Correction of the deterministic part of space-charge interaction in momentum microscopy of charged particles.

Ultrahigh spectral brightness femtosecond XUV and X-ray sources like free electron lasers (FEL) and table-top high harmonics sources (HHG) offer fascinating experimental possibilities for analysis of transient states and ultrafast electron dynamics. For electron spectroscopy experiments using illumination from such sources, the ultrashort high-charge electron bunches experience strong space-charge interactions. The Coulomb interactions between emitted electrons results in large energy shifts and severe broadening of photoemission signals. We propose a method for a substantial reduction of the effect by exploiting the deterministic nature of space-charge interaction. The interaction of a given electron with the average charge density of all surrounding electrons leads to a rotation of the electron distribution in 6D phase space. Momentum microscopy gives direct access to the three momentum coordinates, opening a path for a correction of an essential part of space-charge interaction. In a first experiment with a time-of-flight momentum microscope using synchrotron radiation at BESSY, the rotation in phase space became directly visible. In a separate experiment conducted at FLASH (DESY), the energy shift and broadening of the photoemission signals were quantified. Finally, simulations of a realistic photoemission experiment including space-charge interaction reveals that a gain of an order of magnitude in resolution is possible using the correction technique presented here.

Compression of subrelativistic space-charge-dominated electron bunches for single-shot femtosecond electron diffraction.

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.

Ultracold electron source for single-shot, ultrafast electron diffraction.

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.

Radiation sources based on laser-plasma interactions.

Plasma waves excited by intense laser beams can be harnessed to produce femtosecond duration bunches of electrons with relativistic energies. The very large electrostatic forces of plasma density wakes trailing behind an intense laser pulse provide field potentials capable of accelerating charged particles to high energies over very short distances, as high as 1GeV in a few millimetres. The short length scale of plasma waves provides a means of developing very compact high-energy accelerators, which could form the basis of compact next-generation light sources with unique properties. Tuneable X-ray radiation and particle pulses with durations of the order of or less than 5fs should be possible and would be useful for probing matter on unprecedented time and spatial scales. If developed to fruition this revolutionary technology could reduce the size and cost of light sources by three orders of magnitude and, therefore, provide powerful new tools to a large scientific community. We will discuss how a laser-driven plasma wakefield accelerator can be used to produce radiation with unique characteristics over a very large spectral range.

How to realize uniform three-dimensional ellipsoidal electron bunches.

Uniform three-dimensional ellipsoidal distributions of charge are the ultimate goal in charged particle accelerator physics because of their linear internal force fields. Such bunches remain ellipsoidal with perfectly linear position-momentum phase space correlations in any linear transport system. We present a method, based on photoemission by radially shaped femtosecond laser pulses, to actually produce such bunches.

Nonlinear electrostatic emittance compensation in kA, fs electron bunches.

Nonlinear space-charge effects play an important role in emittance growth in the production of kA electron bunches with a bunch length much smaller than the bunch diameter. We propose a scheme employing the radial third-order component of an electrostatic acceleration field, to fully compensate the nonlinear space-charge effects. This results in minimal transverse root-mean-square emittance. The principle is demonstrated using our design simulations of a device for the production of high-quality, high-current, subpicosecond electron bunches using electrostatic acceleration in a 1 GV/m field. Simulations using the GPT code produce a bunch of 100 pC and 73 fs full width at half maximum pulse width, resulting in a peak current of about 1.2 kA at an energy of 2 MeV. The compensation scheme reduces the root-mean-square emittance by 34% to 0.4pi mm mrad.