A longitudinal space charge amplifier (LSCA), operating in soft x-ray regime, was recently proposed. Such an amplifier consists of a few amplification cascades (focusing channel and chicane) and a short radiator undulator in the end. The broadband nature of LSCA supports generation of few-cycle pulses as well as wavelength compression. In this paper we consider an application of these properties of LSCA for generation of attosecond x-ray pulses. It is shown that a compact and cheap addition to the soft x-ray free-electron laser facility FLASH would allow one to generate 60 attosecond (FWHM) long x-ray pulses with the peak power at the 100 MW level and a contrast above 98%.

Finding a high quality of lattice that simultaneously meets low beam emittance performance and acceptable dynamic aperture is a challenging task for the storage ring-based light source, especially for the next generation storage ring which is characterized with ultralow beam emittance. This paper presents an alternative method, based on the concept of genetic algorithm, to simultaneously optimize the beam emittance and dynamic aperture for low emittance lattice. Instead of analyzing the nonlinear indicators extracted from the high order nonlinear map, the algorithm can globally optimize the nonlinear performance by the direct dynamic aperture tracking result. So this method is more straightforward and efficient than analyzing the nonlinear driving terms. In order to illustrate this method, the quadrupole and sextupole strengths of a five-bend-achromatic lattice are simultaneously optimized by nondominated sorting genetic algorithm II (NSGA-II). Finally, the optimal linear optics for ultralow emittance lattices with better dynamic aperture are obtained. The result shows that the algorithm is particularly useful for the low emittance lattice design, where the beam emittance and the dynamic aperture always conflict with each other.

The future electron-ion collider (eRHIC) at Brookhaven National Laboratory demands a high-current, polarized, bunched electron beam [http://www.bnl.gov/cad/eRhic]. One of the challenges here is to combine the bunched beams generated by multiple cathodes so to address the issue of designing and prototyping a combiner with high-frequency (700 kHz) rotational magnetic fields. This article presents its design, and simulation, and details some of the test results from this unprecedented device.

The local energy spread of electron beam is a very important parameter for high-gain free-electron lasers (FELs), especially the seeded FELs. Highly accurate measurement of the extremely small local energy spread is rather challenging. In this paper, a simple method to accurately measure the average local energy spread based on the coherent harmonic generation is proposed. A one-dimensional analytical estimation is given to show the principle of this method, and three-dimensional simulation codes have been used to verify the theory. This method has been demonstrated on the Shanghai deep ultraviolet FEL, and the results show that the average local energy spread is about only 1.2 keV at the exit of the 136 MeV linac, which agrees well with the numerical simulations.

Coherent transverse beam oscillations in the Tevatron were analyzed with the model-independent analysis (MIA) technique. This allowed one to obtain the model-independent values of coupled betatron amplitudes, phase advances, and dispersion function around the ring from a single dipole kick measurement. In order to solve the MIA mode mixing problem which limits the accuracy of determination of the optical functions, we have developed a new technique of rotational MIA mode untangling. The basic idea is to treat each beam position monitor (BPM) as two BPMs separated in a ring by exactly one turn. This leads to a simple criterion of MIA mode separation: the betatron phase advance between any BPM and its counterpart shifted by one turn should be equal to the betatron tune and therefore should not depend on the BPM position in the ring. Furthermore, we describe a MIA-based technique to locate vibrating magnets in a storage ring.

The combined effect of impedance and higher order chromaticity can act on the beam in a nontrivial manner which can cause a tune shift which depends on the relative momenta with respect to the “on momentum” particle ($\Delta p/p$). Experimentally, this tune shift affects the measurement of the linear chromaticity which is traditionally measured with a change of $\Delta p/p$. The theory behind this effect will be derived in this paper. Computer simulations and experimental data from the Tevatron will be used to support the theory.

A general architecture of an emittance exchanger (EEX) is considered, where the horizontal and longitudinal phase spaces are exchanged. A family of designs is described which can lead to extremely short final longitudinal lengths, even subfemtosecond. Using higher-order particle simulations, a preferred configuration is found, which has better compression capability and less emittance growth than the standard EEX design at high beam energy. An alternative design is also found which eliminates any final energy-phase coupling. Features of compression using an EEX are significantly different than with a chicane because the final longitudinal phase space is decoupled from the initial longitudinal phase space. Advantages of using an EEX for compression include less susceptibility to the coherent synchrotron radiation (CSR) microbunch instability, less susceptibility to bunch length broadening from CSR effects, and elimination of the initial energy-phase correlation that is needed for compression using a chicane as well as any residual energy-phase correlation after compression. A key disadvantage of using an EEX is that the final horizontal emittance tends to strongly depend on the initial bunch length and beam energy.

In the Compact Linear Collider (CLIC) now being studied at CERN, the rf power which accelerates the main beam is provided by decelerating a high current drive beam. The drive-beam linac has to accelerate a 4.2 A electron beam up to 2.4 GeV in almost fully loaded structures. The pulse contains about 70 000 bunches, one in every second rf bucket, and has a length of $140\mu \mathrm{s}$. The beam stability along the beam line is of concern for such a high current and pulse length. We present different options for the lattice of the linac based on FODO, triplet, and doublet cells and compare the transverse instability for each lattice including the effects of beam jitter, alignment, and beam-based correction.

