Laser plasma accelerators (LPA) can sustain $\mathrm{GeV}/\mathrm{m}$ accelerating fields offering outstanding new possibilities for compact applications. The LPA beam brightness can now be comparable to radio-frequency accelerators’ (RFA), thanks essentially to the beams short duration ($<3\mathrm{fs}$) and low emittance (as small as 0.1 micron). Still, the mrad level divergence and few percent level energy spread, remain limiting parameters in the cases of demanding applications such as free electron lasers (FELs). Several concepts of transfer line were proposed to mitigate those intrinsic properties targeting undulator radiation applications. We study here the robustness of the chromatic matching strategy for FEL amplification at 200 nm in a dedicated transport line, and analyze its sensitivity to several parameters. We consider not only the possible LPA source jitters, but also various realistic defaults of the equipment such as magnetic elements misalignments or focussing strength errors, imperfect undulator fields, etc.

We report on the application of machine learning (ML) methods for predicting the longitudinal phase space (LPS) distribution of particle accelerators. Our approach consists of training a ML-based virtual diagnostic to predict the LPS using only nondestructive linac and e-beam measurements as inputs. We validate this approach with a simulation study for the FACET-II linac and with an experimental demonstration conducted at LCLS. At LCLS, the e-beam LPS images are obtained with a transverse deflecting cavity and used as training data for our ML model. In both the FACET-II and LCLS cases we find good agreement between the predicted and simulated/measured LPS profiles, an important step towards showing the feasibility of implementing such a virtual diagnostic on particle accelerators in the future.

We measured the interferences of the coherent radiation, produced by a train of electron bunches in an undulator and stored in an optical cavity. The device used here is a free-electron laser, whose optical cavity stores the radiation pulse for a multipass amplification process. The amplification process is avoided by detuning the optical cavity. This is done by increasing the cavity length, producing a desynchronism between the radiation pulses and electron bunches. Then, the output intensity is dependent on the cavity lengthening and exhibits periodical interferences. An analytical analysis makes a description of this effect. In practice, it allows a direct and simple measurement of the position of the “zero detuning,” corresponding to the perfect synchronism of the light pulse and electron bunch in the free-electron laser process.

Completely integrable nonlinear Hamiltonian systems offer attractive models for novel accelerator systems that are being developed to push the frontiers of high intensity and high power beam applications. However, all known numerical methods used to discretize and simulate these systems spoil the integrability property of the original, time-continuous, theoretically integrable system. Since the hallmark of integrability is the existence of sufficiently many functionally independent first integrals in involution, we investigated several methods’ performance in terms of invariant preservation over longer timescales. We conclude that, while novel structure preserving integrators would be ideal, symplectic methods under mild restrictions offer adequate invariant preservation. We especially highlight the very good performance of the Stormer-Verlet method at low accuracy and a modified Lobatto method at high accuracy for the Integrable Optics Test Accelerator (IOTA). However, we show that symplectic integrators are limited by numerically induced resonances, which make interpretation of some simulation results challenging. These limitations and workarounds are discussed.

Interaction of hadrons with electron beam in a modulator is an important part of coherent electron cooling (CeC), a novel cooling method for hadron beams. Being an untested technique, the CeC is undergoing a proof-of-principle test at Brookhaven National Laboratory (BNL). Simulation of this process for a realistic electron beam propagating through a realistic quadrupole beamline constitutes a very challenging problem. We successfully used the code space for these simulations and obtained accurate dependences of the modulation process on the position and velocity of ions. We obtained good numerical convergence of simulations and performed verification tests using theoretical predications available for a uniform infinite plasma with $\kappa -2$ velocity distribution. In this paper, we describe simulation methods and results, and report our findings for the CeC modulator in the BNL experiment.

The method of enhanced self-amplified spontaneous emission (eSASE) is one of the strongest candidates for the generation of sub-femtosecond x-ray pulses in a free-electron laser (FEL). The optimization of an eSASE experiment involves many independent parameters, which makes the exploration of the parameter space with thee-dimensional (3D) simulations computationally intensive. Therefore, a robust theoretical analysis of this problem is extremely desirable. We provide a self-consistent, analytical treatment of such a configuration using a one-dimensional (1D), time-dependent FEL model that includes the key effects of electron beam chirp and undulator taper. Verified via comparison with numerical simulation, our formalism is utilized in parameter studies that seek to determine the optimum setup of the FEL.

