We recently developed a reliable hydrogenation procedure for the diamond amplifier that assures the generation of a high-current, high-brightness beam. In this paper, we compare room-temperature hydrogenation with that at high temperatures. We identified the factors leading to the decay of quantum efficiency. The optimum temperature for heat treatment ranged from $400–450°\mathrm{C}$; its superiority was proven in the gain test. Hydrogenated diamond amplifiers exposed to ${\mathrm{N}}_2$ and air exhibited a good emission after being heated to $350°\mathrm{C}$; the highest gain we registered in emission scanning was 178. Our systematic study of hydrogenation resulted in the reproducible fabrication of diamond amplifiers.

We developed a two-beam interdigital-H-type radio frequency quadrupole (IH-RFQ) linac as a prototype of a multibeam IH-RFQ for high intensity heavy ion acceleration in the low energy region. This linac has two sets of RFQ electrodes within an IH-type resonant frequency cavity that is a power-efficient structure for low energy beam acceleration. The linac can accelerate two beams in parallel in one cavity with a reduction in the coulomb repulsive force (the space charge effect) between the accelerated heavy ion particles. The resonance frequency and the $Q$ factor of the linac were found to be 47 MHz and 5900, respectively. We also developed a two-beam laser ion source with a direct plasma injection scheme as an injection system for the two-beam IH-RFQ linac and built a system to demonstrate the use of the two-beam IH-RFQ linac. Using this linac system, we were able to accelerate carbon ions from 5 to $60\mathrm{keV}/\mathrm{u}$ and generate an output beam current of about 108 mA ($2\times 54\mathrm{mA}/\mathrm{channel}$). A coherency between the two beams, derived from the imbalance of the beam loading, was observed in the acceleration test with carbon ions.

It is demonstrated in a numerical simulation that an intense fs keV x-ray pulse can be generated by the interaction of a tightly focused femtosecond laser copropagating with an electron bunch. In general, the interaction of a loosely focused (focal spot $\sim 100\mu \mathrm{m}$ in diameter) laser with a copropagating electron is rather weak so that the radiation is not only weak but also produced in the vicinity of laser wavelength. However, in the case of tight focus (focal spot on the order of wavelength), the radiation characteristics turn out to be drastically different so that a keV x-ray pulse can be produced at high flux. This is due to the nonparaxial fields induced in a tight-focus regime. The radiation characteristics are discussed for different beam waists and electron beam energies. This simulation suggests that the interaction of a tightly focused laser with a copropagating electron bunch can be a unique source for an x-ray pulse of the photon energy from 10 to 100 keV that lasts a few femtoseconds.

Switch for modulation of the $Q$ factor of a multimode rf energy storage resonator is described. The rf energy is stored in a cylindrical resonator in the ${\mathrm{TE}}_{02}$ mode, and switched out from it in the ${\mathrm{TE}}_{01}$ mode. The operating principle of the switch is based on effecting a fast increase in the ${\mathrm{TE}}_{02}\to {\mathrm{TE}}_{01}$ mode coupling coefficient when an electron beam is injected into the switch cavity. This paper focuses on use of the switch in an active rf pulse compressor. Use of the electron beam allows one to separate the switching element from the compressor itself, and therefore to eliminate limitations to the level of switched power that exists for other switches. The pulse compressor was tested at low power at a frequency of 11.424 GHz. The level of peak power gain achieved in these experiments was 19–21, with compressed pulse widths of 30–40 ns. Good stability and repeatability of the shape of compressed pulses was observed. High power versions of this switch appear promising for use in active GW-level microwave compressors.

The well-known Haissinski distribution provides a stable equilibrium of longitudinal beam distribution in electron storage rings below a threshold current. Yet, how to accurately determine this threshold, above which the Haissinski distribution becomes unstable, is not firmly established in theory. In this paper, we will show how to apply the Laguerre polynomials in an analysis of this stability that are associated with the potential-well distortion. Our approach provides an alternative to the discretization method proposed by Oide and Yokoya. Moreover, it reestablishes an essential connection to the theory of mode coupling originated by Sacherer. Our new and self-consistent method is applied to study the microwave instability driven by commonly known impedances, including coherent synchrotron radiation in free space.

Two dipole doglegs are widely used to translate the beam axis horizontally or vertically. Quadrupoles are placed between the two consecutive dipoles to match first-order dispersion and provide betatron focusing. Similarly, a four dipole chicane is usually employed to form a bypass region, where the beam axis is transversely shifted first, then translated back to the original axis. In order to generate an isochronous section, quadrupoles are again needed to tune the first-order transfer matrix element $R_{56}$ equaling zero. Usually sextupoles are needed to correct second-order dispersion in the bending plane, for both the dogleg optics and the chicane (with quad) optics. In this paper, an alternative optics design is introduced, which is based on a simple FODO cell and does not need sextupole assistance to form a second-order achromat. It may provide a similar function of either a dogleg or a bypass, by using two or four of such combined supercells.

