Increasing the transformer ratio at the Argonne wakefield accelerator

C Jing,A Kanareykin,J G Power,Z Yusof,M Conde,W Liu,W Gai

doi:10.1103/physrevstab.14.021302

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.

Highlights

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
The transformer ratio (R) in a collinear wakefield accelerator is defined as the ratio of the maximum energy gained by the witness bunch to the maximum energy lost by the drive bunch, or, in the case of a bunch train, the maximum energy lost by a bunch in the drive train
The theoretical derivation of the single bunch transformer ratio (R1) and the transformer ratio along the ramped bunch train (RBT) (Rn) are provided in Ref. [3] and we only recap the main results here. (Note, the analysis we present is for a dielectric wakefield accelerator but the conclusion can be generalized to metallic structures and linear plasma wakefield accelerators.) Numerical simulations [5] reveal the dependence of the peak acceleration gradient (W1þ) and R1 on the bunch length z (Fig. 2, taken from Ref. [5])

Summary

INTRODUCTION

The transformer ratio (R) in a collinear wakefield accelerator is defined as the ratio of the maximum energy gained by the witness bunch to the maximum energy lost by the drive bunch, or, in the case of a bunch train, the maximum energy lost by a bunch in the drive train. There was a mismatch between the wavelength of the fundamental mode of the wakefield ( 1⁄4 22 mm) and the bunch length (z 1⁄4 2 mm) produced by the Argonne Wakefield Accelerator (AWA) drive gun [4]. To understand how z effects the transformer ratio enhancement (Rn:RnÀ1) numerical simulations were performed for the previous experiment [3] [13.625 GHz dielectric-loaded accelerating (DLA) structure, 1⁄4 2 mm]. The AWA drive bunch length was increased with the newly implemented method based on a UV laser pulse stacking technique [6]. A shorter bunch will excite a larger wakefield, making it easier to measure the acceleration of the witness bunch in the experiment, but decrease the transformer ratio. IV we summarize the pros and cons of the RBT technique based on experiences gained from the two experiments

ELECTRON BUNCH LENGTHENING TECHNIQUE

EXPERIMENT TO ENHANCE TRANSFORM RATIO

Findings

DISCUSSION