Experimental characterization and numerical simulations of the electron source at PITZ

Silica aerogel radiators for bunch length measurements

The use of a transverse gradient undulator (TGU) is viewed as an attractive option for free-electron lasers (FELs) driven by beams with a large energy spread. By suitably dispersing the electron beam and tilting the undulator poles, the energy spread effect can be substantially mitigated. However, adding the dispersion typically leads to electron beams with large aspect ratios. As a result, the presence of higher-order modes in the FEL radiation can become significant. To investigate this effect, we study the eigenmode properties of a TGU-based, high-gain FEL, using both an analytically-solvable model and a variational technique. Our analysis, which includes the fundamental and the higher-order FEL eigenmodes, can provide an estimate of the mode content for the output radiation. This formalism also enables us to study the trade-off between FEL gain and transverse coherence. Numerical results are presented for a representative soft X-ray, TGU FEL example.

Transverse emittance is one of the most important properties for high-brightness electron beams used for X-Ray free-electron lasers. The photo injector test facility at DESY in Zeuthen (PITZ) focuses on the development of high-brightness electron sources. The two main methods to measure the emittance are the quadrupole scan and the slit scan. Combining either of these methods with a transverse deflecting cavity allows the measurement of the slice emittance. At PITZ, space-charge effects at the low beam momentum of 24 MeV/c complicate in particular the quadrupole scan. This has to be considered in the emittance measurements. First slit-scan based slice emittance results will be shown next to studies on the beam transport for quadrupole scans.

The Photo Injector Test facility at DESY, Zeuthen site (PITZ), develops high brightness electron sources for modern linac-based Free Electron Lasers (FELs). The PITZ accelerator can also be considered as a suitable machine for the development of an IR/THz source prototype for pump-probeexperiments at the European XFEL. Calculations of THz radiation by means of a SASE FEL based on the simulated and the measured beam profiles at PITZ for the radiation wavelength of 100 μm were performed by using the GENE-SIS1.3 code. The results of these simulations are presented and discussed in this paper.

Developments on high power tunable THz source for pump-probe experiments at the European XFEL are ongoing at the Photo Injector Test facility at DESY in Zeuthen (PITZ). A LCLS-I undulator is used for proof-of-principle experiments at PITZ. Narrow-band 3 THz pulses with pulse energy of ~100 microjoules were generated in a single pass using 2-3 nC 17 MeV electron bunches from the high brightness photo injector. First characterization results of the THz FEL at PITZ are reported.

The effects of the imposed uniform magnetic field, ranging from 1 MG up to 50 MG, on the production and transport of relativistic electron beams (REBs) in overdense plasmas irradiated by ultraintense laser pulse are investigated with two-dimensional particle-in-cell numerical simulations. This study gives clear evidence that the imposed magnetic field is capable of effectively confining the relativistic electrons in space even when the source is highly divergent since it forces the electrons moving helically. In comparison, the spontaneous magnetic fields, generated by the helically moving electrons interplaying with the current filamentation instability, are dominant in scattering the relativistic electrons. As the imposed magnetic field was increased from 1 MG to 50 MG, overall coupling from laser to the relativistic electrons which have the potential to heat the compressed core in fast ignition was found to increase from 6.9% to 21.3% while the divergence of the REB increases significantly from 64° to 90°. The simulations show that imposed magnetic field of the value of 3–30 MG could be more suitable to fast-ignition inertial fusion.

Production of high-brightness electron beams for the ELSA-FEL

Coherent synchrotron radiation (CSR) is of great interest to those designing accelerators as drivers for free-electron lasers (FELs). Although experimental evidence is incomplete, CSR is predicted to have potentially severe effects on the emittance of high-brightness electron beams. The performance of an FEL depends critically on the emittance, current, and energy spread of the beam. Attempts to increase the current through magnetic bunch compression can lead to increased emittance and energy spread due to CSR in the dipoles of such a compressor. The code elegant was used for design and simulation of the bunch compressor for the Low-Energy Undulator Test Line (LEUTL) FEL at the Advanced Photon Source (APS). In order to facilitate this design, a fast algorithm was developed based on the 1-D formalism of Saldin and coworkers. In addition, a plausible method of including CSR effects in drift spaces following the chicane magnets was developed and implemented. The algorithm is fast enough to permit running hundreds of tolerance simulations including CSR for 50 thousand particles. This article describes the details of the implementation and shows results for the APS bunch compressor.

