Self-injection-locked magnetron as an active ring resonator side coupled   to a waveguide with a delayed feedback loop
  The theoretical analysis and numerical simulations of the magnetron operation with a feedback loop were performed assuming that the delay of the electromagnetic wave propagating in the loop is constant whereas the phase of the complex feedback reflection coefficient is varied. Results of simulations showed that by a proper adjustment of values of the time delay and phase of reflection coefficient that determines phase matching between the waves in the resonator and feedback loop, one can increase the magnetron's output power significantly without any other additional measures. 
Y.P. Bliokh, Ya.E. Krasik, and J. Felsteiner
physics.class-ph