A relativistic magnetron with a thermionic cathode

Abstract

A 500-kV, S-band magnetron has been designed and tested. An oxide cathode was used in an attempt to achieve 1 μs pulse lengths by using the pre-pulse space charge to shield the cathode from vacuum dc breakdown. The magnetron was designed according to conventional, nonrelativistic scaling laws and is unstrapped, utilizing a symmetric, axial output into a circular vacuum waveguide. Initially, focus electrodes were used to prevent axial electron losses but arcing between these and the anode limited the voltage below the π-mode oscillation threshold. Operation occurred in various space harmonics, as predicted by the relativistic Buneman-Hartree oscillation threshold equation. Removal of the focus electrodes allowed π-mode operation at voltages of ∼400 kV, powers of ∼20 MW, and pulse lengths of ∼35 ns. It is believed that high energy electrons strike the anode, ejecting ionized copper which neutralizes the space charge and creates a low impedance plasma channel, thereby preventing operation at pulse lengths longer than ∼35 ns for this design. The pulse lengths correlate well with a simple estimate of the ion transit time across the anode-cathode gap rather than the 1–5 cm/μs plasma diode closure rate frequently observed. The low powers obtained are a consequence both of the reduced efficiencies characteristic of relativistic magnetrons and operation at voltages well below the relativistic optimum.