We analyze the nonlinear classical effects of the $\mathrm{X}/\gamma$ radiation produced by Thomson/Compton sources. We confirm the development of spectral fringes of the radiation on axis, which comports broadening, shift, and deformation of the spectrum. For the nominal parameters of the SPARC-LAB Thomson scattering and of the European Proposal for the gamma source ELI-NP, however, the radiation, when collected in the suitable acceptance angle, does not reveal many differences from that predicted by the linear model and the nonlinear redshift is subdominant with respect to the quantum recoil. An experiment aimed to the study of the nonlinearities is proposed on the SPARC-LAB source.

Stable operation of a laser-plasma accelerator near the threshold for electron self-injection in the blowout regime has been demonstrated with 25–60 TW, 30 fs laser pulses focused into a 3–4 millimeter length gas jet. Nearly Gaussian shape and high nanosecond contrast of the focused pulse appear to be critically important for controllable, tunable generation of 250–430 MeV electron bunches with a low-energy spread, $\sim 10\mathrm{pC}$ charge, a few-mrad divergence and pointing stability, and a vanishingly small low-energy background. The physical nature of the near-threshold behavior is examined using three-dimensional particle-in-cell simulations. Simulations indicate that properly locating the nonlinear focus of the laser pulse within the plasma suppresses continuous injection, thus reducing the low-energy tail of the electron beam.

The dynamics of laser ionization-based electron injection in the recently introduced plasma photocathode concept is analyzed analytically and with particle-in-cell simulations. The influence of the initial few-cycle laser pulse that liberates electrons through background gas ionization in a plasma wakefield accelerator on the final electron phase space is described through the use of Ammosov-Deloine-Krainov theory as well as nonadiabatic Yudin-Ivanov (YI) ionization theory and subsequent downstream dynamics in the combined laser and plasma wave fields. The photoelectrons are tracked by solving their relativistic equations of motion. They experience the analytically described transient laser field and the simulation-derived plasma wakefields. It is shown that the minimum normalized emittance of fs-scale electron bunches released in mulit-$\mathrm{GV}/\mathrm{m}$-scale plasma wakefields is of the order of ${10}^{-2}\mathrm{mm}\mathrm{mrad}$. Such unprecedented values, combined with the dramatically increased controllability of electron bunch production, pave the way for highly compact yet ultrahigh quality plasma-based electron accelerators and light source applications.

Coupling impedances and wakefields are fundamental quantities to characterize the electromagnetic interaction of a particle beam with the surrounding environment. In particular, collective effects, triggered by these self-induced fields, may play an important role in beam stability and machine performance. Within the framework of the LHC Injectors Upgrade project, since a significantly higher beam intensity is planned for the CERN Proton Synchrotron, wakefields are expected to increase their influence on the beam dynamics, and their evaluation is becoming important. In this paper we present the results of recent measurements of the longitudinal broadband coupling impedance by means of the incoherent quadrupole synchrotron frequency shift as a function of beam intensity. A detailed evaluation of the contribution of several machine installations to the total impedance budget is also presented and compared with the measurements.

Based on single particle tracking in the framework of classical Thomson scattering with incoherent superposition, we developed a relativistic, three-dimensional numerical model that calculates and quantifies the characteristics of emitted radiation when a relativistic electron beam interacts with an intense laser pulse. This model has been benchmarked against analytical expressions, based on the plane wave approximation to the laser field, derived by Esarey et al. [Phys. Rev. E 48, 3003 (1993)]. For laser pulses of sufficient duration, we find that the scattered radiation spectrum is broadened due to interferences arising from the pulsed nature of the laser. We find that by appropriately chirping the scattering laser pulse, spectral broadening can be minimized, and the peak on-axis brightness of the emitted radiation is increased by a factor of approximately 5.

An electron beam halo monitor using diamond-based detectors, which are operated in the ionization mode, has been developed for the SPring-8 Angstrom compact free-electron laser (SACLA) to protect its undulator magnets from radiation damage. Diamond-based detectors are inserted in a beam duct to measure the intensity of the beam halo directly. To suppress the degradation of the electron beam due to the installation of the beam halo monitor, rf fingers with aluminum windows are newly employed. We evaluated the effect of radiation from the Al windows on the output signal both experimentally and by simulation. The operational results of the beam halo monitor employed in SACLA are presented.

