The National User Facility for Advanced Accelerator Experimental Tests II (FACET-II) at SLAC National Accelerator Laboratory expands upon the experiments conducted at FACET. Its purpose is to build upon the decades-long experience developed conducting accelerator R&D at SLAC in the areas of advanced acceleration and coherent radiation techniques with high-energy electron and positron beams. This paper summarizes the motivations for the design and resulting capabilities of the FACET-II facility.

We report on the application of machine learning (ML) methods for predicting the longitudinal phase space (LPS) distribution of particle accelerators. Our approach consists of training a ML-based virtual diagnostic to predict the LPS using only nondestructive linac and e-beam measurements as inputs. We validate this approach with a simulation study for the FACET-II linac and with an experimental demonstration conducted at LCLS. At LCLS, the e-beam LPS images are obtained with a transverse deflecting cavity and used as training data for our ML model. In both the FACET-II and LCLS cases we find good agreement between the predicted and simulated/measured LPS profiles, an important step towards showing the feasibility of implementing such a virtual diagnostic on particle accelerators in the future.

We outline the fundamental coherent radiation emission processes from a bunched charged particles beam. In contrast to spontaneous emission of radiation from a random electron beam that is proportional to the number of particles, a pre-bunched electron beam can emit spontaneously coherent radiation proportional to the number of particles -squared, through the process of (spontaneous) superradiance (SP-SR) (in the sense of Dicke's). The coherent SP-SR emission of a bunched electron beam can be even further enhanced by a process of stimulated-superradiance (ST-SR) in the presence of a seed injected radiation field. In this review, these coherent radiation emission processes for both single bunch and periodically bunched beams are considered in a model of radiation mode expansion.We also analyze here the SP-SR and ST-SR processes in the nonlinear regime, in the context of enhanced undulator radiation from a uniform undulator (wiggler) and in the case of wiggler Tapering-Enhanced Stimulated Superradiant Amplification (TESSA).The processes of SP-SR and TESSA take place also in tapered wiggler seed-injected FELs. In such FELs, operating in the X-Ray regime, these processes are convoluted with other effects. However these fundamental emission concepts are useful guidelines in efficiency enhancement strategy of wiggler tapering. Based on this model we review previous works on coherent radiation sources based on SP-SR (coherent undulator radiation, synchrotron radiation, Smith-Purcell radiation etc.), primarily in the THz regime and on-going works on tapered wiggler efficiency-enhancement concepts in various frequency regimes.

In the field of beam physics, two frontier topics have taken center stage due to their potential to enable new approaches to discovery in a wide swath of science. These areas are: advanced, high gradient acceleration techniques, and x-ray free electron lasers (XFELs). Further, there is intense interest in the marriage of these two fields, with the goal of producing a very compact XFEL. In this context, recent advances in high gradient radio-frequency cryogenic copper structure research have opened the door to the use of surface electric fields between 250 and 500 MV m−1. Such an approach is foreseen to enable a new generation of photoinjectors with six-dimensional beam brightness beyond the current state-of-the-art by well over an order of magnitude. This advance is an essential ingredient enabling an ultra-compact XFEL (UC-XFEL). In addition, one may accelerate these bright beams to GeV scale in less than 10 m. Such an injector, when combined with inverse free electron laser-based bunching techniques can produce multi-kA beams with unprecedented beam quality, quantified by 50 nm-rad normalized emittances. The emittance, we note, is the effective area in transverse phase space (x, p x /m e c) or (y, p y /m e c) occupied by the beam distribution, and it is relevant to achievable beam sizes as well as setting a limit on FEL wavelength. These beams, when injected into innovative, short-period (1–10 mm) undulators uniquely enable UC-XFELs having footprints consistent with university-scale laboratories. We describe the architecture and predicted performance of this novel light source, which promises photon production per pulse of a few percent of existing XFEL sources. We review implementation issues including collective beam effects, compact x-ray optics systems, and other relevant technical challenges. To illustrate the potential of such a light source to fundamentally change the current paradigm of XFELs with their limited access, we examine possible applications in biology, chemistry, materials, atomic physics, industry, and medicine—including the imaging of virus particles—which may profit from this new model of performing XFEL science.

The dynamics of intense electron bunches in free electron lasers and plasma wakefield accelerators are dominated by complex collective effects such as wakefields, space charge, coherent synchrotron radiation, and drift unpredictably with time, making it difficult to control and tune beam properties using model-based approaches. We report on a first of its kind combination of automatic, model-independent feedback with a neural network for control of the longitudinal phase space of relativistic electron beams with femtosecond resolution based only on transverse deflecting cavity measurements.