A novel magnet configuration for permanent magnet undulators is proposed together with the operational procedures to improve the wavelength tunability of free electron lasers (FELs). The magnetization axes of the magnet blocks in the new configuration are inclined at a certain angle to those of the conventional Halbach configuration according to a particular rule. As a result, a composite-period magnetic field is generated, which contains two field components having two different periods, i.e., the fundamental and double periods. Moving the top and bottom magnet arrays toward the opposite directions along the undulator axis, it is possible to eliminate either of the two field components and thus the period is selectable. Simple analysis shows that the operation with the composite field is also useful for FELs despite that the gain length grows slightly due to the effects of the nonsinusoidal field profile. An example of the new undulator scheme applied to expanding the tunability of the SPring-8 x-ray FEL is presented together with discussions on technical issues.

Dynamic aperture (DA) is one of the key nonlinear properties for a storage ring. Although there have been both analytical and numerical methods to find the aperture, the reverse problem of how to optimize it is still a challenging problem. A general and flexible way of optimizing the DA is highly demanded in accelerator design and operation. In this paper, we discuss the use of multiobjective optimization for DA. First we consider using objective functions based only on numerical tracking results. Data mining of these results demonstrated a correlation between DA and low-order nonlinear driving terms. Next we considered using objective functions which included both numerical tracking results and analytical estimates of low-order nonlinear driving terms. This resulted in faster convergence. The National Synchrotron Light Source II (NSLS-II) lattice was taken as an example to illustrate this method. This multiobjective approach is not limited by particular linear or nonlinear lattice settings, and can also be applied for optimizing other properties of a storage ring.

A one-dimensional Vlasov-Poisson model for sheet beams is reviewed and extended to provide a simple framework for analysis of space-charge effects. Centroid and rms envelope equations including image-charge effects are derived and reasonable parameter equivalences with commonly employed 2D transverse models of unbunched beams are established. This sheet-beam model is then applied to analyze several problems of fundamental interest. A sheet-beam thermal equilibrium distribution in a continuous focusing channel is constructed and shown to have analogous properties to two- and three-dimensional thermal equilibrium models in terms of the equilibrium structure and Debye screening properties. The simpler formulation for sheet beams is exploited to explicitly calculate the distribution of particle oscillation frequencies within a thermal equilibrium beam. It is shown that as space-charge intensity increases, the frequency distribution becomes broad, suggesting that beams with strong space-charge can have improved stability relative to beams with weak space-charge.

The vacuum diode which is used to generate relativistic electron beams is one of the most important parts of a pulsed-power modulator. In this paper, the electron emission of cathode holder of a vacuum diode and its effect on the output voltage is investigated by experiments on an intense electron-beam accelerator with 180 ns full width at half maximum and 200–500 kV output voltage. First, the field emission is analyzed and the electric field of the vacuum chamber is calculated. Then, the flatness of the output voltage is discussed before and after adding an insulation plate when a water load is used. It is found that the electron emission at the edges of the cathode holder is the main reason to cause the change of the flatness. Last, a piece of polyester film is used as a target to further show the electron emission of the cathode holder. This analysis shows that decreasing the electron emission of the cathode holder in such a pulse power modulator could be a good way to improve the quality of the output voltage.

The in-vacuum undulator with a permanent magnet at room temperature is a mature technology and is widely used; with a short period length in a medium-energy facility, it can enhance photon brilliance in the hard x-ray region. A cryogenic permanent magnet has been investigated as an in-vacuum undulator; this undulator will become the best prospective device to satisfy the requirements of a photon source with great brilliance in the hard x-ray region. For the further hard x-ray region, a superconducting wiggler can provide great flux with a continuous spectrum, whereas a superconducting undulator will provide great brilliance with a discrete spectrum. High-temperature superconducting wires are highly promising for use in the development of superconducting undulators; unique algorithms for their development with an extremely short period in a small-magnet gap have been devised. Some out-of-vacuum planar undulators with special functions must also be fabricated to enable diverse applications in various light-source facilities. This article describes current and future developments for insertion devices in storage-ring and free-electron-laser facilities and discusses their feasibility for use therein.

