A free electron laser (FEL) facility utilizing a recirculated superconducting radio frequency (SRF) electron linear accelerator (linac) provides the opportunity to achieve about 5 times greater photon energy than an unrecirculated linac facility of similar cost. An electron linac configuration utilizing a 4 GeV unrecirculated, SRF linac could be used to drive a FEL producing 5 keV photons. However, for a similar cost, a recirculated SRF linac system can deliver the 4 GeV electrons for photon energies of 5 keV and provide an upgrade path to photon energies of 25 keV. Further support amounting to about a third of the initial investment would provide additional recirculated SRF linac and cryogenic capacity sufficient to deliver electron energies appropriate for 25 keV photons matching the European XFEL.

We describe the experimental observation of highly nonlinear energy striations generated by two lasers in a relativistic electron beam in an echo-enabled harmonic generation (EEHG) setup. The rich energy banding structure results from strong dispersion of the sinusoidally modulated beam, and measurements of the banding spectrum enable benchmarking, optimization, and characterization of the concomitant EEHG process. Results are found to be in good agreement with theory, and suggest that the presented technique can facilitate the practical implementation of EEHG to generate intense, fully coherent light in future advanced accelerator-based light sources.

A fast-cycling induction synchrotron was demonstrated. Ions with extremely low energies and mass-to-charge ratios ($A/Q$) in the range from 2 to 10 were injected, captured by barrier voltages, and accelerated to the end of the acceleration cycle of 50 ms by flat pulse voltages generated by pulse transformers referred to as induction cells. Induction acceleration in a wide dynamic frequency range of 56 kHz to 1 MHz was also demonstrated. This accelerator is expected as the next generation of a heavy ion driver for cancer therapy, where a large scale injector is not required. A wide variety of ions for ion energy implantation experiments needing novel materials will be delivered from this compact circular accelerator.

The field flatness of any radio frequency quadrupole (RFQ) is an important parameter that needs to be carefully tuned because it can affect beam transmission efficiency. In four-rod RFQs, the heights of a set of tuning plates determine the quality of the field flatness. The goals of this paper are (a) to show that by using a lumped circuit model of a four-rod RFQ, the field flatness profile for any tuning plate height distribution can be quickly calculated, (b) to derive a perturbative solution of the model so that insights into the physics of the tuning process and its effects can be understood, and (c) to compare the predicted field profiles to measurements.

Farey sequences were discovered in the beginning of the nineteenth century proving very useful in number theory. These series are found to be tightly related to tune resonance diagrams in accelerators. So far, no connection has been made between these two fields. This paper aims to serve as a bridge between these two areas of knowledge introducing the concept of resonance sequences for the most efficient exploration of the tune space.

Harmonic generation becomes increasingly important as x-ray free-electron lasers push to shorter wavelengths. Recent studies have pointed to the possibility of enhancing harmonic generation by detuning the fundamental. In x-ray free-electron lasers, the wiggler line is composed of multiple wiggler segments with magnetic quadrupoles in the gaps to provide for increased focusing. In this paper, we study the effect on harmonic generation in simulation by (1) varying the gap lengths between the wiggler segments and (2) varying the electron beam $\beta$ function. In studying the harmonic we find enhanced harmonic generation is periodic in the gap length and peaks are found as the wiggler separation varies by $\lambda /3$ (where $\lambda$ is the fundamental wavelength), which corresponds to a phase shift of $2\pi /3$. As a consequence, enhanced harmonic generation is found both when the fundamental emission is strong by the nonlinear harmonic generation mechanism and by linear harmonic generation when the fundamental is detuned.

In storage rings, when the number of electrons in a bunch exceeds a threshold value, microstructures appear spontaneously in the bunch. Experimental observations of these microstructures and their evolution are indirect to obtain. In practice, most information is obtained from the terahertz coherent synchrotron radiation (THz CSR), since microstructures in the millimeter range radiate THz CSR. Here we show, thanks to a simple analytical formula associated to numerical simulations, that a frequency component in the THz CSR signal permits one to obtain information about the structure of the electron bunch longitudinal phase space during the microbunching instability. The proposed formula permits one also to link experimental information together, in particular the bunch length and the THz CSR spectrum.

