One of the major criteria for designing superconducting niobium resonant cavities is to minimize the peak surface electric and magnetic fields to maximize the achievable accelerating electric gradient. Even after addressing the extrinsic effects adequately, a large number of cavities perform below the theoretical gradient limit. The peak magnetic field for the first flux-line penetration in the superconducting state of niobium, which either severely degrades the cavity quality factor or results in complete thermal breakdown, is an important limitation. The flux-line penetration is known to depend on the microstructural properties of niobium which may get altered in the process of cavity fabrication. The most common technique of fabricating niobium cavities is to form their components using standard sheet metal techniques and join them by electron beam welding in vacuum. We present results of a study on the superconducting response through magnetization measurements in the electron beam welded region of niobium to understand the limitations (if any) posed by the welding in achieving the highest gradient. We also present and discuss results on the performance of niobium quarter wave resonators incorporating such electron beam welds in the high magnetic field region.

During 2008 and 2009 dedicated beam experiments with crab cavities were performed in the KEKB. The goal was to measure the impact of crab-cavity radio frequency (rf) noise on the beam quality. These experiments were performed as a validation of the crab-cavity beam dynamics models in view of the possible use of crab cavities in the upgrade of the CERN Large Hadron Collider (LHC). An unexpected strong beam-beam instability was observed during the course of the experiments as a kind of frequency response. Understanding this finding required extensive multiparticle and single particle simulations plus an extra experimental session to consolidate the observations.

A method is presented that enables one to compute the parameters of matched beams with space charge in cyclotrons with emphasis on the effect of the transverse-longitudinal coupling. Equations describing the transverse-longitudinal coupling and corresponding tune shifts in first order are derived for the model of an azimuthally symmetric cyclotron. The eigenellipsoid of the beam is calculated and the transfer matrix is transformed into block-diagonal form. The influence of the slope of the phase curve on the transverse-longitudinal coupling is accounted for. The results are generalized and numerical procedures for the case of an azimuthally varying field cyclotron are presented. The algorithm is applied to the PSI injector II and ring cyclotron and the results are compared to TRANSPORT.

A hydrogen-terminated diamond is a proven efficient electron emitter that can support emission of high average current. Several factors dictate the amplifier’s gain: the number of secondary electrons created at their point of entry into the diamond, the fraction of created electrons transmitted to the emitting face, and the fraction of transmitted electrons emitted. In this paper, we present a model detailing the impact of charge trapping at the surface on the instantaneous electric field inside the diamond, and its effect on the transmission gain. The ratio of instantaneous emitted electrons to the transmitted electrons depends on the electron’s energy distribution and the surface barrier. We calculated the latter by evaluating the magnitude of the negative-electron affinity that is modified by the Schottky effect due to the presence of the external applied field. The instantaneous values then were time integrated to yield the time-averaged ratio of the number of emitted electrons to the transmitted ones. The findings from the model agree very well with our experimental measurements. As an application of the model, we estimate the energy spread of the electrons inside the diamond from the measured secondary-electron emission.

Minimizing the electron-beam emittance of photoinjectors is an important task for maximizing the brightness of the next-generation x-ray facilities, such as free-electron lasers and energy recovery linacs. Optimally shaped laser pulses can significantly reduce emittance. A reliable diagnostic for the laser pulse intensity is required for this purpose. We demonstrate measurement of three-dimensional spatiotemporal intensity profiles, with spatial resolution of $20\mu \mathrm{m}$ and temporal resolution of 130 fs. The capability is illustrated by measurements of stacked soliton pulses and pulses from a dissipative-soliton laser.

The mechanism of steady-state microbunching (SSMB) has been proposed [D. F. Ratner and A. W. Chao, Phys. Rev. Lett. 105, 154801 (2010)] to produce steady-state microbunched beams by using laser modulations in a storage ring for generating coherent radiation with high repetition rate at wavelengths from the submillimeter to extreme ultraviolet range. In the present paper, we analyze the dynamics of the SSMB system with a Hamiltonian and Jacobi matrix approach and identify the original proposal of SSMB as a mechanism with period-1 fixed point in phase space. We then propose an alternative SSMB mechanism with period-2 fixed point, which is able to produce microbunched beams with shorter bunch length and, hence, higher harmonic. Taking the SPEAR3 storage ring as an example, we illustrate the application of the period-2 SSMB to generate terahertz (THz) steady-state coherent radiation in a storage ring using an $X$-band radio-frequency (rf) system instead of a more technically demanding laser system. Issues covered include choice of rf parameters, system errors, beam lifetime, collective effects, and radiation power evaluation. Compared to the more traditional low-momentum-compaction operation mode, the proposed SSMB scheme potentially promises higher beam current, larger bunching factor, and hence brightness increase of at least 1 order of magnitude.

