TTX-II (Tsinghua Thomson Scattering X-Ray Source Phase II) is a Thomson scattering based x-ray source currently under development at the accelerator laboratory in Tsinghua University. The circumference of the storage ring is 5.668 m. The compact nature of the ring gives rise to several design challenges such as chromaticity correction and collective instability, which can be suppressed by carefully optimizing ring parameters. The lattice design and the beam dynamics of TTX-II are presented in this work.

Optical stochastic cooling (OSC) is a promising technique for the cooling of dense particle beams. Its operation at optical frequencies enables obtaining a much larger bandwidth compared to the well-known microwave-based stochastic cooling. In the OSC undulator radiation generated by a particle in an upstream “pickup” undulator is amplified and focused at the location of a downstream “kicker” undulator. Inside the kicker, a particle interacts with its own radiation field from the pickup. The resulting interaction produces a longitudinal kick with its value depending on the particles momentum which, when correctly phased, yields longitudinal cooling. The horizontal cooling is achieved by introducing a coupling between longitudinal and horizontal degrees of freedom. Vertical cooling is achieved by coupling between horizontal and vertical motions in the ring. In this paper, we present formulas for computation of the corrective kick and validate them against numerical simulations performed with a wave-optics computer program.

Radio frequency breakdown rate is a crucial performance parameter that ensures that the design luminosity is achieved in the CLIC linear collider. The required low breakdown rate for CLIC, of the order of ${10}^{-7}\text{breakdown}{\text{pulse}}^{-1}{\mathrm{m}}^{-1}$, has been demonstrated in a number of 12 GHz CLIC prototype structures at gradients in excess of the design $100\mathrm{MV}/\mathrm{m}$ accelerating gradient, however without the presence of the accelerated beam and associated beam loading. The beam loading induced by the approximately 1 A CLIC main beam significantly modifies the field distribution inside the structures, and the effect on breakdown rate is potentially significant so needs to be determined. A dedicated experiment has been carried out in the CLIC Test Facility CTF3 to measure this effect, and the results are presented.

This paper details the development of a single-shot diagnostic technique for the 4D average and core phase space densities of low-charge, high-brightness electron beams, based on the analysis of shadow point-projection images of metal grids. This technique is similar to the standard pepper pot method, although it allows much greater transmission of the beam and therefore is more suitable for low-charge electron beams. Transverse coupling terms are included in the analysis, allowing the complete 4D transverse beam matrix to be reconstructed. The 4D beam phase space information is extremely important for the characterization of nonround beams. An analysis of the resolution limits and experimental benchmarking of the technique with pepper-pot emittance measurements are presented.

A new type of fast luminosity separation scans (“emittance scans”) was introduced at the CERN Large Hadron Collider (LHC) in 2015. The scans were performed systematically in every fill with full-intensity beams in physics production conditions at the interaction point (IP) of the compact muon solenoid (CMS) experiment. They provide both transverse emittance and closed orbit measurements at a bunch-by-bunch level. The precise measurement of beam-beam closed orbit differences allowed a direct, quantitative observation of long-range beam-beam PACMAN effects, which agrees well with numerical simulations from an improved version of the TRAIN code.

We generalize the temporally encoded spectral shifting (TESS) analysis for measuring plasma wakefields using spectral interferometry to dissimilar probe pulses of arbitrary spectral profile and to measuring nonlinear wakefields. We demonstrate that the Gaussian approximation used up until now results in a substantial miscalculation of the wakefield amplitude, by a factor of up to two. A method to accurately measure higher amplitude quasilinear and nonlinear wakefields is suggested, using an extension to the TESS procedure, and we place some limits on its accuracy in these regimes. These extensions and improvements to the analysis demonstrate its potential for rapid and accurate on-shot diagnosis of plasma wakefields, even at low plasma densities.

Transverse mode-coupling instability (TMCI) is known to limit bunch intensity. Since space charge (SC) changes coherent spectra, it affects the TMCI threshold. Generally, there are only two types of TMCI with respect to SC: the vanishing type and the strong space charge (SSC) type. For the former, the threshold value of the wake tune shift is asymptotically proportional to the SC tune shift, as it was first observed twenty years ago by M. Blaskiewicz for exponential wakes. For the latter, the threshold value of the wake tune shift is asymptotically inversely proportional to the SC, as it was shown by one of the authors. In the presented studies of various wakes, potential wells, and bunch distributions, the second type of instability was always observed for cosine wakes; it was also seen for the sine wakes in the case of a bunch within a square potential well. The vanishing TMCI was observed for all other wakes and distributions we discuss in this paper: always for the negative wakes, and always, except the cosine wake, for parabolic potential wells. At the end of this paper, we consider high-frequency broadband wake, suggested as a model impedance for CERN SPS ring. As expected, TMCI is of the vanishing type in this case. Thus, SPS Q26 instability, observed at strong SC almost with the same bunch parameters as it would be observed without SC, cannot be TMCI.

