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.

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At present, the smallest spot size which can be achieved with state-of-the-art focused ion beam ͑FIB͒ technology is mainly limited by the chromatic aberrations associated with the 4.5 eV energy spread of the liquid-metal ion source. Here we numerically investigate the performance of an ultracold ion source which has the potential for generating ion beams which combine high brightness with small energy spread. The source is based on creating very cold ion beams by near-threshold photoionization of a laser-cooled and trapped atomic gas. We present ab initio numerical calculations of the generation of ultracold beams in a realistic acceleration field and including all Coulomb interactions, i.e., both space charge effects and statistical Coulomb effects. These simulations demonstrate that with existing technology reduced brightness values exceeding 10 5 A m −2 sr −1 V −1 are feasible at an energy spread as low as 0.1 eV. The estimated spot size of the ultracold ion source in a FIB instrument ranges from 10 nm at a current of 100 pA to 0.8 nm at 1 pA.

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Absolute charge calibration of scintillating screens for relativistic electron detection Review of Scientific Instruments 81, 033301 (2010);

Ultrafast electron diffraction (UED) enables studies of structural dynamics at atomic length and timescales, i.e., 0.1 nm and 0.1 ps, in single-shot mode. At present UED experiments are based on femtosecond laser photoemission from solid state cathodes. These photoemission sources perform excellently, but are not sufficiently bright for single-shot studies of, for example, biomolecular samples. We propose a new type of electron source, based on near-threshold photoionization of a laser-cooled and trapped atomic gas. The electron temperature of these sources can be as low as 10 K, implying an increase in brightness by orders of magnitude. We investigate a setup consisting of an ultracold electron source and standard radio-frequency acceleration techniques by GPT tracking simulations. The simulations use realistic fields and include all pairwise Coulomb interactions. We show that in this setup 120 keV, 0.1 pC electron bunches can be produced with a longitudinal emittance sufficiently small for enabling sub-100 fs bunch lengths at 1% relative energy spread. A transverse root-mean-square normalized emittance of epsilon(x) = 10 nm is obtained, significantly better than from photoemission sources. Correlations in transverse phase-space indicate that the transverse emittance can be improved even further, enabling single-shot studies of biomolecular samples.

Since the recent publication of a practical recipe to create ''pancake'' electron bunches which evolve into uniformly filled ellipsoids, a number of papers have addressed both an alternative method to create such ellipsoids as well as their behavior in realistic fields. So far, the focus has been on the possibilities to preserve the initial ''thermal'' transverse emittance. This paper addresses the linear longitudinal phase space of ellipsoidal bunches. It is shown that ellipsoidal bunches allow ballistic compression at subrelativistic energies, without the detrimental effects of nonlinear space-charge forces. This in turn eliminates the need for the large correlated energy spread normally required for longitudinal compression of relativistic particle beams, while simultaneously avoiding all problems related to magnetic compression. Furthermore, the linear space-charge forces of ellipsoidal bunches can be used to reduce the remaining energy spread even further, by carefully choosing the beam transverse size, in a process that is essentially the time-reversed process of the creation of an ellipsoid at the cathode. The feasibility of compression of ellipsoidal bunches is illustrated with a relatively simple setup, consisting of a half-cell S-band photogun and a two-cell booster compressor. Detailed GPT simulations in realistic fields predict that 100 pC ellipsoidal bunches can be ballistically compressed to 100 fs, at a transverse emittance of 0:7 m, with a final energy of 3.7 MeV and an energy spread of only 50 keV.

The "Rijnhuizen" Fusion Free-Electron Maser (FEM) is the pilot experiment for a high power, mm-wave source, tunable in the range 130-260 GHz. The FEM has generated 730 kW output power during 10 µs pulses.To increase the overall efficiency to over 50 % and to reach a pulse length of at least 100 ms, an electron beam charge and energy recovery system is currently being designed and installed. This system consists of an electrostatic decelerator, which decels the beam from 2 MeV to an average of 200 keV, and a depressed collector. The EM-wave interaction inside the undulator can result in an energy spread of 300 keV behind the decelerator.The multi-stage collector is designed so that electrons fall on the backside of one of three electrodes, thus ensuring that secondary particles will immediately be accelerated back towards the electrodes. However, scattered primary electrons can cause back streaming, hereby reducing the efficiency and possibly damaging the machine.To reduce this back streaming to below a tolerable 0.1 %, the General Particle Tracer (GPT) code is being used to calculate primary and scattered particle trajectories inside the collector. It will be shown that an off-axis bending scheme, using a rotating perpendicular magnetic field lowers the back streaming and hereby increases the pulse length of the machine. The bending scheme also improves the power dissipation in the collector.