Fast negative ion thermometer for 3He superfluids

A thermometer based on the temperature dependence of the negative ion mobility in superfluid 3 He is described. The device can be used for monitoring the temperature of an open geometry sample of 3 He in the range 0.4 ≲ T / T c ≤ 1. The ion thermometer is most sensitive immediately below T c , in the Ginzburg-Landau region; above T ≈ 0.95 T c a resolution of 0.1 μK can be achieved. The measuring technique provides an on-line display of the relative mobility, μ ( T )/ μ ( T c ), and the ions probe the temperature of the 3 He sample directly, which leads to a short time constant τ versus T / T c is insensitive to the external magnetic field and the pressure of the sample. Only a low electric field, E ≲ 10 V cm −1 , with essentially no homogeneity requirements is needed. Parasitic heating due to the flow of ions is of the order of 1 pW.