The normalized Townsend first ionization coefficient α/N and normalized attachment coefficient η/N in pure C4F7N were measured by using the steady-state Townsend (SST) method for a range of reduced electric fields E/N from 750 to 1150 Td at room temperature (20 °C). Meanwhile, the effective ionization coefficients are obtained. All SST experimental results show good agreement with pulsed Townsend (PT) experiment results. Comparisons of the critical electric fields of C4F7N with SF6 and other alternative gases such as c-C4F8 and CF3I indicate that C4F7N has a better insulation performance with a much higher normalized critical electric field at 959.19 Td.

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Only the first-order Doppler frequency shift is considered in current laser dual-frequency interferometers; however; the second-order Doppler frequency shift should be considered when the measurement corner cube (MCC) moves at high velocity or variable velocity because it can cause considerable error. The influence of the second-order Doppler frequency shift on interferometer error is studied in this paper, and a model of the second-order Doppler error is put forward. Moreover, the model has been simulated with both high velocity and variable velocity motion. The simulated results show that the second-order Doppler error is proportional to the velocity of the MCC when it moves with uniform motion and the measured displacement is certain. When the MCC moves with variable motion, the second-order Doppler error concerns not only velocity but also acceleration. When muzzle velocity is zero the second-order Doppler error caused by an acceleration of 0.6g can be up to 2.5 nm in 0.4 s, which is not negligible in nanometric measurement. Moreover, when the muzzle velocity is nonzero, the accelerated motion may result in a greater error and decelerated motion may result in a smaller error.

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