The use of stripping foils for charge exchange injection can cause a number of operational problems in high intensity hadron accelerators. A recently proposed three-step method of laser-assisted injection is capable of overcoming these problems. This paper presents advances in the physical model of laser-assisted charge exchange injection of ${\mathrm{H}}^{-}$ beams and covers a wide field of atomic physics. The model allows the calculation of the evolution of an ${\mathrm{H}}^0$ beam taking into account spontaneous emission, field ionization, and external electromagnetic fields. Some new data on the hydrogen atom related to the problem are calculated. The numerical calculations in the model use realistic descriptions of laser field and injection beam. Generally, the model can be used for design and optimization of a laser-assisted injection cell within an accelerator lattice. Example calculations of laser-assisted injection for an intermediate experiment at SNS in Oak Ridge and for the PS2 accelerator at CERN are presented. Two different schemes, distinctively characterized by various magnetic fields at the excitation point, are discussed. It was shown that the emittance growth of an injected beam can be drastically decreased by moving the excitation point into a strong magnetic field.

We have developed a new compact high average current dc superconducting rf (DC-SRF) photoinjector. A 3.5-cell superconducting cavity with a mid-beta structure is the key component of the DC-SRF photoinjector. Herein, we present the new design, fabrication, and vertical test results of the 3.5-cell large grain niobium superconducting cavity. After preliminary surface preparation and room temperature rf measurements, the cavity was sent to Jefferson Lab for a vertical cold test. The gradient of the cavity reaches $23.5\mathrm{MV}/\mathrm{m}$, and the $Q_0$ value is higher than $1.2\times {10}^{10}$. The high-quality 3.5-cell cavity will guarantee the good performance of the DC-SRF photoinjector.

A 6-m cryomodule including four Tesla-like cavities was developed, and was tested in the Superconducting RF Test Facility phase-I at KEK. The performance as a total superconducting cavity system was checked in the cryomodule tests at 2 K with high rf power. One of the four cavities achieved a stable pulsed operation at $32\mathrm{MV}/\mathrm{m}$, which is higher than the operating accelerating gradient in the ILC. The maximum accelerating gradient ($E_{\mathrm{acc},\max }$) obtained in the vertical cw tests was maintained or slightly improved in the cryomodule tests operating in a pulse mode. Compensation of the Lorentz force detuning at $31\mathrm{MV}/\mathrm{m}$ was successfully demonstrated by a piezo tuner and predetuning.

This paper studies the impact of transverse dispersion on the free-electron laser (FEL) performance in two main aspects: radiation power sensitivity to electron beam energy offset and radiation wavelength spectrum. Since these issues are related to electron trajectories, studies on the FEL power dependence to the beam orbit are also presented. The experiments were done at the free-electron laser in Hamburg (FLASH). A method to measure and control the dispersion is also described.

The crossbar-$H$-mode (CH) structure is the first superconducting multicell drift tube cavity for the low and medium energy range operated in the $H_{21}$ mode. Because of the large energy gain per cavity, which leads to high real estate gradients, it is an excellent candidate for the efficient acceleration in high power proton and ion accelerators with fixed velocity profile. A prototype cavity has been developed and tested successfully with a gradient of $7\mathrm{MV}/\mathrm{m}$. A few new superconducting CH cavities with improved geometries for different high power applications are under development at present. One cavity ($f=325\mathrm{MHz}$, $\beta =0.16$, seven cells) is currently under construction and studied with respect to a possible upgrade option for the GSI UNILAC. Another cavity ($f=217\mathrm{MHz}$, $\beta =0.059$, 15 cells) is designed for a cw operated energy variable heavy ion linac application. Furthermore, the EUROTRANS project (European research program for the transmutation of high level nuclear waste in an accelerator driven system, 600 MeV protons, 352 MHz) is one of many possible applications for this kind of superconducting rf cavity. In this context a layout of the 17 MeV EUROTRANS injector containing four superconducting CH cavities was proposed by the Institute for Applied Physics (IAP) Frankfurt. The status of the cavity development related to the EUROTRANS injector is presented.

