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Physical Review

Negative-Ion Capture by Vortex Lines in Helium II

D. J. Tanner

Published Dec 2, 1966• 58 citations

Abstract

We have studied negative-ion absorption by rotating He II from 0.98\ifmmode^\circ\else\textdegree\fi{}K to 1.7\ifmmode^\circ\else\textdegree\fi{}K. Measurements of vortex-line capture cross sections are reported as a function of temperature (0.98\ifmmode^\circ\else\textdegree\fi{}K to 1.7\ifmmode^\circ\else\textdegree\fi{}K at three electric fields (16.3, 49.0, and 97.9 V/cm) and as a function of electric field (10 to 200 V/cm) at two temperatures. Great care was taken to insure that the reported cross-section values were not affected by space-charge effects. The results are found in good agreement with the ion-capture theories developed by Donnelly and Roberts. The decrease in cross section above 1.6\ifmmode^\circ\else\textdegree\fi{}K is in agreement with the escape calculation of Parks and Donnely and supports their value of 15.96 \AA{} for the negative-ion radius.

DOI: 10.1103/physrev.152.121

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Mobility of Negative Ions Along Vortices in Rotating Superfluid Helium

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Physical Review•Oct 5, 1968•56 citations

We have studied negative-ion absorption by rotating He II from 0.98\ifmmode^\circ\else\textdegree\fi{}K to 1.7\ifmmode^\circ\else\textdegree\fi{}K. Measurements of vortex-line capture cross sections are reported as a function of temperature (0.98\ifmmode^\circ\else\textdegree\fi{}K to 1.7\ifmmode^\circ\else\textdegree\fi{}K at three electric fields (16.3, 49.0, and 97.9 V/cm) and as a function of electric field (10 to 200 V/cm) at two temperatures. Great care was taken to insure that the reported cross-section values were not affected by space-charge effects. The results are found in good agreement with the ion-capture theories developed by Donnelly and Roberts. The decrease in cross section above 1.6\ifmmode^\circ\else\textdegree\fi{}K is in agreement with the escape calculation of Parks and Donnely and supports their value of 15.96 \AA{} for the negative-ion radius.

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.

Measurements have been made of the lifetime for the escape of negative ions trapped in rotating helium II as a function of temperature and pressure. These measurements concern lifetimes in the range from 10 to 1000 sec, and have been made between the temperatures of 1.09 and 1.68\ifmmode^\circ\else\textdegree\fi{}K from the vaporization pressure to the solidification pressure. At constant pressure the lifetime decreases rapidly with increasing temperature. The curves of constant lifetime in the pressure-versus-temperature plane have negative slopes

\frac{\mathrm{dP}}{\mathrm{dT}}

and positive second derivatives

\frac{{d}^{2}P}{d{T}^{2}}

. The results have been interpreted in terms of the bubble model of the negative ion and the vortex-line model of the rotating superfluid. The interpretation suggests that the ion radius equals approximately 19.5 A at saturated vapor pressure and decreases to a value between 12 and 14 \AA{} at the solidification pressure. However, a significant discrepancy between theory and experiment casts some doubt on the radius values obtained.

The mobility of negative ions trapped on linear vortices in rotating superfluid helium has been measured by a time-of-flight technique in the temperature range $0.8\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}<T<1.6\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}$. Measurements could not be extended below 0.8\ifmmode^\circ\else\textdegree\fi{}K because, contrary to the present theory, the ion escape rate became too large. No evidence of direct trapping of positive ions by vortex lines was observed down to 0.4\ifmmode^\circ\else\textdegree\fi{}K. The trapped-ion mobility is interpreted in terms of scattering of the ion by thermal distributions of quantized vortex waves and rotons localized about the vortex axis.