Prystupa V. Excitation of coherent wakefields in dielectric, plasma and plasma-dielectric multibunch wakefield accelerators

The results of experimental investigations of the coherence of wakefields excited by a regular sequence of short relativistic electron bunches in dielectric, plasma and hybrid plasma-dielectric structures are presented. For a waveguide segment, the excited field leaves the waveguide, so that the number of bunches of the sequence, whose wakefields are added, is determined by the length of the waveguide. An experimental proof of the coherence of the excited fields is the linear growth of the total wakefield with the number of bunches of the sequence. By varying the plasma density, detuning of plasma frequency and bunch repetition frequency was created, that allowed not only to excite plasma wakefield, but also to accelerate bunches of the same sequence. In a dielectric structure the similar detuning was achieved by varying the bunch repetition frequency if changing the master oscillator frequency of the klystron amplifier. In the hybrid plasma-dielectric structure, both the dielectric wakefield for acceleration and the plasma wakefield for focusing were excited. Plasma density 1011cm-3 in the transit channel of the hybrid plasma-dielectric structure provides focusing of bunches by diameter in 1.8 times, which was fixed experimentally by dual Faraday cylinders, which measured the peripheral and axial parts of the beam current.