Compact photon sources based on laser-plasma accelerators J. van Tilborg 1* , S. K. Barber 1 , C. B. Schroeder 1 , A. Gonsalves 1 , K. Nakamura 1 , S. Steinke 1 , J.-L. Vay 1 , A. Huebl 1 , C. G. R. Geddes 1 , E. Esarey 1 , D. Umstadter 2 , M. Fuchs 2 , F. Albert 3 , A. Pak 3 , N. Lemos 3 , C. Siders 3 , J. Palastro 4 , J. Shaw 4 , A. G. R. Thomas 5 , A. Maksimchuk 5 , K. Krushelnick 5 , H. Milchberg 6 , M. Downer 7 , R. Zgadzaj 7 , M. Hogan 8 , and C. Joshi 9 1 Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA * JvanTilborg@lbl.gov 2 University of Nebraska, Lincoln, NE 68588 USA 3 Lawrence Livermore National Laboratory, Livermore, CA 94550 USA 4 University of Rochester, Laboratory for Laser Energetics, Rochester, NY 14623, USA 5 University of Michigan, Ann Arbor, Michigan 48109, USA 6 University of Maryland, College Park, MD 20742, USA 7 University of Texas, Austin, TX 78712, USA 8 SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA 9 University of California, Los Angeles, CA 90095, USA August 31, 2020 Introduction Laser Plasma Accelerators (LPAs), see Refs. [1, 2], offer an attractive technology towards production of short (few-fs) and energetic (GeV-class) electron beams. The quality of the electron beams can be assessed based on the six-dimensional phase space, including final energy, energy spread, charge, bunch length, beam divergence, transverse source size, repetition rate, as well as long-term and shot-to-shot stability. Recently, 8 GeV energy gain has been reported [3], as well as percent-level energy spread [4], 10-100 pC charge, few-fs beam duration [5], sub-mrad divergence, few-μm source size [6], and repetition rates up to 10 Hz (and planned for kHz). These beam parameters not only reinforce the role for LPAs to play for future collider concepts, they also spur the development of unique radiation sources in the THz, XUV, X-ray and gamma ray regime, offering compactness (due to the high accelerating gradient in the plasma), few-fs radiation pulse lengths for high temporal resolution, strong fluxes ), and intrinsic femtosecond synchronization to a host of hyper-spectral pump-probe beams. These novel sources have attracted interest for applications throughout medicine [7], industry [8], material science [9], nuclear science, and nuclear nonproliferation [10]. Diagnosing and optimizing the light sources through precision and control will not only enable the high societal impact applications, but also provide novel insight and a handle on the rich physics at play during the complex laser-plasma interaction. The latter is critical for elevating laser-plasma acceleration to the ultimate goal of supporting the next generation of linear colliders. Coherent undulator emission (FEL radiation) from LPAs One of the key indicators in validating the quality of LPA electron beams for the collider application is the ability to drive a free-electron laser (FEL). To do so, the peak current, transverse emittance, energy spread, and longitudinal phase space, need to be optimized and controlled to meet minimal FEL lasing requirements, making FEL emission a perfect diagnostic to the six-dimensional phase space. 1