Coherent electron beams for ultrafast structural dynamics

Abstract only

A laser-cooled electron source for single-shot femtosecond X-ray and electron diffraction

In 2009 the first hard X-ray Free Electron Laser (XFEL) has become operational – LCLS at Stanford University – which enables recording the full diffraction pattern of a tiny protein crystal in a single, few-femtosecond shot. This is an enormously important development but it requires a large-scale facility and investments at the national, if not the international, level. For reasons of size, costs and accessibility of the setup a small-scale XFEL, affordable by a university laboratory, would be highly desirable. A promising route towards a small-scale XFEL is the development of low-emittance electron sources, which enable lasing at reduced electron energies. We have developed a new, ultracold pulsed electron source, based on near-threshold photo-ionization of a laser-cooled gas. The source is characterized by an effective electron temperature of ~10 K, almost three orders of magnitude lower than conventional sources. This should enable normalized RMS emittances 1-2 orders of magnitude lower than photocathode sources, at comparable bunch charge. I will discuss the properties of this new source and the possible implications for XFELs. Another route we are investigating is to use electrons directly. Electrons and X-rays both enable the study of structural dynamics at atomic length scales, yet the information that can be extracted by probing with either electrons or X-rays is quite different and, in fact, complementary. A pulsed electron source with the XFEL capability of performing single-shot, femtosecond diffraction would therefore be highly desirable as well. The primary obstacle facing the realization of such an electron source is the inevitable Coulomb expansion of the bunch, leading to loss of temporal resolution. We have developed a method, based on radio-frequency (RF) techniques, to invert the Coulomb expansion. We will report on the first experiments demonstrating RF compression of 0.25 pC, 100 keV electron bunches to sub-100 fs bunch lengths. We have used these bunches to produce high-quality, single-shot diffraction patterns of poly-crystalline gold. By combining the laser-cooled, ultracold electron source with RF acceleration and bunch compression techniques, single-shot, femtosecond studies of the structural dynamics of macromolecular crystals will become possible with electrons as well

Atomen in actie

Met elektronenmicroscopen is het mogelijk om afzonderlijke atomen in beeld te brengen, maar alleen als ze absoluut stilstaan. Wat er feitelijk gebeurt op atomaire schaal, bijvoorbeeld tijdens een chemische reactie, een faseovergang in een metaal, of als een eiwit zich vouwt, is nog nauwelijks in beeld gebracht. Dat komt doordat atomen meestal veel te snel bewegen om ze met conventionele technieken te kunnen volgen. Om atomen in actie te kunnen filmen, zo laat Luiten zien, zijn de ontwikkeling van exotische bundeltechnieken en het verkennen van uiterste grenzen vereist. De toevoeging van de tijdsdimensie aan het onderzoek van de nanowereld realiseert een wetenschappelijke droom, met belangrijke implicaties voor de maatschappij

Design and optimization of a 100 keV DC/RF ultracold electron source

An ultracold electron source based on near-threshold photoionization of a laser-cooled and trapped atomic gas is presented in this work. Initial DC acceleration to 10 keV and subsequent acceleration of the created bunches to 100 keV by RF fields makes the design suitable to serve as injector for accelerator-based light sources, single-shot ultrafast protein crystallography, applications in dielectric laser acceleration schemes, and potentially as an injector for free electron lasers operating in the quantum regime. This paper presents the design and properties of the developed DC/RF structure. It is shown that operation at a repetition frequency of 1 kHz is achievable and detailed particle tracking simulations are presented showing the possibility of achieving a brightness that can exceed conventional RF photosources

Photon yield of superradiant inverse Compton scattering from microbunched electrons

