Towards a Tunable sub-THz Gyrotron for Spectroscopy of Positronium

We propose a simple method of increasing the bandwidth of smooth frequency tuning in gyrotron by the use of a short cavity driven by an electron beam with high current. Analytical estimates of the achievable band of frequency tuning are presented. Numerical simulations based on both averaged equations and PIC-code show a feasibility of the gyrotron with the output power of 1 kW within a 10-GHz band at the frequency of 0.2 THz. The radiation source with such parameters is needed for testing quantum electrodynamics predictions through the spectroscopy of positronium.