Observation of High Transformer Ratio of Shaped Bunch Generated by an Emittance-Exchange Beam Line

Collinear wakefield acceleration has been long established as a method capable of generating ultrahigh acceleration gradients. Because of the success on this front, recently, more efforts have shifted towards developing methods to raise the transformer ratio (TR). This figure of merit is defined as the ratio of the peak acceleration field behind the drive bunch to the peak deceleration field inside the drive bunch. TR is always less than 2 for temporally symmetric drive bunch distributions and therefore recent efforts have focused on generating asymmetric distributions to overcome this limitation. In this Letter, we report on using the emittance-exchange method to generate a shaped drive bunch to experimentally demonstrate a $\mathrm{TR}\approx 5$ in a dielectric wakefield accelerator.

Figure 1

Schematic of beam line, B, TDC, Q, DLS, and SPEC stand for dipole, transverse deflecting cavity, quadrupole, dielectric-loaded structure, and spectrometer. YAG represents the beam image observation station.

Figure 2

Beam image from experiment: (a) Drive and witness transverse distribution at YAG1, the red line refers to projection. (b) Longitudinal profile measurement result of drive bunch at YAG2. (c)–(d) Drive and witness bunch at YAG3 with wakefield on-off. (e)–(f) Witness bunch at YAG3 with wakefield on-off.

Figure 3

Reconstructed longitudinal phase space and measured wakefield: (a) Longitudinal phase space full envelope. (b) Average profile in LPS. (c) Average profiles error bar in LPS. (d) Measured energy gain induced by wakefield.

Figure 4

(a)–(b) Comparison between measured and analytical wakefield induced by drive bunch when the gap is 2.5 (a) or 2.1 mm (b).