Compact x-ray sources offering high-brightness radiation for advanced imaging applications are highly desired. We investigate, analytically and numerically, the photon yield of superradiant inverse Compton scattering from microbunched electrons in the linear Thomson regime, using a classical electrodynamics approach. We show that for low electron beam energy, which is generic to inverse Compton sources, the single electron radiation distribution does not match well to collective amplification pattern induced by a density modulated electron beam. Consequently, for head-on scattering from a visible laser, the superradiant yield is limited by the transverse size of typical electron bunches driving Compton sources. However, by simultaneously increasing the electron beam energy and introducing an oblique scattering geometry, the superradiant yield can be increased by orders of magnitude

Single-cycle surface plasmon polaritons on a bare metal wire excited by relativistic electrons

Terahertz (THz) pulses are applied in areas as diverse as materials science, communication and biosensing. Techniques for subwavelength concentration of THz pulses give access to a rapidly growing range of spatial scales and field intensities. Here we experimentally demonstrate a method to generate intense THz pulses on a metal wire, thereby introducing the possibility of wave-guiding and focussing of the full THz pulse energy to subwavelength spotsizes. This enables endoscopic sensing, single-shot subwavelength THz imaging and study of strongly nonlinear THz phenomena. We generate THz surface plasmon polaritons (SPPs) by launching electron bunches onto the tip of a bare metal wire. Bunches with 160 pC charge and ≈6 ps duration yield SPPs with 6-10 ps duration and 0.4±0.1 MV m-1 electric field strength on a 1.5 mm diameter aluminium wire. These are the most intense SPPs reported on a wire. The SPPs are shown to propagate around a 90° bend.</p

A tabletop soft X-ray source based on 5-10 MeV LINACs

We are investigating the feasibility of a novel, tabletop, high-brightness soft X-ray source

High-brightness, narrowband, compact soft x-ray Cherenkov sources in the water window

Narrowband, soft x-ray Cherenkov radiation at energies of 453 and 512 eV has been generated by 10 MeV electrons in, respectively, titanium and vanadium foils. The measured spectral and angular distribution of the radiation, and the measured total yield (10–4 photon per electron) are in agreement with theoretical predictions based on refractive index data. We show that the brightness that can be achieved using a small electron accelerator is sufficient for practical x-ray microscopy in the water-window spectral region

A bright ultracold atoms-based electron source

An important application of pulsed electron sources is Ultrafast Electron Diffraction [1]. In this technique, used e.g. in chemistry, biology and condensed matter physics, one can observe processes that take place at the microscopic level with sub-ps resolution. To reach the holy grail of UED, single-shot diffraction images of biologically relevant molecules, electron bunches of 1pC charge, 100fs length and 10nm coherence length are required. Conventional pulsed electron sources cannot fulfil these requirements, but according to the simulations reported in [2] and [3] a new type of source can.The new source combines the use of magneto-optical atom trapping with fast high voltage technology. We start by cooling and trapping rubidium atoms, followed by ionisation just above threshold, leading to an ultracold plasma. Another possibility is to excite the atoms into a high Rydberg level, from which they spontaneously evolve into an ultracold plasma. Applying a fast high voltage pulse, electron bunches can be extracted. In an initial study [2] it has been shown that this type of source can provide a very high brightness. Depending on the initial particle distribution, the reduced brightness can be in the order of 1x109 A/(rad2m2V), which is orders of magnitude higher than established technology such as an electron photogun can provide.Here we report the first experiments toward realisation of the source. Here, a simple accelerator structure consists of four bars surrounding a MOT, on which an 800V pulsed voltage with a rise time of 1ƒÝs is applied. An MCP together with a phosphor screen and a CCD camera are used as detection system. The bunch size obtained from the phosphor screen is fitted with a Gaussian distribution, from which the electron temperature is extracted. For small extracted charges, the electron temperature is found to have an upper limit of 500K, the measurement being limited by stray magnetic fields due to the low electron energy (10eV). We have also extracted a pulsed ion beam by reversing the sign of the accelerating voltage. Since ions are heavier, they obtain higher energy and are less influenced by the magnetic fields. The temperature in this case is found to b