Performance of adiabatic melting as a method to pursue the lowest possible temperature in 3^3He and 3^3He-4^4He mixture at the 4^4He crystallization pressure

We studied a novel cooling method, in which $^3$He and $^4$He are mixed at the $^4$He crystallization pressure at temperatures below $0.5\,\mathrm{mK}$. We describe the experimental setup in detail, and present an analysis of its performance under varying isotope contents, temperatures, and operational modes. Further, we developed a computational model of the system, which was required to determine the lowest temperatures obtained, since our mechanical oscillator thermometers already became insensitive at the low end of the temperature range, extending down to $\left(90\pm20\right)\,\mathrm{\mu K\approx}\frac{T_{c}}{\left(29\pm5\right)}$ ($T_{c}$ of pure $^3$He). We did not observe any indication of superfluidity of the $^3$He component in the isotope mixture. The performance of the setup was limited by the background heat leak of the order of $30\,\mathrm{pW}$ at low melting rates, and by the heat leak caused by the flow of $^4$He in the superleak line at high melting rates up to $500\,\mathrm{\mu mol/s}$. The optimal mixing rate between $^3$He and $^4$He, with the heat leak taken into account, was found to be about $100..150\,\mathrm{\mu mol/s}$. We suggest improvements to the experimental design to reduce the ultimate achievable temperature further.Comment: 39 pages, 24 figure

Thermodynamics of adiabatic melting of solid He 4 in liquid He 3

In the cooling concept by adiabatic melting, solid He4 is converted to liquid and mixed with He3 to produce cooling power directly in the liquid phase. This method overcomes the thermal boundary resistance that conventionally limits the lowest available temperatures in the helium fluids and hence makes it possible to reach for the temperatures significantly below 100μK. In this paper we focus on the thermodynamics of the melting process, and examine the factors affecting the lowest temperatures achievable. We show that the amount of He3-He4 mixture in the initial state, before the melting, can substantially lift the final temperature, as its normal Fermi fluid entropy will remain relatively large compared to the entropy of superfluid He3. We present the collection of formulas and parameters to work out the thermodynamics of the process at very low temperatures, study the heat capacity and entropy of the system with different liquid He3, mixture, and solid He4 contents, and use them to estimate the lowest temperatures achievable by the melting process, as well as compare our calculations to the experimental saturated He3-He4 mixture crystallization pressure data. Realistic expectations in the execution of the actual experiment are considered. Further, we study the cooling power of the process, and find the coefficient connecting the melting rate of solid He4 to the dilution rate of He3.Peer reviewe

Thermal Conductivity of Superfluid 3 He-B in a Tubular Channel Down to 0.1 Tc at the 4 He Crystallization Pressure

We studied the thermal conductivity of superfluid 3He in a 2.5-mm effective diameter and 0.15-m-long channel connecting the two volumes of our experimental assembly. The main volume contained pure solid 4He, pure liquid 3He and saturated liquid 3He–4He mixture at varying proportions, while the separate heat-exchanger volume housed sinter and was filled by liquid 3He. The system was cooled externally by a copper nuclear demagnetization stage, and, as an option, internally by the adiabatic melting of solid 4He in the main volume. The counterflow effect of superfluid just below the transition temperature Tc resulted in the highest observed conductivity about five times larger than that of the normal fluid at the Tc. Once the hydrodynamic contribution had practically vanished below 0.5 Tc, we first observed almost constant conductivity nearly equal to the normal fluid value at the Tc. Finally, below about 0.3 Tc, the conductivity rapidly falls off toward lower temperatures.Peer reviewe

Effects of 4He Film on Quartz Tuning Forks in 3He at Ultra-low Temperatures

| openaire: EC/H2020/694248/EU//TOPVACIn pure superfluid 3He–B at ultra-low temperatures, the quartz tuning fork oscillator response is expected to saturate when the dissipation caused by the superfluid medium becomes substantially smaller than the internal dissipation of the oscillator. However, even with a small amount of 4He covering the surfaces, we have observed saturation already at significantly higher temperatures than anticipated, where we have other indicators to prove that the 3He liquid is still cooling. We found that this anomalous behavior has a rather strong pressure dependence, and it practically disappears above the crystallization pressure of 4He. We also observed a maximum in the fork resonance frequency at temperatures where the transition in quasiparticle flow from the hydrodynamic to the ballistic regime is expected. We suggest that such anomalous features derive from the superfluid 4He film on the oscillator surface.Peer reviewe