M. J. van der Wiel

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.

Femtosecond synchronization of a 3 GHz RF oscillator to a mode-locked Ti:sapphire laser

We have synchronized a 3 GHz electronic oscillator to a 75 MHz femtosecond self-mode-locked Ti : sapphire laser with a relative root-mean-square phase-jitter of less than 20 fs in the frequency range of 0:05 Hz–100 kHz and a drift of 20 fs=h: The jitter has been measured by time and frequency domain analysis. Potential applications include synchronization of lasers and RFpower sources in particle accelerator experiments and high-resolution pump–probe experiments using, e.g. electrons and lasers. r 2001 Elsevier Science B.V. All rights reserved. PACS: 29.17.+W

Observation of narrow-band Si L-edge Čerenkov radiation generated by 5 MeV electrons

Narrow-band Cerenkov radiation at 99.7 eV has been generated by 5 MeV electrons in a silicon foil, with a yield ∼1×10−3 photon/electron. These measurements demonstrate the feasibility of a compact, narrow-band, and intense soft x-ray source based on small electron accelerators. The observed yield and dependence of the photon spectrum on emission angle are in agreement with theoretical predictions for Cerenkov radiation based on refractive index data of silicon.

High-brightness, narrowband, and 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.

Direct measurement of synchronization between femtosecond laser pulses and a 3 GHz radio frequency electric field inside a resonant cavity

We demonstrate a method to measure synchronization between femtosecond laser pulses and the electric field inside a resonant 3u2009GHz radio frequency (RF) cavity. The method utilizes the Pockels effect in a crystal inside the RF cavity by measuring the retardation of the components of polarization as a function of RF phase. Resolution of the setup used is shown to be 29u2009±u20092u2009fs (root-mean-square, rms), with timing jitter between the laser pulses and the RF field inside the cavity of 96u2009±u20097u2009fs (rms). The method provides a tool to reduce jitter and improve time-resolution in ultrafast electron diffraction experiments.

Laser wakefield acceleration: the injection issue. Overview and latest results

External injection of electron bunches into laser-driven plasma waves so far has not resulted in ‘controlled’ acceleration, i.e. production of bunches with well-defined energy spread. Recent simulations, however, predict that narrow distributions can be achieved, provided the conditions for properly trapping the injected electrons are met. Under these conditions, injected bunch lengths of one to several plasma wavelengths are acceptable. This paper first describes current efforts to demonstrate this experimentally, using state-of-the-art radio frequency technology. The expected charge accelerated, however, is still low for most applications. In the second part, the paper addresses a number of novel concepts for significant enhancement of photo-injector brightness. Simulations predict that, once these concepts are realized, external injection into a wakefield accelerator will lead to accelerated bunch specs comparable to those of recent ‘laser-into-gasjet’ experiments, without the present irreproducibility of charge and final energy of the latter.

Experimental validation of a radio frequency photogun as external electron injector for a laser wakefield accelerator

A purpose-built RF-photogun as external electron injector for a laser wakefield accelerator has been thoroughly tested. Different properties of the RF-photogun have been measured such as energy, energy spread and transverse emittance. The focus of this study is the investigation of the smallest possible focus spot and focus stability at the entrance of the plasma channel. For an electron bunch with 10 pC charge and 3.7 MeV kinetic energy, the energy spread was 0.5% with a shot-to-shot stability of 0.05%. After focusing the bunch by a pulsed solenoid lens at 140 mm from the middle of the lens, the focal spot was 40 μm with a shot-to-shot stability of 5 μm. Higher charge leads to higher energy spread and to a larger spot size, due to space charge effects. All properties were found to be close to design values. Given the limited energy of 3.7 MeV, the properties are sufficient for this gun to serve as injector for one particular version of laser wakefield acceleration, i.e., injection ahead of the laser puls...

Production of ultra-short, high charge, low emittance electron bunches using a 1 GV/m DC gun

Advanced acceleration schemes, for example those based on wake fields of laser pulses traveling through plasma, require the injection of very high quality relativistic femtosecond electron bunches. Such bunches can be produced by a photoexcited RF gun followed by longitudinal bunch compression. Currently we are investigating a different pre-acceleration scheme, which avoids the necessity of magnetic compression and the associated potential emittance growth due to coherent synchrotron radiation.

The use of linear superconducting electron accelerator for subcritical reactor driving

At the National Science Centre, Kharkiv Institute of Physics and Technology (NSC KIPT) the possibility of creating an installation with a subcritical reactor driven by an electron accelerator is examined. To obtain the maximal stream of neutrons from a neutron-producing target at a minimal density of energy emission, the electron energy should be in the range of 100–200 MeV and the size of the target should be as large as possible. Other important requirements are beam continuity with time and long-term stability of the accelerator parameters. The variants of using the superconducting linear accelerator on the basis of a TESLA accelerating structure as of subcritical reactor driver are considered. The basic design parameters and characteristics of this installation are presented.

Transverse self-fields within an electron bunch moving in an arc of a circle

As a consequence of motions driven by external forces, self-fields (which are different from the static case) originate within an electron bunch. In the case of magnetic external forces acting on an ultrarelativistic beam, the longitudinal self-interactions are responsible for Coherent Synchrotron Radiation-related phenomena, which have been studied extensively. On the other hand, transverse self-interactions are present too. At the time being, existing theoretical analysis of transverse self-forces deal with the case of a bunch moving along a circular orbit only, without considering the situation of a bending magnet with a finite length. In this paper, we propose an electrodynamical analysis of transverse self-fields which originate, at the position of a test particle, from an ultrarelativistic electron bunch moving in an arc of a circle. The problem is first addressed within a two-particle system. We then extend our consideration to a line bunch with a stepped density distribution, a situation which can be easily generalized to the case of an arbitrary density distribution. Our approach turns out to be also useful in order to obtain a better insight in the physics involved in the case of simple circular motion.