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INTERACTION OF IONS WITH VORTEX LINES

A. F. Borghesani

Published May 31, 2007• 1 citation
Publisher

Abstract

Abstract If the superfluid is contained in a rotating bucket, it resembles a solid-body rotation by developing an uniform array of quantized vortex lines. These can capture ions and, thus, rotating superfluid He II shows a strong anisotropy to the motion of ions. This chapter is concerned with the interaction of ions with vortex lines. The ion capture experiments and their results are described. Concepts such as capture width are discussed. A theoretical model of Brownian diffusion has been developed to understand the capture of ions by vortex lines. The competition between the attractive hydrodynamic suction and the thermal diffusion leads to an effective capture rate of ions. The effect of the pressure on the capture of an electron bubble is also discussed.

DOI
10.1093/acprof:oso/9780199213603.003.0011

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Abstract If the superfluid is contained in a rotating bucket, it resembles a solid-body rotation by developing an uniform array of quantized vortex lines. These can capture ions and, thus, rotating superfluid He II shows a strong anisotropy to the motion of ions. This chapter is concerned with the interaction of ions with vortex lines. The ion capture experiments and their results are described. Concepts such as capture width are discussed. A theoretical model of Brownian diffusion has been developed to understand the capture of ions by vortex lines. The competition between the attractive hydrodynamic suction and the thermal diffusion leads to an effective capture rate of ions. The effect of the pressure on the capture of an electron bubble is also discussed.
Abstract Ions and electron bubbles are captured by the vortex lines which occur in a rotating superfluid sample. Once captured, however, the charges can move along the quantized vortex lines. On one hand, this degree of freedom allows the researcher to use ions to detect the appearance of single, quantized vortex lines. On the other hand, the study of the ion motion along the lines gives important pieces of information on the structure and dynamics of vortices. In fact, the ion motion along the lines is limited by scattering on 3He impurities captured by the vorticity field and by vortex waves, i.e., columnar oscillation of the vortex lines. Experiments and theory of vortex wave-, roton, and 3He impurity scattering on vortex lines are described.
Abstract The interaction of ions and quantized vortex lines in helium II is treated by imagining that the motion of the superfluid near a vortex line acts on an ion so as to produce a potential well. The ion itself is looked upon as a Brownian particle immersed in a gas of quasi-particles. The capture of ions is pictured as a process of sedimentation into the well. The escape of the ions from the well is treated as a problem of the escape of a Brownian particle over a potential barrier. A solution of the Fokker-Planck equation for a two-dimensional well is given. Comparison of the theory with a variety of experiments is presented.
Negative-ion trapping has been investigated in rotating liquid helium at temperatures above 1.2\ifmmode^\circ\else\textdegree\fi{}K. We report measurements of the ion-capture width versus temperature and electric field (from 5 to 100 V/cm). The theory of the effect is reviewed and discussed and is shown to be in excellent agreement with experiment. Owing mainly to the insensitivity, at low ratios of electric field to temperature, of the capture width to the properties of the ion-vortex potential well, we are able to deduce only that the well must be larger than the ionic mean free path.
Abstract This chapter is devoted to discussing some key superfluid properties exhibited by dilute Bose gases confined in external traps. Key issues concern the behaviour of the critical velocity and the applicability of Landau’s criterion for superfluidity, the moment of inertia and its quenching caused by the irrotationality of the superfluid motion, the scissors mode, and the expansion of a rotating superfluid cloud. Rotational effects at high angular velocities, with the appearance of quantized vortices and vortex lattices, are also considered. The chapter also discusses the consequences for vortices on the collective oscillations of the trapped gas, the stability and the precession of vortex lines, and the nature of the superfluid flow in toroidal traps.