A new design principle of a nonscaling fixed field alternating gradient accelerator is proposed. It is based on optics that produce approximate scaling properties. A large field index $k$ is chosen to squeeze the orbit shift as much as possible by setting the betatron oscillation frequency in the second stability region of Hill’s equation. Then, the lattice magnets and their alignment are simplified. To simplify the magnets, we expand the field profile of $r^k$ into multipoles and keep only a few lower order terms. A rectangular-shaped magnet is assumed with lines of constant field parallel to the magnet axis. The lattice employs a triplet of rectangular magnets for focusing, which are parallel to one another to simplify alignment. These simplifications along with fringe fields introduce finite chromaticity and the fixed field alternating gradient accelerator is no longer a scaling one. However, the tune excursion of the whole ring can be within half an integer and we avoid the crossing of strong resonances.

A $B$-dot monitor that measures the current 6 cm from the axis of dynamic loads fielded on ${10}^7$-A multiterawatt pulsed-power accelerators has been developed. The monitor improves upon the multimegampere load-current gauge described in Phys. Rev. ST Accel. Beams 11, 100401 (2008). The design of the improved monitor was developed using three-dimensional particle-in-cell simulations that model vacuum electron flow in the transmission line near the monitor. The simulations include important geometric features of the $B$-dot probe and model the deposition of electron energy within the probe. The simulations show that the improved design reduces by as much as a factor of 5 the electron energy deposition to the interior of the monitor. Data taken on accelerator shots demonstrate that the improved monitor works as well as the original monitor on shots with low-impedance loads, and delivers superior performance on higher-impedance-load shots.

ATF2 is a final-focus test beam line which aims to focus the low emittance beam from the ATF damping ring to a vertical size of about 37 nm and to demonstrate nanometer level beam stability. Several advanced beam diagnostics and feedback tools are used. In December 2008, construction and installation were completed and beam commissioning started, supported by an international team of Asian, European, and U.S. scientists. The present status and first results are described.

This paper addresses debris mitigation techniques for two different kinds of debris sources that are found in the high-energy density community. The first debris source stems from the laser-target interaction and this debris can be mitigated by avoiding a direct line of sight to the debris source (e.g. by using a sacrificial fold mirror) or by inserting a thin debris shield. Several thin film debris shields have been investigated and nitrocellulose was found to be the best suited. The second debris source originates from an external high-energy density driver or experiment. In our specific case, this is the $Z$ accelerator, a $Z$-pinch machine that generates 2 MJ of x rays at 300 TW. The center section of the $Z$ accelerator is an extremely violent environment which requires the development of novel debris mitigation approaches for backlighting with petawatt lasers. Two such approaches are presented in this paper. First, a self-closing focusing cone. In our facility, the focused beam on target is fully enclosed inside a solid focusing cone. In the first debris mitigation scenario, the last part of the cone has a “flapper” that should seal the cone when the pressure wave from the $Z$-pinch explosion hits it. In the second scenario, an enclosed target assembly is used, with the last part of the focusing cone connected to a “target can” which houses the laser target. The laser produced x rays for backlighting escape through a 3 mm diameter hole that is protected by an x-ray filter stack. Both techniques are discussed in detail and have been successfully tested on the $Z$ accelerator.

The superconducting radio frequency photoinjector (SRF gun) is one of the latest applications of superconducting rf technology in the accelerator field. Since superconducting photocathodes with high quantum efficiency are yet unavailable, normal conducting cathode material is the main choice for SRF photoinjectors. However, the compatibility between the photocathode and the cavity is one of the challenges for this concept. Recently, a SRF gun with ${\mathrm{Cs}}_2\mathrm{Te}$ cathode has been successfully operated in Forschungszentrum Dresden-Rossendorf. In this paper, we will present the physical properties of ${\mathrm{Cs}}_2\mathrm{Te}$ photocathodes in the SC cavity, such as the quantum efficiency, the lifetime, the rejuvenation, the charge saturation, and the dark current.