The use of both spatial premodulation and negative mass instability is promising for the energy enhancement and spectrum narrowing of the undulator radiation from dense electron bunches formed in modern laser-driven photoinjectors. Combining these two methods will permit the formation of long-lived trains of short and dense bunches with very large charges, which are able to efficiently emit a much more powerful and narrow-band terahertz radiation than single bunches with the same total charge, or premodulated bunches in the positive mass oscillation regime. The development of radiation instability in the negative mass regime can enable significant quasiperiodic self-modulation of relatively long nonmodulated bunches resulting in their fairly efficient and selective terahertz superradiance.

Stable, with a narrow energy band $<3\%$, and energy selectable, from 1 to 40 MeV, electron beams with charges $\sim 1\mathrm{pC}$ are produced from broadband electron bunches accelerated in the wakefield of femtosecond laser pulses with pulse energy $\sim 0.4\mathrm{J}$. The beams are extracted with use of energy filtering based on a compact and controllable pulse-driven solenoid. Such a plasma cathode based on laser wakefield acceleration can be used as an injector for the multistage laser wakefield electron acceleration as well as for ultrafast electron imaging systems.

A new optimization algorithm is introduced for online optimization applications. The algorithm was modified from the popular Nelder-Mead simplex method to make it noise aware and noise resistant. Simulation with an analytic function is used to demonstrate its performance. The algorithm has been successfully tested in experiments, which showed that the algorithm is robust for optimization problems with complex functional dependence, high cross-coupling between parameters, and high noise. Advantages of the new algorithm include high efficiency and that it does not require prior knowledge of the parameter space such as an initial conjugate direction set.

Laser wakefield acceleration of electrons usually offers an axisymmetry around the laser propagation axis. Thus, the accelerating electrons that are focused on axis often execute small transverse oscillations. In this article, we propose a simple scheme to break this symmetry, which enhances the transverse wiggling of electrons and boosts the betatron radiation emission. Through 3D particle-in-cell simulations, we show that sending the laser with a small angle of incidence on a transverse plasma density gradient generates an asymmetric wakefield. It first provokes injection and then increases the wiggling of the electrons through the transverse shifting of the wakefield axis which occurs when the laser pulse leaves the gradient. Consequently, we show that the radiated energy per unit of charge can increase by a factor $>20$ when using this scheme, and that the critical energy of the radiation quintuples compared with a reference case without the transverse density gradient.

Contemporary particle injector technologies provide different advantages depending on the chosen design. In the case of copper rf injectors these is primarily the high accelerating field, enabling the generation of high charge bunches with very low emittance. However, the cost of that is a comparably low repetition rate. DC guns, on the other hand, can provide higher repetition rates and consequently increased beam currents at lower beam quality, i.e., increased emittance. The concept of a superconducting rf injector offers the opportunity to combine the advantages of both these concepts. However, it demands special concepts for emittance compensation, as the common approach with overlapping magnetic fields during the rf acceleration interferes with the limitations of superconductivity. The ELBE SRF Gun project is one of the most advanced in this field. Gun II, the second SRF injector at the Electron Linear accelerator with high Brilliance and low Emittance (ELBE), introduces new features for emittance compensation which were studied in detail over the last years. One of these methods is the integration of a superconducting solenoid into the cryostat. Another method uses rf focusing by retracting the photocathode’s tip from the last cell of the resonator. This paper discusses both of these schemes by briefly outlining their setups, discussing results of numerical simulations of their impact, and presenting results of initial experimental beam measurements with Gun II.

Knowledge of the longitudinal beam parameters is important for understanding beam dynamics in hadron linacs. Recently, a noninterceptive method suitable for measuring the longitudinal rms Twiss parameters in the high energy part of a hadron linac was developed and experimentally verified at the Spallation Neutron Source (SNS). This work presents an extension and modification of the method to allow measurements in the low energy part of a linac. Like the original method, amplitude of the beam rf harmonic measured by the beam position monitor is used to derive the information on the bunch length. The new method was tested at the medium energy beam transfer line of the SNS accelerator. The results are in good agreement with an independent measurement of the bunch length using the longitudinal acceptance scan of the drift tube linac. The suggested method is fast, and it can be a useful part of the initial longitudinal beam tuning at the front end of hadron linacs.

The performance of free electron laser x-ray light sources, and systems for ultrafast electron diffraction and ultrafast electron microscopy, is limited by the brightness of the electron sources used. The intrinsic emittance, or equivalently, the mean transverse energy (MTE) of electrons emitted from the photocathode determines the maximum possible brightness in such systems. With ongoing improvements in photocathode design and synthesis, we are now at a point where the physical and chemical surface roughness of the cathode can become a limiting factor. Here we show how measurements of the spatially dependent variations in height and surface potential can be used to compute the electron beam mean transverse energy (MTE), one of the key determining factors in evaluation of brightness.

