Design and Operation of a Second Harmonic Dual Mode Gyrotron for OAM Beam Generation

A novel scheme of Orbital angular momentum (OAM) beam generation in radio frequency (RF) range using Gyrotron is demonstrated. The Gyrotron generates few kilowatts (~30-40 kW) of power at 190 GHz frequency in a second harmonic mode-pair TE_8,3/TE₁₁,₂ from 32/37 kV, 7A electron beam. Switching between the operating modes is achieved by changing the cathode voltage at constant magnetic field. It incorporates the two sinusoidal perturbations in the cylindrical cavity to eliminate the fundamental mode competition. This mode-pair is propagated in free space in the form of OAM beams having the order of l = 8/11 same as the azimuthal mode index. These high-power OAM beams can be used for free space RF communication and RF beam plasma interaction. These waveguide modes have been further radiated from an overmoded waveguide section into free space to reduce its divergence which increases the propagation distance of these OAM RF beams. This technique of natural generation of high-power OAM RF beams opens a wide research area for study of the OAM communication and plasma interaction with vortex RF beams.