Formation of a novel shaped bunch to enhance transformer ratio in collinear wakefield accelerators

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 MV=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.

Upper limit for the accelerating gradient in the collinear wakefield accelerator as a function of the transformer ratio

“…This is accomplished by using the drive bunch with an asymmetric electron distribution along the bunch length [9]. In this paper we give a rigorous proof (in a full agreement with [9,10]) that the highest ratio of the maximum accelerating field to the maximum decelerating field is obtained using the drive bunch electron distribution that produces an identical decelerating field for all electrons inside the drive bunch. Expanding on this result, we also show that for a given environment and a charge of the drive bunch, the upper value of the accelerating field is strictly bounded by the ratio of the maximum accelerating field to the maximum decelerating field.…”

“…1. The case (a) has a characteristic triangular shaped distribution with a linear growth from s ¼ 0 to the end of the bunch at s ¼ l previously considered in a number of publications (see, i.e., [9,10]). In this case E − is not constant along the electron bunch length, which makes obtaining high R (and high max jE þ j for a large R) more difficult (see, Figs.…”

Tailored electron bunches with smooth current profiles for enhanced transformer ratios in beam-driven acceleration

“…We note that N does not have to be an integer number (it is generally a real-positive number) but this choice leads to simpler final equations and adheres to similar choice in the literature; see, e.g., [6,10]. Finally, the transformer ratio can be calculated by taking the ratio of the maximum accelerating field (see Appendix A) over the maximum decelerating field which yields…”

“…Consequently, there has been regained interests in devising alternative techniques to shape the current distribution of drive bunches. Most recently a piecewise "double-triangle" current profile was suggested as an alternative to the linear-ramp distribution and its possible realization was numerically demonstrated [10] by implementing a transverse-to-longitudinal phase-space exchanger [11]. A limitation common to the formation of all the proposed shapes resides in their discontinuous character which makes their experimental realization either challenging or relying on complicated beam-manipulation techniques [11,12].…”

Suppressing parasitic effects in a long dielectric wakefield accelerator

“…The accelerating gradient in a DWA is created by a low-energy high-charge drive beam traveling on-axis. A high transformer ratio wakefield can be realized with a custom (double triangular) shape of the drive bunch which experiences almost uniform deceleration [2]. In addition, by proper shaping and timing of a witness bunch one can minimize the growth of its energy spread [3].…”