Optical lithography has been actively used over the past decades to produce more and more dense integrated circuits. To keep with the pace of the miniaturization, light of shorter and shorter wavelength was used with time. The capabilities of the present 193-nm UV photolithography were expanded time after time, but it is now believed that further progress will require deployment of extreme ultraviolet (EUV) lithography based on the use of 13.5-nm radiation. However, presently no light source exists with sufficient average power to enable high-volume manufacturing. We report here the results of a study that shows the feasibility of a free-electron laser EUV source driven by a multiturn superconducting energy-recovery linac (ERL). The proposed $40\times 20{\mathrm{m}}^2$ facility, using MW-scale consumption from the power grid, is estimated to provide about 5 kW of average EUV power. We elaborate the self-amplified spontaneous emission (SASE) option, which is presently technically feasible. A regenerative-amplifier option is also discussed. The proposed design is based on a short-period (2–3 cm) undulator. The corresponding electron beam energy is about 0.5–1.0 GeV. The proposed accelerator consists of a photoinjector, a booster, and a multiturn ERL.

Compton scattering of a laser beam with a relativistic electron beam has been used to generate intense, highly polarized and nearly monoenergetic x-ray or gamma-ray beams at many facilities. The ability to predict the spatial, spectral, and temporal characteristics of a Compton gamma-ray beam is crucial for the optimization of the operation of a Compton light source as well as for the applications utilizing the Compton beam. In this paper, we present two approaches, one based upon analytical calculations and the other based upon Monte Carlo simulations, to study the Compton scattering process for various electron and laser-beam parameters as well as different gamma-beam collimation conditions. These approaches have been successfully applied to characterize Compton gamma-ray beams, after being benchmarked against experimental results at the High Intensity Gamma-ray Source ($\mathrm{HI}\gamma \mathrm{S}$) facility at Duke University.

GaAs-based dc high voltage photoguns used at accelerators with extensive user programs must exhibit long photocathode operating lifetime. Achieving this goal represents a significant challenge for proposed high average current facilities that must operate at tens of milliamperes or more. This paper describes techniques to maintain good vacuum while delivering beam, and techniques that minimize the ill effects of ion bombardment, the dominant mechanism that reduces photocathode yield of a GaAs-based dc high voltage photogun. Experimental results presented here demonstrate enhanced lifetime at high beam currents by: (a) operating with the drive laser beam positioned away from the electrostatic center of the photocathode, (b) limiting the photocathode active area to eliminate photoemission from regions of the photocathode that do not support efficient beam delivery, (c) using a large drive laser beam to distribute ion damage over a larger area, and (d) by applying a relatively low bias voltage to the anode to repel ions created within the downstream beam line. A combination of these techniques provided the best total charge extracted lifetimes in excess of 1000 C at dc beam currents up to 9.5 mA, using green light illumination of bulk GaAs inside a 100 kV photogun.

Plasma wakefield acceleration (PWFA) has demonstrated the ability to produce very high gradients to accelerate electrons and positrons. In PWFA, a drive bunch of charged particles passes through a uniform plasma, thereby generating a wakefield that accelerates a witness bunch traveling behind the drive bunch. This process works well for electrons, but much less so for positrons due to the positive charge attracting rather than repealing the plasma electrons, which leads to reduced acceleration gradient, halo formation, and emittance growth. This problem can be alleviated by having the positron beam travel through a hollow plasma channel. Presented are modeling results for producing 10–100 cm long hollow plasma channels suitable for positron PWFA. These channels are created utilizing laser-induced gas breakdown in hydrogen gas. The results show that hollow channels with plasma densities of order ${10}^{16}{\mathrm{cm}}^{-3}$ and inner channel radii of order $20\mu \mathrm{m}$ are possible using currently available terawatt-level lasers. At these densities and radii, preliminary positron PWFA modeling indicates that longitudinal electric fields on axis can exceed $3\mathrm{GV}/\mathrm{m}$.

