Sarita Prasad

Frequency switching in a relativistic magnetron with diffraction output

Symmetric axial extraction of radiation from a relativistic magnetron with diffraction output (MDO) facilitates the use of any eigenmode as the operating one. As a consequence, a relatively small input RF signal can be used for mode switching, unlike the case for asymmetric extraction when only non-degenerate modes (the π- mode or the 2π- mode) can be used as the operating one. Using the MAGIC particle-in-cell code we demonstrate that about 180 MW is required to switch these non-degenerate modes in the well-known 400 kV A6 magnetron with extraction of radiation from one of its cavities when driven by a solid cathode, and about 30 MW is required for the same device when driven by a transparent cathode. For the gigawatt A6 MDO with a transparent cathode, however, only 200–300 kW is sufficient for mode switching and the switched mode continues to be generated after elimination of the input short RF signal when the amplitude of the applied axial magnetic field is near the critical value corresponding to the b...

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 Ethetas 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 Ethetas ~ sinh[g(r-rc)], we provide a much larger field Ethetas ~ In(gr) near the cathode with radius rc. Here In 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

Prospects for Long Pulse Generation in a Magnetron using a Transparent Cathode

Summary form only given. The gap between the cathode and anode in a relativistic magnetron is, as a rule, much smaller than the radii of the electrodes in order to provide synchronous interaction between the electrons and the operating wave. However, the inadvertent production of plasma on the cathode during explosive electron emission quickly fills the anode-cathode (A-K) gap, thereby limiting the time of interaction. By using a transparent cathode, it is possible to increase the gap distance several times and facilitate longer pulse operation. In the transparent cathode, longitudinal strips are removed from a thin-walled tubular cathode, thereby increasing the amplitude of the synchronous azimuthal electric field in the electron flow region. As an example, computer simulations using the particle-in-cell code MAGIC of an A6 relativistic magnetron with 18 cathode strips show that the gap distance can be increased by a factor of three without significant degradation in the output characteristics. In addition to the possibility of larger gap spacings, a novel feature of the transparent cathode is that plasma can now propagate in both radial directions, unlike the case of a traditional solid cathode, where plasma can allow to propagate toward the interaction region. Simulation results, as well as plans for experiments, will be presented.

Design of a Magnetron with a Transparent Cathode for Experimental Demonstration of Fast Start of Microwave Oscillations

Summary form only given. To provide fast start microwave oscillatios in relativistic magnetrons, we proposed the transparent cathode, which is essentially a longitudinally slit cylindrical cathode comprising multiple strip electron emitters arranged in a circle. The fast start is achieved due to both cathode and magnetic priming, and the presence of strong electromagnetic field within the electron flow near the electron emitters. We have performed a comprehensive analysis of magnetron operation using computer simulations with the particle-in-cell code MAGIC that have showed the advantage of the transparent cathode implementation over the use of a conventional solid cylindrical cathode.