We present a power-fit formula, obtained from a variational analysis using three-dimensional free-electron laser theory, for the gain length of a high-gain free-electron laser’s fundamental mode in the presence of diffraction, uncorrelated energy spread, and longitudinal space-charge effects. The approach is inspired by the work of Xie [Nucl. Instrum. Methods Phys. Res., Sect. A 445, 59 (2000)], and provides a useful shortcut for calculating the gain length of the fundamental Gaussian mode of a free-electron laser having strong space-charge effects in the 3D regime. The results derived from analytic theory are in good agreement with detailed numerical particle simulations that also include higher-order space-charge effects, supporting the assumptions made in the theoretical treatment and the variational solutions obtained in the single-mode limit.

The average temporal (longitudinal) and spatial (transverse) coherence of free-electron-laser pulses in the extreme ultraviolet at FLASH is measured by interfering two time-delayed partial beams directly on a CCD camera. Wavelengths between $\lambda =32\mathrm{nm}$ and $\lambda =8\mathrm{nm}$ are examined. A decrease of the coherence time for the fundamental wavelengths from ${\tau }_c=(6\pm 0.5)\mathrm{fs}$ at 32 nm to ${\tau }_c=(2.9\pm 0.5)\mathrm{fs}$ at 8 nm is measured. At $\lambda =8\mathrm{nm}$ the fundamental wavelength and the third harmonic of 24 nm are compared to each other. For 8 nm radiation as third harmonic of 24 nm a coherence time of ${\tau }_c=(2.5\pm 0.5)\mathrm{fs}$ is observed. The spatial coherence of 24 and 8 nm fundamental pulses are found to be very similar. The visibility decreases to 50% of the maximum visibility at about 3.2 mm overlap of the partial beams, which corresponds to 42% of the beam diameter at a distance of 90 m from the exit of the undulator. These results are analyzed in terms of the Gaussian Schell model resulting in six contributing modes to the total radiation. In addition, the correlation of the visibility between the fundamental radiation at 24 nm and its third harmonic at $\lambda =8\mathrm{nm}$ is investigated for identical shots.

We discuss $X$-band rf power production and deceleration in the two-beam test stand of the CLIC test facility at CERN. The rf power is extracted from an electron drive beam by a specially designed power extraction structure. In order to test the structures at high-power levels, part of the generated power is recirculated to an input port, thus allowing for increased deceleration and power levels within the structure. The degree of recirculation is controlled by a splitter and phase shifter. We present a model that describes the system and validate it with measurements over a wide range of parameters. Moreover, by correlating rf power measurements with the energy lost by the electron beam, as measured in a spectrometer placed after the power extraction structure, we are able to identify system parameters, including the form factor of the electron beam. The quality of the agreement between model and reality gives us confidence to extrapolate the results found in the present test facility towards the parameter regime of CLIC.

The ion-caused beam instability in the future light sources and electron damping rings can be serious due to the high beam current and ultrasmall emittance of picometer level. One simple and effective mitigation of the instability is a multibunch train beam filling pattern which can significantly reduce the ion density near the beam, and therefore reduce the instability growth rate up to 2 orders of magnitude. The suppression is more effective for high intensity beams with low emittance. The distribution and the electric field of trapped ions are benchmarked to validate the model used in the paper. The wakefield of ion cloud and the beam-ion instability is investigated both analytically and numerically. We derived a simple formula for the buildup of ion cloud and instability growth rate with the multibunch train filling pattern. The simulation in NSLSII, PEPX, SuperKEKB, and the observation in SPEAR3 are used to compare with our analyses. The analyses agree well with simulations and observations.

Ions released from target surfaces by impact of a high intensity and current electron beam can be accelerated and trapped in the beam potential, and further destroy the beam focus. By solving the 2D Poisson equation, we found that the charge neutralization factor of the ions to the beam under space charge limited condition is $1/3$, which is large enough to disrupt the spot size. Therefore, the ion emission at the target in a single-pulse beam/target system must be source limited. Experimental results on the time-resolved beam profile measurement have also proven that. A new focus scheme is proposed in this paper to focus the beam to a small spot size with the existence of back-streaming ions. We found that the focal spot will move upstream as the charge neutralization factor increases. By comparing the theoretical and experimental focal length of the Dragon-I accelerator (20 MeV, 2.5 kA, 60 ns flattop), we found that the average neutralization factor is about 5% in the beam/target system.

