Generation of attosecond x-ray pulses is attracting much attention within the x-ray free-electron laser (FEL) user community. We propose a novel scheme for the generation of coherent stable attosecond x-ray pulse trains in a seeded FEL, via a process of mode-locked amplification. Three modulators and two chicanes are used for generating separated attosecond scale microstructures in the electron beam using the beam echo effect. Such electron beam will produce high harmonic radiation with a comb of longitudinal modes at the very beginning of the radiator. By using a series of spatiotemporal shifts between the copropagating radiation and electron beam in the radiator, all these modes can be preserved and amplified to saturation. Using a representative realistic set of parameters, three-dimensional simulation results show that trains of 200 attosecond soft x-ray pulses with stable peak powers at gigawatt level can be generated directly from ultraviolet seed lasers. The even spacing between the attosecond pulses can be easily altered from subfemtosecond to tens of femtoseconds by slightly changing the wavelength of one seed laser.

A laser pulse propagating in a corrugated plasma channel is composed of spatial harmonics whose phase velocities can be subluminal. The phase velocity of a spatial harmonic can be matched to the speed of a relativistic electron resulting in direct acceleration by the guided laser field in a plasma waveguide and linear energy gain over the interaction length. Here we examine the fully self-consistent interaction of the laser pulse and electron beam using particle-in-cell (PIC) simulations. For low electron beam densities, we find that the ponderomotive force of the laser pulse pushes plasma channel electrons towards the propagation axis, which deflects the beam electrons. When the beam density is high, the space charge force of the beam drives the channel electrons off axis, providing collimation of the beam. In addition, we consider a ramped density profile for lowering the threshold energy for trapping in a subluminal spatial harmonic. By using a density ramp, the trapping energy for a normalized vector potential of $a_0=0.1$ is reduced from a relativistic factor ${\gamma }_0=170$ to ${\gamma }_0=20$.

The behavior of free-electron laser amplification in the small signal linear regime can be understood by studying the dispersion relation derived from the electron energy distribution. A thorough understanding of the growth modes in this regime is of great value in understanding numerical results obtained using simulations. In this paper we show that for a typical bell-shape energy distribution in the electron beam there is not more than one growing mode. We also derive an analytical expression, which determines the bandwidth of the free-electron laser. We also discuss the limitation on the number of growing modes for the case of beam energy distributions with multiple peaks.

In plasma-based accelerators, electrons are accelerated by ultrahigh gradient of $1–100\mathrm{GV}/\mathrm{m}$ and undergo the focusing force with the same order as the accelerating force. Heated electrons are injected in a plasma wake and exhibit the betatron oscillation that generates synchrotron radiation. Intense betatron radiation from laser-plasma accelerators is attractive x-ray/gamma-ray sources, while it produces radiation loss and significant effects on energy spread and transverse emittance via the radiation reaction force. In this article, electron beam dynamics on transverse emittance and energy spread with considering radiation reaction effects are studied numerically. It is found that the emittance growth and the energy spread damping initially dominate and balance with radiative damping due to the betatron radiation. Afterward the emittance turns to decrease at a constant rate and leads to the equilibrium at a nanometer radian level with growth due to Coulomb scattering at PeV-level energies. A constant radiation loss rate $R_T=2/3$ is found without regard to the electron beam and plasma conditions. Self-cooling of electron beams due to betatron radiation may guarantee TeV-range linear colliders and give hints on astrophysical ultrahigh-energy phenomena.