The possibility of using electron-beam phase-space manipulations to support a free-electron laser accelerator design optimization has motivated our research. An ongoing program demonstrating the exchange of transverse horizontal and longitudinal emittances at the Fermilab A0 photoinjector has benefited recently from the upgrade of several of the key diagnostics stations. Accurate measurements of these properties upstream and downstream of the exchanger beam line are needed. Improvements in the screen resolution term and reduced impact of the optical system’s depth of focus by using YAG:Ce single crystals normal to the beam direction will be described. The requirement to measure small energy spreads ($<10\mathrm{keV}$) in the spectrometer and the exchange process which resulted in bunch lengths less than 800 fs led to other diagnostics performance adjustments and upgrades as well. Application of these diagnostics in demonstrating the exchange of transverse ($x$) and longitudinal phase spaces is also reported.

The harmonics field effect of a planar undulator on free-electron laser (FEL) harmonic generation has been analyzed. For both the linear case and the nonlinear case, the harmonic fraction of the radiation can be characterized by the coupling coefficients. The modification of the coupling coefficients is given when the third harmonics magnetic field component exists, thus the enhancement of the harmonic radiation can be predicted. The numerical results show that with the third harmonics magnetic field component that has the opposite sign to the fundamental, the intensity of third-harmonic radiation can be increased distinctly for both the small signal gain and the nonlinear harmonic generation. The increase is larger for the smaller undulator deflecting parameter.

The linac coherent light source (LCLS) is a self-amplified spontaneous emission (SASE) free-electron laser (FEL) operating at fundamental photon energies from 0.5 to 10 keV. Characterization of the higher harmonics present in the FEL beam is important to users, for whom harder x rays can either extend the useful operating wavelength range or increase experimental backgrounds. We present measurements of the power in both the second and third harmonics, and compare the results to expectations from simulations. We also present studies of the transport of harmonics to the users, and the harmonic power as a function of electron beam quality.

Design of a muon collider interaction region (IR) presents a number of challenges arising from low ${\beta }^{*}<1\mathrm{cm}$, correspondingly large beta-function values and beam sizes at IR magnets, as well as the necessity to protect superconducting magnets and collider detectors from muon decay products. As a consequence, the designs of the IR optics, magnets and machine-detector interface are strongly interlaced and iterative. A consistent solution for the 1.5 TeV center-of-mass muon collider IR is presented. It can provide an average luminosity of ${10}^{34}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ with an adequate protection of magnet and detector components.

At the laser undulator compact x-ray source at KEK, our group has designed and built a new rf gun with a larger mode separation of 8.6 MHz and higher $Q$ value than earlier guns. This paper presents details on the design and fabrication, low-power measurements, and tuning procedures followed in the fabrication of the gun cavity. We also give a detailed account of experiments performed with this gun and present measurement results. We have successfully generated 100 bunches per train with a 50 nC charge at 41 MeV and 300 bunches per train with a 160 nC charge at 5 MeV, with low peak-to-peak energy difference. In the future, we plan to go up to a 0.5 nC charge per bunch in an 8000-bunch train at low energy and a 2 nC charge per bunch in a 100-bunch train at high energy. This will make it possible to have higher charge available for laser-beam collisions, thus enabling the generation of high flux soft x rays using the low-energy electron beam and approximately 30 keV x rays using the high-energy electron beam.

A model is developed to study the eddy current induced in a thin conducting but nonmagnetic plate of finite size when exposed to a time varying magnetic field. The applied field may be uniform or vary in space. This model can accurately estimate the eddy current contour in the plate and loss due to eddy current. Power losses for plates of various dimensions and at different frequencies are calculated to establish the accuracy of the model. We have also calculated the magnetic field generated by the induced eddy current when the plate of finite size is placed between the two parallel poles of a dipole magnet made of magnetic material of very high permeability. The force acting on the plate due to the interaction of the induced eddy current and the applied external field is also calculated. The model can predict the time variation of force and eddy current. The model may be applicable to understand the effect of eddy current on the vacuum chamber of an accelerator. Various other applications, where this model is useful, are also reported. The results are compared against the results obtained by a simulation using a finite element based code. Here the rectangular plate is considered but the model can be applicable for other geometries as well.

