Design Considerations for a 240-GHz Gyromonotron

One of the most promising high-power, near-millimeter-wave (NMMW) sources at present is the cyclotron resonance maser (CRM), also known as the gyrotron. The attractiveness of this tube is due both to the high efficiency predicted for it theoretically [1] and to the remarkable performance of such tubes built by the Soviets. Examples of this performance are 1500 W cw at 9.9 mm [2] and 1.1 MW pulsed at 3 mm [3] with efficiencies around 10%. In a joint effort at the Harry Diamond Laboratories and the Naval Research Laboratory (NRL), a gyrotron oscillator, or gyromonotron, is being built which will have an output power at 240 GHz of about 3.5 kW with an overall efficiency of 13%. Figure 1 is a schematic of the overall layout of the device. The highly uniform axial magnetic field required for the CRM interaction is to be produced by a superconducting magnet system available at NRL. Since the maximum magnetic field produced by this magnet is 60 kG, the minimum possible cyclotron harmonic, S, and the one chosen, was 2.