Impurity ions in liquid helium

A technique for introducing a wide variety of positive ions into liquid helium is described and used to investigate positive atomic ions of K, Rb, Cs, Ca, Sr, and Ba. The zero-field mobilities of the impurity ions were measured in He II in the temperature range 1.2 < T < 1.4 K. The dependence of mobility on ion mass alone was found directly for Ca⁺ ions by investigating two isotopes. Measurements were made of the ion drift velocities in high electric fields, where nucleation of quantized vortex rings occurs. All the data and its analysis are consistent with the assumptions that all the impurity ions are singly charged, that the size of the cluster surrounding the ion is significantly different for the different ions, and that the ion-roton cross section is approximately geometric. A possible alternative model for the structure of the ion complex is discussed and a mechanism is suggested which may account for the observed variations in ionic radii.     

The influence of electrostatic fields on films of liquid helium

Prompted by the recent striking experimental results reported by Babkin and Hakonen that appeared to show that liquid helium-II does not wet magnesium fluoride, we have examined the effects that an inhomogeneous electrostatic field has on thin films of liquid helium at temperatures below 0.5K. Our model includes the influence of gravity, surface tension, the electric field and the van der Waals interaction between the helium and its supporting substrate. We show that, an inhomogeneous charge on the substrate can produce effects that mimic the surface profiles between wetted and non-wetted areas. The calculations also indicate that some special precautions may be necessary when studying films of liquid or solid helium on insulators.     

Forward electric field calculation using BEM for time-varying magnetic field gradients and motion in strong static fields

A boundary element method for evaluating the electric fields induced in conducting bodies exposed to magnetic fields varying at low frequency has been developed and applied to sources of magnetic field variation that are of relevance in magnetic resonance imaging. An integral formulation based on constant boundary elements which can be used to study the effects of both temporally varying magnetic field gradients and rigid body movement in a static magnetic field is presented. The validity of this approach has been demonstrated for simple geometries with known analytical solutions and it has also been applied to the evaluation of the induced fields in more realistic models of the human head.     

Relaxation time and effective mass of ions in liquid helium

A measurement of momentum relaxation times for charged particles in liquid helium is reported. The method is to measure the change in the reflection coefficient of a microwave cavity caused by the admission of ions. The measured relaxation times can be combined with the known dc mobilities of the ions to yield values for the effective masses. Most extensive measurements were performed using positive ions in He II. A temperature-dependent effective mass, increasing from approximately 40 helium masses at 1.3 K to over twice this value near the lambda point is found. The data are shown to be consistent with the electrostriction model of the ion, including contributions to the mass from both normal and superfluid components. A core radius of approximately 6 Å is shown to account both for the mass and for the mobility in the viscous flow region. Data on the negative carrier near 1.8 K yield an effective mass of 100 to 200 helium masses, and are consistent with the bubble model. The radius implied both by the mass and an analysis of mobility data in the viscous flow regime is 14±3 Å, quite consistent with the value deduced from other measurements. A measurement on the positive carrier in saturated (NBP) helium vapor indicates a mass of 75±20 helium masses. Both the mass and the mobility of this carrier are consistent with a droplet of radius 11±2 Å forming on the ion. The theoretical droplet radius is 12.6 Å.This work was supported in part by the Air Force Office of Scientific Research, The U.S. Atomic Energy Commission, and Research Corporation.     

Self-organization in a layer of magnetic fluid in strong electric fields

The effect of a polarizing voltage on the electrical properties of a magnetic fluid confined between the plates of a plane capacitor connected to a series resonance circuit has been studied. The magnetic fluid layer features the formation, development, and self-organization of aggregates with dimensions on the order of several millimeters. These processes influence the physical properties of the magnetic fluid layer.     

