Pure 4He Research Articles

Bubble nucleation in a superfluid He3–He4 mixture induced by acoustic wave

The nucleation dynamics of bubbles induced by an acoustic wave pulse in He3–He4 liquid mixtures is reported. The experiment is performed mixtures with fixed He3 concentrations of about 4% and 25% and for pure superfluid He4. When a pulse of 1ms duration is applied to the mixture at the saturated vapor pressure, a spherical bubble is nucleated on the active area of a piezoelectric transducer. For the case of pure He4 it is not a spherically shaped bubble but an irregularly shaped, larger one that is observed. Pictures of the bubble expansion and contraction motions after the nucleation are taken with a high speed CCD camera. The temperature dependence of the bubble nucleation is also investigated by changing the acoustic wave power. The results show that the nucleation and growth dynamics of a cavitation bubble depend greatly on the He3 concentration and temperature.

Osmotic Pressure of 3He-4He Solutions at 25.3 Bar and Low Temperatures

The osmotic pressures of dilute 3He-4He solutions were determined at 25.3 bar from measurements of crystallization curves at temperatures from 5 mK to 60 mK, when the 3He component of the solution obeying the Fermi-Dirac statistics was deep in the degenerate state. We determine the shift of the crystallization pressure of the solution of interest relative to pure 4He when both these substances are present in the cell in two separate volumes at the same temperature. We used our novel ultra-sensitive capacitive gauge for measurements of small pressure differences between the two substances. We used a quartz resonator for determination of solution’s concentration in situ. The difference between the crystallization pressure of the saturated solution and pure 4He, both extrapolated to zero temperature, is (339±2) mbar.

Energy Loss of Fourth Sound in Superfluid 3He within Small Pores

We performed a high-sensitivity fourth sound resonance experiment for pure superfluid 3He at 29.0 bar in sintered silver powder cell to clarify the hydrodynamic property of 3He in the aerogel-sintered silver system. We discuss the energy loss of fourth sound. An anomaly in the energy loss at the AB phase transition as found in the aerogel system was not observed. Our analysis shows that the energy loss Q−1, which monotonically decreases with decreasing temperature, can be understood by the hydrodynamic theory in the B phase qualitatively and quantitatively. We estimated the effective pore radius R in the powder cell.

Resonance transition 795-nm rubidium laser using 3He buffer gas

We report a demonstration of a 795-nm rubidium optical resonance transition laser using a buffer gas consisting of pure 3He. This follows our recent demonstration of a hydrocarbon-free 795-nm rubidium resonance laser which used naturally-occurring He as the buffer gas. Using He gas that is isotopically enriched with 3He yields enhanced mixing of the Rb fine-structure levels. This enables efficient lasing at reduced He buffer gas pressure, improved thermal management in high average power Rb lasers and enhanced power scaling potential of such systems.

Critical Casimir effect in superfluid wetting films

Recent experimental data for the complete wetting behavior of pure 4He and of 3He-4He mixtures exposed to solid substrates show that there is a change of the corresponding film thicknesses L upon approaching thermodynamically the lambda transition and the tricritical end point, respectively, which can be attributed to critical Casimir forces fC. We calculate the scaling functions theta of fC within models representing the corresponding universality classes. For the mixtures our analysis provides an understanding of the rich behavior of theta deduced from the experimental data and predicts the crossover behavior between the tricritical point and the lambda transition of pure 4He which are connected by a line of critical points. The formation of a "soft-mode" phase within the wetting films gives rise to a pronounced maximum of fC below the tricritical point as observed experimentally. Near the tricritical point we find logarithmic corrections approximately L(-3)(ln L)1/2 for the leading behavior of theta dominating the contributions from the background dispersion forces.

