Demands of high intensity beams are increasing for material science and engineering applications. To control the high intensity beam dynamics, the nonlinear space charge forces, produced by the beam and the residual plasma at low energy, have to be carefully studied. Interactions between the beam and the residual gas tend to neutralize the beam space charge. The temporal and spatial evolutions of these interactions complicate the beam dynamics, even into a drift section. A 1.5D ($xy$ for motion, $r$ for space charge) PIC code, CARTAGO, has been written in order to simulate the beam dynamics in a low energy beam transport line including the space charge compensation effects. This paper relates the structure and the numerical methods used in the code and shows studies of the space charge compensation regime for ${\mathrm{H}}^{+}$ and ${\mathrm{H}}^{-}$ beams in a drift section.

We study the transverse geometric impedance of elliptical cross-section tapers in the low-frequency “inductive regime.” We have followed a dual approach: computer simulations have been carried out using the finite element electromagnetic code GDFIDL and analytic results for the dipolar and quadrupolar components of the impedance have been derived extending a perturbation technique introduced by Stupakov. Our work provides new insight into the behavior of the impedance of axially asymmetric tapered structures at low frequency. In particular, we clarify the frequency range characterizing the inductive regime, suggesting new criteria relating the extent of the inductive regime for dipolar and quadrupolar components of the impedance to the dimensions of the minimal cross section of a tapered transition.

Optics correction is an important issue in the KEKB B-factory. Especially, the correction of beta functions is performed so as to reach near half-integer resonance of the horizontal betatron tune as closely as possible. Consequently, the luminosity can be improved during experiments of the B-meson in $e^{+}{e}^{-}$ collisions. Therefore, we have developed a correction method of the beta function not only at the designed beam energy, but also at an energy deviated from the nominal value. We present the procedures used to measure the behavior of the beta function, which depends on the momentum deviation, and the results compared with the model optics in the KEKB rings.

A $1\AA$ x-ray free electron laser can be operated at 6 GeV provided a very high brightness electron beam. Therefore, Paul Scherrer Institute has initiated development of the low-emittance gun based on field emission. A field-emitter array (FEA) is expected to deliver high peak current while reducing the source emittance to levels close to the thermal limit. A 100 keV gun test stand has been designed in order to gain experience with electron bunches emitted by pulsed FEAs, investigate emittance compensation, and develop diagnostic procedures to characterize the beam. First FEA samples have been installed and pulsed in the gun; the emitted bunches have been accelerated and characterized. We present results acquired with the first FEA samples.

The charged particles nonlinear dynamics in the field of a strong electromagnetic wave pulse of finite duration and certain form of the envelope, in the refractive medium with a constant and variable refraction indexes, is investigated by means of numerical integration of the classical relativistic equations of motion. The particle energy dependence on the pulse intensity manifests the nonlinear threshold phenomenon of a particle reflection and capture by actual laser pulses in dielectric-gaseous media that takes place for a plane electromagnetic wave in the induced Cherenkov process. Laser acceleration of the particles in the result of the reflection from the pulse envelope and in the capture regime with the variable refraction index along the pulse propagation direction is investigated.

A new tungsten single-crystal target has been successfully employed at the positron source of the KEKB injector linac. The crystal thickness was determined to be 10.5 mm based on previous systematic measurements of the positron-production efficiency. The crystal axis, $⟨111⟩$, was precisely aligned to the direction of the 4-GeV primary electron beam. The positron yield increased by $\sim 25\%$ compared to that for a conventional tungsten plate with a thickness of 14 mm. On the contrary, the steady-state heat load on the crystal target decreased by $\sim 20\%$. After a two-month operation, no degradation of the positron-production efficiency was observed, and the crystal target has been stably operating at the KEK B factory.

In a companion report, we have derived a method for finding the impedance at high frequencies of vacuum chamber transitions that are short compared to the catch-up distance, in a frequency regime that—in analogy to geometric optics for light—we call the optical regime. In this report we apply the method to various nonaxisymmetric geometries such as irises/short collimators in a beam pipe, step-in transitions, step-out transitions, and more complicated transitions of practical importance. Most of our results are analytical, with a few given in terms of a simple one-dimensional integral. Our results are compared to wakefield simulations with the time-domain, finite-difference program ECHO, and excellent agreement is found.

Recent measurements of resonance terms suggested the extension of the existing technique to measure magnet strengths from turn-by-turn beam position monitor data. This article describes the algorithm to infer the magnet strength from the variation along the ring of the resonance terms and reports on the first measurement. Both the algorithm and the software written for the analysis of the data can be particularly useful in the commissioning period of an accelerator in order to find magnets with wrong strengths or polarities as well as magnets with large magnetic errors.

Stored beams of polarized protons, electrons, or deuterons can be spin flipped by sweeping an rf dipole’s or solenoid’s frequency through an rf spin resonance. Fitting such data to the modified Froissart-Stora equation’s spin resonance strength ${\mathcal{E}}_{\mathrm{FS}}$ gave very large deviations from the ${}^{*}\mathcal{E}_{Bdl}$ obtained from each rf magnet’s $\int B_{rms}dl$. We recently varied an rf dipole’s frequency sweep range $\Delta f$, and the momentum spread $\Delta p/p$ and betatron tune ${\nu }_y$ of stored $1.85\mathrm{GeV}/c$ polarized deuterons. We found a sharp constructive interference when ${\nu }_y$ was near an intrinsic spin resonance. Moreover, over large $\Delta f$ and $\Delta p/p$ ranges, ${\mathcal{E}}_{\mathrm{FS}}$ was about 7 times smaller than the predicted ${}^{*}\mathcal{E}_{Bdl}$.

