Abstract
The capability of accelerating electron bunches at high repetition rate is one of the key performance criteria for all high average power particle accelerator applications. High gradient laser-driven acceleration holds the potential for greatly reducing size and costs of future machines, but typically requires very high peak laser powers. On the other hand, MHz pulse trains of TW-class laser beams are much beyond the state of the art, so that laser recycling and recirculation is a necessary step to bridge that gap. In this experiment we demonstrate for the first time an inverse free electron laser accelerator (IFEL) operating within an active optical cavity showing the ability to laser-accelerate electron bunch trains in burst mode at $g20\text{ }\text{ }\mathrm{MHz}$ repetition rate. The experimental setup, synchronization challenges and acceleration results are presented. It is found that careful control of the dispersive properties of the cavity is required in order to sustain high accelerating gradients over many passes in the laser pulse train.
Highlights
The last decade has seen major advances in high gradient laser-driven particle acceleration mostly using plasma wakefield, and dielectric structure, laser-generated THz-driven or even vacuum interactions [1,2,3,4]
The scalability of laser-driven particle accelerators remains fundamentally linked to the progress in the development of high average power high peak power lasers
While there are few scientific applications where operating at low repetition rates would be acceptable and even desired in some cases, any power-hungry, industrial or medical deployment will require to scale up the accelerated beam average current/power to levels which put demands on the drive power essentially beyond the reach of current laser technology
Summary
The last decade has seen major advances in high gradient laser-driven particle acceleration mostly using plasma wakefield, and dielectric structure, laser-generated THz-driven or even vacuum interactions [1,2,3,4]. Since IFEL acceleration is mainly determined by the static magnetic field profile in the undulator, the accelerated beam longitudinal phase space can be fully controlled by properly designing the interaction, resulting in small energy spread, and, even in presence of large laser energy jitters, much reduced output energy fluctuations, especially compared to other laser acceleration schemes which rely on nonlinear effects Many of these properties have been demonstrated by a series of experiments at BNL which, utilizing an intense CO2 laser. The experiment constitutes the first proof-of-principle test of a recirculating laser acceleration concept and required addressing many key issues such as the design of a long recirculating cavity to accommodate e-beam (focusing quadrupoles, steering magnets, prebuncher and IFEL undulator) as well as laser (refocusing mirrors, CO2 amplifier, vacuum windows) optics elements and the synchronization of electron and laser bunches to achieve an efficient operation essential for repetition rate multiplication. We first describe the details of the laser cavity, we discuss how we achieved synchronization and timing, show the experimental acceleration results and discuss the challenges associated with laser pulse degradation inside the active cavity, before drawing conclusions and future outlook
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