Rapid Onset of Oscillations in a Magnetron with a Transparent Cathode

Summary form only given. The relativistic magnetron is one of the most powerful sources of microwaves developed to-date. However, its typically slow onset of oscillations limits its use in many promising systems such as, for example, in high resolution radars. The slow start is caused by the small amplitude of the synchronous field near the cathode, which hampers the capturing of electrons into spokes to form the anode current. We propose to increase the E
_thetas
wave field in the interaction region by removing longitudinal strips from a thin-walled tubular cathode, thus making it transparent to this field. Therefore, instead of a field distribution E
_thetas
~ sinh[g(r-r
_c
)], we provide a much larger field E
_thetas
~ I
_n
(gr) near the cathode with radius r
_c
. Here I
_n
is the modified Bessel function of order n, the azimuthal index of the operating wave, and g is the transverse wave number. Simulations using the fully 3D electromagnetic particle-in-cell code MAGIC of an A6 magnetron with solid and transparent cathodes show a marked difference in the time for build-up of oscillations. Moreover, in a magnetron with a transparent cathode the emission centers are automatically located periodically on the cathode perimeter, which also promotes rapid onset of oscillations. This was recently demonstrated in [M.C. Jones et al., Rev. Sci. Instrum., vol. 75, 2976 (2004)] for a magnetron using a solid cathode with nonuniform emission centers. Longitudinal currents along the cathode strips produce azimuthal magnetic fields around them that lead to the azimuthally varying tangential magnetic field. The drift velocity, height of electron cyclotron orbits, and, therefore, the synchronism and efficiency depend on this field. As was shown in [M.C. Jones et al., Appl. Phys. Lett., vol. 84, 1016 (2004)], the varying magnetic field also promotes rapid onset of oscillations