Two ${\mathrm{K}}_2\mathrm{CsSb}$ photocathodes were manufactured at Brookhaven National Lab and delivered to Jefferson Lab within a compact vacuum apparatus at pressure $\sim {10}^{-11}\mathrm{Torr}$. These photocathodes were evaluated using a dc high voltage photogun biased at voltages up to 200 kV, and illuminated with laser light at wavelengths 440 or 532 nm, to generate dc electron beams at currents up to 20 mA. Some conditions produced exceptionally large photocathode charge lifetimes, without measurable quantum efficiency decay, even from the center of the photocathode where operation using GaAs photocathodes is precluded due to ion bombardment. Under other conditions the charge lifetime was poor due to extensive ion bombardment under severely degraded vacuum conditions, and as a result of localized heating via the laser beam. Following beam delivery, the photocathodes were evaluated using a scanning electron microscope to determine surface morphology.

For a planar free electron laser (FEL) configuration we study self-amplified coherent spontaneous emission driven by a gradient of the bunch current in the presence of different levels of noise in bunches. The longitudinal granularity of the electron bunch density originating from shot noise is maintained throughout the analysis. For the FEL model with the SwissFEL injector bunch parameters, we calculate the probability density distribution of the maximum power of the radiation pulses for different levels of shot noise. It turns out that the temporal coherence quickly increases as the noise level reduces. We also show that the FEL based on coherent spontaneous emission produces almost Fourier transform limited pulses. The analysis indicates that the time-bandwidth product is mainly determined by the bunch length and the interaction distance in an undulator. Calculations of the FEL characteristics for different rise times of the front of the current pulse are performed, and it is found that a reduced level of the power fluctuations is preserved for the bunch current pulse with a front duration up to several FEL wavelengths. We also propose a novel scheme that permits the formation of electron bunches with a reduced level of noise and a high gradient of the current at the bunch tail to enhance coherent spontaneous emission. The presented scheme uses effects of noise reduction and controlled microbunching instability and consists of a laser heater, a bunch compressor, and a shot noise suppression section. We show that shot noise reduction by 2 orders of magnitude in electron bunches produced by the SwissFEL injector can be achieved in a compact noise suppression section. The noise factor and microbunching gain of the overall proposed scheme with and without laser heater are estimated.

We present a numerical study that characterizes the dependence on the radiator length of the output power produced by a free-electron laser (FEL) operated in the high gain harmonic generation (HGHG) configuration. Using the main parameters of the FERMI@Elettra FEL, numerical simulations of the FEL process have been performed for different lengths of the radiator. Our results show that in the case of HGHG the achievable output power has a dependence on the radiator length that is linear. The impact of the electron beam parameters on the achievable maximum power vs radiator length dependence is also studied. A normalization of the results to the FEL saturation power and to the FEL gain length shows that this dependence can be expressed by a universal linear equation that, in some conditions, is independent on the electron beam current and brightness. The reported results could be useful for the design of future FELs based on the HGHG scheme and could be used for a quick estimate of the best undulator length.

We consider the dynamics of a coherent seed undergoing a free electron laser high gain amplification. We discuss the dispersion of the optical packet during the interaction and study the problem using different formalisms, including that of the Wigner distribution. The analysis we develop is mainly based on a 1D procedure; the inclusion of transverse longitudinal interplay is also briefly discussed.

In heavy-ion synchrotrons such as the SIS18 at Helmholtzzentrum für Schwerionenforschung, Helmholtz Centre for Heavy Ion Research (GSI), coherent oscillations of the particle bunches are damped by rf feedback systems to increase the stability and to improve the beam quality. In the longitudinal direction, important modes are the coherent longitudinal dipole and quadrupole oscillation. In this paper we present a new and rigorous approach to analyze the longitudinal feedback to damp these modes. The results are applied to the rf feedback loop at GSI that damps the quadrupole mode. The stability analysis is compared with simulations and is in good agreement with results of a beam experiment. Finally, we summarize practical implications for the operation of the feedback system regarding performance and stability.

