Pressurized Liquid Helium Research Articles

Hot-wire anemometry for superfluid turbulent coflows.

We report the first evidence of an enhancement of the heat transfer from a heated wire to an external turbulent coflow of superfluid helium. We used a standard Pt-Rh hot-wire anemometer and overheat it up to 21 K in a pressurized liquid helium turbulent round jet at temperatures between 1.9 K and 2.12 K. The null-velocity response of the sensor can be satisfactorily modeled by the counterflow mechanism, while the extra cooling produced by the forced convection is found to scale similarly as the corresponding extra cooling in classical fluids. We propose a preliminary analysis of the response of the sensor and show that-contrary to a common assumption-such sensor can be used to probe local velocity in turbulent superfluid helium.

Laser spectroscopic study of Mg atoms in pressurized liquid helium

Emission and excitation spectra of the 3s 2 1S 0–3s3p 1P 1 transition of Mg atom in pressurized liquid helium-4 and helium-3 have been measured. In the emission spectra we have found that their transition wavelengths, which are significantly red shifted, remain constant or slightly increase with increasing the liquid helium pressure, whereas for other alkali–earth atoms the increase of the liquid pressure always shifts their emission spectra toward shorter wavelength. Our theoretical calculations based on a bubble model have successfully reproduced this unique spectral property, and have suggested the possibility of the formation of a Mg(3s3p 1P 1)He n exciplex in a bubble.

Insulation performance of pressurized liquid helium under quench-induced thermal bubble disturbance for superconducting power apparatus

We have been studying quench-induced 'dynamic' breakdown characteristics of LHe and have already found that the electrical insulation performance of LHe was degraded drastically by the thermal bubble disturbance due to quench of superconductors. In this paper, in order to improve the insulation performance of LHe under quench conditions, we measured dynamic breakdown and prebreakdown characteristics of pressurized LHe. Experimental results revealed that breakdown voltage of LHe under quench conditions at 0.2 MPa was greatly improved, reaching 2 to 4/spl times/ compared with that at atmospheric pressure under both uniform and non-uniform electric fields. Moreover, for practical and efficient insulation design of superconducting power apparatus, we investigated the dynamic breakdown voltage of LHe as functions of pressure and gap length.

Influence of quench-induced thermal bubble disturbance on insulation performance of liquid helium for superconducting power apparatus

Quench of superconductor is an unavoidable phenomenon in superconducting power apparatus. The insulation performance of liquid helium (LHe) may be drastically degraded in thermal bubble disturbance under quench conditions. In this paper, we investigated the influence of quench-induced thermal bubble disturbance on insulation performance of LHe for practical design of superconducting power apparatus. Experimental results revealed that the dynamic breakdown voltage of LHe under uniform and non-uniform electric fields could be quantitatively evaluated by injected thermal energy during the quench period of superconductors. Moreover, suppression of thermal bubbles in pressurized LHe was verified to be effective to improve the insulation performance of LHe under quench conditions.

Optical properties of alkali-metal atoms in pressurized liquid helium

The excitation and emission spectra of alkali-metal atoms (Cs and Rb) in liquid helium are studied. The helium pressure is changed from the saturated vapor pressure to about 25 atm at 1.6 K, under which conditions the liquid helium is in a superfluid state. The pressure shift and broadening of the observed spectra corresponding to the D lines are explained qualitatively by a theoretical calculation based on the spherical atomic bubble model. The Cs ${\mathit{D}}_{2}$ excitation spectra are found to have double peaks, indicating the existence of anisotropic oscillations of the bubble surface. The disappearance of the ${\mathit{D}}_{2}$ emission lines and the pressure-dependent quenching of the Rb ${\mathit{D}}_{1}$ emission lines are also discussed.

An optical window with thermal contraction free seal for low temperature use

A new type window seal for low temperature use is reported. In the present seal, a normal component of thermal stresses to the quartz window is essentially eliminated by using an expansion coefficient difference between iron and brass. This seal is found to be leak tight to pressurized liquid helium, at least up to 0.5 MPa and withstands many cooling and warming cycles.

Motion of negative ions at supercritical drift velocities in liquidHe4at low temperatures

The motion of negative ions at supercritical drift velocities in pressurized liquid helium is governed at low temperatures by the spontaneous emission of rotons. Assuming a constant matrix element the transition probability for two-roton emission processes is calculated using perturbation theory. The effect of recoil on the ionic motion is studied qualitatively by means of a simple kinetic approach and quantitative calculations are performed using the Boltzmann equation. The calculated dependence of drift velocity on electric field $E$ shows that an ${E}^{\frac{1}{3}}$ variation occurs over a substantial range of fields as found experimentally. Comparison with experimental data for fields $2<E<{10}^{3}$ kV ${\mathrm{m}}^{\ensuremath{-}1}$ is made and deviations from the ${E}^{\frac{1}{3}}$ law at high fields are accurately accounted for. The role of vortex nucleation is discussed and further experiments are suggested.