For the acceleration of very high intensity protons in the 3 GeV rapid cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC), the beam loading compensation in the rf accelerating cavities is implemented. The magnetic-alloy (MA) loaded cavities, which have a wide-band frequency response, are employed in the J-PARC RCS. The wake voltage in the wide-band MA cavity consists of not only a component of the fundamental accelerating rf, but also the higher harmonics. We developed a multiharmonic feedforward compensation system, which handles the most important three harmonics ($h=2,4,6$). The function of the multiharmonic feedforward is described. Adjustments of the amplitude and phase patterns for the cancellation of the wake voltage during the acceleration period are not trivial. We developed the commissioning methodology of the feedforward. We describe the methodology in the cases without and with driving the accelerating rf. The commissioning of the systems for 11 cavities in the RCS was successful. We describe the commissioning results and the beneficial effects of the feedforward compensation in the beam operation.

In this work, we present a technique to measure the acceptance of a circular accelerator extending a previous method by the measurement of particle loss. Here, we apply an rf voltage to transversely excite a coasting heavy ion beam in the SIS-18 synchrotron at GSI. The resulting growth of the transverse beam width causes particle loss when the beam width exceeds the limiting aperture. The acceptance is determined from the measured time evolution of the beam current after particles start to hit the aperture. In doing so, the result will not depend on the initial width of the ion beam.

Design and some performance results of the pulsed digital low level radio frequency (LLRF) for the radio frequency quadrupole (RFQ) systems of Rutherford Appleton Laboratory–front end test stand and the future European Spallation Source Bilbao linac are presented. For rf field regulation, the design is based on direct rf-to-baseband conversion using an analog in-phase quadrature (IQ) demodulator, high-speed sampling of the I/Q components, baseband signal processing in a field-programmable gate array (FPGA), conversion to analog, and IQ modulation. This concept leads to a simple and versatile LLRF system which can be used for a large variety of rf frequencies and virtually any LLRF application including cw, ramping, and pulsed. In order to improve the accuracy of the probe voltage measurement, errors associated with the use of analog IQ demodulators have been identified and corrected by FPGA algorithms and proper setting of the feedback loop parameters. Furthermore, a baseband-equivalent model for the rf plant is developed in MATLAB-Simulink to study the RFQ transient response under beam loading in the presence of phase and delay errors. The effect of the unwanted resonant modes on the feedback loop stability and the LLRF considerations to avoid such instabilities are discussed and compared to some other machines such as the ILC and the European free electron laser . The practical results obtained from tests with a mock-up cavity and an RFQ cold model verify that amplitude and phase stabilities down to a fraction of one percent and one degree and phase margins larger than $\pm 50°$ can be achieved with this method preserving the linearity and bandwidth of the feedback loops.

Beam position monitors are an important diagnostics tool for particle accelerator operation and related beam dynamics research. The measurement of the beam deflection angle, or moving direction of a charged particle beam with respect to the beam pipe axis, can provide useful additional information. Beam monitors sensitive to the beam’s azimuthal $B$-dot field (sometimes referred as $B$ dots) are used to measure the displacement (position) of the beam centroid, as the beam generates a dipole term of the azimuthal magnetic field. Similarly, a dipole term of the axial magnetic field will be generated by the beam moving in a direction not parallel to the axis of the beam pipe. In this paper, a new method using the axial $B$-dot field is presented to measure the beam deflection angle directly, including the theoretical background. Simulations using the MAFIA numerical code have been performed, demonstrating a good agreement to the new established analytical model.

The charge-state distributions of ${}^{238}\mathrm{U}$ with energies of 14 and $15\mathrm{MeV}/\mathrm{nucleon}$ were measured using nitrogen (${\mathrm{N}}_2$) gas and carbon foil (C-foil) charge strippers. The most probable charge states of ${}^{238}\mathrm{U}$ with energies of 14 and $15\mathrm{MeV}/\mathrm{nucleon}$ in ${\mathrm{N}}_2$ gas attain equilibrium at 60.8 and 62.4, whereas those in C-foils attain equilibrium at 75.8 and 76.7, respectively. Novel empirical formulas for the accurate prediction of the equilibrium charge states of ${}^{238}\mathrm{U}$ with energies $10–20\mathrm{MeV}/\mathrm{nucleon}$ in gases and C-foil were derived by fitting the data in the energy range of $1–20\mathrm{MeV}/\mathrm{nucleon}$. The charge states predicted by using the formulas for gases and C-foil are in good agreement with the data in the energy range of $10–20\mathrm{MeV}/\mathrm{nucleon}$ within an average of 0.23 and 0.21, respectively.

We present the application of a short rise ($\sim 150\mathrm{ns}$) 250 kA linear transformer driver (LTD) to wire array $z$-pinch loads for the first time. The generator is a modification of a previous driver in which a new conical power feed provides a low inductance coupling to wire loads. Performance of the new design using both short circuit and plasma loads is presented and discussed. The final design delivers $\sim 200\mathrm{kA}$ to a wire array load which is in good agreement with SCREAMER calculations using a simplified representative circuit. Example results demonstrate successful experiments using cylindrical, conical, and inverse wire arrays as well as previously published work on $x$-pinch loads.