7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Recent progresses of plasma wakefield acceleration studies at IHEP
Ming Zeng, Dazhang Li, Jia Wang, Yulong Liu
Institute of High Energy Physics, Chinese Academy of Sciences, and University of Chinese
Academy of Sciencese-mail (speaker): zengming@ihep.ac.cn
Laser or charged particle beam driven plasma wakefield
accelerators (PWAs) are considered to be the acceleratorsof the next generation. Because of their extremely highacceleration gradient in the order of 1~100 GeV/m whichis 3 to 4 orders of magnitude higher than conventional
radio-frequency (RF) accelerators, PWA provides us thepossibility to accelerate particles exceeding the energy
frontier using facilities with affordable sizes.
Although PWA has such huge advantage in theacceleration gradient, its output beam quality is not as
good as RF accelerators. Especially, the energy spread isusually larger and the charge is lower for the output beam
in PWAs compared with RF accelerators. In Figure 1(a)we show the comparison of the energy spread vs. beam
charge from typical PWAs and the RF accelerators. Wecan see that the energy spread and beam charge arecontradictory parameters for PWAs. Although one single
parameter of the output beam from a PWA may havereached that from RF accelerators, to achieve
simultaneous high charge and low energy spread in a PWA
still remains challenging. This drawback strongly limits
the application range of PWAs.
Our plasma acceleration group at Institute of HighEnergy Physics (IHEP), Chinese Academy of Sciences,
has been doing works for high quality PWAs, especiallyfor achieving both high charge and low energy spread inPWAs. Based on the injection theory [1], we haveproposed a few new injection schemes for theoptimization of the beam quality. The first is the scissorcross ionization injection scheme [2], in which we use a
trigger laser colliding with the drive laser with an acuteangle 𝜃, so that the ionization injection is occurred only
during their collision as illustrated in Figure 1(b), and thebeam with ∼ 1% energy spread and ∼ 40 pC charge
can be produced. The second is the interference injection
by coaxial lasers [3], in which we use a tightly focused
trigger laser co-propagating with the drive laser to trigger
the injection of background plasma electrons as illustrated

in Figure 1(c). The trigger laser and the drive laser have
similar intensities at focus to generate interference rings,
so that the onion-like wakefield is created. The fastevolution of the wave front curvature of the trigger laserchanges the effective phase velocity of the wakefield totrigger injection. The beam with ∼ 0.3% energy spreadand ∼ 170 pC charge can be produced. The third is theinjection by tightly-focused drive laser [4], in which theinjection is triggered by the defocusing process of thedrive laser, and the beam with ∼ 1% energy spread and∼ 1 nC charge can be produced. These new injection
schemes produce beams with the energy spread and thebeam charge approaching that of the RF accelerators asshown in Figure 1(a).
We have also studied the long-term behavior of electrons
in a PWA for its future applications in high energy physics.
Based on the 3D betatron oscillation model, we havederived the long-term equations of motion without
resolving the betatron frequency [5, 6]. We have found
new phenomena including the precession of the ellipticaltrajectory in the betatron phase shift dominant regime asshown in Figure 2(a) and the elliptical trajectory
narrowing in the radiation reaction dominant regime asshown in Figure 2(b) [6]. This study is useful for future
PWAs with extremely long distances and high energies.
References[1] Ming Zeng et al., New J. Phys. 22, 123003 (2020).
[2] Jia Wang et al., Plasma Phys. Control. Fusion 64,045012 (2022).
[3] Jia Wang et al., Matter Radiat. Extremes 7, 054001(2022).
[4] Jia Wang et al., arXiv:2304.10730 (2023).
[5] M. Zeng and K. Seto, New J. Phys. 23, 075008 (2021).
[6] Yulong Liu and Ming Zeng, Phys. Rev. Accel. Beams
26, 031301 (2023).

(a)

RF(b)

(c)

Figure 1. (a) The energy spread vs. beam charge for typical PWA output
beams from previous results, and for our injection schemes including (b)scissor-cross ionization injection [2], (c) interference injection by coaxial
lasers [3], and injection by tightly-focused drive laser [4] (not illustratedhere). The typical beam quality for a RF accelerator is in the gray region in(a).

(a)

(b)

Figure 2. Evolution of the particle
trajectory (red ellipse) in the transverse
plane of a PWA, in (a) the betatronphase shift dominant regime and (b) theradiation reaction dominant regime.