Search Results (10 total)

Electron bunches produced in self-modulated laser wakefield experiments usually have a broad energy distribution, with most electrons at low energy (1–3 MeV) and only a small fraction at high energy. We propose and investigate further acceleration of such bunches in a channel-guided resonant laser wakefield accelerator. Two-dimensional simulations with and without the effects of self-consistent beam loading are performed and compared. These results indicate that it is possible to trap about 40% of the injected bunch charge and accelerate this fraction to an average energy of about 50 MeV in a plasma channel of a few mm.

A new regime of laser wakefield acceleration of an injected electron bunch is described. In this regime, the bunch charge is so high that the bunch wakefields play an important role in the bunch dynamics. In particular, the transverse bunch wakefield induces a strong self-focusing that suppresses the transverse emittance growth arising from misalignment errors. The decelerating longitudinal bunch wakefield, however, is not so strong that it completely cancels the accelerating laser wakefield. In fact, the induced energy spread can be compensated by exploiting phase slippage effects. These features make the new regime interesting for high beam quality laser wakefield acceleration.

The merging process of current filaments in a strongly magnetized plasma is described. The evolution is calculated using a contour dynamics method, which accurately tracks piecewise constant distributions of the conserved quantities. In the interaction of two screened currents, both develop dipolar vortical flows, bringing the currents together. This is the manifestation of the Lorentz force between aligned currents. Currents will merge into single filaments. Reconnection of the magnetic field takes place, converting the magnetic topology from a figure eight to a circle.

A new method for the calculation of the magnetic field of beam guiding elements is presented. The method relates the calculation to measurement data of the magnetic field in a direct way. It can be applied to single beam guiding elements as well as to clusters of elements. The presented description of the magnetic field differs from the classical approach in that it does not rely on power series approximations. It is also both divergence free and curl free, and takes fringe field effects up to any desired order into account. In the field description, pseudodifferential operators described by Bessel functions are used to obtain the various multipole contributions. Magnetic field data on a two-dimensional surface, e.g., a cylindrical surface or median plane, serve as input for the calculation of the three-dimensional magnetic field. A boundary element method is presented to fit the fields to a discrete set of field data, obtained, for instance, from field measurements, on the two-dimensional surface. Relative errors in the field approximation do not exceed the maximal relative errors in the input data. Methods for incorporating the obtained field in both analytical and numerical computation of transfer functions are outlined. Applications include easy calculation of the transfer functions of clusters of beam guiding elements and of generalized field gradients for any multipole contribution up to any order.

The dynamics of the acceleration of a short electron bunch in a strong plasma wave excited by a laser pulse in a plasma channel is studied both analytically and numerically in slab geometry. In our simulations, a fully nonlinear, relativistic hydrodynamic description for the plasma wave is combined with particle-in-cell methods for the description of the bunch. Collective self-interactions within the bunch are fully taken into account. The existence of adiabatic invariants of motion is shown to have important implications for the final beam quality. Similar to the one-dimensional case, the natural evolution of the bunch is shown to lead, under proper initial conditions, to a minimum in the relative energy spread.

A one-dimensional model for fast electron generation by an intense, nonevolving laser pulse propagating through an underdense plasma has been developed. Plasma wave breaking is considered to be the dominant mechanism behind this process, and wave breaking both in front of and behind the laser pulse is discussed. Fast electrons emerge as a short bunch, and the electrostatic field of this bunch is shown to limit self-consistently the amount of generated fast electrons.

The statistics of uncorrelated point vortices in a plane is studied analytically and numerically. Theoretical distributions are obtained with the general method developed by Holtsmark [Ann. Phys. 58, 577 (1919)] and Chandrasekhar [Rev. Mod. Phys. 15, 1 (1943)]. They are found to agree with the results of numerical tests. Randomly placed Euler vortices have nearly Gaussian velocity distributions and Lorentzian distributions of the velocity difference. Statistics of other types of point vortices is essentially non-Gaussian.

Dipole drift vortices in the Hasegawa-Mima-Charney equation are studied by means of particle-in-cell (PIC) calculations. Apart from providing an efficient and accurate solution of the equations, PIC provides additional information about the fluid flow such as exchange of fluid between regions interior and exterior to the dipoles. Several cases of perturbed dipoles are studied with particular emphasis on the evolution of the fluid that is initially trapped inside the separatrix of the co-moving stream function of each unperturbed dipole. In particular, the effect of a finite tilt of the dipole axis is analyzed. Here, asymmetric losses from the two dipole halves are found to play a crucial role in the qualitative evolution of the dipole trajectory: dipoles initially moving in the unstable direction are found to reverse their average velocity perpendicular to the density gradient. Very large perturbations are obtained in dipole collisions. Here symmetry of the initial conditions plays an important role: collisions of aligned dipoles appear almost solitonlike, while for nonaligned dipoles the collision at least generates a tilt of the axes of the dipoles, but may also lead to a complete destruction of one of the poles. In all cases a significant loss of initially trapped fluid is demonstrated.

The spectral behavior of a high-power, high-gain free-electron maser (FEM) is investigated. The maser has a step-tapered undulator consisting of two sections with different strengths and lengths and equal periodicities. The sections are separated by a field-free gap. The configuration is enclosed within a low quality cavity. The millimeter wave beam is guided within a rectangular corrugated waveguide. The purpose of this undulator setup is to enhance the efficiency at high output power. The associated high gain in the linear, as well as in the nonlinear regime provides a unique oscillator. The spectral dynamics of this device is analyzed with a multipass, multifrequency code. The radiation field of the code is described as a sum over discrete frequency components. The linear gain curve of the step-tapered undulator is not the sum of the curves of two single undulators and has a completely different spectrum. The gain of the FEM is so high that nonlinear interaction occurs within a few passes. In the fully nonlinear regime the gain is still relatively high. The power spectrum evolves towards a state in which the power at the resonance of the second undulator section is suppressed. In the final state, where the frequency spectrum hardly changes from pass to pass, the power spectrum exhibits two peaks at frequencies that are determined by the first section of the undulator. The main peak is related to its resonance frequency, while the second peak is a lower sideband. The dependence of the sideband on the gap length, the relative polarization of both sections, and the reflection coefficient is investigated.

The synchronous interaction between a light pulse and a pulsed relativistic electron beam in a hole-coupled resonator free-electron laser (FEL) is investigated. The spatial structure of the light pulse inside the cavity and the fraction of power lost through the aperture are strongly influenced by the overlap between the light and the electron beam pulses, both in the transverse and in the longitudinal directions. The pulse shape is determined by the competition between power loss and scattering at the aperture, by the gain due to the resonant interaction with the electrons, and by the slippage with respect to the electron pulse. At the back of the optical pulse, where the main interaction with the electron pulse occurs, gain and the associated focusing are dominant. The front of the optical pulse tends to overtake the electrons and will finally propagate in vacuum. In this front region of the pulse, the on-axis field intensity is reduced only due to scattering. The influence of these competing mechanisms on the intracavity field distribution and the extracted power is analyzed. The full spatial structure of the optical pulse is taken into account, whereas the electrons are considered to move in a density averaged ponderomotive potential. The emittance and betatron oscillations of the electron beam are included insofar as they lead to a variation of the beam envelope. The phenomena are demonstrated numerically for FEL parameters close to those of the the free-electron laser for infrared experiments.