A horizontal cylindrical cavity bounded by isothermal walls was partially filled with liquid tritium which was then frozen by reduction of the temperature to 1.0 K below the triple point. Visual observations revealed that the solid subsequently redistributed itself into a layer of uniform thickness covering the complete interior of the cavity. The time constant for this effect depends on the age (or 3 He content) of the tritium and not on the initial filling fraction. Time constants of 14.9, 92, 219, and 234 min were measured for tritium 0. 04, 7, 16, and 17 days old, respectively. PACS numbers: 52.55.Pi, 05.70.Fh, 23.20.Nx, 64.70.Hz Radioactive isotopes are classic examples of materials which exhibit internal self-heating. Tritium decays to 3 He, emitting a p particle and an antineutrino. Because the /3*s are reabsorbed within approximately 10 ^m, condensed tritium samples have a nearly uniform selfheating rate q and a quadratically increasing temperature profile in the direction away from the containing boundary. The interior surface of a thick layer of tritium can thus be warmer than the interior surface of a nearby, thinner layer, as long as the exterior surfaces of these layers are equal in temperature or are radiating with equal emissivities to an infinite thermal sink. Being warmer, the interior surface of the thicker layer has a higher vapor pressure than the interior of the thinner layer, and a preferential sublimation-condensation effect can occur, tending to make the layers uniform in thickness. This effect, dubbed the "/? heating effect," was first proposed by Martin, Simms, and Musinski 1 as a method of preparing the "ideal" inertial-confinement fusion target, namely a uniform spherical shell of DT.Martin and Simms 2 have constructed a one-dimensional model of the p heating effect which predicts that layer-thickness equilibration proceeds exponentially in time with a minimum time constant T m { n =H s /q, where H s is the heat of sublimation of the solid. For pure T2 at 19.6 K, T m in = 14.4 min, representing the rate constant for the hypothetical case where there is no impedance to the flow of vapor in the cavity. Without repeating the arguments used in Ref. 2, we can easily derive the above expression for one-dimensional slabs of material bounded by semi-infinite isothermal plane surfaces. Imagine a layer of radioactively heated solid completely filling a finite space between two such semi-infinite surfaces, both at the same temperature. At steady-state conditions, the solid will have a parabolic temperature profile with a maximum at the exact center. Take a thin sliver of the solid starting at a distance 8 from the center and expand it many times its original width, turning the solid into vapor and simultaneously creating a space between two layers of solid of unequal thickness, as shown in Fig. 1. We now assume that the impedance to the flow of vapor is so small that the process of sublimation and condensation can transport heat just as effectively as did the original solid sliver. In other ...

Capsules with beryllium ablators have long been considered as alternatives to plastic for the National Ignition Facility laser ; now the superior performance of beryllium is becoming well substantiated . Beryllium capsules have the advantages of relative insensitivity to instability growth, low opacity, high tensile strength, and high thermal Zimmerman and W. L. h e r , Comments Plasmas Phys. Controlled Thermonucl. Fusion, 2 , 5 1 (2975)l results that particular beryllium capsule designs are several times less sensitive than the CH point design to instability growth from DT ice roughness. These capsule designs contain more ablator mass and leave some beryllium unablated at ignition. By adjusting the level of copper dopant, the unablated mass can increase or decrease, with a corresponding decrease or increase in sensitivity to perturbations . A plastic capsule with the same ablator mass as the beryllium and leaving the same unablated mass also shows this reduced perturbation sensitivity. Beryllium's low opacity permits the creation of 250 eV capsule designs. Its high tensile strength allows it to contain DT fuel at room temperature. Its high thermal conductivity simplifies cryogenic fielding.

The constant-volume heat capacity of '*He has been measured at molar volumes from 20.45 to 29.71 cma/mole in the temperature range from 0.3 to 4 K. The entropy has been obtained as a function of volume and temperature by extrapolation of the data to 0 K. The P-T equilibrium curves below 2 K were obtained from the volume dependence of the entropy in the two-phase regions. The V-T curves were obtained above 1.25 K by observation of heat capacity discontinuities at the phase boundaries and below 1.25 K from the equilibrium pressure-temperature data and the properties of the pure phases. The minimum in the melting pressure occurs at 0.774 K and is 8.04 • 10-3 atm below the 0 K value. The corresponding maxima in the molar volumes of the solid and liquid were also determined. In the bcc phase (~S/t~V) x and (OCv/t~V) r are everywhere positive. Both the temperature and volume dependence of the heat capacity are similar to those of bcc 3He, in the very limited ranges of volume and temperature in which the phase could be studied. An unexpected rise in heat capacity in the 20 mK interval below the melting temperature was observed.