Witness electron beam injection using an active plasma lens for a proton beam-driven plasma wakefield accelerator

An active plasma lens (APL) focuses the beam in both the horizontal and vertical planes simultaneously using a magnetic field generated by a discharge current through the plasma. A beam size of <math display="inline"><mrow><mn>5</mn><mi>–</mi><mn>10</mn><mtext> </mtext><mtext> </mtext><mi>μ</mi><mi mathvariant="normal">m</mi></mrow></math> can be achieved within a short distance using a focusing gradient on the order of <math display="inline"><mrow><mn>100</mn><mtext> </mtext><mtext> </mtext><mi mathvariant="normal">T</mi><mo>/</mo><mi mathvariant="normal">m</mi></mrow></math>. The APL is therefore an attractive element for plasma wakefield acceleration, because an ultrasmall size of the witness electron beam is required for injection into the plasma wakefield to minimize emittance growth and to enhance the capturing efficiency. When the drive beam and witness electron beam copropagate through the APL, interactions between the drive and witness beams, and the plasma must be considered. In this paper, through particle-in-cell simulations, we discuss the possibility of using an APL for the final focusing of the electron beam for the AWAKE RUN 2 experiments. It is confirmed that the amplitude of the plasma wakefield excited by proton bunches remains the same even after propagation through the APL. The emittance of the witness electron beam increases rapidly in the plasma density ramp regions of the lens. Nevertheless, when the witness electron beam has a charge of 100 pC, emittance of 10 mm mrad, and bunch length of <math display="inline"><mrow><mn>60</mn><mtext> </mtext><mtext> </mtext><mi>μ</mi><mi mathvariant="normal">m</mi></mrow></math>, its emittance growth is not significant along the active plasma lens. For small emittance, such as 2 mm mrad, the emittance growth is found to be strongly dependent on the rms beam size, plasma density, and multiple Coulomb scattering.