Of particular interest to x-ray free-electron laser light source facilities is the enhanced self-amplified spontaneous emission (ESASE) technique. ESASE requires an ultrafast (20–50 fs), high peak power, high repetition rate, reliable laser system working in the midinfrared spectral range ($\ge 2\mu \mathrm{m}$). These requirements can be met by a novel ultrafast midinfrared laser system based on optical parametric chirped-pulse amplification (OPCPA). OPCPA is a technique ideally suited for production of ultrashort laser pulses at the center wavelength of $2\mu \mathrm{m}$. Some of the key features of OPCPA are the wavelength agility, broad spectral bandwidth, and negligible thermal load. The ESASE-compatible laser technology analysis and the preliminary OPCPA simulation results are presented.

The acceleration of polarized electron beams in the blowout regime of plasma-based acceleration is explored. An analytical model for the spin precession of single beam electrons, and depolarization rates of zero emittance electron beams, is derived. The role of finite emittance is examined numerically by solving the equations for the spin precession with a spin tracking algorithm. The analytical model is in very good agreement with the results from 3D particle-in-cell simulations in the limits of validity of our theory. Our work shows that the beam depolarization is lower for high-energy accelerator stages, and that under the appropriate conditions, the depolarization associated with the acceleration of 100–500 GeV electrons can be kept below 0.1%–0.2%.

Occasionally, it is possible to bring together experiment, theory, and simulation in detail. Such an occasion occurred during a high intensity beam physics study in the Spallation Neutron Source (SNS). A transverse dipole instability in the vertical direction has been observed in the accumulator ring for a coasting beam that was stored for 10 000 turns. This instability was observed at a beam intensity of about $12\mu \mathrm{C}$ and was characterized by a frequency spectrum peaking at about 6 MHz. The probable cause of the instability is the impedance of the ring extraction kickers. We carry out here a detailed benchmark of the observed instability, uniting an analysis of the experimental data, a precise ORBIT code tracking simulation, and a theoretical estimate of the observed beam instability.

Beam steering in superconducting quarter-wave resonators (QWRs), which is mainly caused by magnetic fields, has been pointed out in 2001 in an early work [A. Facco and V. Zviagintsev, in Proceedings of the Particle Accelerator Conference, Chicago, IL, 2001 (IEEE, New York, 2001), p. 1095], where an analytical formula describing it was proposed and the influence of cavity geometry was discussed. Since then, the importance of this effect was recognized and effective correction techniques have been found [P. N. Ostroumov and K. W. Shepard, Phys. Rev. ST Accel. Beams 4, 110101 (2001)]. This phenomenon was further studied in the following years, mainly with numerical methods. In this paper we intend to go back to the original approach and, using well established approximations, derive a simple analytical expression for QWR steering which includes correction methods and reproduces the data starting from a few calculable geometrical constants which characterize every cavity. This expression, of the type of the Panofski equation, can be a useful tool in the design of superconducting quarter-wave resonators and in the definition of their limits of application with different beams.

A means for noninvasive measurement of transverse and longitudinal characteristics of bunched beams in synchrotrons has been developed based on high sensitivity slotted waveguide pickups. The pickups allow for bandwidths exceeding hundreds of MHz while maintaining good beam sensitivity characteristics. Wide bandwidth is essential to allow bunch-by-bunch measurements by means of a fast gate. The Schottky detector system is installed and successfully commissioned in the Fermilab Tevatron, Recycler and CERN LHC synchrotrons. Measurement capabilities include tune, chromaticity, and momentum spread of single or multiple beam bunches in any combination. With appropriate calibrations, emittance can also be measured by integrating the area under the incoherent tune sidebands.

We describe a new technique for low-emittance tuning of electron and positron storage rings. This technique is based on calibration of the beam position monitors (BPMs) using excitation of the normal modes of the beam motion, and has benefits over conventional methods. It is relatively fast and straightforward to apply, it can be as easily applied to a large ring as to a small ring, and the tuning for low emittance becomes completely insensitive to BPM gain and alignment errors that can be difficult to determine accurately. We discuss the theory behind the technique, present some simulation results illustrating that it is highly effective and robust for low-emittance tuning, and describe the results of some initial experimental tests on the CesrTA storage ring.

The High Energy Accelerator Research Organization KEK digital accelerator (KEK-DA) is a renovation of the KEK 500 MeV booster proton synchrotron, which was shut down in 2006. The existing 40 MeV drift tube linac and rf cavities have been replaced by an electron cyclotron resonance (ECR) ion source embedded in a 200 kV high-voltage terminal and induction acceleration cells, respectively. A DA is, in principle, capable of accelerating any species of ion in all possible charge states. The KEK-DA is characterized by specific accelerator components such as a permanent magnet X-band ECR ion source, a low-energy transport line, an electrostatic injection kicker, an extraction septum magnet operated in air, combined-function main magnets, and an induction acceleration system. The induction acceleration method, integrating modern pulse power technology and state-of-art digital control, is crucial for the rapid-cycle KEK-DA. The key issues of beam dynamics associated with low-energy injection of heavy ions are beam loss caused by electron capture and stripping as results of the interaction with residual gas molecules and the closed orbit distortion resulting from relatively high remanent fields in the bending magnets. Attractive applications of this accelerator in materials and biological sciences are discussed.