Abstract
Conventional metal-wall waveguides support waveguide modes with phase velocities exceeding the speed of light. However, for infrared frequencies and guide dimensions of a fraction of a millimeter, one of the waveguide modes can have a phase velocity equal to or less than the speed of light. Such a metal microchannel then acts as a slow-wave structure. Furthermore, if it is a transverse magnetic mode, the electric field has a component along the direction of propagation. Therefore, a strong exchange of energy can occur between a beam of charged particles and this slow-waveguide mode. Moreover, the energy exchange can be sustained over a distance limited only by the natural damping of the wave. This makes the microchannel metal waveguide an attractive possibility for high-gradient electron laser acceleration because the wave can be directly energized by a long-wavelength laser. Indeed the frequency of ${\mathrm{CO}}_{2}$ lasers lies at a fortuitous wavelength that produces a strong laser-particle interaction in a channel of reasonable macroscopic size (e.g., $\ensuremath{\sim}0.6\text{ }\text{ }\mathrm{mm}$). The dispersion properties including phase velocity and damping for the slow wave are developed. The performance and other issues related to laser accelerator applications are discussed.
Highlights
A conventional metal-wall waveguide supports a series of waveguide modes
The laser-energized metal microchannel seems ideally suited for CO2 lasers, which are capable of terawatt peak power levels [10]
The microchannel example has a gradient in the GV=m range, but it is slightly lower than the laser wakefield accelerator (LWFA) examples
Summary
A conventional metal-wall waveguide supports a series of waveguide modes One class of these is transverse magnetic (TM) waves, which have a forward component of the electric field. Under special conditions (e.g., proper frequency and channel size) a metal waveguide supports one slow-wave mode that can have a phase velocity equal to or less than c. We note this same mechanism can PACS numbers: 41.75.Jv, 42.55.Lt be used for the inverse process whereby coherent light is extracted from a charged particle beam This inverse process will not be discussed in this paper. This begins with a discussion of where this approach fits within the family of accelerator concepts.
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