An x-ray diffraction reciprocal spacing mapping (RSM) technique was applied to investigate crystal quality of Nb films and bulk materials, relevant to superconducting radio frequency applications. The RSM study used different x-ray penetration depths to study material processes after two different surface treatments: mechanical polishing or energetic condensation (a plasma process). The study revealed the material’s microstructure evolution for the two different processes. The RSM plots revealed crystal quality of the materials at different thickness. The novel, differential-depth RSM technique presented in this study substantiated the following facts: (1) For a heteroepitaxial Nb film $\mathrm{Nb}(100)/\mathrm{MgO}(100)$, a film thickening process, via a cathodic arc-discharge Nb ion deposition (energetic condensation), created a near-perfect single crystal Nb on the surface top layer. (2) For a fine mechanically polished single-crystal bulk Nb material, the microstructure on the top surface layer is more disordered than that at a greater depth.

Neutron capture cross sections of unstable isotopes are important for neutron induced nucleosynthesis as well as for technological applications. A combination of a radioactive beam facility, an ion storage ring and a high flux reactor would allow a direct measurement of neutron induced reactions over a wide energy range on isotopes with half lives down to minutes.

High-quality, stable electron beams are produced from self-injected laser wakefield acceleration using the interaction of moderate 3 TW, 45 fs duration Ti:sapphire laser pulses with high density ($>5\times {10}^{19}{\mathrm{cm}}^{-3}$) helium gas jet plasma. The electron beam has virtually background-free quasimonoenergetic distribution with energy ${35.6}_{-2.5}^{+3.9}\mathrm{MeV}$, charge ${3.8}_{-1.2}^{+2.8}\mathrm{pC}$, divergence and pointing variation $\sim 10\mathrm{mrad}$. The stable and high quality of the electron beam opens an easy way for applications of the laser wakefield accelerator in the future, particularly due to the widespread availability of sub-10 TW class lasers with a number of laser plasma laboratories around the world.

Quasiequilibrium power flow in two radial magnetically insulated transmission lines (MITLs) coupled to a vacuum post-hole convolute is studied at $50\mathrm{TW}–200\mathrm{TW}$ using three-dimensional particle-in-cell simulations. The key physical dimensions in the model are based on the ZR accelerator [D. H. McDaniel, et al., Proceedings of 5th International Conference on Dense Z-Pinches, edited by J. Davis (AIP, New York, 2002), p. 23]. The voltages assumed for this study result in electron emission from all cathode surfaces. Electrons emitted from the MITL cathodes upstream of the convolute cause a portion of the MITL current to be carried by an electron sheath. Under the simplifying assumptions made by the simulations, it is found that the transition from the two MITLs to the convolute results in the loss of most of the sheath current to anode structures. The loss is quantified as a function of radius and correlated with Poynting vector stream lines which would be followed by individual electrons. For a fixed MITL-convolute geometry, the current loss, defined to be the difference between the total (i.e. anode) current in the system upstream of the convolute and the current delivered to the load, increases with both operating voltage and load impedance. It is also found that in the absence of ion emission, the convolute is efficient when the load impedance is much less than the impedance of the two parallel MITLs. The effects of space-charge-limited (SCL) ion emission from anode surfaces are considered for several specific cases. Ion emission from anode surfaces in the convolute is found to increase the current loss by a factor of 2–3. When SCL ion emission is allowed from anode surfaces in the MITLs upstream of the convolute, substantially higher current losses are obtained. Note that the results reported here are valid given the spatial resolution used for the simulations.