We are performing experiments with a 92-kV, 810-kA, 74.6-GW linear transformer driver (LTD) cavity. This cavity generates a $\sim 100\mathrm{ns}$ power pulse from DC-charged capacitors in a single step. Our experiments start with an existing 100-kV, 490-kA LTD cavity and are making a number of improvements to it that are aimed at increasing the cavity’s peak output power and better understanding its operation. We are making improvements to the gas switches, the capacitors, and the magnetic toroids as well as heavily instrumenting the cavity. These experiments have increased the cavity’s output current into a matched load by 65% without increasing its volume.

The oblate spheroidal shape is close to the commonly used elliptical rf cavity shape employed in accelerators. Here we solve the oblate spheroidal radial and angular wave functions to obtain the frequencies of the axisymmetric TM and TE modes. We develop a semianalytic formalism to calculate the characteristic parameters, such as shunt impedance, of higher order modes (HOMs). Our formulation is applied to calculate the HOM frequencies of the INDUS-2 and ILC cavities, and the agreement with three-dimensional finite element calculations is excellent. Using this formalism we investigate the effect of changing the oblate shape, and predict an optimized range of ${\xi }_0$ (one of the key parameters to define the geometry), to reduce the number of significant HOMs.

In this paper we present the experience at Diamond Light Source in the design, implementation, and operation of low momentum compaction factor lattices for the generation of short x-ray pulses and coherent THz radiation. The effects of higher-order terms in the expansion of the momentum compaction factor on beam dynamics are reviewed from a theoretical point of view, and the details of both high- and low-emittance solutions at Diamond are discussed. Measurements taken to characterize the lattices under a variety of machine conditions are presented, along with the practical limitations that exist as the momentum compaction factor is made to approach zero.

We derive a new scaling law for the photon spectral density in nonlinear Thomson/Compton scattering, extending the findings of Heinzl, Seipt, and Kämpfer [Phys. Rev. A 81, 022125 (2010)]. This allows one to easily include the effects of general scattering geometries, e.g., side injection, and of a finite-size detector on the photon spectrum. The scaling law is employed to study substructures emerging in the nonlinear Thomson/Compton spectra due to temporally shaped laser pulses scattering off relativistic electrons. We determine optimum scattering geometries for an experimental verification of these substructures.

Cyclic thermal stresses produced by rf pulsed heating can be the limiting factor on the attainable reliable gradients for room temperature linear accelerators. This is especially true for structures that have complicated features for wakefield damping. These limits could be pushed higher by using special types of copper, copper alloys, or other conducting metals in constructing partial or complete accelerator structures. Here we present an experimental study aimed at determining the potential of these materials for tolerating cyclic thermal fatigue due to rf magnetic fields. A special cavity that has no electric field on the surface was employed in these studies. The cavity shape concentrates the magnetic field on one flat surface where the test material is placed. The materials tested in this study have included oxygen free electronic grade copper, copper zirconium, copper chromium, hot isostatically pressed copper, single crystal copper, electroplated copper, Glidcop®, copper silver, and silver plated copper. The samples were exposed to different machining and heat treatment processes prior to rf processing. Each sample was tested to a peak pulsed heating temperature of approximately $110°\mathrm{C}$ and remained at this temperature for approximately $10\times {10}^6$ rf pulses. In general, the results showed the possibility of pushing the gradient limits due to pulsed heating fatigue by the use of copper zirconium and copper chromium alloys.

