Abstract
We study the longitudinal wake field components which are induced in a rectangular, dielectric‐lined structure having micron‐scale dimensions by the passage of one or more charge bunches having femtosecond duration. The bunches would be obtained from a 500 MeV LACARA “chopper” which uses a TW optical wave from a CO2 laser [1]; the bunches are chopped from a macrobunch having duration ∼1 psec obtained from a high brightness 500 MeV rf linac. The high intensity laser wave accomplishes the chopping of the macrobunch into slices which are roughly 10% of the 10.6 μm radiation wavelength. These microbunches can be shaped into a rectangular cross section, approximately 10 μm × 150 μm in dimension, and will excite wake fields when injected into a rectangular dielectric wake field accelerating structure. We compute sample 3D wake fields, using the PIC code KARAT, as well as by means of an analytic method. The passage of just one pC bunch will set up a longitudinal wake field ∼ 40 MeV/m, and a train of ten properly‐timed such bunches can produce a cumulative wake field ∼ 600 MeV/m. The choice of dimensions causes the wave solutions to approximate a single‐mode excited by an infinitely‐tall bunch in a 2D structure; a highly uniform longitudinal wake field in the cross‐sectional plane of the structure results, suitable for accelerating a correctly positioned “test bunch”. KARAT includes the effect of interference between the Cerenkov radiation of the bunch with the transition radiation emitted as the bunch enters the structure. The wake field structure is several cm in length, and is both rigid and capable of microfabrication accuracy; it could accordingly be a reproducible module in a staged array. The stability of the bunches and the analytic formulation are dealt with in a companion paper [2].
Published Version
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