In this paper, we address the energy spread and slice energy spread of an externally injected electron beam in plasma wakefield accelerators operating in the linear or quasilinear regime. The energy spread is first derived taking into account the phase dependence of the wakefield along the finite-length bunch together with the dephasing during acceleration and found to be strongly dependent on the bunch length. This could be compensated by the beam loading effect, the energy spread from which is then derived and found to be nearly independent of the bunch length. However, the transverse dependence of the beam loading effect also makes the particles at the same longitudinal position experience different accelerating fields, introducing a significant slice energy spread. To estimate the slice energy spread, a theoretical analysis was conducted by taking the transverse betatron motion into account. As a study case, 3D simulations for the 5 GeV laser-plasma acceleration stage of the European Plasma Research Accelerator with eXcellence in Applications project have been performed. Careful optimization of the parameters allows one to obtain an energy spread of $\le 1\%$ and a slice energy spread of $\le 0.1\%$, with good agreement between theories and simulations.

Many numerical solvers model the general nonlinear problem of particle motion in self-generated fields through the computation of particle trajectories and fields in a Lagrangian and Eulerian framework, respectively. The particle-weighting step mapping the source terms between these frameworks is a primary source of noise and computation time. Herein we eliminate this costly step by treating the trajectories as bounds on the particle flow, applying charge conservation to interpolate the source distribution. The result is an order of magnitude improvement in accuracy and an 80 to 120 fold speed-up.

As the multibend achromats had been applied to the lattice design in latest light sources to achieve ultralow beam emittance, whereas its performance is restricted by the strong sextupoles and concomitant nonlinearities to a certain extent. Contractive dynamic aperture becomes an inevitable but pivotal characteristic of future diffraction-limited storage rings, which lead to the exclusion of the conventional off-axis injection schemes. In this paper we will present a new on-axis injection scheme in allusion to future light sources, which is based on a triple-frequency rf system. By means of delicate superposition of rf voltages with fundamental, 2nd and 3rd hamonic frequencies, a commodious and rational main bucket which can accommodate an injected bunch and a circulating bunch simultaneously is able to be formed. The injected bunch can be captured by the longitudinal acceptance on-axis in a reasonable time offset with respect to the circulating bunch, utilizing the ultrafast pulse kicker, and ultimately merging to the center of the main bucket through synchrotron radiation damping. Detailed construction of the triple-frequency rf system and application of this scheme to the High Energy Photon Source will be discussed in the paper, relevant simulation studies and discussions are also presented.

Femtosecond electron bunches with complex temporal structures play a crucial role in terahertz generation, free-electron lasers, and plasma wakefield accelerators. However, diagnostics of electron beam current profiles with femtosecond resolution are still challenging. Coherent transition radiation (CTR) has been proven to have the capability to reconstruct current profiles based on prior knowledge or complete information of spectra. In this paper, a weighted greedy sparse phase retrieval (WGESPAR) algorithm is developed to reduce the ambiguities during reconstruction of complex current profiles. This algorithm can achieve better performance than the Kramers-Kronig method and the Bubblewrap algorithm in the case of reconstructing complex temporal structures from truncated noisy CTR spectra. Based on the WGESPAR algorithm, the complex temporal structures of femtosecond electron bunches generated from laser wakefield accelerators can be successfully reconstructed.

A problem of coupled-beam instability is solved for two multibunch beams with slightly different revolution frequencies, as in the Fermilab Recycler Ring (RR). Sharing of the interbunch growth rates between the intrabunch modes is described. The general analysis is applied to the RR; possibilities to stabilize the beams by means of chromaticity and feedback are considered.

To qualify electron cyclotron resonance charge breeders, the method that is traditionally used to evaluate the charge breeding time consists in generating a rising edge of the injected beam current and measuring the time in which the extracted multicharged ion beam reaches 90% of its final current. It is demonstrated in the present paper that charge breeding times can be more accurately measured by injecting short pulses of $1+\mathrm{ions}$ and recording the time resolved responses of $\mathrm{N}+$ ions. This method is used to probe the effect of the $1+\mathrm{ion}$ accumulation in the plasma known to disturb the buffer gas plasma equilibrium and is a step further in understanding the large discrepancies reported in charge breeding times. The experiments are conducted injecting a ${}^{85}{\mathrm{Rb}}^{+}$ ion beam into the Laboratoire de Physique Subatomique et de Cosmologie (LPSC) charge breeder operated with helium as a buffer gas. The time needed for the extraction of 90% of the multicharged ions in short pulse mode is found slightly shorter (9%) than the charge breeding time measured with the traditional method. The charge breeding efficiency is identical with both methods. The pulse width and amplitude of the $1+$ injection pulses have been varied to study their influence on the $\mathrm{N}+$ response and temporal parameters are proposed to qualify the time response. The short pulse method can be used to study the influence of the ion source tuning parameters on the charge breeding temporal characteristics. For example, it is shown here that an increase of the minimum magnetic field strength of the LPSC charge breeder in the range 0.432–0.444 T improves the multicharged ion confinement in the electron cyclotron resonance plasma, and so increases the charge breeding efficiency. The short pulse method is also used to estimate the charge breeding efficiencies of radioactive Rb isotopes taking into account their half-lives and charge states. The neutron-rich heavy isotopes have short half-lives, which makes the charge state distributions shift to lower charge states and the estimated charge breeding efficiencies of high charge states being close to three times less in comparison to the stable ions.

