Efficient and Stable Acceleration of Electrons in Laser Wakefield Accelerators

Prof. Navid Vafaei-Najafabadi, Stony Brook University (Principal Investigator)

Dr. Mikhail Fedurin, Brookhaven National Laboratory (Co-Investigator)

Laser wakefield accelerators (LWFAs) are capable of generating a much larger accelerating force than the typical structures used in particle accelerators today. These forces arise within large-amplitude waves produced from the interaction of a high-power laser pulse with an ionized, plasma medium. While LWFA-based electron sources would in principle find diverse areas of application including particle colliders and free electron lasers (FELs), such particle sources must satisfy stringent beam quality criteria to be viable for these applications. In particular, nearly all applications require high luminosity beams with a low energy spread. High beam luminosity at a practical power consumption requires the LWFAs to operate at a similar or greater power efficiency compared to conventional accelerators. However, recent theoretical work has suggested that a low-energy-spread beam that is accelerating at high efficiency in an LWFA becomes unstable, with the beam breaking up into pieces as it is accelerating. The breaking up of the accelerating beam due to this transverse beam break up (BBU) instability will significantly compromise its utility. While only a simplified model of a plasma wakefield was used in the aforementioned theoretical study, this proposed efficiency-instability relation places a fundamental limitation on the achievable energy spread that can efficiently be accelerated in an LWFA, and thus poses a serious challenge to the future applicability of plasma accelerators to high-luminosity applications. Since operating at high luminosity is a requirement for the next generation particle accelerators, validation of this theory is critical to ascertaining whether LWFAs can be used as the basis for future compact particle sources. 

The objective of this proposal is to evaluate the relationship between efficiency and instability in LWFAs. This objective will be attained via theoretical, experimental, and simulation studies of the correlation between efficiency and instability. State of the art simulations based on the Particle-in-cell (PIC) methods will enable a rigorous investigation of the fundamental components of BBU, namely the growth rate of the transverse motion of the accelerating electrons in LWFA. The accuracy of the modern PIC simulation tools has made them ideal for benchmark investigations of the physics in LWFA. Using parameters of the LWFA in various intensity regimes, the transverse motion of the accelerated beam will be compared with theory. Thus, these simulations will provide the required data for the examination of the theory and will be used to validate or provide the basis for proposing modifications and revisions to it. 

Experimental evaluation of transverse stability of LWFAs will be carried out at the Accelerator Test Facility (ATF) of Brookhaven National Laboratory (BNL). As the facility is equipped with an independently controlled particle accelerator that produces well-characterized electron beams as well as a high-power laser pulse capable of driving high-amplitude plasma waves, the facility is ideally suited for the first-ever experimental characterization of the BBU instability in LWFAs. In these experiments, the ATF electron beam will be injected into the LWFA driven by the ATF laser. Using advanced electron beam and laser diagnostics, the transverse force experienced by the electron beam will be measured, which will allow for the characterization of the growth of the BBU instability. Because higher charge electron beam extracts more energy from the LWFA, leading to higher energy extraction efficiency, the independent control over the electron beam is vital for examining the transverse behavior of the beam at various efficiencies. Finally, the experiments will be supported by simulations, which will assist in the interpretation of the data. Thus, the proposed research leads to the examination of a potentially significant obstacle in the path of high-luminosity applications of LWFAs.