[en] We report the design of a laser wakefield accelerator (LWA) with external injection by a rf photogun and acceleration by a linear wakefield in a capillary discharge channel. The design process is complex due to the large number of intricately coupled free parameters. To alleviate this problem, we performed front-to-end simulations of the complete system. The tool we used was the general particle-tracking code, extended with a module representing the linear wakefield by a two-dimensional traveling wave with appropriate wavelength and amplitude. Given the limitations of existing technology for the longest discharge plasma wavelength (∼50 μm) and shortest electron bunch length (∼100 μm), we studied the regime in which the wakefield acts as slicer and buncher, while rejecting a large fraction of the injected bunch. The optimized parameters for the injected bunch are 10 pC, 300 fs at 6.7 MeV, to be injected into a 70 mm long channel at a plasma density of 7x10²³ m^-3. A linear wakefield is generated by a 2 TW laser focused to 30 μm. The simulations predict an accelerated output of 0.6 pC, 10 fs bunches at 90 MeV, with energy spread below 10%. The design is currently being implemented. The design process also led to an important conclusion: output specifications directly comparable to those reported recently from 'laser-into-gas jet' experiments are feasible, provided the performance of the rf photogun is considerably enhanced. The paper outlines a photogun design providing such a performance level

[en] 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

[en] We propose a technique for producing electron bunches that has the potential for advancing the state-of-the-art in brightness of pulsed electron sources by orders of magnitude. In addition, this method leads to femtosecond bunch lengths without the use of ultrafast lasers or magnetic compression. The electron source we propose is an ultracold plasma with electron temperatures down to 10 K, which can be fashioned from a cloud of laser-cooled atoms by photoionization just above threshold. Here we present results of simulations in a realistic setting, showing that an ultracold plasma has an enormous potential as a bright electron source

[en] The fusion free-electron maser (FEM) is the prototype of a high power, electrostatic mm-wave source, tunable in the range 130-260 GHz. In order to achieve a high overall efficiency, the charge and energy of the spent electron beam, i.e. the beam which leaves the undulator after interaction with the EM-wave, has to be recovered. A 50% overall efficiency is achieved, even for the maximum energy spread of 320 keV generated in the undulator, using a collection system consisting of a decelerator and a depressed collector. The general particle tracer code (GPT) is being used as the major design tool for the whole fusion FEM beam line, from the accelerator to the depressed collector. The high accuracy, ability to include FEL interaction and full 3D treatment make GPT the ideal choice for such a project. An overview of the separate sections and the use of GPT for each part of the FEM is presented. GPT is currently being applied to the design of the energy recovery system of the fusion FEM. The first simulation results, including a 3D on-axis bending scheme and scattered incident electrons, are shown. (orig.)

[en] Experiments have been performed with the free-electron maser (FEM) at Rijnhuizen, a high-power mm-wave source. A unique feature of the FEM is the possibility to tune the frequency over the entire range from 130 to 260 GHz at an output power exceeding 1 MW. In the so-called inverse set-up, where the electron gun is mounted inside the high-voltage terminal, a peak power of 730 kW was measured at 200 GHz and of 350 kW at 167 GHz . Furthermore, we made the design work to extend the pulse-length to 1 s. Detailed thermal behavior of the critical components is studied. Both the cavity mirrors and the depressed-collector electrodes seem to have adequate cooling

[en] The realization of high quality ultrashort pulsed beams requires ultrafast time-dependent electron optics. We present derivations of closed expressions both for the longitudinal and transverse focusing powers of resonant microwave TM₀₁₀ cavities. The derived expressions are validated by particle tracking simulations using realistic cavity fields. For small field amplitudes, in which case the “weak lens” approximation holds, the focusing powers obtained from simulations are in good agreement with the derived expressions. Furthermore, the required phase and temperature stability for synchronization of electron bunches generated by femtosecond photoemission are discussed. - Research highlights: ► Closed expressions are derived for focusing powers of microwave TM₀₁₀ cavities. ► Expressions are validated by particle tracking simulations using realistic fields. ► Analytical theory and simulations agree for small field amplitudes. ► Phase and temperature stability requirements for synchronization are discussed

[en] Self-consistent simulation of a LINAC-driven FEL by time-domain particle tracking can give very detailed results on both the produced radiation and the evolution of the electron bunch. We show that when special subsets are tracked, instead of individual macro-particles, only a few of these subsets are required to obtain converging results. The subsets used are short longitudinal arrays of macro-particles, of the order of a few ponderomotive waves, distributed longitudinally in such a way that they are almost only sensitive to stimulated emission. This new approach has been carried out with the 3D General Particle Tracer code and a set of axisymmetric Gaussian waves propagating in free space. Due to the first-principles approach, it can be used for a variety of radiation problems, including studies of FEL start-up and saturation effects. The model and two applications will be presented

[en] The shorted air line stub used for subnanosecond pulsing of the grounded-grid cathode gun structure of the IRI 3 MV Van de Graaff electron accelerator is revised. Three-dimensional high-frequency field propagation calculations provide better insight into the performance of different geometrical shapes. Effects on rise- and decay time, and ringing on the output pulses are considered. Practical possibilities for improvement are discussed. Comparison with sampling measurements on several device modifications confirms the reliability of the calculations. The calculation method is subsequently used as design aid for the construction of a '1 ns' device using a quartz loaded shorted stub to fit into the geometry of the existing variable pulse length unit. Capabilities for short pulsing of the accelerator are improved and extended by application of the results obtained

[en] 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 0.4π mm mrad