Passively Driving X-band Structures to Achieve Higher Beam Energies

Particle accelerators at X-band frequencies have been shown to reach gradients of greater than 100 MV/m [1,2]. Such technology permits more compact accelerators. One of our aims at the Colorado State University (CSU) Accelerator Laboratory is to adapt this technology to our L-band (1.3 GHz) accelerator system to increase our overall beam energy; however, we would like to do this in a passive manner, i.e. one that does not require investment in an expensive, custom, and high-power klystron system. In this paper we explore using the beam from our L-band 6-MeV photo-injector to power an X-band structure tuned to the 9 harmonic of our 1.3 GHz L-band system, 11.7 GHz. Electron bunches will be generated at a repetition rate of 81.25 MHz and passed through a high shunt impedance X-band accelerating structure where they will resonantly excite the fundamental field. Once the peak gradient is achieved a single electron bunch can be passed through the system at a phase that places it near the crest of the X-band accelerating wave thereby increasing the electron bunch energy without need for additional external power sources.