Volume ignition of 3He pellets

During the past 15 years, using the volume ignition model, a computational code has been designed and gradually developed with the purpose of simulating the laser or ion projectile driven nuclear fusion. Up to now using this code various calculations have been done in different conditions for evaluating fusion gain and time dependence of temperature of fuels such as DD, DT, H 11B and D 3He, respectively, and the results have been published in several papers [R. Khoda-Bakhsh, et al., Fusion Technol. 22 (1992) 50; R. Khoda-Bakhsh, Nucl. Instr. and Meth. A 330 (1993) 268; G.H. Miley, et al., Fusion Technol. 19 (1991) 50 [3]; R. Khoda-Bakhsh, et al., Fusion Technol. 24 (1993) 28; R. Khoda-Bakhsh, AIP Conf. Proc. 369 (Part One) (1996) 289 [5]]. Here, using this extended code and for the first time, we have simulated the conditions under which pure 3He fuel pellets with different volumes and initial compressions begin to ignite. Fusion gains of more than 50 are obtained with an ignition temperature decreased to only 22.9 keV at an initial compression to 500 000 times the solid-state density. Also calculations showed that for spin-polarized fuel, there is a driver energy reduction which for maximum gain condition, is less for low initial densities and a gain increase which varies irregularly for different initial conditions.

Adiabatic Melting of 4He Crystal in Superfluid 3He at Sub-millikelvin Temperatures

Adiabatic melting of 4He crystal to phase separated 3He–4He solution (at T< 2 mK) is probably the most promising method to cool the dilute phase down to temperatures substantially below 0.1 mK. When started well below the superfluid transition temperature Tc of pure 3He, this process allows, in principle, to get the final temperature (Tf) several orders of magnitude less than the initial one (Ti). This work is the first practical implementation of the method below the Tc of 3He. The observed cooling factor was Ti/Tf=1.4 at 0.9 mK, being mainly limited by the bad performance of the superleak filling line, by incomplete solidification of 4He in the cell, and by the improper thermal contact between the cell wall and the liquid.

Studying creation of bulk elementary excitation by heaters in superfluid helium-II at low temperatures

In this paper, the obtained experimental results concerning creation of bulk elementary excitations (BEEs) in isotopically pure liquid 4He at low temperatures ∼60 mK are discussed. Positive rotons’ (R+-rotons) creation by a pulsed heater was studied. Signals were recorded for the following quantum processes: quantum evaporation of 4He-atoms from the free liquid-helium surface by the BEEs of the liquid helium-II, and BEEs reflection from the free surface back into the bulk liquid. Typical signals are shown, and ratios of signal amplitudes are evaluated. For long heater pulses from 5 to 10 μs, appearance of the second atomic cloud consisting of evaporated 4He-atoms was observed in addition to the first atomic cloud. It is thought that the first atomic cloud of the evaporated helium atoms consists of very fast 4He-atoms with energies ∼35 K evaporated by positive rotons with the special energies ∼17 K (∼2ER∼2×8.6 K with ER representing the roton minimum energy) corresponding to the third non-dispersive Zakharenko wave. The second cloud of slower 4He-atoms was created by surface elementary excitations (SEEs or ripplons) possessing the special energies ∼7.15 K representing the binding energy. It was assumed that such SEEs can be created by phonons incoming to the liquid surface with special energies ∼6.2 K corresponding to the first non-dispersive Zakharenko wave, which can interact at the liquid surface with the same phonons already reflected from the surface for long heater pulses. Also, some pulsed-heater characteristics were studied in order to better understand the features of such heaters in low temperature experiments.

Acoustic Properties of Normal Liquid 3He in 98% Aerogel

We have performed longitudinal ultrasound (9.5 MHz) attenuation and sound velocity measurements in the normal state of liquid 3He in 98% aerogel. The absolute attenuation and sound velocity were determined by direct propagation of sound pulses through the medium in a wide range of temperatures, 2 mK<T<200 mK. Due to the scattering off the aerogel, the sound excitation remains as first sound over the entire range of temperatures and pressures studied. Unlike pure liquid 3He, the sound attenuation shows a minimum around 30–50 mK, depending on the pressure. We report our results of absolute sound attenuation measurements at 29 bars of sample pressure.

Time-Dependent Nuclear Measurements of Mix in Inertial Confinement Fusion

The first time-dependent nuclear measurements of turbulent mix in inertial confinement fusion have been obtained. Implosions of spherical deuterated-plastic shells filled with pure 3He gas require atomic-scale mixing of the shell and gas for the D-3He nuclear reaction to proceed. The time necessary for Rayleigh-Taylor (RT) growth to induce mix delays peak nuclear production time, compared to equivalent capsules filled with a D2-3He mixture, by 75+/-30 ps, equal to half the nuclear burn duration. These observations indicate the likelihood of atomic mix at the tips of core-penetrating RT spikes.

