Albert Reitsma

Evidence of photon acceleration by laser wake fields

Photon acceleration is the phenomenon whereby a light wave changes color when propagating through a medium whose index of refraction changes in time. This concept can be used to describe the spectral changes experienced by electromagnetic waves when they propagate in spatially and temporally varying plasmas. In this paper the detection of a large-amplitude laser-driven wake field is reported for the first time, demonstrating photon acceleration. Several features characteristic of photon acceleration in wake fields, such as splitting of the main spectral peak and asymmetries between the blueshift and redshift for large shifts, have been observed. The experiment is modeled using both a novel photon-kinetic code and a three-dimensional particle-in-cell code. In addition to the wide-ranging applications in the field of compact particle accelerators, the concept of wave kinetics can be applied to understanding phenomena in nonlinear optics, space physics, and fusion energy research.

Orbital and spin physics in LiNiO2 and NaNiO2

We derive a spin–orbital Hamiltonian for a triangular lattice of eg orbital degenerate (Ni3+) transition-metal ions interacting via 90° superexchange involving (O2−) anions, taking into account the onsite Coulomb interactions on both the anions and the transition metal ions. The derived interactions in the spin–orbital model are strongly frustrated, with the strongest orbital interactions selecting different orbitals for pairs of Ni ions along the three different lattice directions. In the orbital-ordered phase, favoured in mean field theory, the spin–orbital interaction can play an important role by breaking the U(1) symmetry generated by the much stronger orbital interaction and restoring the three-fold symmetry of the lattice. As a result, the effective magnetic exchange is non-uniform and includes both ferromagnetic and antiferromagnetic spin interactions. Since ferromagnetic interactions still dominate, this offers yet insufficient explanation for the absence of magnetic order and the low-temperature behaviour of the magnetic susceptibility of stoichiometric LiNiO2. The scenario proposed to explain the observed difference in the physical properties of LiNiO2 and NaNiO2 includes small covalency of Ni–O–Li–O–Ni bonds inducing weaker interplane superexchange in LiNiO2, insufficient to stabilize orbital long-range order in the presence of stronger intraplane competition between superexchange and Jahn–Teller coupling.

Coupling of longitudinal and transverse motion of accelerated electrons in laser wakefield acceleration

The acceleration dynamics of electrons in a laser wakefield accelerator is discussed, in particular the coupling of longitudinal and transverse motion. This coupling effect is important for electrons injected with a velocity below the laser pulse group velocity. It is found that the electron bunch is adiabatically focused during the acceleration and that a finite bunch width contributes to bunch lengthening and growth of energy spread. These results indicate the importance of a small emittance for the injected electron bunch.

A method of determining narrow energy spread electron beams from a laser plasma wakefield accelerator using undulator radiation

In this paper a new method of determining the energy spread of a relativistic electron beam from a laser-driven plasma wakefield accelerator by measuring radiation from an undulator is presented. This could be used to determine the beam characteristics of multi-GeV accelerators where conventional spectrometers are very large and cumbersome. Simultaneous measurement of the energy spectra of electrons from the wakefield accelerator in the 55–70 MeV range and the radiation spectra in the wavelength range of 700–900 nm of synchrotron radiation emitted from a 50 period undulator confirm a narrow energy spread for electrons accelerated over the dephasing distance where beam loading leads to energy compression. Measured energy spreads of less than 1% indicates the potential of using a wakefield accelerator as a driver of future compact and brilliant ultrashort pulse synchrotron sources and free-electron lasers that require high peak brightness beams.

Photon kinetic modeling of laser pulse propagation in underdense plasma

This paper discusses photon kinetic theory, which is a description of the electromagnetic field in terms of classical particles in coordinate and wave number phase space. Photon kinetic theory is applied to the interaction of laser pulses with underdense plasma and the transfer of energy and momentum between the laser pulse and the plasma is described in photon kinetic terms. A comparison is made between a one-dimensional full wave and a photon kinetic code for the same laser and plasma parameters. This shows that the photon kinetic simulations accurately reproduce the pulse envelope evolution for photon frequencies down to the plasma frequency.

Propagation of a weakly nonlinear laser pulse in a curved plasma channel

In this paper, the propagation of a high intensity laser pulse in a curved plasma channel is studied. The matching conditions, which are different from straight plasma channels, are examined. The analysis includes a study of centroid and spot size oscillations, relativistic self-focusing, and the effect of wakefields on the propagation of the laser pulse. The possible application of curved plasma channels in laser wakefield acceleration is discussed.

Envelope equations and conservation laws describing wakefield generation and electron acceleration

Previous authors have proposed various envelope equations to describe the behavior of an electromagnetic pulse generating a wakefield. In general these retain second-order derivatives, the reason being that the eikonal contains the initial wave frequency. Here it is shown that if the evolution of the wave frequency is followed using ray-tracing equations, a first-order evolution equation is obtained. It can be shown with this formalism that wave action is conserved and the energy lost from the electromagnetic wave can be explicitly accounted for in terms of energy gained by the plasma. The energy balance equations suggest that an electron bunch which will extract energy efficiently from a wakefield can be at least as efficiently accelerated by direct interaction with the electromagnetic pulse.

Transport of ultra-short electron bunches in a free-electron laser driven by a laser-plasma wakefield accelerator

Focussing ultra-short electron bunches from a laser-plasma wakefield accelerator into an undulator requires particular attention to be paid to the emittance, electron bunch duration and energy spread. Here we present the design and implementation of a focussing system for the ALPHA-X beam transport line, which consists of a triplet of permanent magnet quadrupoles and a triplet of electromagnetic quadrupoles.

Propagation of a Short Intense Laser Pulse in a Curved Plasma Channel

In this paper, the propagation of a short intense laser pulse in a curved plasma channel is considered. The effects of the shape of the plasma density profile and feedback from the wakefield on the pulse envelope dynamics are studied, with particular attention being paid to the conditions for avoiding laser spot size and centroid oscillations. A possible application to laser wakefield acceleration in the nonlinear regime is discussed.

Limits of validity of photon-in-cell simulation techniques

A comparison is made between two reduced models for studying laser propagation in underdense plasma; namely, photon kinetic theory and the slowly varying envelope approximation. Photon kinetic theory is a wave-kinetic description of the electromagnetic field where the motion of quasiparticles in photon coordinate-wave number phase space is described by the ray-tracing equations. Numerically, the photon kinetic theory is implemented with standard particle-in-cell techniques, which results in a so-called photon-in-cell code. For all the examples presented in this paper, the slowly varying envelope approximation is accurate and therefore discrepancies indicate the failure of photon kinetic approximation for these cases. Possible remedies for this failure are discussed at the end of the paper.