Do Fluctuations Determine the Ion Mobility in He II near Tλ?
Measurements of the zero-field ionic mobility µ in the roton region and near T λ of superfluid helium have been obtained from the characteristics of space-chargelimited discharges. The results for negative ions were obtained from the dc current I = CµV 2 of a planar diode using an 8-Ci tritium ion source. The source was plated on a 1-in.-diameter by 0.005-in.-thick OFHC copper disk which faced a collector at a distance of 0.5 cm. The source was maintained at a fixed voltage, V = − 8.00 V, which ensured space charge limitation for all currents measured, a condition which we checked at various temperatures by observing that I ∝ V 2. The geometric constant C was determined by matching the data to a known value of µ at a single temperature. The results for positive ions were obtained from tungsten field emission points and have been reported previously.1 These data have been compared to the recent data of Schwarz2 and Ahlers and Gamota.3 Over the range from 1.4°K to T λ tabular comparison with Schwarz’s data shows a point-by-point variation of no more than ± 1%.
KeywordsSuperfluid Helium Geometric Constant Fixed Voltage Planar Diode Inverse Mobility
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