

Comments on the paper by R.C. Kirkpatrick ‘An overview of design space for small fusion targets’, Nucl. Fusion 19 1(1979) 69–79

Recently, Kirkpatrick published a paper (Nucl. Fusion 19 1 (1979) 69) in which he presented a twelve-parameter burn code to gain an overview of the design space available for laser and E-beam fusion targets.

Wave-front shearing interferometer for cryogenic laser-fusion targets

The operating principles of a wave-front shearing interferometer designed by Murty are described. The interferometer has simple, independent adjustment for both phase and shear and is stable mechanically. It has been used to assess the quality of cryogenic laser-fusion targets immediately prior to their irradiation by a high-power laser. It has also been used to study the changes in cryogenic targets caused by energy emitted by the high-power laser prior to the main pulse.

Neon spectral line broadening as a diagnostic for compressed laser-fusion targets

Experiments have been conducted on the JANUS laser system to measure fuel density in imploded laser-fusion targets filled with deuterium, tritium, and neon. An x-ray crystal spectrometer was used to record the line emission of the 1s-2p line of the Ne+9 ion. Simultaneous measurement of the linewidth and the neutron yield indicated that symmetrically irradiated exploding pusher targets reached fuel densities of approximately 1 g/cm3.

Exploding pusher performance − A theoretical model

A simple model of ’’exploding-pusher’’ laser fusion targets is presented. The peak ion temperature is calculated by assuming that the ion heating mechanism is a shock followed by isentropic compression. The compression is calculated by conservation of mass and reasonable assumptions on the density profile at peak compression time. The neutron yield predictions of this model agree over a broad range of parameter space with those of complex one-dimensional numerical simulation, which in two dimensions have tracked experimentally measured yields quite accurately.

Temporally and spatially resolved harmonic emission from spherical laser fusion targets

An electro-optic streak camera was used to record 2ω0 and (3/2) ω0 harmonic emission from glass microsphere targets irradiated with a 0.3-TW Nd : glass laser. These two harmonic frequencies are produced at the critical and quarter-critical surfaces, respectively, and thus provide a measure of these electron-density trajectories. After applying a refraction correction to the data, the trajectories were compared with predictions from the KMSF TRHYD code. It was found that hydrodynamic development in the critical-density region was accurately depicted by the temporal behavior of the 2ω0 emission. In addition, observed periodic fluctuations in (3/2) ω0 emission along a meridian give direct evidence of density rippling in the underdense corona.

Microradiographs of laser fusion targets: 2-D modeling and analysis

A new contact microradiographic system for analyzing laser fusion targets with 2-D modeling and image analysis techniques is described. This system, which uses a monochromatic x-ray source and Kodak highresolution plate emulsion, is sensitive to spherical wall thickness variations (eccentricities) as small as +/-200 A in hollow shells with a mean wall thickness of 1 microm. Measurements of wall thickness and of local and spherical wall thickness variations by radiographic techniques, using 2-D video, digital image analysis, and optical interferometry, are compared.

Measurement of Return Current in a Laser-Produced Plasma

A laser-fusion target's support fiber is heated at large distances from the laser plasma principally by Ohmic heating due to an electrical return current. Optical and electrical measurements demonstrate that return-current heating dominates over other heating mechanics such as hot-electron propagation along the fiber, thermal conduction, and line-of-sight plasma radiation.

Magnetron sputter coating of microspherical substrates

The fabrication of laser fusion targets requires the deposition of various metal and dielectric layers on microspherical substrates (100–500 μm diameter). The continuous motion for uniformly coating these spherical substrates has been accomplished for rf sputtering applications. In one approach, the spheres being coated are bounced and laterally swept across a table as the frequency of vibration of the table is changed continously. In another approach, the sputtering gas is also a levitation gas. The argon is fed through a collimated hole structure and keeps the microsphere both levitated and rotating during the sputter deposition. The uniformity and surface quality of the deposited films obtained by these two techniques are compared.