Linear and nonlinear resonant instabilities of charged-particle beams traveling in periodic quadrupole focusing channels are studied experimentally with a compact non-neutral plasma trap. The present experiments are based on the idea that the collective motion of a beam in an accelerator is physically similar to that of a one-component plasma in a trap. A linear Paul trap system named “S-POD” (simulator for particle orbit dynamics) was developed to explore a variety of space-charge-induced phenomena. To emulate lattice-dependent effects, periodic perturbations are applied to quadrupole electrodes, which gives rise to additional resonance stop bands that shift depending on the plasma density. It is confirmed that an $m$th-order resonance takes place when the corresponding tune of an $m$th-order collective mode ${\Omega }_m$ is close to a half integer.

A weak transverse resonance in the Spallation Neutron Source (SNS) superconducting linac is identified in computer simulations, and is believed to be one of the mechanisms behind measured beam losses. This weak resonance is induced by the nonlinear dodecapole component of the linac quadrupole magnets. It occurs when the linac focusing lattice has a transverse phase advance close to 60°. By reducing the phase advance to approximately 50° to avoid the resonance, we observe significant reduction in beam loss in the SNS superconducting linac. We present theory and computer simulation results supporting the notion that a suppression of the 60° resonance may contribute to reduction in the beam loss.

Proton beam driven plasma wakefield acceleration was recently proposed as a way to bring electrons to TeV energy range in a single plasma section. Here we present a detailed numerical analysis of this acceleration scheme. We identify the main effects limiting acceleration efficiency and ultimate energy gain, and formulate optimum conditions for acceleration.

We have constructed and tested 1D-photonic crystal (PhC) mirrors with tunable reflectivity to be used as efficient, broadband outcouplers for THz free electron lasers (FELs). The test mirrors cover a spectral range between 0.5 and 1.5 THz. They are assembled by stacking up quarter-wave dielectric layers separated by vacuum. The adopted PhC mirror design enables dynamical (while lasing) adjustment of individual layer spacing. Single as well as multiple defects in the periodicity are introduced to invoke a continuous, well-defined tuning of reflectivity and outcoupling ratio. The scheme allows one to vary also the PhC period while the equidistant spacing between the layers is maintained. This feature is used to shift the photonic band gap (center) for achieving an effective extension in the reflectance spectrum. Because of the exceptional flexibility provided by the scheme in tailoring the characteristics of the PhC defects, additional features such as tunable (narrow) bandpass filtering as well as (fast) THz intensity modulation can be combined with the reflectivity/coupler properties of the proposed PhC mirrors for FELs.

Free electron lasers (FELs) are favored in many research applications due to their ability to operate primarily in the fundamental (Gaussian) mode. In oscillator FELs, the appropriate basis set for decomposition is uniquely determined by the cavity parameters. Since FEL amplifiers generate and amplify the optical field over the length of the undulator influenced only by the electron beam, the appropriate basis set is not uniquely determined. A methodology is presented to decompose the optical fields with a Hermite-Gaussian basis set chosen such that the number of higher-order modes is minimized. The methodology is used to study amplifier FEL optical beams for comparison with experiment.

We report on the possibility to passively linearize the bunch compression process in electron linacs for the next generation x-ray free electron lasers. This can be done by using the monopole wakefields in a dielectric-lined waveguide. The optimum longitudinal voltage loss over the length of the bunch is calculated in order to compensate both the second-order rf time curvature and the second-order momentum compaction terms. Thus, the longitudinal phase space after the compression process is linearized up to a fourth-order term introduced by the convolution between the bunch and the monopole wake function.

Because of the long synchrotron period between the two barriers, bunch compression in a barrier bucket is usually relatively slow. The conservation of the phase space area requires that the compression is performed adiabatically, that is much slower than the synchrotron period. We will show that the presence of longitudinal space charge can considerably improve the efficiency of the barrier compression. An optimized scheme using a novel “shock” compression is presented. The corresponding analytic model relies on the exact solution for shock waves in a cold beam. Results obtained from this model are compared with Vlasov simulations.