An electron bunch passing through a dielectric-lined waveguide generates Čerenkov radiation that can result in a high-peak axial electric field suitable for acceleration of a subsequent bunch. Axial fields beyond gigavolt-per-meter are attainable in structures with sub-mm sizes depending on the achievement of suitable electron bunch parameters. A promising configuration consists of using a planar dielectric structure driven by flat electron bunches. In this paper we present a three-dimensional analysis of wakefields produced by flat beams in planar dielectric structures thereby extending the work of Tremaine, Rosenzweig, and Schoessow, Phys. Rev. E 56, 7204 (1997)] on the topic. We especially provide closed-form expressions for the normal frequencies and field amplitudes of the excited modes and benchmark these analytical results with finite-difference time-domain particle-in-cell numerical simulations. Finally, we implement a semianalytical algorithm into a popular particle-tracking program thereby enabling start-to-end high-fidelity modeling of linear accelerators based on dielectric-lined planar waveguides.

This study investigates the dynamics of a compressed electron layer (CEL) when a circularly polarized laser pulse with a sharp front irradiates a high-density foil. A time-dependent model for CEL motion during the hole-boring stage is proposed to describe details of the interaction for any shape of laser pulse. The opacity case, where the laser pulse is totally reflected, is investigated using this model. The results obtained are consistent with the results from particle-in-cell (PIC) simulations. A relaxation distance determined by the laser-front steepness is necessary to build a stable CEL state before ions rejoin into the CEL. For the transparent case, the laser-front steepness is important for the formation of the stable CEL state at the back surface of the target. Considering the motion of ions, both the CEL and ion dynamics are important to rebalance the laser pressure and electrostatic charge-separation force as the hole-boring stage changes to the light-sail stage.

Reproducible operation at high performances of superconducting cavities is required for linear accelerators. High beta elliptical cavities are thus of concern and, to achieve required performances for such resonators, surface preparation including electropolishing is recommended. We have designed and operate a setup for electropolishing in the vertical position of multicell cavities in order to: (i) obtain high yield with large elliptical cavities for Superconducting Linac (SPL) or European Spallation Source projects; (ii) develop a reference installation demonstrating that this process is appropriate for the large scale treatment of cavities in industry. The setup described here is the first one able to electropolish vertically multicell cavities with circulating acid and high safety standards. This equipment makes it possible to use a wide range of parameters such as voltage, acid flow rate, temperature, and nitrogen injection with an R&D purpose in mind. Optimization is studied using modeling with COMSOL software for different cavities. As examples, we present some results for the 704 MHz high-beta SPL cavity and the 1300 MHz International Linear Collider cavity and show the influence of cathode shape on both acid flow and electric field distribution during the process. Importance of the size of the cavity and first results achieved on single-cell and nine-cell cavities will be discussed.

A one-dimensional numerical model to study the evolution of longitudinal electrostatic wakefields, generated by propagation of a circularly polarized laser pulse in magnetized plasma has been presented. The direction of the external magnetic field is considered to be along as well as opposite to the axis of propagation of the laser pulse. Further, two-dimensional particle-in-cell code is used to obtain the generated wakefields. Separatrix curves are plotted to study the trapping and energy gain of an externally injected test electron, by the generated electrostatic wakefields, in the relativistic regime. Under appropriate conditions, an enhancement in the peak energy of an externally injected electron in magnetized plasma, as compared to the unmagnetized case, has been observed.

A technological gap exists between the $\mu \mathrm{m}$-scale wiggling periods achieved using electromagnetic waves of high intensity laser pulses and the mm scale of permanent-magnet and superconducting undulators. In the sub-mm range, surface-micromachined soft-magnetic micro-electro-mechanical system inductors with integrated solenoidal coils have already experimentally demonstrated 100 to 500 mT field amplitude across air gaps as large as $15\mu \mathrm{m}$. Simulations indicate that magnetic fields as large as 1.5 T across $50\mu \mathrm{m}$ inductor gaps are feasible. A simple rearranging of the yoke and pole geometry allows for fabrication of 10+ cm long undulator structures with period lengths between $12.5\mu \mathrm{m}$ and 1 mm. Such undulators find application both in high average power spontaneous emission sources and, if used in combination with ultrahigh-brightness electron beams, could lead to the realization of low energy compact free-electron lasers. Challenges include electron energy broadening due to wakefields and Joule heating in the electromagnet.

