The breakdown potential dependence on the voltage rise rate in a long discharge tube at low pressure

The dynamical breakdown voltage in a long (80 cm) discharge tube is measured for neon, argon, their mixture, and argon with mercury vapors at a pressure of 80 – 400 Pa in a wide range of anode voltage rise rate ($dU/dt\sim$ 10$^{-4}$ – 10$^2$ kV/ms). Non-monotonous dependence of the breakdown potential on the $dU/dt$ magnitude is found, with the minimum at 0.1 – 10 kV/ms values. The growth of the breakdown voltage at higher $dU/dt$ is caused by existence of the breakdown delay time. The increase in the breakdown potential in a lower voltage rise rate is presumably due to specialty of the breakdown in long tubes and is a result of the deposit of the surface charge on the tube wall. The charge diminishes potential difference between the anode and the wall, so obstructing the initial breakdown between them. The data of additional experiments with the pulses raised to the constant level pedestal confirm the possibility of such explanation.