Vacuum transmission lines for pulse sharpening and diagnostics applications

We investigated the propagation of MV electrical pulses along coaxial transmission lines (TL) in vacuum with network simulations and experiments. One goal was to establish how well a 3 m-long TL would sharpen the output pulse of a relativistic electron beam accelerator. Sharpening occurs as the cathode of the TL emits electrons and the current flow forces the TL into magnetic cut-off. The other goal was to determine how well field emission must be suppressed in a TL to avoid distortion of a propagating pulse. Simulations predict a four-fold risetime improvement (8 ns to 2 ns) through magnetic insulation in a TL with an electrical length (10 ns) comparable to the risetime of the input pulse. In the laboratory we have shown a five-fold improvement (15 to 3 ns) with a velvet covered 3-m line and a 7.5 fold improvement (2 ns) when a vacuum flashover switch was incorporated between the first third and the last two thirds of the TL. Simple arguments and TL simulations suggest that even a small fraction (1 or 2%) of Child-Langmuir (CL) space charge limited emission will distort a propagating voltage pulse. This result is of particular importance when the TL is part of a voltage diagnostic system.