Nuclear measurements of fuel-shell mix in inertial confinement fusion implosions at OMEGA

Direct drive spherical implosions on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have shown that increased capsule convergence results in increased susceptibility to fuel-shell mix. Mix results from saturation of the Rayleigh-Taylor instability, leading to small-scale, turbulent eddies and atomic-level mixing of the high-density compressed shell with hot, low-density fuel from the core. To sensitively probe the extent of mix, nuclear yields were measured from implosions of capsules filled with pure He3. The plastic capsule shell contains a deuterated plastic (CD) layer either on the inner surface or offset from the inner surface by 1μm. Mixing of D from the shell with hot He3 in the core is necessary to produce 14.7MeV DHe3 protons in such capsules. DHe3-proton spectral measurements have been used to constrain the amount of mix at shock time, to demonstrate that some of the fuel mixes with the offset CD layer, and that capsules with a higher initial fill density or thicker shell are less susceptible to the effects of mix.

Molar volume of solid isotopic helium mixtures

Solid isotopic helium mixtures have been studied by path-integral Monte Carlo simulationsin the isothermal–isobaric ensemble. This method allowed us to study the molar volumeas a function of temperature, pressure, and isotopic composition. At 25 K and0.2 GPa, the relative difference between molar volumes of isotopically pure crystals of3He and 4He is found to be about 3%. This difference decreases under pressure, and for 12 GPa it issmaller than 1%. For isotopically mixed crystals, a linear relation between latticeparameters and concentrations of helium isotopes is found, in agreement withVegard’s law. The virtual crystal approximation, valid for isotopic mixtures ofheavier atoms, does not give reliable results for solid solutions of helium isotopes.

Measurement of helium-3 and deuterium stopping power ratio for negative muons

The measurement method and results measuring of the stopping power ratio of helium-3 and deuterium atoms for muons slowed down in the D/3He mixture are presented. Measurements were performed at four values of pure 3He gas target densities, ϕHe = 0.0337, 0.0355, 0.0359, 0.0363 (normalized to the liquid hydrogen density) and at a density 0.0585 of the D/3He mixture. The experiment was carried out at PSI muon beam μE4 with the momentum Pμ= 34.0 MeV/c. The measured value of the mean stopping ratio S$_{3^He/D}$ is 1.66±0.04.

Tests of the hydrodynamic equivalence of direct-drive implosions with different D2 and He3 mixtures

Direct drive implosions of targets filled with different mixtures of D2 and He3 gas on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have shown an unexpected scaling of experimental nuclear yields. At temperatures above a few electron volts, D2 and He3 gases are fully ionized, and hydrodynamically equivalent fuels with different ratios of D2 and He3 can be chosen to have the same mass density, total particle density, and equation of state. Implosions with a 50/50 mixture of D:He3 by atom consistently result in measured nuclear yields half of that anticipated by scaling from measured yields of implosions with pure D2 and nearly pure He3. This observation is seen over a wide range of experimental configurations, including targets with a variety of shell thicknesses and fill pressures, simultaneously for two different nuclear yields (DD and DHe3), and for shock and compression yields. A number of possible mechanisms to cause the scaling are considered, but no dominant mechanism has been identified.

Strontium isotopes in melt inclusions from Samoan basalts: Implications for heterogeneity in the Samoan plume