The plasma instabilities play an important role in an electron cyclotron resonance (ECR) ion source for the production of intense heavy ion beams in high charge states for particle accelerators. The geometrical and operational constraints of ECR sources hinder the trapping of ions for a sufficient time to get fully ionized with maximum efficiency. This problem is looked at in detail by studying the plasma instabilities in ECR ion sources. The ECR environment is full of complex rearrangements of various electric and magnetic fields to define a sustainable trap for the ions. The maximum frequency of plasma instability has been observed to be of 122.5 kHz under a set of sustainable plasma parameters. However, this limit may be pushed further if the plasma is overdriven in terms of source parameters. The instabilities cover a full regime of few tens of Hz to few hundreds of kHz under various operating conditions of radio frequency (rf), negative bias voltage, rf power and injection gas pressure. The rigorous details of frequencies and amplitudes of plasma instabilities are being reported by studying the Fourier spectrum of extracted and analyzed beam intensity. The plasma instabilities are attributed as drift waves in an inhomogeneous ECR plasma generated by the application of radio-frequency fields.

Experiments with high gradient traveling-wave mm-wave metallic accelerating structures were performed to research the physics of vacuum rf breakdowns. The accelerating structures are open, composed of two identical halves separated by an adjustable gap. The electromagnetic fields were induced by an ultrarelativistic electron beam at the Facility for Advanced Accelerator Experimental Tests (FACET) at SLAC National Accelerator Laboratory. These accelerating structures have a fundamental beam-synchronous mode with a frequency that varies from 95 to 140 GHz depending on the gap width. When the gap is opened by more than half of a freespace wavelength, the corresponding synchronous mode remains trapped demonstrating behavior consistent with the so called “surface wave.” This regime is of potential interest for femtosecond electron beam diagnostics and beam manipulation techniques. The behavior of the surface wave was characterized and the measured synchronous frequency, pulse length and beam deflection voltage are found to be in good agreement with simulations. The wave magnitude was changed by positioning the beam at different distances from the surface of a structure-half and the corresponding rf breakdown rate was recorded. The breakdown rate was $4\times {10}^{-3}$ per pulse at a peak surface electric field of $0.3\mathrm{GV}/\mathrm{m}$ and a pulse length of 60 ps.

The microbunching instability is known to be detrimental to x-ray free electron laser performance. At the Linear Coherent Light Source, the microbunching instability is suppressed with a laser heater, which increases the uncorrelated energy spread of e-beam in the injector. While the current system has been shown to improve x-ray brightness, other laser architectures could further enhance performance. In this study, we model the interaction between a laser and e-beam with arbitrary transverse profiles and examine the effect of various laser designs on the energy distribution of the electrons after the injector and laser heater, as well as their ability to suppress microbunching instability. This simulation incorporates random transverse jitter in order to reproduce physically representative operation of the Linac Coherent Light Source. We compare Gaussian and Laguerre-Gaussian modes, and explore composite beams in the form of an array of Gaussian beamlets. We conclude that the Gaussian laser profile is highly susceptible to e-beam ellipticity and random transverse jitter. The Laguerre-Gaussian profile, a mathematically ideal solution to suppressing microbunching, is less susceptible to these effects and can provide effective suppression even with a distorted e-beam, though performance can be improved by increasing stabilization. The array of beamlets presents a solution that produces consistent and smooth energy distributions with significantly less variance in heating than the Laguerre-Gaussian profile.

In this paper, the system identification and feedback controller design for a normal conducting standing wave radio frequency cavity operated in pulsed mode of a free electron laser is presented. The system identification, essential to achieve high performance control, is based on a grey box model, with a priori knowledge about the physical behavior. The feedback regulation concept is separated into two basic controller parts. A radio frequency controller design, based on the identified radio frequency model, is followed by a system identification and regulation of the pulse width to overcome the limitation of the underlying temperature regulation. It will be shown by measurements at the free electron laser FLASH at DESY, Germany, that the combination of both cascaded control schemes lead to high radio frequency field performance.

The treatment of flue gases from power plants and municipal or industrial wastewater using electron beam irradiation technology has been successfully demonstrated in small-scale pilot plants. The beam energy requirement is rather modest, on the order of a few MeV; however, the adoption of the technology at an industrial scale requires the availability of high beam power, of the order of 1 MW, in a cost effective way. In this article we present the design of a compact superconducting accelerator capable of delivering a cw electron beam with a current of 1 A and an energy of 1 MeV. The main components are an rf-gridded thermionic gun and a conduction cooled $\beta =0.5$ elliptical ${\mathrm{Nb}}_3\mathrm{Sn}$ cavity with dual coaxial power couplers. An engineering and cost analysis shows that the proposed design would result in a processing cost competitive with alternative treatment methods.

Linear optics errors in the linac, the transport line, and the undulator section of a free electron laser (FEL) can significantly impact the FEL performance. In this paper we propose two methods to measure and correct the linear optics of one-pass systems using trajectory scan data. The methods have been successfully applied to data taken on the Linac Coherent Light Source (LCLS) to find the quadrupole errors and beam position monitor (BPM) gains.