The 3-GeV rapid cycling synchrotron (RCS) of Japan Proton Accelerator Research Complex (J-PARC) simultaneously delivers high intensity beam to the Material and Life Science Experimental Facility (MLF) as well as to the main ring (MR) at a repetition rate of 25 Hz. The RCS is designed for a beam power of 1 MW. RCS has to meet not only the need of power upgrade but also the specific requirement of each downstream facility. One of the issues, especially for high intensity operation, is to maintain two different transverse sizes of the extracted beam for MLF and MR; namely, a wider beam for MLF in order to reduce damage on the neutron production target but reversely a narrower one for the MR in order to ensure a permissible beam loss in the beam transport line of 3-GeV to MR and also in the MR. We proposed pulse-to-pulse direct control of the transverse painting area during the RCS beam injection process in order to get an extracted beam profile as desired. In addition to two existing dc septum magnets used for fixing injected beam trajectory for MLF beam, two additional dipoles named pulse steering magnets are designed for that purpose in order to control injected beam trajectory for a smaller painting area for the MR. The magnets are already installed in the injection beam transport line and successfully commissioned well in advance before they will be put in normal operation in 2014 for the 400 MeV injected beam energy upgraded from that of the present 181 MeV. Their parameters are found to be consistent to those expected in the corresponding numerical simulations. A trial one cycle user operation run for a painting area of $100\pi \mathrm{mm}\mathrm{mrad}$ for the MR switching from the MLF painting area of $150\pi \mathrm{mm}\mathrm{mrad}$ has also been successfully carried out. The extracted beam profile for the MR is measured to be sufficiently narrower as compared to that for the MLF, consistent with numerical simulation successfully demonstrating validity of the present principle.

We have developed a new full 3D approach for the electromagnetic and beam dynamics design and simulation of a radio-frequency quadrupole (RFQ). A detailed full 3D model including vane modulation was simulated, which was made possible by the ever advancing computing capabilities. The electromagnetic (EM) design approach was first validated using experimental measurements on an existing prototype RFQ and more recently on the actual full size RFQ. Two design options have been studied, the original with standard sinusoidal modulation over the full length of the RFQ; in the second design, a trapezoidal modulation was used in the accelerating section of the RFQ to achieve a higher energy gain for the same power and length. A detailed comparison of both options is presented supporting our decision to select the trapezoidal design. The trapezoidal modulation increased the shunt impedance of the RFQ by 34%, the output energy by 15% with a similar increase in the peak surface electric field, but practically no change in the dynamics of the accelerated beam. The beam dynamics simulations were performed using three different field methods. The first uses the standard eight-term potential to derive the fields, the second uses 3D fields from individual cell-by-cell models, and the third uses the 3D fields for the whole RFQ as a single cavity. A detailed comparison of the results from TRACK shows a very good agreement, validating the 3D fields approach used for the beam dynamics studies. The EM simulations were mainly performed using the CST Microwave-Studio with the final results verified using other software. Detailed segment-by-segment and full RFQ frequency calculations were performed and compared to the measured data. The maximum frequency deviation is about 100 kHz. The frequencies of higher-order modes have also been calculated and finally the modulation and tuners effects on both the frequency and field flatness have been studied. We believe that with this new full 3D approach, the enhanced computing capabilities and the calculation precision the electromagnetic design software offer, we may be able to skip the prototyping phase and build the final product at once, although we recognize that prototyping is still needed to establish and validate the fabrication procedure.

Photocathodes are a limiting factor for the next generation of ultrahigh brightness photoinjectors. We studied the behavior of a superconducting Pb cathode in the cryogenic environment of a superconducting rf gun cavity to measure the quantum efficiency, its spatial distribution, and the work function. We will also discuss how the cathode surface contaminants modify the performance of the photocathode as well as the gun cavity and we discuss the possibilities to remove these contaminants.

In the coherent electron cooling, the modern hadron beam cooling technique, each hadron receives an individual kick from the electric field of the amplified electron density perturbation created in the modulator by this hadron in a copropagating electron beam. We developed a method for computing the dynamics of these density perturbations in an infinite electron plasma with any equilibrium velocity distribution—a possible model for the modulator. We derived analytical expressions for the dynamics of the density perturbations in the Fourier-Laplace domain for a variety of 1D, 2D, and 3D equilibrium distributions of the electron beam. To obtain the space-time dynamics, we employed the fast Fourier transform algorithm. We also found an analytical solution in the space-time domain for the 1D Cauchy equilibrium distribution, which serves as a benchmark for our general approach based on numerical evaluation of the integral transforms and as a fast alternative to the numerical computations. We tested the method for various distributions and initial conditions.