A Cherenkov wake confined by perfectly reflecting transverse walls is amplified if the dielectric medium is active. Because of the multiple-reflections process, the effective gain of the wake is enhanced compared to a ray propagating in a straight line. Higher enhancement occurs when the electron velocity is close to the Cherenkov velocity. This Cherenkov wake can then accelerate a second bunch of electrons trailing the first. Gradients larger than 1 GV/m are predicted before saturation becomes a major impediment.

No abstract

Cerenkov wake amplification can be used as an accelerating scheme, in which a trigger bunch of electrons propagating inside a cylindrical waveguide filled with an active medium generates an initial wake field. Due to the multiple reflections inside the waveguide, the wake may be amplified significantly more strongly than when propagating in a boundless medium. Sufficiently far away from the trigger bunch the wake, which travels with the same phase velocity as the bunch, reaches saturation and it can accelerate a second bunch of electrons trailing behind.For a CO 2 gas mixture our numerical and analytical calculations indicate that a short saturation length and a high gradient can be achieved with a large waveguide radius filled with a high density of excited atoms and a trigger bunch that travels at a velocity slightly above the Cerenkov velocity. To obtain a stable level of saturated wake that will be suitable for particle acceleration, it is crucial to satisfy the single-mode resonance condition, which requires high accuracy in the waveguide radius and the ratio between the electron phase velocity and the Cerenkov velocity. For single-mode propagation our model indicates that it is feasible to obtain gradients as high as GV m −1 in a waveguide length of cm.

We present detailed development of the linear theory of wakefield amplification by active medium and its possible application to a two-beam accelerator (TBA) is discussed. A relativistic train of triggering microbunches traveling along a vacuum channel in an active medium confined by a cylindrical waveguide excites Cherenkov wake in the medium. The wake is a superposition of azimuthally symmetric transverse magnetic modes propagating along a confining waveguide, with a phase velocity equal to the velocity of the triggering bunches. The structure may be designed in such a way that the frequency of one of the modes is close to active-medium resonant frequency, resulting in amplification of the former and domination of a single mode far behind the trigger bunches. Another electron bunch placed in proper phase with the amplified wakefield may be accelerated by the latter. Importantly, the energy for acceleration is provided by the active medium and not the drive bunch as in a traditional TBA. Based on a simplified model, we analyze extensively the impact of various parameters on the wakefield amplification process.

Abstract. While a Cherenkov wake propagates in a perfectly reflecting waveguide loaded with active medium, a part of its spectrum is amplified. Due to gain enhancement the amplified frequency components become dominant making the wake an effectively monochromatic wave. As in a two-beam accelerator, this intense wake may accelerate a different (trailing) train of microbunches.