The interference of stray electrons (also called “electron clouds”) with accelerator beams is important in modern intense-beam accelerators, especially those with beams of positive charge. In magnetic wigglers, used, for instance, for transverse emittance damping, the intense synchrotron radiation produced by the beam can generate an electron cloud of relatively high density. In this paper the complicated dynamics of electron clouds in wigglers is examined using the example of a wiggler in the Cornell Electron Storage Ring Test Accelerator experiment at the Cornell Electron Storage Ring. Three-dimensional particle-in-cell simulations with the WARP-POSINST computer code show different density and dynamics for the electron cloud at locations near the maxima of the vertical wiggler field when compared to locations near the minima. Dynamics in these regions, the electron cloud distribution vs longitudinal position, and the beam coherent tune shift caused by the wiggler electron cloud will be discussed.

For the electron devices with oversized interaction space, the Bragg structures of different types are effective instruments for controlling and redistribution of the electromagnetic energy fluxes to provide a spatial coherence of radiation. In this paper a powerful long-pulse free-electron laser of the terahertz waveband is studied in which an advanced Bragg structure based on the coupling between the propagating waves and the cutoff mode is used as an upstream reflector in the two-mirror resonator. In the case of planar geometry, this structure provides effective mode selection over the “narrow” transverse coordinate directed between the plates. Under the assumption that the system is open over a second “wide” coordinate, the diffraction effects are sufficient for synchronization of radiation from a sheet electron beam over this direction. An alternative way of radiation synchronization for a case of more extended electron beam by exploiting a 2D Bragg structure as a downstream reflector is discussed.

In first approximation storage ring multipole magnets are described as simple two-dimensional magnet structures and many linear and nonlinear beam optic features of a magnet lattice can already be derived from this model. In contrast, undulators, and in particular variably polarizing devices, employ complicated three-dimensional magnetic fields which may have a severe impact on the electron beam, in particular, in low energy third generation storage rings. A Taylor expanded generating function method is presented to generate a fast, flexible, and symplectic mapping routine for particle tracking in magnetic fields. This method is quite general and is based on the solution of the Hamilton-Jacobi equation. It requires an analytical representation of the fields, which can be differentiated and integrated. For undulators of the APPLE II type, an accurate analytic field model is derived which is suitable for the tracking routine. This field model is fully parametrized representing all operation modes for the production of elliptical or linear polarized light with an arbitrary inclination angle or even arbitrary polarization. Based on this field model, analytic expressions for 2nd order kicks are derived. They are used to estimate the influence of APPLE II undulators on the electron beam dynamic. Furthermore, an analytic model for the description of shims is given. The shims are needed for field and performance optimization. Passive and active shimming concepts for the compensation of linear and nonlinear effects of variably polarizing undulators are discussed.

Experiments to investigate copper surface fatigue caused by pulsed rf radiation were carried out using the 30 GHz free electron maser. The copper surface of a special test cavity was exposed to $15–20\mathrm{MW}/150–200\mathrm{ns}$ rf pulses with a repetition rate of 1 Hz, providing a temperature rise of up to $250°\mathrm{C}$ in each pulse. An electron microscope was used to study the copper surface both before and after exposure to ${10}^4–{10}^5$ rf pulses. An examination of the copper microstructure and cracks which developed during the experiment was made. Dramatic degradation of the copper surface and causes of very frequent breakdown were observed when the total number of rf pulses reaches $6\times {10}^4$.

To change the wavelength of undulator radiation, variation of the undulator magnetic field amplitude is frequently applied. Another option is changing the undulator period. A corresponding scheme is described. Both the period and number of periods can be changed. For a set of undulator sections (as in the x-ray free-electron lasers ), the mechanical motion of the periods allows doing without phase shifters between the undulator sections. Magnetic field calculations for some undulator parameters of interest were performed. Numerous advantages of the new undulators (fixed gap, strong dependence of the undulator radiation wavelength on the period, relatively low field amplitude variation, and variable number of periods) look very attractive. The prospects for this new type of undulators are discussed.