Kerr electro-optic field mapping measurements in liquid nitrogen

Kerr electrooptic field mapping measurements were made in liquid nitrogen at 70 psi and -177°C. A sensitive measurement technique was used whereby a small-signal AC voltage was superimposed on a DC level. The nonlinear optical effect then gave an optical signal with a DC level and AC harmonic at the same frequency as the AC voltage. The amplitude of the AC optical signal was proportional to the local electric field and was measured with a lock-in amplifier. For 6330-nm-wavelength light the Kerr constant was measured to be B≈10^-16 m/V ². Steady-state electric fields between parallel stainless steel electrodes of gap 7 mm and area 31.6 cm² were shown to be essentially uniform for field strengths up to 90 kV/cm with no significant space charge distortion. The DC voltage-current characteristic was also measured and found to be nonohmic, reaching constant current of ≈13 nA over the voltage range of ≈30-60 kV