General scaling of pulse shortening in explosive-emission-driven microwave sources

Microwave generation in devices that depend on synchronization between an electron beam and a resonant cavity or slow wave structure can be disrupted by changes in either. Explosive-emission-driven microwave sources use plasma as the electron source in the diode. This plasma is conductive enough to act as the boundary for both the applied diode voltage and the microwave electric field. The motion of this plasma can effectively change the dimensions of either the electron beam diode or the cavity and will thereby cause resonance destruction. This shortens the microwave pulse length τ_μ. Using simple models of cathode plasma motion and plasma speed dependence on diode current, we derive a scaling relation between microwave power and microwave pulse length. This general model of the process predicts that, for a Child-Langmuir diode, microwave power falls as P∝τ_μ^-5/3 and that pulse energy falls as E∝τ_μ^-2/3. Therefore, energy efficiency declines as the pulse length is extended. We compare with data from magnetrons, MILOs and BWOs, with good agreement. Explosive-emission-driven microwave sources are fundamentally limited by the speed of the diode plasma and can be improved by finding cathode materials that generate slower plasmas