High power RF amplifiers for accelerator applications

High power microwave tubes that can generate power in MW level even at frequency above 100 GHz sources are known for their potential to power advanced accelerators other than their applications like high data rate ground and space communications, defence and civilian radars, plasma heating, and e-bomb. Among the high power microwave tubes, the large orbit gyrotron amplifier and the relativistic klystron amplifier (RKA) offers to be the promising candidates for future linear colliders. Since the gradient limit due to RF breakdown increases as the square root of frequency, linear collider projects aimed at the high energy frontier chose high RF frequencies to take advantage of the cost reduction associated with the higher possible gradients and the recent colliders are based on X-band frequency (11.4 GHz). Higher energy efficiency (HE) power systems is a must for all accelerators. Gyrotrons have given very good performance such as pulse output of more than a megawatt at frequencies higher than 100 GHz. Operation in fundamental mode at millimeter wave frequencies requires the use of superconducting magnets. By using harmonic operation, there is a lower requirement of magnetic field intensity though at a slightly lower RF efficiency. Large orbit gyrotrons amplifiers are particularly suited for high harmonic operation, in which the electron beam is of axis-encircling type which is generated by passing an axially streaming annular beam through a magnetic cusp. The electronic efficiency of a gyrotron can be increased by suitably tapering the magnetic field in the resonator and also by depressed collector techniques (a considerable fraction of energy is in spiralling motion) since high efficiency is required to reduce the power supply and cooling requirements. The microwave signal is generated by the interaction, of the rotating beam with the magnetron-like structure and travels through the remainder, of the downstream region in a smooth cylindrical waveguide. These problems can be significantly mitigated in the large orbit gyrotron (LOG), where all the electrons describe, helical trajectories with the axes coinciding with the cavity axis or close to it. Due to such a configuration stronger coupling with operating modes and better mode selectivity at high harmonics can be obtained