Measurements of harmonic wake fields excited by rough surfaces

Superradiant Cherenkov-wakefield radiation as THz source for FEL facilities

An electron beam passing through a tube of small inner diameter which is lined on the inside with a dielectric layer will radiate energy in the THz range due to the interaction with the boundary. The resonant enhancement of certain frequencies is conditioned by structure parameters such as tube radius and the permittivity and thickness of the dielectric layer. In low-loss structures narrow-band radiation is generated which can be coupled out by suitable antennas. For higher frequencies, the coupling to the resistive outer metal layer becomes increasingly important. The losses in the outer layer prohibit reaching higher frequencies with narrow-band conditions. Instead, short broad-band pulses can be generated with still attractive power levels. In the first section of the paper, a general theory of the impedance of a two-layer structure is presented and the coupling to the outer resistive layer is discussed. Approximate relations for the radiated energy, power and pulse length for a set of structure parameters are derived and compared with numerical results in the following section. Finally, the first numerical result of the out-coupling of the radiation by means of a Vlasov antenna and estimates of the achieved beam quality are presented.

Surface-roughness wakefield measurements at brookhaven accelerator test facility

An experiment has been carried out at the Brookhaven Accelerator Test Facility to investigate the effect of a surface-roughness wakefield in narrow beam tubes with artificially created bumps. The measurements show that the synchronous modes decay significantly due to the randomization of the roughness pattern. It is pointed out that this decay mechanism has not been investigated in the previous experiment at DESY and the investigators' conclusion does not apply for surface-roughness wakefields in real surfaces.