Abstract
Plasma accelerators driven by particle beams are a very promising future accelerator technology as they can sustain high accelerating fields over long distances with high energy efficiency. They rely on the excitation of a plasma wave in the wake of a drive beam. To generate the plasma, a neutral gas can be field-ionized by the head of the drive beam, in which case the distance of acceleration and energy gain can be strongly limited by head erosion. Here we overcome this limit and demonstrate that electrons in the tail of a drive beam can be accelerated by up to 27 GeV in a high-ionization-potential gas (argon), boosting their initial 20.35 GeV energy by 130%. Particle-in-cell simulations show that the argon plasma is sustaining very high electric fields, of ∼150 GV m−1, over ∼20 cm. The results open new possibilities for the design of particle beam drivers and plasma sources.
Highlights
Plasma accelerators driven by particle beams are a very promising future accelerator technology as they can sustain high accelerating fields over long distances with high energy efficiency
The head erosion rate vhe was previously found to be strongly dependent on the gas ionization potential, Ei, and it was shown that forÀelectrÁon beams that are matched to the plasma, uhe / E1i :73EN= gI3=2, where EN is the normalized emittance of the beam, g is the Lorentz relativistic factor and I is the beam current at the ionization front[17]
In the experiment reported here, where a mismatched beam propagates in an argon gas, the conditions are expected to be strongly unfavourable for plasma acceleration because of the higher erosion rate in a high-ionization-potential gas (Ei 1⁄4 15.8 eV for argon) and because of the emittance growth resulting from the large mismatch, and it was anticipated that such high-ionizationpotential plasma accelerators would produce small energy gains
Summary
Plasma accelerators driven by particle beams are a very promising future accelerator technology as they can sustain high accelerating fields over long distances with high energy efficiency. The use of a particle beam driver (the so-called Plasma Wakefield Accelerator scheme) presents crucial advantages for future high-energy accelerator technology It promises very competitive energy efficiency from the wall plug to the accelerated beam[10], as well as a long interaction distance per acceleration stage because of the absence of dephasing between the particles and the plasma wave. It allows the production of long, uniform and high-density plasmas with no alignment or timing issues This self-ionization scheme leads to a rather fast erosion of the head of the drive bunch, which can limit the acceleration length. The simulations provide evidence for the role of the electron beam self-focusing in making the beam denser and able to drive nonlinear wakes at very high density They support the conclusion that long interaction distances are achieved when focusing the electron beam with large diffraction lengths, despite the anticipated high head erosion rate. The results provide a more complete understanding of the beam–plasma interaction, which will guide future optimizations of plasma acceleration stages
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