A new technique for fabricating cryogenic laser-fusion targets using cold-gas jets

A new technique of fabricating a uniform layer of solid DT inside a glass microshell has been developed using a heater-wire–cold-gas-jets combination. A D2-filled glass microsphere was continuously cooled by two cold-helium-gas jets, and a controlled amount of current was pulsed through a heater wire surrounding the microshell. This gave rise to fast evaporation and refreezing of the D2 inside the glass shell, resulting in a uniform layer of frozen D2. This new scheme has potential application in the development of a target-fabrication system which would allow for continuous fabrication, inspection, and delivery of cryogenic laser-fusion targets.

An overview of design space for small fusion targets

A twelve-parameter burn code has been used to gain an overview of the design space available for laser and E-beam fusion targets. The results of a few thousand implosion calculations are presented here in terms of an initial-condition space. The initial conditions include temperature, density and pusher jump-off velocity. For marginal driving energy there is an isolated region in the initial-condition space (θ0, ρ0) for which ignition may be achieved.

Conical targets for implosion studies with a CO2 laser

The design, fabrication, and testing of conical laser fusion targets is described. These are DT-filled conical holes in lead, 190 μm deep and covered with a 132-μm-diam plastic cap. The results of two-dimensional simulations of the cones are displayed. Experimentally, up to 2.8×105 neutrons have been obtained from the exploding pusher implosion of such a target under single-beam CO2 illumination of the cap at 0.13 TW.

Fuel content characterization and pressure retention measurements of DT-filled laser fusion microballoon targets

We have developed a nondestructive assay of the fule content of deuterium-tritium (DT) -filled microballoon laser fusion targets, which is based on β-particle counting rates. Using a model employing transmission measurements of kilovolt electrons through thin films, observed count rates are correlated with the amount of tritium in the glass walls and hollow interior of the microballoons. It has been shown that gas pressure in balloons can be calculated from tritium content and a knowledge of the initial gas composition since D and T were found to leak at the same rate. This assay technique is primarily applicable for balloons with glass wall thicknesses <1.5 μm where the number of escaping β particles is large compared with the number of x-ray photons generated in the glass. The technique has been applied to measure the pressure retention characteristics of individual targets. At room temperature, the balloons exhibited widely diverse and rapid leakage rates which could not be correlated with a model based on molecular diffusion and the assumption that all balloons had a homogeneous composition. Cryogenic storage greatly reduced the leakage rates with pressure retention half-lives ranging from 5 to approximately 12 years. Finally, it was determined that tritium in the glass wall is trapped and evidence is presented to support the hypothesis that it is uniformly distributed across the shell.

Prepulse damage to targets and alignment verification

We measured the damage threshold of 10.6-μm light incident on glass microballoon laser fusion targets. The threshold is several dozen microjoules, depending on target size and laser pulse width, and the damage mechanism appears to be thermal heating and rupture. Perforating glass microballoons proves to be a useful alignment verification technique.

Diagnostics for the laser fusion program - Plasma physics on the scale of microns and picoseconds

Diagnostic techniques are reviewed in the context of insights they provide to physical processes dominating the implosion of laser fusion targets. Specific examples chosen from recent experiments demonstrate that adequate diagnostic techniques, resolved to microns and picoseconds, have been developed for the demanding requirements of laser exploding pusher targets. Future diagnostic needs are related to the special problems forseen with ablatively driven high density implosions.

Interferometric measurement of laser fusion targets

We describe techniques for the quantitative interferometric characterization of wall thickness variations of hollow glass microspheres. By using a combination of techniques, one can rapidly form a picture of the total configuration for most defect combinations. These techniques require only very simple calculations and do not involve detailed ray tracing through the sphere. We also show how ray tracing calculation can be done very simply for careful analysis of the interference phenomenon in a perfect sphere. These calculations show that if both a large illumination angle and a large aperture objective are used, the nonparallel illumination causes degradation of both the spatial and the phase resolution of the interferometer. These problems can be overcome by properly aperturing the illumination.