We propose and demonstrate an all-optical method to measure laser energy modulation in a relativistic electron beam. In this scheme the time-dependent energy modulation generated from the electron-laser interaction in an undulator is converted into time-dependent density modulation with a chicane, which is measured to infer the laser energy modulation. The method, in principle, is capable of simultaneously providing information on femtosecond time scale and ${10}^{-5}$ energy scale not accessible with conventional methods. We anticipate that this method may have wide applications in many laser-based advanced beam manipulation techniques.

The Courant-Snyder theory for two-dimensional coupled linear optics is presented, based on the systematic use of the real representation of the Dirac matrices. Since any real $4\times 4$ matrix can be expressed as a linear combination of these matrices, the presented ansatz allows for a comprehensive and complete treatment of two-dimensional linear coupling. A survey of symplectic transformations in two dimensions is presented. A subset of these transformations is shown to be identical to rotations and Lorentz boosts in Minkowski space-time. The transformation properties of the classical state vector are formulated and found to be analog to those of a Dirac spinor. The equations of motion for a relativistic charged particle—the Lorentz force equations—are shown to be isomorph to envelope equations of two-dimensional linear coupled optics. A universal and straightforward method to decouple two-dimensional harmonic oscillators with constant coefficients by symplectic transformations is presented, which is based on this isomorphism. The method yields the eigenvalues (i.e., tunes) and eigenvectors and can be applied to a one-turn transfer matrix or directly to the coefficient matrix of the linear differential equation.

We demonstrate magnetic resonance imaging (MRI) visualization of a 6 MeV electron beam in ferrous-doped water; a 25 mm penetration depth was measured. Time domain nuclear magnetic resonance was used to investigate the effect of generated free radicals on the free induction decay (FID) in nondoped water; no apparent effects to the FID were observed. We show that MRI visualization of charged particle beams used in medical applications will require exogenous agents to provide contrast enhancement.

We present the first experimental studies of the initial source of electron beam microbunching instability in a free electron laser (FEL) injector. By utilizing for the studies a transform-limited laser pulse at the photocathode, we eliminated laser-induced microbunching at the National Synchrotron Light Source Source Development Laboratory (SDL). The detailed measurements of the resulting electron beam led us to conclude that, at SDL, microbunching arising from shot noise is not amplified to any significant level, thereby allowing us to set an upper limit on the initial modulation depth of microbunching arising from shot noise. Our analysis demonstrated that the only significant source of microbunching instability under normal operational conditions at SDL is the longitudinal modulation of the photocathode laser pulse. Our work shows that assuring a longitudinally smoothed photocathode laser pulse allows mitigating microbunching instability at a typical FEL injector with a moderate microbunching gain.

Alternative chicane-type beam lines are proposed for exact emittance exchange between transverse phase space $(x,x^{\prime })$ and longitudinal phase space $(z,\delta )$, where $x$ is the transverse position, $x^{\prime }$ is the transverse divergence, and $z$ and $\delta$ are relative longitudinal position and energy deviation with respect to the reference particle. Methods to achieve exact phase space exchanges, i.e., mapping $x$ to $z$, $x^{\prime }$ to $\delta$, $z$ to $x$, and $\delta$ to $x^{\prime }$, are suggested. Schemes to mitigate and completely compensate for the thick-lens effect of the transverse cavity on emittance exchange are studied. Some applications of the phase space exchange for advanced beam manipulation and diagnostics are discussed.

The crossbar $H$-mode (CH) cavity is an accelerating structure operated in the $H_{21(0)}$ mode. The robustness of the crossbar geometry allows one to realize room temperature as well as superconducting linac cavities. The shunt impedance characteristics of this structure are attractive to develop proton and heavy ion linacs in the low and medium beta range. A first room temperature eight-cell prototype has proven the feasibility of the crossbar design in terms of mechanical construction, copper plating, and cooling. An innovative rf coupling concept has been developed where two CH cavities are connected by a two gap $E_{010}$-mode resonator which, at the same time, provides transverse focusing by a quadrupole triplet. The concept has been applied in the design of the new FAIR proton linac and a scaled model of the second cavity of this injector has been built and tested too. The full scale prototype is now under construction at the University of Frankfurt. In this paper, the room temperature CH cavity development as well as the general layout of the FAIR proton injector (70 MeV, 325 MHz, 70 mA) is presented and discussed.

A study of target parameters for a mercury jet target for a neutrino factory or muon collider is presented. We simulate particle production initiated by incoming protons with kinetic energies between 2 and 100 GeV. For each proton beam kinetic energy, we maximize production by varying the geometric parameters of the target: the mercury jet radius, the incoming proton beam angle, and the crossing angle between the mercury jet and the proton beam. With an 8-GeV proton beam, we study the variation of meson production with the entry direction of the proton beam relative to the jet. We also examine the influence on the meson production by the focusing of the proton beam. The number of muons surviving through the neutrino factory front end channel is determined as a function of the proton beam kinetic energy.

