CYCLOTRON AUTORESONANCE ACCELERATION WITH 7TH-HARMONIC COGENERATION

The TE72 mode in a cylindrical waveguide at frequency 7 omega has its group velocity upsilon(gr) nearly equal to that of the TE11 mode at fr equency omega, In consequence, the two modes have nearly equal resonan t magnetic fields for strongly coupled interactions at electron cyclot ron resonance. This allows coherent radiation in the TE72 mode to be g enerated at 7 omega as a gyrating electron beam is energized using the TE11 mode at frequency omega in the same waveguide. It is shown analy tically that the electron gyration radius can approach 0.5431 times th e waveguide radius during cyclotron autoresonance acceleration, for an axis-encircling beam injected with zero transverse momentum. This fac t allows electron acceleration to relativistic energies using the TE11 mode to lead to large enough axis-encircling orbit radii for strong c oupling with the TE72 fields. Results of a particle simulation study a re discussed for a 300-kV, 30-A finite-emittance pencil beam injected into a 3,27-cm-radius waveguide, and driven with 20 MW of radio freque ncy (RF) power at 2.856 GHz, The results show that careful choice of t he magnetic field profile, and suppression of the TE11 mode after its power is depleted, can allow seventh-harmonic output to reach 10 MW at 20 GHz. Furthermore, it is shown that injection at the input of harmo nic power can also benefit co-generation, both increasing seventh-harm onic output up to 16 MW and improving spent beam quality. Higher spent beam quality is advantageous for efficient beam energy recovery.