Plasma-based acceleration (PBA) can sustain an accelerating gradient on the order of GV/cm, which is over three orders of magnitude higher than that achievable with conventional RF cavities [1]. Upon full development, this technology promises to substantially mitigate the current challenges associated with the excessive size and cost of high-energy accelerators and colliders. The Institute of High Energy Physics, Chinese Academy of Sciences (IHEP), since 2005 has studied PBA and since 2017 has mainly focused on the CEPC plasma injector (CPI). CPI was introduced to mitigate the low-field dipole magnet issue in the CEPC booster by increasing the electron and positron energy from 10 to 30 GeV prior to injection into the booster. Figure 1 illustrates the conceptual design of CPI.

CPI’s conceptual design V3.0

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Gun1, Gun3, and Gun4 are S-band photocathode electron guns, producing 4 nC, 1 nC, and 3.6 nC electrons per bunch, respectively. Gun2 is an L-band photocathode gun delivering 12 nC electrons per bunch, while Gun5 is a thermionic cathode gun capable of generating 1 ns/10 nC electrons per bunch. The linacs-labeled L1–L8 utilize S-band technology to accelerate both electron and positron beams. The “target” is designated for positron production and shares materials and structures with the baseline positron source. The acronyms “DR” and “FFS” stand for damping ring and final focusing system, respectively. PWFA-I and PWFA-II represent the plasma wakefield acceleration sections for electrons and positrons, respectively. Among these components, “e1” to “e5” denote electron bunches, while “p1” represents a positron bunch.

In PWFA, the transformer ratio (TR) is characterized as the average energy gain (per single particle) of the trailer beam relative to the energy loss of the driver beam [2]. In the case of the CPI, the TR should be maintained at a minimum value of 2, which means the hosing instability must be considered. Simulation results show that with the assistance of ion motion, the hosing instability will not significantly impact beam quality, provided that the driver-trailer’s transverse misalignment and tilt angle are limited to 20 μm and 1°, respectively. Although high-quality electron acceleration is possible, positron acceleration in a plasma wakefield is even more difficult. In the PWFA community, the most significant breakthroughs have been achieved using high-intensity electron bunches to generate a nonlinear wakefield that accelerates a second electron bunch. However, this method is not effective for positron acceleration due to the extremely limited volume at the rear of such wakes, where the wakefield accelerates and focuses positrons. Positron acceleration in plasmas is a recognized and challenging problem, and various approaches have been proposed to overcome this limitation. One such approach involves using hollow plasma channels generated by readily available electron beams, as they offer a uniform accelerating field in transverse planes and zero focusing force within the channels. However, any misalignment of the drive and trailing bunches can trigger a pronounced beam-breakup instability, resulting in beam emittance growth and, ultimately, the loss of positrons. During CPI studies, we introduced a nonlinear scheme that offered field structures suitable for stable acceleration and the focusing of a positron bunch within a hollow plasma channel [3]. In this scheme, by using an asymmetry electron driver, the focusing field exhibits nearly linear variation, and the longitudinal field remains largely independent of the transverse dimensions throughout the channel region where the positron beam is situated. This particular field structure facilitates guided propagation and efficient acceleration of a high charge positron beam. Additionally, the positron beam effectively loads the wake, acquiring energy while maintaining a narrow energy spread. Although the efficiency of this scheme is not yet on par with electron PWFA, it currently stands as the most promising approach for achieving high efficiency, high-quality positron acceleration within PWFA.

Since 2017, we have completed a series of simulation studies on the key issues of CPI. However, we were unable to achieve any categorical conclusions due to the lack of experimental tests. In 2022, we proposed a dedicated test facility based on BEPC-II linac and obtained funding (of approximately US $14 million) from CAS in 2023. The PBA TF will be in operation in 2025.

Except for PWFA, we also carried out laser wakefield acceleration (LWFA) studies at IHEP, especially on the investigation of novel controlled injection schemes. Three injection schemes, named “scissor-crossing ionization injection,” “coaxial laser interference induced injection,” and “tightly focused laser defocusing-induced injection,” respectively, were proposed by the IHEP team [4,5,6]. All of these schemes aim to improve the LWFA produced beam qualities to the standard of conventional accelerators.