Methodical studies to improve the existing e-beam Longitudinal Phase Space (LPS) tomography were performed at the Photo Injector Test facility at DESY in Zeuthen. Proof-of-principle simulations were done to address some core concerns e.g. booster phase range, space charge effects and noisy artefacts in results. Phase advance analysis was done with the help of an analytical model that determined the booster phase range and step size. A slit was introduced before the booster to truncate the beam and reduce space charge forces. The reconstruction method adopted was image space reconstruction algorithm owing to its assurance of non-negative solution. An initial scientific presumption of LPS from low energy momentum measurements was established to reduce artefacts in the phase space. This paper will explain the proof-of-principle simulations highlighting the key aspects to obtain accurate results. Reconstructed LPS for different experimental cases will be presented to demonstrate the diagnostic capability.

Preliminary study for the laboratory experiment of cosmic-rays driven magnetic field amplification

The SPring-8 project is in progress in Japan. The main facility is an 8 GeV storage ring dedicated to synchrotron radiation [H. Kamitsubo et al. (this conference)]. The main feature of the ring is in the free straight sections 30 m long. In this paper the design of the long straight sections and how to use them is described. The long free straights are achieved through two phases to make the commissioning of the ring smooth. In the second phase, the long free straight sections are available. These long straight sections have the following potential to realize (1) extremely high brilliance, (2) multiundulators with broadband, and (3) free electron lasers. The emittance of the stored electron beam can be reduced to subnano m rad at 4 GeV with damping wigglers. Three-order higher brilliance can be obtained around 4 keV. With these optics, three-dimensional simulation was performed for free electron lasers (FEL). The calculated gain was not so large in the practical peak current level.

Planetary bow shocks, interplanetary shocks, and other heliospheric shocks are remotely observable at radio frequencies due to plasma emission. In this process, shocks are a source of energetic electron beams which excite electrostatic plasma oscillations (Langmuir waves) near the shock. Langmuir waves are then converted into electromagnetic emission at the local plasma frequency (fp) and its harmonic (2fp). While the production of electron beams by shocks and the subsequent generation of Langmuir waves are well understood, the mechanism which converts electrostatic Langmuir waves into electromagnetic radiation at fp and 2fp remains a subject of debate. A conversion mechanism is presented based on the idea that a substantial fraction of Langmuir waves are localized as eigenmodes of ambient plasma density fluctuations. Thus localized, the fp and 2fp currents associated with Langmuir waves radiate as antennas. Evidence, in the form of observations from the STEREO and WIND spacecraft, is presented for the localization of Langmuir waves by density structures and for radio frequency emission by the eigenmode‐antenna mechanism.

High temporal resolution, single-shot electron bunch-length measurements

The expression ‘linear accelerator technology’ usually addresses all technological topics related to linear accelerators’ (linac) realization. These topics comprise, for instance, accelerating structures, Radio Frequency (RF) sources, magnets, diagnostics systems, controllers, power supplies, vacuum systems, particle sources, etc. Obviously, the extent of all these subjects is too wide to be covered in a single one-hour lecture and, therefore, this paper will only present an verview of the RF structures (including their fabrication technology) with a few examples of particle sources, RF sources and waveguide components. Moreover, the focus will be on linac technology for Free-Electron Lasers (FELs) and Energy Recovery Lasers (ERLs), which are electron accelerators. Electrons, in fact, even at very low energies (a few MeV) travel very close to the speed of light, and this fact has a direct consequence in the accelerating structures design, since they can be identical all along the linac.

A method of angular streaking with a rotating terahertz electric field for photoelectrons produced by femtosecond extreme ultraviolet pulses is suggested and theoretically analyzed. The method can be used for free electron laser (FEL) pulse characterization on a shot-to-shot basis. It is shown that in related measurements an interesting phenomenon appears: formation of very bright and sharp features in the angular resolved electron spectra measured in the plane perpendicular to the collinear beam direction. These features are similar to the conventional caustics in the wave propagation. The caustics are accompanied by a well-developed interference structure. The intensity distribution along the caustic is determined by the envelope of the FEL pulse.