Modern synchrotron light sources are competing intensively to increase x-ray brightness and, eventually, approach the diffraction limit, which sets the final goal of lattice emittance. All recent synchrotron facility upgrades follow the multibend achromat approach by arranging small horizontal beta function and dispersion inside discrete bending dipoles. In this paper we propose a concept of a lattice element that we call “complex bend,” which has the potential of becoming a main building block for low emittance lattices. The complex bend is a sequence of dipole poles interleaved with strong alternate focusing so as to maintain the beta function and dispersion oscillating at low values. Comprising the ring lattice with complex bends, instead of regular dipoles, will minimize the $H$-function and reduce horizontal emittance while localizing bending to a small fraction of the storage ring circumference, which should provide more space for insertion devices. In this paper we present the details of the complex bend, considerations regarding the choice of optimal parameters, and thoughts for its practical realization. We focus here on complex bend physics and engineering design, rather than integration of this complex bend into a specific ring lattice.

This paper describes a reliable, handy, and inexpensive measurement system to characterize, in the sub-THz frequency range, the coating materials used in the beam pipes of accelerators. The method is based on time domain measurements of an electromagnetic wave passing through a waveguide having a thin central slab, where the coating material is deposited on both sides. Two horn antennas are integrated on both sides of the device to optimize the signal collection and detection. This novel technique is tested on slabs coated with a nonevaporable getter (NEG) and allows to evaluate the surface impedance in the frequency range from 0.1 to 0.3 THz.

Collisionless shock acceleration of protons and ${\mathrm{C}}^{6+}$ ions has been achieved by the interaction of a ${10}^{20}\mathrm{W}/{\mathrm{cm}}^2$, $1\mu \mathrm{m}$ laser with a near-critical density plasma. Ablation of the initially solid density target by a secondary laser allows for systematic control of the plasma profile. This enables the production of beams with peaked spectra with energies of $10–18\mathrm{MeV}/\mathrm{amu}$ and energy spreads of 10%–20% with up to $3\times {10}^9$ particles within these narrow spectral features. The narrow energy spread and similar velocity of ion species with different charge-to-mass ratios are consistent with acceleration by the moving potential of a shock wave. Particle-in-cell simulations show shock accelerated beams of protons and ${\mathrm{C}}^{6+}$ ions with energy distributions consistent with the experiments. Simulations further indicate the plasma profile determines the trade-off between the beam charge and energy and that with additional target optimization narrow energy spread beams exceeding $100\mathrm{MeV}/\mathrm{amu}$ can be produced using the same laser conditions.

Vacuum radio-frequency (rf) breakdown is one of the major factors that limit operating accelerating gradients in rf particle accelerators. The occurrence of rf breakdowns was shown to be probabilistic, and can be characterized by a breakdown rate. Experiments with hard copper cavities showed that harder materials can reach larger accelerating gradients for the same breakdown rate. We study the effect of cavity material on rf breakdowns with short $X$-band standing wave accelerating structures. Here we report results from tests of a structure at cryogenic temperatures. At gradients greater than $150\mathrm{MV}/\mathrm{m}$ we observed a degradation in the intrinsic cavity quality factor, $Q_0$. This decrease in $Q_0$ is consistent with rf power being absorbed by field emission currents, and is accounted for in the determination of accelerating gradients. The structure was conditioned up to an accelerating gradient of $250\mathrm{MV}/\mathrm{m}$ at 45 K with $10^8$ rf pulses and a breakdown rate of $2\times {10}^{-4}/\mathrm{pulse}/\mathrm{m}$. For this breakdown rate, the cryogenic structure has the largest reported accelerating gradient. This improved performance over room temperatures structures supports the hypothesis that breakdown rate can be reduced by immobilizing crystal defects and decreasing thermally induced stresses.

To understand and control dynamics in the longitudinal phase space, time-resolved measurements of different bunch parameters are required. For a reconstruction of this phase space, the detector systems have to be synchronized. This reconstruction can be used for example for studies of the microbunching instability which occurs if the interaction of the bunch with its own radiation leads to the formation of substructures on the longitudinal bunch profile. These substructures can grow rapidly—leading to a sawtooth-like behavior of the bunch. At KARA, we use a fast-gated intensified camera for energy spread studies, Schottky diodes for coherent synchrotron radiation studies as well as electro-optical spectral decoding for longitudinal bunch profile measurements. For a synchronization, a synchronization scheme is used which compensates for hardware delays. In this paper, the different experimental setups and their synchronization are discussed and first results of synchronous measurements presented.