The parameters of electron avalanches and the runaway of electrons in strong electric fields

A simple hydrodynamic model is suggested for the calculation of parameters of electron avalanches in a strong field, where the mean energy of electrons exceeds the ionization energy; this model is used to calculate the electron drift velocities and the coefficient of ionization and intensity of electric fields for cesium, helium, and xenon atoms in a wide range of variation of the mean energy of electron flow. It is demonstrated that, in a strong field, inelastic collisions make a determining contribution both to the loss of energy by electrons and to the loss of momentum. A criterion is suggested for the determination of the conditions of runaway of electrons. The dependence is obtained of the voltage, at which the runaway of electron with mean energy occurs, on the field intensity, mean energy, and the value of pd.     

Agitation of a liquid in a strong external electric field

This article considers the condition for instability of the surface waves in a liquid system in a strong external electric field. As a result of the viscosity, such surface waves lead to vigorous volume agitation, accelerating chemical and physicochemical processes occurring in the system.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 17, No. 6, pp. 981–988, December, 1969.     

Conductivity of electrons on a liquid helium surface in high magnetic fields

The elementary processes of electron scattering on a liquid helium surface in the limit of a high magnetic field are considered. For the scattering on helium vapor atoms, the adiabatic approach is developed, which takes into account the slow thermal motion of atoms. The scattering on ripplons is considered in a lowest, nonvanishing order of the perturbation theory.     

Kinetic properties of surface electrons over liquid helium under strong electron-electron interaction

The mobility and lifetime of ground-level electrons are studied experimentally in a two-dimensional electron system at the surface of liquid helium at temperatures of0.4 to1.4 K and charge concentrations of (2.8–12.0) × 10⁸ cm^–2. It is shown that for fairly low temperature and high concentration where the frequency of interelectronic collisions is much higher than that of electron-ripplon ones, the so-called complete control condition is realized in the electron system, i.e., when the average drift velocity of electrons and the effective electron temperature can be introduced. This model is found to describe well the kinetic properties of surface electrons in the range of charge concentrations up to 4×10⁸ cm^–2, where the one-particle approach is no longer applicable. For the value of parameter =e²(n_s)^1/2/T=47 that corresponds to a strong electron-electron interaction, the lifetime is found to increase sharply and the short-range order typical of a liquid state appears in the two-dimensional electron layer.     

Trapped-ion motion in helium II

We present the results of a series of experiments on the mobility of negative ions trapped on quantized vortex lines in superfluid helium. Trapped-ion mobilities were measured as a function of temperature, pressure, and He³ concentration. Some of the experimental data have been previously reported.^1,2 The temperature and pressure dependence of trapped-ion mobilities is qualitatively different than that of free ions. The He³ data strongly suggest that the negative ion bubble does not become deformed on the vortex line. We present a model for a vortex line having a central core of normal fluid extending over a distance of several angstroms surrounded by a tail, a region of excess roton density whose momenta are predominantly aligned opposite to the direction of circulation of the superfluid. This model is used to calculate the drag on a negative ion trapped on a vortex line. The model appears to account for the experimental results satisfactorily.Submitted to the Department of Physics of the University of Chicago in partial fulfillment of the requirements for the PhD degree.     

Vacancy motion in solid helium

A model calculation of the vacancy tunneling frequency in bcc ³ He, hcp ³ He, and hcp ⁴ He is presented. Only the Debye temperature (and its volume dependence) and vacancy activation energies (from NMR data) are used; comparison with experiment is made. The relative tunneling rates in the three systems along with NMR and specific heat data provide evidence for the nature of vacancy motion in each of these systems.This work submitted in partial fulfillment of the requirements for Ph.D. degree, University of Massachusetts (1975).     

Characteristic length and enhancement time of a runaway electron avalanche in strong electric fields

The dependences of the length of exponential enhancement of the runaway electron avalanche on the electric field strength and air and helium pressure were calculated using the Monte Carlo method. The calculation results allow one to arrive at the conclusion that the length and time of the exponential enhancement of the runaway avalanche electrons decrease with decreasing pressure and atomic number of the discharge gas, provided that the electric field strengths are equal.     