A new model to compensate for the transient beam loading in the CLIC main linac is developed. It takes into account the CLIC specific power generation scheme and the exact 3D geometry of the accelerating structure including couplers. A new method of calculating unloaded and loaded voltages during the transient is proposed and a dedicated optimization scheme of the rf pulse to compensate the transient beam-loading effect is presented. It is demonstrated that the 0.03% limit on the rms relative bunch-to-bunch energy spread in the main beam after acceleration can be reached. The optimization technique has been used to increase the rf to beam efficiency while preserving the CLIC requirements and to compensate for the energy spread caused by the Balakin-Novokhatski-Smirnov damping and transient process in the subharmonic buncher. Effects of charge jitters in the drive and main beams are studied. It is shown that within the 0.1% CLIC specification limit on the rms spread in beams charge the energy spread is not significantly affected.

In recent years, the use of diffraction radiation (DR), emitted when a charged particle beam passes through a rectangular slit, has been proposed and successfully tested as a nonintercepting diagnostic of high brightness beams. However, some problems related to the control of the particle trajectory through the slit still remain. If an additional slit is placed in front of the first one, at a distance shorter than the radiation formation length, interference between the forward diffraction radiation from the upstream slit and the backward diffraction radiation from the downstream slit can be observed. In this paper we report the first experimental observation of this effect, which we call here optical diffraction radiation interference (ODRI). If the two slits have different dimensions and are not aligned on the same axis, the properties of the ODRI pattern can be effectively used for nonintercepting beam diagnostics, especially for the unambiguously determination of the beam size. Indeed, the advantage of ODRI compared with a single aperture DR screen is due to the reduction of synchrotron radiation background, the increase of sensitivity for transverse beam dimensions, and the possibility to separate effects caused by the beam size and by beam offset within the slit.

Cylindrical asymmetry of an rf gun cavity excites the multipole rf field which will result in emittance growth of electron beams. With demanding of electron beams of ultralow emittance for the x-ray free electron laser, it is necessary to study the effect of the multipole rf field to the emittance growth and to eliminate the multipole field effect on electron beam emittance in an rf gun. In this paper, we present both theoretical analysis and numerical simulations of the multipole field effect to the emittance growth. We proposed a four-hole scheme to minimize the emittance growth from the multipole field.

Obtaining basic knowledge of breakdowns is desirable in many fields of physics; however, often the energy absorbed by a breakdown is not known or can vary by large amounts. We have therefore investigated how processing and breakdown properties scale with available energy for two materials, Cu and Mo, in the energy range of about 1 mJ to 1 J. A central result obtained is that there appears to be an optimum energy for processing and thus the highest possible electric field a processed material can sustain without breaking down depends on energy accordingly; what this implies for radio frequency cavity processing is discussed as well. For Cu, both the local field and the field enhancement factor showed only a weak dependence on energy; the average of the local field over the entire energy range investigated was $(9.6\pm 0.4)\mathrm{GV}/\mathrm{m}$ at breakdown. For Mo, the local field increased with increasing energy, while the field enhancement factor remained constant at $34\pm 2$. Finally, a possible explanation of the direct current (DC) processing mechanism—at least in the case of Mo—as an oxide removal process is presented.

We are commissioning a 2.5 GeV synchrotron radiation source (SRS) where electrons travel in high vacuum inside the vacuum chambers made of aluminum alloys. These chambers are kept between the pole gaps of magnets and are made to facilitate the radiation coming out of the storage ring to the experimental station. These chambers are connected by metallic bellows. During the commissioning phase of the SRS, the metallic bellows became ruptured due to the frequent tripping of the dipole magnet power supply. The machine was down for quite some time. In the case of a power supply trip, the current in the magnets decays exponentially. It was observed experimentally that the fast $B$ field decay generates a large eddy current in the chambers and consequently the chambers are subjected to a huge Lorentz force. This motivated us to develop a theoretical model to study the force acting on a metallic plate when exposed to an exponentially decaying field and then to extend it for a rectangular vacuum chamber. The problem is formulated using Maxwell’s equations and converted to the inhomogeneous Helmholtz equation. After taking the Laplace transform, the equation is solved with appropriate boundary conditions. Final results are obtained after taking the appropriate inverse Laplace transform. The expressions for eddy current contour and magnetic field produced by the eddy current are also derived. Variations of the force on chambers of different wall thickness due to spatially varying and exponentially time decaying field are presented. The result is a general theory which can be applied to different geometries and calculation of power loss as well. Comparisons are made with results obtained by simulation using a finite element based code, for quick verification of the theoretical model.