Kerr constant frequency dependence in liquid nitrogen

Kerr electro-optic field mapping measurements using the sensitive AC modulation method were made in liquid nitrogen. The steady-state electric fields between parallel stainless steel electrodes that had a gap of 7 mm are shown to be essentially uniform for voltages up to 70 kV with no significant space charge distortion. The Kerr constant B of liquid nitrogen is constant over the AC modulation frequency range of 1-20 kHz, while increasing above 20 kHz. This change of B with the modulation frequency coincides with the frequency-amplitude gain characteristic of the lock-in amplifier used, so that B is independent of the AC modulation frequency within the range of this study. B monotonically decreases with increasing temperature, T, thus decreasing liquid nitrogen density, ρ, so that B is related linearly to ρ/T . The polarizability anisotropy of liquid nitrogen is evaluated to be 8.3×10^-41 F-m² from the slope of the B-(ρ/T) characteristic, obtained from a simple physical model