Anisotropic instability of dielectric liquids and decay to vapor-liquid system in strong electric fields

The results of a linear analysis for the stability of a homogeneous dielectric liquid with respect to density perturbations in a uniform electric field are presented. The electric field increases the instability increment for the stratification along the field and decreases this increment in the transverse direction. Thus, a strong electric field can induce anisotropic decay into liquid and vapor phases for a fluid that is initially both in a labile state and in a metastable or stable state. Theoretical calculations are confirmed by the results of numerical modeling of the fluid dynamics. The new regions of a low-density phase have the form of narrow cylindrical channels oriented along the field. This new mechanism of gas phase formation in strong local electric fields probably plays a key role in the inception and ultra-fast propagation of streamers during the breakdown in dielectric liquids in a nanosecond range.     

The λ-temperature of rotating liquid helium

The -temperature of liquid He⁴ in an annulus was found to be independent of rotation to ±10^–5 °K for 35 sec^–1, even if the rotating liquid is allowed only 0.5 min. in the He II state to come to rotational equilibrium. This result is in disagreement with a recent publication.     

The sonically induced cavitation of liquid helium

The onset of sonically induced cavitation in liquid helium at frequencies between 30 and 40 kHz has been studied. In helium II, two types of cavitation activity were identified: acoustic cavitation whose characteristic noise can be detected, and visible cavitation in which vaporous cavities grow to visible size. The onset of acoustic cavitation is statistical in nature with increasing event rates as the sound pressure amplitude is increased and whose threshold depends on the waiting time at that particular amplitude. The acoustic threshold sound pressure amplitude in helium II between 1.8° K andT was found to lie within 0.15 mb of 0.3 mb, the variation of ±0.15 mb occurring from one determination to another, whereas the sound pressure amplitude corresponding to the visible threshold was about a hundred times larger. These two distinct types of sonically induced cavitation appear to be unique to liquid helium. However, aboveT the two thresholds were found to coincide at a sound pressure amplitude within 0.4 mb of 0.8 mb. The characteristics of the onset of acoustic cavitation were found to be independent of applied static pressure of up to 1.5 atm above and belowT and in helium II they were unaffected by filtering, heat flushing, or rotating the liquid. The results suggest that liquid helium is nucleated by random events initiated by the ambient cosmic radiation or by vortices generated in the liquid, and they imply that at ultrasonic frequencies this liquid cannot withstand a tensile stress and behaves in this respect like water saturated with gas and containing dust motes. Attempts to determine the onset of acoustic cavitation by scattering light off the bubbles or by detecting sonoluminescence were not successful: The upper limit to the size of these bubbles was shown to be about 30 µm and the intensity of any sonoluminescence must have been less than 10^–4 of that from cavitating water. The possibilities of exploiting the two types of cavitation activity in liquid helium in the construction of a posttriggerable ultrasonic bubble chamber for visualizing the tracks of ionizing particles are discussed, as are the theoretical background and future development of the work presented in this paper.     

A method for applying strong electric fields in electron paramagnetic resonance at low temperatures

A method is described for studying epr spectra in crystals under the action of an external electric field which is applicable to a RE-1301 spectrometer, in the resonator of which a quartz cryostat extension is housed. A viscous electrically insulating grease is used, making it possible to carry out investigations in electric fields up to 700 kV cm^?1 at T = 77 and 4.2 K.     

The surface tension meniscus of liquid helium

A technique is described for calculating the shape of the surface tension meniscus at a vertical wall, in the presence of Van der Waals forces. The method of analytic continuation is used to compute a solution of the relevant differential equation. Results (believed accurate to better than 0.5%) are presented for He³ and for He⁴ at a variety of film flow rates. It is found that the shape of the classical meniscus remains unaltered over most of its length, but it is displaced horizontally relative to the wall by a flow dependent amount. The results are contrasted with those from a previous calculation.