Abstract
The transformer ratio, characterizing the ratio of maximum accelerating field behind the drive bunch to the maximum decelerating field inside the drive bunch, is one of the key parameters for characterizing the performance of collinear wakefield accelerators. In this paper the use of electron drive bunches possessing a particular temporal profile (the double-triangular bunch) is shown to significantly increase the transformer ratio beyond 2, the limit for a symmetric bunch. The double-triangular bunch is generated using the emittance exchange technique [P. Emma et al., Phys. Rev. ST Accel. Beams 9, 100702 (2006)]. Complete beam simulations have been performed for a collinear wakefield acceleration experiment planned at the Argonne Wakefield Accelerator facility including the effects of the physical emittance increase of the bunch in the wakefield device. A transformer ratio of 6.5 and gradient of $20\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ is expected in this proof-of-principle experiment for a 3 nC double-triangular bunch traversing a 10 cm long, 200 GHz quartz based dielectric wakefield accelerator structure.
Highlights
Future high energy colliders as well as future free electron laser light sources will require high accelerating gradients, making wakefield acceleration a very attractive candidate technology [1,2,3,4]
Complete beam simulations have been performed for a collinear wakefield acceleration experiment planned at the Argonne Wakefield Accelerator facility including the effects of the physical emittance increase of the bunch in the wakefield device
Note that drive bunch is fixed, i.e. Iave is a constant, and we assume that the R=Q of the structure does not change for different frequencies which is generally valid if the radius a of the dielectric wakefield accelerator (DWA) is fixed
Summary
Future high energy colliders as well as future free electron laser light sources will require high accelerating gradients, making wakefield acceleration a very attractive candidate technology [1,2,3,4]. It appears to be a great challenge to achieve a higher R using a longer bunch train due to the tight requirements on the space and charge ratio among bunches in the train [8] These technical challenges can be greatly reduced in an approach using a triangular shaped drive bunch. It should be pointed out that the concept of transformer ratio is much less important in a two-beam-accelerator (TBA) scheme, where the wakefield energy of the drive bunch is extracted and coupled into another external powered accelerator. A coaxial wakefield device in principle can combine the advantages of collinear and TBA schemes [16,17], but it faces great challenges in drive beam formation, structure construction, beam transport, etc. The paper concludes with a discussion of planned future research directions
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