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Register a new userNucleation of helium in liquid lithium
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Fusion energy stands out as a promising alternative for a future decarbonised energy system. To be sustainable, future fusion nuclear reactors will have to produce their own tritium. In the so-called breeding blanket of a reactor, the neutron bombardment of lithium will produce the desired tritium, but also helium, which can trigger nucleation mechanisms owing to the very low solubility of helium in liquid metals. An understanding of the underlying microscopic processes is important for improving the efficiency, sustainability and reliability of the fusion energy conversion process. A spontaneous creation of helium drops or bubbles in the liquid metal used as breeding material in some designs may be a serious issue for the performance of the breeding blankets. This phenomenon has yet to be fully studied and understood. This work aims to provide some insight on the behavior of lithium and helium mixtures at experimentally corresponding operating conditions (843 K and pressures between 0.1 and 7 GPa). We report a microscopic study of the thermodynamic, structural and dynamical properties of lithium-helium mixtures, as a first step to the simulation of the environment in a nuclear fusion power plant. We introduce a microscopic model devised to describe the formation of helium drops in the thermodynamic range considered. A transition from a miscible homogeneous mixture to a phase-separated one, in which helium drops are nucleated, is observed as the pressure is increased above 0.175 GPa. The diffusion coefficient of lithium (2 {AA} 2 /ps) is in excellent agreement with reference experimental data, whereas the diffusion coefficient of helium is in the range of 1 {AA} 2 /ps and tends to decrease as pressure increases. The radii of helium drops have been found to be between 1 and 2 {AA}.
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The second-layer phase diagrams of $^4$He and $^3$He adsorbed on graphite are investigated. Intrinsically rounded specific-heat anomalies are observed at 1.4 and 0.9 K, respectively, over extended density regions in between the liquid and incommensurate solid phases. They are identified to anomalies associated with the Kosterlitz-Thouless-Halperin-Nelson-Young type two-dimensional melting. The prospected low temperature phase (C2 phase) is a commensurate phase or a $textit{quantum hexatic}$ phase with quasi-bond-orientational order, both containing $textit{zero}$-$textit{point}$ defectons. In either case, this would be the first atomic realization of the $textit{quantum liquid crystal}$, a new state of matter. From the large enhancement of the melting temperature over $^3$He, we propose to assign the observed anomaly of $^4$He-C2 phase at 1.4 K to the hypothetical supersolid or superhexatic transition.
On the basis of first-principle Monte Carlo simulations we find that the screw dislocation along the hexagonal axis of an hcp He4 crystal features a superfluid core. This is the first example of a regular quasi-one-dimensional supersolid, and one of the cleanest cases of a regular Luttinger-liquid system. In contrast, the same type of screw dislocation in solid Hydrogen is insulating.
We study two techniques to create electrons in a liquid helium environment. One is thermionic emission of tungsten filaments in a low temperature cell in the vapor phase with a superfluid helium film covering all surfaces; the other is operating a glowing filament immersed in bulk liquid helium. We present both the steady state and rapid sweep I-V curves and the electron current yield. These curves, having a negative dynamic resistance region, differ remarkably from those of a vacuum tube filament. A novel low temperature vapor-phase electron collector for which the insulating helium film on the collector surface can be removed is used to measure emission current. We also discuss our achievement of producing multi-electron bubbles (MEBs) in liquid helium by a new method.
Nucleation at large metastability is still largely an unsolved problem, although is a problem of tremendous current interest, with wide practical value. It is well-accepted that the classical nucleation theory (CNT) fails to provide a qualitative picture and gives incorrect quantitative values for such quantities as activation free energy barrier and supersaturation dependence of nucleation rate, especially at large metastability. In this article, we present a powerful alternative formalism to treat nucleation at large supersaturation. This formalism goes over to the classical picture at small supersaturation where CNT is expected to be valid. The new theory is based on an extended set of order parameters in terms of k-th largest liquid-like clusters where k=1 is the largest cluster in the system, k=2 is the second largest cluster and so on. We derive an analytic expression for the free energy of formation of the k-th largest cluster which shows that at large metastability the barrier of growth for the few largest liquid-like clusters disappear, the nucleation becomes collective and the approach to the critical size occurs by barrierless diffusion in the cluster size space. The expression for the rate of barrier crossing predicts a weaker supersaturation dependence than that of CNT at large metastability. Such a cross-over behavior has indeed been observed in recent experiments but eluded an explanation till now. In order to understand the large numerical difference between simulation predictions and experimental results, we carried out a study of the dependence on the range of intermolecular interaction of both the surface tension of an equilibrium planar gas-liquid interface and the free energy barrier of nucleation. Both are found to depend significantly on the range of interaction for a Lennard-Jones potential, both in two and three dimensions.
We present neutron scattering measurements of the dynamic structure factor, $S(Q,omega)$, of amorphous solid helium confined in 47 $AA$ pore diameter MCM-41 at pressure 48.6 bar. At low temperature, $T$ = 0.05 K, we observe $S(Q,omega)$ of the confined quantum amorphous solid plus the bulk polycrystalline solid between the MCM-41 powder grains. No liquid-like phonon-roton modes, other sharply defined modes at low energy ($omega<$ 1.0 meV) or modes unique to a quantum amorphous solid that might suggest superflow are observed. Rather the $S(Q,omega)$ of confined amorphous and bulk polycrystalline solid appear to be very similar. At higher temperature ($T>$ 1 K), the amorphous solid in the MCM-41 pores melts to a liquid which has a broad $S(Q,omega)$ peaked near $omega simeq$ 0 characteristic of normal liquid $^4$He under pressure. Expressions for the $S(Q,omega)$ of amorphous and polycrystalline solid helium are presented and compared. In previous measurements of liquid $^4$He confined in MCM-41 at lower pressure the intensity in the liquid roton mode decreases with increasing pressure until the roton vanishes at the solidification pressure (38 bars), consistent with no roton in the solid observed here.
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