Christopher Leach

Compact Relativistic Magnetron With Gaussian Radiation Pattern

A compact A6 relativistic magnetron is proposed which operates in the π-mode and whose radiation is extracted axially as a TE11 mode through a cylindrical waveguide with the same cross section as that of the anode block. This radiated mode is similar to a Gaussian microwave beam. The advantages of this magnetron include the minimal volume of the applied magnetic field and, as a consequence, the proximity of the electron dump to the anode block for the electrons leaking from the interaction space that minimizes both the diameter and the axial length of the magnetron. By using MAGIC particle-in-cell (PIC) simulations, we demonstrate the possibility of generating a Gaussian radiation pattern with power of about 0.5 GW when the applied voltage is 350 kV. This compact magnetron is easier to implement than the magnetron with diffraction output (MDO), although with reduced efficiency.

X-band relativistic BWO with frequency tuning

Recently, considerable attention has been given to the development of relativistic high-power microwave sources with the capability of broad-band frequency tuning. We have demonstrated frequency tunability via computer simulations and experiments in an X-band backward wave oscillator (BWO) with a modified cavity reflector.

Compact relativistic magnetron with gaussian beam radiation pattern

We present a compact relativistic magnetron with a simple mode converter that radiates the TE11-mode when the operating mode of the magnetron is the π-mode. The magnetron used in this study has the standard A6 magnetron [1] dimensions except that the microwaves are extracted axially from a cylindrical waveguide whose radius is the same as the cavity radius. The proposed mode converter implements an anode endcap that electrically opens those cavities whose electric field distribution corresponds to the polarization of the TE11- mode and closes the remaining cavities. This configuration has several advantages: 1) The TE11 output mode can be easily converted into a Gaussian-like beam by means of a conical horn antenna, 2) Axial extraction of microwaves in this manner allows one to use a compact solenoid mounted directly onto the magnetron tube, thereby requiring a small volume for the applied axial magnetic field and reducing the energy requirement for the magnet system, 3) The rapidly diverging magnetic field lines from the solenoid enables a quick deposition of the downstream leakage electrons onto the waveguide, thereby protecting the dielectric window of the radiating horn antenna from electron bombardment [ 2].

Suppression of leakage current in a relativistic magnetron using a novel cathode endcap design

Upcoming experimental verification of MAGIC particle-in-cell (PIC) code simulation of a high-power, 70%-efficient relativistic magnetron with diffraction output (MDO) requires leakage electrons to be insulated axially from a dielectric antenna window [1]. Endcaps in relativistic magnetron systems have been explored before with great success in increasing output power and efficiency and greatly reducing leakage current, but none has yet to completely suppress leakage current [2, 3]. In our experiments the radius of a disk endcap was extended slightly beyond the inner radius of an A6 anode slow-wave structure, such that electrons from the interaction space drifting downstream along the axial magnetic field lines are stopped. On the endcap surface, which is not bombarded by these electrons, a thin dielectric coating was added to lower the endcap surface electric fields and to capture any emitted electrons. Preliminary results of experiments on a radial-extraction relativistic A6 magnetron system (260 kV, 16 ns paraboloidal pulse) with a solid cathode almost entirely suppressed the leakage current. Extension of these experimental results through MAGIC will provide a predictive mechanism for the success of endcap designs in the MDO system.

Compact non-invasive bunch length monitor ICEPIC study

A novel non-invasive, near real-time cavity detector used to monitor the bunch length and profile of photo-emitted electron bunches was simulated in ICEPIC [1] for the purpose of providing a meaningful comparison to experimental data [2] and for predicting the cavity response to a variety of bunch lengths and profiles.

Recent advances in relativistic A6 magnetron research — Improvements in start-up and efficiency

Summary form only given. The A6 magnetron was introduced by Palevsky and Bekefi at MIT in 1979 [1]. It is the most studied relativistic magnetron in the literature. Since 2005 UNM has been studying novel advances to the A6 and to the A6 with diffraction output (MDO) that achieve higher power, greater efficiency (η), fast start of oscillations, and eliminate mode competition. This presentation summarizes recent advances pertaining to improved start-up and increased efficiency.

ICEPIC study of metamaterial-like rodded cathode in a relativistic A6 magnetron

A metamaterial-like rodded cathode in a relativistic A6 magnetron was simulated using ICEPIC [1] to study the effect that a grid of fine rods have on increasing space-charge-limited current emission from the cathode, on affecting the microwave power, and on encouraging a beneficial interaction with the RF mode.

Microwave breakdown of air at low pressure with a 5 ns, 35 GHz pulse

Summary form only given. This research was inspired by the need for a microwave source powerful enough to lead to enhancement in the air breakdown volume, when synchronized in time and space with a small region of pre-ionized air created with an intense laser pulse. We used the compact and portable RADAN pulser that drives a backward wave oscillator to produce microwaves for this purpose. The microwave pulse duration and power are 5 ns and 2 MW, respectively. The frequency of operation is 35 GHz. It is well known that air breakdown at atmospheric pressure requires an electric field amplitude of 30 kV/cm. Thus, an output power of 2 MW simply radiated in air does not have sufficient electric field to assist in air breakdown. For this reason, a parabolic dish was used to focus the microwaves into a small spot, thereby enhancing the electric field. The 2-dimensional, fully relativistic and fully electromagnetic code MAGIC was used to simulate the amplitude of the electric field and the axial position of the focal point of the parabolic dish. The parabolic dish used for this purpose was 30 cm across and 4.5 cm deep. It was observed in the simulations that the focal spot was located 15 cm from the dish depth and the radial electric field was 20 kV/cm. Despite this promising value of the electric field, experimental verification was necessary due to the following arguments that 5 ns was too short a pulse and that 35 GHz was too high a frequency for breakdown. In order to confirm the fidelity of the simulation results a low pressure chamber transparent to the 35 GHz was manufactured. The parabolic dish was placed opposite the radiating horn antenna with enough distance between them to allow breakdown to occur within the low pressure chamber. Time integrated photographs were captured for every microwave pulse. A very well defined breakdown was observed at 180 Torr of air. These results together with some theoretical development of the breakdown mechanism will be presented.

PPPS-2013: UNM transparent cathode experiments revisited

The transparent cathode (TC) invented at the University of New Mexico (UNM) showed high output powers, high efficiency and stable operation in the 2pi-mode over a wide range of magnetic field compared to the solid cathode when tested in the standard A6 magnetron.

Mode purification in a relativistic magnetron with diffraction output

Summary form only given. Previous MAGIC particle-in-cell simulations performed at the University of New Mexico illustrated a mechanism by which a Gaussian TE11 microwave radiation pattern could be generated by a compact relativistic A6 magnetron with diffraction output (axial extraction) [1]. It employed an upstream strap that locked in the π-mode, something new to relativistic magnetrons [2], and a mode converter affixed to the downstream end of the anode block that acted to disrupt the desired π-mode. During portions of the TE11 RF cycle in which the electric field was low, such as when the polarity reversed, it was found that the polarization underwent rotation. Several changes are introduced in simulations to correct this problem at the expense of power and efficiency. These simulation results will be presented.