  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. 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 meters. 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. 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 which
may profit from this new model of performing XFEL science.