The usual practice of constructing quadrupoles from truncated cylindrical hyperbolae is put into question. A new shape is proposed. This shape has an analytic potential function. The exact shape of the analytic quadrupole may be impractical, but in the short case where aspect ratio length/aperture $\approx 1$, pole shapes can be spherical. The optimal spherical radius is found to be 1.65 times the aperture radius. An example is also given demonstrating that for aspect ratio $>1$, the aberrations of order 5 and higher are lower for the optimized shape.

Beam halo is an important factor in any high intensity accelerator. It can cause difficulties in the control of the beam, emittance growth, particle loss, and even damage to the accelerator. It is therefore essential to understand the mechanisms of halo formation and its dynamics. Experimental measurement of the halo distribution is a fundamental tool for such studies. In this paper, we present a new high dynamic range, adaptive masking method to image beam halo, which uses a digital micromirror-array device. This method has been thoroughly tested in the laboratory using standard optical techniques, and with an actual beam produced by the University of Maryland Electron Ring (UMER). A high dynamic range ($\mathrm{DR}\sim {10}^5$) has been demonstrated with this new method at UMER and recent studies, with more intense beams, indicate that this DR can be exceeded by more than an order of magnitude. The method is flexible, easy to implement, low cost, and can be used at any accelerator or light source. We present the results of our measurements of the performance of the method and illustrative images of beam halos produced under various experimental conditions.

A half-cell superconducting rf electron gun is designed to provide 0.5 A, 2 MeV beam for the Brookhaven National Laboratory R&D Energy Recovery Linac. Total rf power of 1 MW must be delivered to beam to meet the beam current and energy specifications, resulting in very strong coupling. Two opposing fundamental power couplers (FPCs) are employed to minimize the transverse kick to beam traversing the structure and to halve the power through the coupler. A single-window coaxial coupler has been designed to meet the average power and rf coupling requirements. The coupler features a planar beryllia rf window for better handling high thermal stresses and a “pringle”-shaped tip of the antenna for enhancing rf coupling. Two FPCs have been fabricated and tested in preparation for the gun cryomodule assembly. A room-temperature test stand was used for conditioning couplers in full reflection regime with variable phase of the reflecting wave. The couplers were tested up to 250 kW in pulse mode and 125 kW in cw mode at different settings of the reflecting wave phase to expose all rf surfaces along the couplers to high fields. Several multipacting barriers were encountered and successfully processed away. The rf power levels, at which multipacting was found, match well those found in computer simulations.

Since October 2007 Sandia National Laboratories has operated the refurbished $Z$ machine at an improved load current of 26 MA yielding 400 TW of x-ray power. The current pulse shape to the load is controlled by 36 independently timed laser triggered gas switches. As part of the refurbishment effort, a fiber coupled laser spark detector system has been installed which is able to detect the laser generated plasma in situ inside the trigger section of the high voltage switch. In this paper we describe how this detection system can be used to characterize the discharge dynamics of these 5.9 MV, 820 kA switches.

The effect of resonance crossing on beam stability is studied systematically by employing a novel tabletop experimental tool and a multiparticle simulation code. A large number of ions are confined in a compact linear Paul trap to reproduce the collective beam behavior. We can prove that the ion plasma in the trap is physically equivalent to a charged-particle beam propagating through a strong focusing channel. The plasma confinement force is quickly ramped such that the trap operating point traverses linear and nonlinear resonance stop bands. Assuming a nonscaling fixed field alternating gradient accelerator composed of many identical FODO cells, we measure how much ion losses occur under diverse conditions. It is experimentally and numerically demonstrated that too slow resonance crossing leads to significant ion losses as expected. Particular attention must be paid to the linear coherent resonance excited at a quarter-integer tune. When the beam intensity is high, this type of linear stop band can seriously affect the beam quality even for rather fast resonance crossing. A scaling law is given of the emittance growth caused by the quarter-integer resonance crossing.

