Increasing the transformer ratio at the Argonne wakefield accelerator

Abstract

The transformer ratio is defined as the ratio of the maximum energy gain of the witness bunch to the maximum energy loss experienced by the drive bunch (or a bunch within a multidrive bunch train). This plays an important role in the collinear wakefield acceleration scheme. A high transformer ratio is desirable since it leads to a higher overall efficiency under similar conditions (e.g. the same beam loading, the same structure, etc.). One technique to enhance the transformer ratio beyond the ordinary limit of 2 is to use a ramped bunch train. The first experimental demonstration observed a transformer ratio only marginally above 2 due to the mismatch between the drive microbunch length and the frequency of the accelerating structure [C. Jing, A. Kanareykin, J. Power, M. Conde, Z. Yusof, P. Schoessow, and W. Gai, Phys. Rev. Lett. 98, 144801 (2007)]. Recently, we revisited this experiment with an optimized microbunch length using a UV laser stacking technique at the Argonne Wakefield Accelerator facility and measured a transformer ratio of 3.4. Measurements and data analysis from these experiments are presented in detail.

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

“…An alternative method involves the use of a train of drive bunches (instead of a shaped single drive bunch) with appropriate charge and separation [5,7,8]. To date enhanced transformer ratios were experimentally demonstrated using the latter bunch-train method [8] and resulted in R ≃ 3.4 [9]. Producing and transporting a train of bunches with different charges has its own sets of complications.…”

High-Gradient Cherenkov Radiation Based on a New Dielectric-Loaded Waveguide

“…By compressing the electron bunch, a short bunch with σ z ∼ 0.6 ps was achieved, and high-gradient coherent Cherenkov radiation was excited in DLW by the short bunch [1]. The ramped electron bunch was applied to excite the high-gradient Cherenkov radiation by enhancing the energy-transmission ratio [14]. The electron bunch trains with a high bunching factor could also excite the high-gradient wakefield in DLW [15].…”

Possibility for ultra-bright electron beam acceleration in dielectric wakefield accelerators

“…For a Gaussianshaped drive beam pulse the transformer ratio cannot exceed two [12]. However, there are techniques that can achieve TR > 2 by breaking the symmetry of the drive beam and creating bunches of triangular shape [13] or its variations (ramped bunch train [14] and double-triangular [15]). Figure 2 illustrates the current profile of the double triangular bunch and its corresponding longitudinal wakefield generated in a single mode wakefield accelerator.…”