Cryogenic Target Characterization

March 2004

Cryogenic Target Characterization

The ultimate goal of inertial confinement fusion is to produce more thermonuclear energy from a laser implosion than the energy required to power the laser. Such a positive gain is a potential source of energy that can be converted to electricity. To reach this goal, laser fusion experiments must use high quality cryogenic targets. The bulk of the research performed at LLE is directed towards understanding the basic physics issues involved in these experiments and includes fabrication and characterization of cryogenic targets.

A laser fusion target is a spherical capsule filled with gaseous deuterium-tritium (DT) mix. Lowering the temperature to below 18K (18 degrees above absolute zero temperature) causes the DT (or D2) gas to solidify inside the capsule. By carefully freezing out the gas/liquid DT it can be made to form a near perfect ice shell attached to the inside of the plastic shell. Such a target can then be very efficiently compressed when uniformly irradiated by a powerful laser. LLE’s cryogenic targets are composed of a thin plastic shell or a thicker plastic foam shell with the deuterium–tritium (DT) mix in the form of ice layers on the inside of the shell. For LLE’s cryogenic experiments to be a success, the ice layers must be very smooth and accurately centered prior to irradiation by the laser system. LLE has developed a process to characterize, or analyze, the quality of our targets prior to fielding them for experiments.

Targets for current OMEGA experiments are filled with approximately ~1000 atm of D2 (a hydrogen isotope) at room temperature and then cooled to ~ 18K (–255°C). Smooth D2-ice layers of approximately 100 µm are thus obtained inside the plastic shell. The target is suspended on four spider silks; this minimizes disturbances of the outer shell surface. A photograph of the target is taken using optical shadowgraphy. Based on these shadowgrams different features of the target are analyzed numerically to describe surface and ice thickness variations and predict potential target performance. A typical shadowgram of a cryogenic target is shown in Fig. 1(a). The outermost bright (white) ring inside the target surface represents the ice layer. The breaks in the ring are due to either outer-surface imperfections, interference from the spider webs, or ice nonuniformities. For a single target, as many as 30 shadowgrams are taken at different angles. These shadowgrams are then analyzed using fast Fournier transforms (FFT) to produce a power spectra, as shown in Fig. 1(b). The same shadowgrams are collated to form a three-dimensional image of the ice-layer thickness as shown in Fig. 1(c). Also shown in this figure are the locations of the great circles for which data were taken. In addition, the X and Y viewing directions are indicated, as are the OMEGA X, Y, and Z-axes locations. The ice thickness variations are shown in color (see color bar). Due to the locations of the viewing ports, there are dead zones near the poles where no data are collected.

The shadowgraphic technique has been analyzed using a 1-D ray-trace simulation (DOE Monthly Report March 2003) and a 3-D optics code (courtesy of Prof. Thomas Brown, UR/The Institute of Optics). The former gave precise information about the locations and origins of the many different rings that may be observed. By contrast, the optics code was used to understand the importance of outer-surface perturbations on perceived inner-ice-surface perturbations. Figure 2 shows simulated bright ring positions for a target with a perfect outer surface and a perturbed inner ice layer. The ice-layer perturbations are fully measurable in this case (a). For a perturbed outer surface and a perfect ice layer, the ring may or may not be distorted depending on the exact location of the outer-surface perturbations[(b) and (c)]. This demonstrates that outer-surface perturbations have to be kept small compared to the expected inner-ice-surface perturbations. It also confirms observations where small outer-surface perturbations or miniscule "dust" particles may lead to partial or complete disruptions of the ring.

The limits of accuracy of the shadowgraphic characterization technique are illustrated in Fig. 3. A sapphire sphere was first characterized with an atomic force microscope (AFM) and then with our optical shadowgraphy. The average spectra obtained by these two techniques are shown in Fig. 3 and demonstrate that the AFM technique is four times more accurate than the optical technique. However, only the optical technique can be used for cryogenic target characterization and its accuracy satisfies our experimental requirements.

The current effort at LLE to characterize cryogenic D2-ice layers prepares the way to future experiments on the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL). In the near term this effort provides characterized cryogenic targets for current implosion experiments on OMEGA and for layering studies necessary to create the requisite D2- or DT-ice layers.

Figure 1

Fig. 1. Shadowgram of a cryogenic target with an ~100 µm ice layer (a). Average power spectra of outer target surface and ring due to the ice layer (b). Three-dimensional view of ice layer thickness variations obtained from a large number of shadowgrams taken from the x-viewing direction (c).

Figure 2

Fig. 2. Results obtained with a 3-D optics imaging code for a perfect outer target surface with inner ice layer perturbations (Y33 with 0.25-µm amplitude) (a). The inner ice surface perturbations are easily measurable. Outer surface perturbations may or may not cause apparent ice surface perturbations as shown in (b) and (c).

Figure 3

Fig. 3. AFM power spectrum of a sapphire sphere (lower trace, blue). Upper trace (red) is an average spectrum of many shadowgrams taken at different angles for the same target.