The nonlinear Thomson scattering of a relativistic electron with an intense laser pulse is calculated numerically. The results show that an ultrashort x-ray pulse can be generated by an electron with an initial energy of 5 MeV propagating across a circularly polarized laser pulse with a duration of 8 femtosecond and an intensity of about $1.1\times {10}^{21}\mathrm{W}/{\mathrm{cm}}^2$, when the detection direction is perpendicular to the propagation directions of both the electron and the laser beam. The optimal values of the carrier-envelop phase and the intensity of the laser pulse for the generation of a single ultrashort x-ray pulse are obtained and verified by our calculations of the radiation characteristics.

Two types of noninterceptive optical monitors, based on gas fluorescence, have been designed for use on the Linear IFMIF Prototype Accelerator (LIPAc) that is currently under development (a 125 mA, 9 MeV, 175 MHz continuous wave deuteron beam). These diagnostics offer a technique to characterize the transverse beam profile for medium to high current hadron beams, without intercepting the beam core. This paper reports on beam tests using the prototype monitors developed for LIPAc. Tests were carried out at an experimental line of the Centro Nacional de Aceleradores cyclotron, using 9 MeV deuterons with beam currents from 0.4 to $40\mu \mathrm{A}$. In addition, transverse beam profile measurements were performed under high background radiation (e.g. gamma dose rate up to $83\mathrm{mSv}/\mathrm{h}$). Preliminary cross-checks with different profilers, as well as a systematic scan of beam current and vacuum pressures and tests with different injected gases (nitrogen and xenon) have been performed. In this work, we present a brief description of the experimental setup and the first measurements obtained with these prototype profilers plus a discussion of the first analysis of the background signal in a detector as a function of radiation background.

Electro-optical detection of THz coherent synchrotron radiation is a nondestructive method for measuring subpicosecond electron bunches or subpicosecond substructures on otherwise longer electron bunches. With a new diagnostic setup at the Swiss Light Source, which combines an amplified Yb fiber laser and a suitable GaP crystal, we demonstrate sampling as well as spectrally resolved single-shot measurements of sliced electron bunches containing as little as a few pC of charge. The single-shot measurements not only allow for a precise electric field characterization but also for a detailed analysis of the timing jitter between the electron bunch and the synchronized Yb fiber laser. The measurements of subsequent turns in the storage ring show distinct deviations from the simulations and we find strong indications that this discrepancy is caused by radiation loss through coherent synchrotron radiation itself, which is not included in many of today’s simulation codes.

We develop an analytical model for the vacuum electric breakdown rate dependence on an external electric field, observed in test components for the compact linear collider concept. The model is based on a thermodynamic consideration of the effect of an external electric field on the formation enthalpy of defects. Although strictly speaking only valid for electric fields, the model also reproduces very well the breakdown rate of a wide range of radio-frequency breakdown experimental data. We further show that the fitting parameter in the model can be interpreted to be the relaxation volume of dislocation loops in materials. The values obtained for the volume are consistent with dislocation loops with radii of a few tens of nanometers.

In the design of beam transport lines, one often meets the problem of constructing a quadrupole lens system that will produce desired transfer matrices in both the horizontal and vertical planes. Nowadays this problem is typically approached with the help of computer routines, but searching for the numerical solution one has to remember that it is not proven yet that an arbitrary four by four uncoupled beam transfer matrix can be represented by using a finite number of drifts and quadrupoles (representation problem) and the answer to this question is not known not only for more or less realistic quadrupole field models but also for the both most commonly used approximations of quadrupole focusing, namely thick and thin quadrupole lenses. In this paper we make a step forward in resolving the representation problem and, by giving an explicit solution, we prove that an arbitrary four by four uncoupled beam transfer matrix actually can be obtained as a product of a finite number of thin lenses and drifts.