In order to calibrate sensors for the very-fast transient overvoltage（VFTO） and determine the upper frequency of the measurement

this paper reports the development of a 2 kV square-wave generator with sub-nanosecond rise time. The works in this paper include:（1） the proposal of the working principles for the generator;（2） the design of the gas switch for the short rise time;（3） the experiments on the effect of gas pressure on the operating stability

in order to determine the switch gas pressure and gap distance;（4） the design of coaxial transmission cavity to stimulate the sensor’s working environments

which is based on the impedance matching and wave impedance continuity;（5） the determination of the structure for the supporting insulators in the cavity on the basis of the simulation results;（6） the design of the resistive voltage divider which also behaves as the impedance matching unit at the cavity end. The experimental results verify the capabilities of the generator on generating the square wave with the amplitude of 2 kV and the rise time shorter than 1 ns. By using the generator for the calibration

the VFTO sensor was reported to offer an upper frequency of 287 MHz and a scale factor of 37 877.