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A relativistic magnetron with a thermionic cathode

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3 Author(s)
Ballard, W.P. ; Institute for Plasma Research, Stanford University, Stanford, California 94305 ; Self, S.A. ; Crawford, F.W.

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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.

Published in:

Journal of Applied Physics  (Volume:53 ,  Issue: 11 )