We measured 87Sr/ 86Sr ratios on 41 olivine-hosted melt inclusions from nine Samoan basalts using laser ablation multi-collector (LA-MC) ICPMS. 87Sr/ 86Sr ratios are corrected for mass bias after eliminating major isobaric interferences from Rb and Kr. The external precision averages ± 320 ppm (2 σ) for the 87Sr/ 86Sr ratios on natural Samoan basalt glass standards of a similar composition to the melt inclusions. All of the Sr-isotope ratios measured by LA-MC-ICPMS on Samoan melt inclusions fall within the range measured on whole-rocks using conventional methods. However, melt inclusions from two Samoan basalt bulk rock samples are extremely heterogeneous in 87Sr/ 86Sr (0.70459–0.70926), covering 70% of the variability observed in ocean island basalts worldwide and nearly all of the variability observed in the Samoan island chain (0.7044–0.7089). Seven melt inclusions from a third high 3He/ 4He Samoan basalt are isotopically homogeneous and exhibit 87Sr/ 86Sr values from 0.70434 to 0.70469. Several melt inclusions yield 87Sr/ 86Sr ratios higher than their host rock, indicating that assimilation of oceanic crust and lithosphere is not the likely mechanism contributing to the isotopic variability in these melt inclusions. Additionally, none of the 41 melt inclusions analyzed exhibit 87Sr/ 86Sr ratios lower than the least radiogenic basalts in Samoa ( 87Sr/ 86Sr = 0.7044), within the quoted external precision. This provides an additional argument against assimilation of oceanic crust and lithosphere as the source of the isotopic diversity in the melt inclusions. The trace element and isotopic diversity in Samoan melt inclusions can be modeled by aggregated fractional melting of two sources: A high 3He/ 4He source and an EM2 (enriched mantle 2) source. Melts of these two sources mix to generate the isotopic diversity in the Samoan melt inclusions. However, the melt inclusions from a basalt with the highest 3He/ 4He ratios in Samoa exhibit no evidence of an enriched component, but can be modeled as melts of a pure high 3He/ 4He mantle source.

Growth of third ferromagnetic solid 3He layer on graphite with and without 4He pre-plating

We have measured the nuclear susceptibility of 3He in Grafoil filled with pure liquid 3He over the pressure region between 0.6 and 31.38 bar and at temperatures down to 0.5 mK with a cw NMR method. The nuclear magnetization corresponding to the adsorbed 3He layers on the Grafoil surface shows a strong ferromagnetic tendency with a periodic behavior as a function of liquid pressure. This observation is attributable to the growth of third and fourth solid 3He layer with the liquid pressure increase. The pressure dependence of the Weiss temperature indicates that the third layer is completed at 19 bar and the fourth probably at 28 bar. The results for Grafoil pre-plated by 2.5 and 3.5 layers of 4He are consistent with this scenario.

3He Impurities in 4He Systems Adsorbed on Curved Substrates

It is shown that 3 He impurities in sufficiently large 4 He systems adsorbed onto substrates with curved geometries form surface bound states, analogous to the Andreev state on a planar liquid--vapor interface. We report the analysis performed for superfluid 4 He adsorbed on the external surface of the nano-fullerene C 60 and on cylindrical nano-wires of Au. It is found that a single 3 He impurity diluted into such adsorbed structures behaves as on films on planar substrates and as on pure 4 He clusters.

Surface Magnetism of Liquid 3He on 4He Pre-plated Graphite

The nuclear susceptibility of liquid 3He in Grafoil pre-plated by 2.5 and 3.5 layers of 4He has been studied with a cw NMR method at temperatures between 0.7 and 100 mK under various liquid pressures. The 3.5 layers of 4He pre-plating suppresses a formation of the first and second solid 3He layer, eliminating most of surface magnetization at saturated vapor pressure. However, with increasing liquid pressure, a magnetization obeying a Curie Weiss law gradually grows in the same way as in the previous experiment for pure liquid 3He. This magnetization, induced by pressurization, is attributable to the formation of solid 3He layer above the pre-plated 4He.

Superfluid Phases of 3He in Aerogel

Formal derivation of criterion for selection of superfluid phases of 3He in aerogel is presented. At the strength of the derived criterion variation of the order parameter of B-like phase in magnetic field differs from that of B-phase of pure 3He. Possible observable consequences of this difference is discussed.

Growth of a Third Ferromagnetic SolidH3eLayer on Graphite

We have measured the nuclear susceptibility of 3He in Grafoil filled with pure liquid 3He over the pressure region between 0.6 and 31.38 bars and at temperatures down to 0.5 mK with a cw NMR method. The nuclear magnetization corresponding to the adsorbed 3He layers on the Grafoil surface shows a strong ferromagnetic tendency with a periodic behavior as a function of liquid pressure. This observation is attributable to the growth of third and fourth solid 3He layers with the liquid pressure increase. The pressure dependence of the Weiss temperature indicates the third layer is completed at 19 bars and the fourth probably at 28 bars. The number of localized spins estimated from the solid magnetization is almost doubled from 0 to 28 bars, being consistent with this scenario.