The conceptual design for a nonscaling fixed field alternating gradient accelerator suitable for charged particle therapy (the use of protons and other light ions to treat some forms of cancer) is described.

The development of models and the “Virtual Detector for Synchrotron Radiation” (vdsr) code that accurately describe the production of synchrotron radiation are described. These models and code are valid in the classical and linear (single-scattering) quantum regimes and are capable of describing radiation produced from laser-plasma accelerators (LPAs) through a variety of mechanisms including betatron radiation, undulator radiation, and Thomson/Compton scattering. Previous models of classical synchrotron radiation, such as those typically used for undulator radiation, are inadequate in describing the radiation spectra from electrons undergoing small numbers of oscillations. This is due to an improper treatment of a mathematical evaluation at the end points of an integration that leads to an unphysical plateau in the radiation spectrum at high frequencies, the magnitude of which increases as the number of oscillation periods decreases. This is important for betatron radiation from LPAs, in which the betatron strength parameter is large but the number of betatron periods is small. The code vdsr allows the radiation to be calculated in this regime by full integration over each electron trajectory, including end-point effects, and this code is used to calculate betatron radiation for cases of experimental interest. Radiation from Thomson scattering and Compton scattering is also studied with vdsr. For Thomson scattering, radiation reaction is included by using the Sokolov method for the calculation of the electron dynamics. For Compton scattering, quantum recoil effects are considered in vdsr by using Monte Carlo methods. The quantum calculation has been benchmarked with the classical calculation in a classical regime.

Laser accelerated proton beams have a considerable potential for various applications including oncological therapy. However, the most consolidated target normal sheath acceleration regime based on irradiation of solid targets provides an exponential energy spectrum with a significant divergence. The low count number at the cutoff energy seriously limits at present its possible use. One realistic scenario for the near future is offered by hybrid schemes. The use of transport lines for collimation and energy selection has been considered. We present here a scheme based on a high field pulsed solenoid and collimators which allows one to select a beam suitable for injection at 30 MeV into a compact linac in order to double its energy while preserving a significant intensity. The results are based on a fully 3D simulation starting from laser acceleration.

Two different tune measurement systems have been installed in the GSI Helmholtzzentrum für Schwerionenforschung heavy-ion synchrotron SIS-18. Tune spectra are obtained with high accuracy using these fast and sensitive systems. Besides the machine tune, the spectra contain information about the intensity dependent coherent tune shift and the incoherent space charge tune shift. The space charge tune shift is derived from a fit of the observed shifted positions of the synchrotron satellites to an analytic expression for the head-tail eigenmodes with space charge. Furthermore, the chromaticity is extracted from the measured head-tail mode structure. The results of the measurements provide experimental evidence of the importance of space charge effects and head-tail modes for the interpretation of transverse beam signals at high intensity.

Transverse emittance is a crucial feature of laser-wakefield accelerators, yet accurately reproducing its value in numerical simulations remains challenging. It is shown here that, when the charge of the bunch exceeds a few tens of picocoulombs, particle-in-cell (PIC) simulations erroneously overestimate the emittance. This is mostly due the interaction of spurious Cherenkov radiation with the bunch, which leads to a steady growth of emittance during the simulation. A new computational scheme is proposed, which is free of spurious Cherenkov radiation. It can be easily implemented in existing PIC codes and leads to a substantial reduction of the emittance growth.

Using a two-dimensional fluid description, we investigate the nonlinear radial-longitudinal dynamics of intense beams in isochronous cyclotrons in the nonrelativistic limit. With a multiscale analysis separating the time scale associated with the betatron motion and the slower time scale associated with space-charge effects, we show that the longitudinal-radial vortex motion can be understood in the frame moving with the charged beam as the nonlinear advection of the beam by the $\mathbf{E}\times \mathbf{B}$ velocity field, where $\mathbf{E}$ is the electric field due to the space charge and $\mathbf{B}$ is the external magnetic field. This interpretation provides simple explanations for the stability of round beams and for the development of spiral halos in elongated beams. By numerically solving the nonlinear advection equation for the beam density, we find that it is also in quantitative agreement with results obtained in particle-in-cell simulations.