We demonstrate the compression of 95 keV, space-charge-dominated electron bunches to sub-100 fs durations. These bunches have sufficient charge (200 fC) and are of sufficient quality to capture a diffraction pattern with a single shot, which we demonstrate by a diffraction experiment on a polycrystalline gold foil. Compression is realized by means of velocity bunching by inverting the positive space-charge-induced velocity chirp. This inversion is induced by the oscillatory longitudinal electric field of a 3 GHz radio-frequency cavity. The arrival time jitter is measured to be 80 fs.

We present a method for producing sub-100 fs electron bunches that are suitable for single-shot ultrafast electron diffraction experiments in the 100 keV energy range. A combination of analytical results and state-of-the-art numerical simulations show that it is possible to create 100 keV, 0.1 pC, 20 fs electron bunches with a spotsize smaller than 500 µm and a transverse coherence length of 3 nm, using established technologies in a table-top set-up. The system operates in the space-charge dominated regime to produce energy-correlated bunches that are recompressed by established radio-frequency techniques. With this approach we overcome the Coulomb expansion of the bunch, providing an entirely new ultrafast electron diffraction source concept.PACS numbers: 61.14. 87.64Bx, 41.75.Fr, 52.59.Sa The development of a general experimental method for the determination of nonequilibrium structures at the atomic level and femtosecond timescale would provide an extraordinary new window on the microscopic world. Such a method opens up the possibility of making 'molecular movies' which show the sequence of atomic configurations between reactant and product during bondmaking and bond-breaking events. The observation of such transition states structures has been called one of the holy-grails of chemistry, but is equally important for biology and condensed matter physics [1, 2].There are two promising approaches for complete structural characterization on short timescales: Ultrafast X-ray diffraction and ultrafast electron diffraction (UED). These methods use a stroboscopic -but so far multi-shot-approach that can capture the atomic structure of matter at an instant in time. Typically, dynamics are initiated with an ultrashort (pump) light pulse and then -at various delay times-the sample is probed in transmission or reflection with an ultrashort electron [3,4] or X-ray pulse [5]. By recording diffraction patterns as a function of the pump-probe delay it is possible to follow various aspects of the real-space atomic configuration of the sample as it evolves. Time resolution is fundamentally limited by the X-ray/electron pulse duration, while structural sensitivity depends on source properties like the beam brightness and the nature of the samples.Electron diffraction has some unique advantages compared with the X-ray techniques, see e.g. Ref.[6]. However, until recently femtosecond electron diffraction experiments had been considered unlikely. It was thought that the strong Coulombic repulsion (spacecharge) present inside of high-charge-density electron bunches produced through photoemission with femtosecond lasers fundamentally limited this technique to picosecond timescales and longer. Several recent developments, however, have resulted in a change of outlook. Three approaches to circumvent the space-charge problem have been attempted by several groups. The traditional way is to accelerate the bunch to relativistic energies to effectively damp the Coulomb repulsion. Bunches of several hundred femtosecond duration containi...

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Taban, G.; Fleskens, B.; Luiten, O.J.; Reijnders, M.P.; Vredenbregt, E.J.D.; de Loos, M.J.; van der Geer, S.B.

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.