Coherent-synchrotron-radiation-free longitudinal shaping of a high-charge electron bunch based on velocity modulation

Beam-driven plasma wakefield acceleration (PWFA) is a promising technique to generate high-energy electron bunches for future electron-positron colliders. Longitudinal shaping of high-charge drive beam is highly desired for achieving high-transformer ratio and loading high-charge witness beam. However, the existing shaping schemes either focused on relatively low-charge bunch shaping or accompanied with significant charge-loss rate (typically over 50%). In this paper, a coherent-synchrotron-radiation-free shaping scheme based on velocity modulation is proposed to generate a high-charge beam with a linearly ramped profile. A <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mo form="prefix" stretchy="false">></a:mo><a:mn>10</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>kA</a:mi></a:mrow></a:math>-peak-current shaped beam containing <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mo form="prefix">></e:mo><e:mn>50</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>nC</e:mi></e:math> charge with a low charge-loss rate is demonstrated by a start-to-end simulation, and the tunabilities of the beam charge and the peak current, and the robustness of the proposed shaping scheme are also discussed. When loaded by a 3 nC witness beam, a <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mo form="prefix">></h:mo><h:mi>GV</h:mi><h:mo>/</h:mo><h:mi mathvariant="normal">m</h:mi></h:math> accelerating electric field with a transformer ratio larger than 4 can be achieved in a uniform plasma for the shaped drive beam, providing the possibility of high-transformer-ratio PWFA for a high-charge beam.