A 6.1-MV, 0.79-MA laser-triggered gas switch (LTGS) is used to synchronize the 36 modules of the $Z$ machine at Sandia National Laboratories. Each module includes one switch, which serves as the last command-fired switch of the module, and hence is used to determine the time at which each module electrically closes relative to the other modules. The switch is $\sim 81\mathrm{\text{−}}\mathrm{cm}$ in length, $\sim 45\mathrm{\text{−}}\mathrm{cm}$ in diameter, and is immersed in mineral oil. The outer switch envelope consists of six corrugated monomer-cast acrylic insulators and five contoured stainless-steel rings. The trigger electrodes are fabricated from copper-infused tungsten. The switch is pressurized with several atmospheres of sulfur hexafluoride (${\mathrm{SF}}_6$), which is turbulently purged within 2 seconds after every shot. Each switch is powered from a 6-MV, 0.78-MJ Marx generator which pulse charges a 24-nF intermediate-store water capacitor in $1.4\mathrm{\text{−}}\mu \mathrm{s}$. Closure of the switch allows power to flow into pulse-forming transmission lines. The power pulse is subsequently compressed by water switches, which results in a total accelerator output power in excess of 70-TW. A previous version of the LTGS performed exceptionally at a 5.4-MV, 0.7-MA level on an engineering test module used for switch development. It exhibited a $1\mathrm{\text{−}}\sigma$ jitter of $\sim 5\mathrm{ns}$, a prefire and flashover rate less than 0.1%, and a lifetime in excess of 150 shots. When installed on the $Z$ accelerator, however, the switch exhibited a prefire probability of $\sim 3\%$, a flashover probability of $\sim 7\%$, and a 15-ns jitter. The difference in performance is attributed to several factors such as higher total charge transfer, exposure to more debris, and more stressful dynamic mechanical loading upon machine discharge. Under these conditions, the replacement lifetime was less than ten shots. Since refurbishment of $Z$ in October 2007, there have been three LTGS design iterations to improve the performance at 6.1-MV. The most recent design exhibits a prefire rate of less than 0.1%, a flashover rate of $\sim 0.2\%$, a single switch jitter of $\sim 6\mathrm{\text{−}}\mathrm{ns}$, and a lifetime of greater than 75 shots. Modifications to achieve the performance improvement are detailed in this article.

The generation of quasimonoenergetic electron beams, with energies greater than 500 MeV, in a laser-plasma accelerator driven by 2.5 J, 80 fs laser pulses guided in a low density plasma channel, is investigated. The laser energy required to achieve electron injection is found to depend strongly on the quality of the input laser focal spot. Simulations show that, although the matched spot size of the plasma channel is greater than the self-focused spot size, the channel assists relativistic self-focusing and enables electron injection to occur at lower plasma densities and laser powers than would be possible without a waveguide.

The low beta of proton or ion beams favors an electrostatic pickup to measure the transverse beam centroid position. Often papers on beam position monitors (BPM) are focused on a particular aspect of the problem; however, it is important to consider all various issues of a position measurement system. Based on our experience at the IPHI (high intensity injector proton) facility at CEA-Saclay, this paper will address all aspects to design, test, and calibrate a BPM for proton linear accelerators, while emphasizing the determination of the absolute beam position. We present details of the readout electronics, and describe the calibration of the BPM using a test station. For calculation and simulation of the electrical signals we developed a Mathematica script. The error analysis presented, on the basis of six BPMs installed in the high energy section of IPHI, demonstrates the expected accuracy of the position measurement. These studies also identify the parameters that could improve the performance of the beam position control. The experience from these developments is currently being used for the BPM design and test stand dedicated to the Spiral2 accelerator at Ganil-Caen which will deliver heavy ion beams.