The most challenging issue for understanding the performance of superconducting radio-frequency (rf) cavities made of high-purity (residual resistivity ratio $>200$) niobium is due to a sharp degradation (“$Q$-drop”) of the cavity quality factor $Q_0(B_p)$ as the peak surface magnetic field ($B_p$) exceeds about 90 mT, in the absence of field emission. In addition, a low-temperature ($100–140°\mathrm{C}$) in situ baking of the cavity was found to be beneficial in reducing the $Q$-drop. In this contribution, we present the results from a series of rf tests at 1.7 and 2.0 K on a single-cell cavity made of high-purity large (with area of the order of few ${\mathrm{cm}}^2$) grain niobium which underwent various oxidation processes, after initial buffered chemical polishing, such as anodization, baking in pure oxygen atmosphere, and baking in air up to $180°\mathrm{C}$, with the objective of clearly identifying the role of oxygen and the oxide layer on the $Q$-drop. During each rf test a temperature mapping system allows measuring the local temperature rise of the cavity outer surface due to rf losses, which gives information about the losses location, their field dependence, and space distribution. The results confirmed that the depth affected by baking is about 20–30 nm from the surface and showed that the $Q$-drop did not reappear in a previously baked cavity by further baking at $120°\mathrm{C}$ in pure oxygen atmosphere or in air up to $180°\mathrm{C}$. These treatments increased the oxide thickness and oxygen concentration, measured on niobium samples which were processed with the cavity and were analyzed with transmission electron microscope and secondary ion mass spectroscopy. Nevertheless, the performance of the cavity after air baking at $180°\mathrm{C}$ degraded significantly and the temperature maps showed high losses, uniformly distributed on the surface, which could be completely recovered only by a postpurification treatment at $1250°\mathrm{C}$. A statistic of the position of the “hot spots” on the cavity surface showed that grain boundaries are not the preferred location. An interesting correlation was found between the $Q$-drop onset, the quench field, and the low-field energy gap, which supports the hypothesis of thermomagnetic instability governing the $Q$-drop and the baking effect.

The influence of linear betatron coupling due to constant-in-time skew quadrupolar fields on the transverse emittances is discussed using both a simplified model of a smooth circular accelerator and a more realistic strong-focusing lattice with localized sources of coupling (thin lens). New formulas for the coupled transverse emittances are derived that include the initial emittances, the coupling strengths, and the tune distance from the resonance. By using the more powerful Lie algebra and the resonance driving terms formalism, equivalent formulas are derived that provide a better understanding of some counterintuitive effects, otherwise not understandable in the smooth approximation. The new formulas have been tested both numerically and experimentally by using data of the CERN Proton Synchrotron showing a remarkable agreement.

The transverse Vlasov equilibrium distribution function of an unbunched ion beam propagating in a continuous-focusing channel is specified by a function $f_{\perp }(H_{\perp })$, where $H_{\perp }$ is the single-particle Hamiltonian. In standard treatments of continuous-focusing equilibria in Vlasov-Poisson electrostatic models, it is assumed that a stable beam equilibrium specified by monotonic $f_{\perp }(H_{\perp })$ with $\partial f_{\perp }(H_{\perp })/\partial H_{\perp }\le 0$ is axisymmetric (no variation in azimuthal angle, i.e., with $\partial /\partial \theta =0$). In this paper a simple, but rigorous, proof is presented that only axisymmetric equilibrium solutions are possible in Vlasov-Poisson models for any physical choice of $f_{\perp }(H_{\perp })$ with $\partial f_{\perp }(H_{\perp })/\partial H_{\perp }\le 0$ if the confining boundary of the system (the beam pipe) is axisymmetric or if the geometry is radially unbounded.

Rotary and displacement tuners are described for multistub superconducting rf resonators. The effectiveness of these tuners is made possible because the resonators have low currents between their outer conductors and tuner elements. Computer simulations and experimental data show that the devices provide a tuning range up to 100 kHz with a frequency resolution of about 1 Hz. As well, only a small driving force is required allowing use of a low-backlash drive mechanism. The use of the rotary tuner is limited to the resonators with two loading elements such as the 2-stub quarter wave resonator, the conventional split loop resonator, and the 2-stub, half wave resonator. The displacement tuner is more versatile and can be used for any TEM-like quarter wave resonator or half wave resonator resonators with two or more loading elements.

The transverse compression of a long charge bunch is investigated in the Paul trap simulator experiment (PTSX), which is a linear Paul trap that simulates the nonlinear transverse dynamics of an intense charged particle beam propagating through an equivalent kilometers-long magnetic alternating-gradient (AG) focusing system. Changing the voltage amplitude at fixed focusing frequency in the PTSX device corresponds to changing the field gradient of the quadrupole magnets with fixed axial periodicity in the AG transport system. In this work, we present experimental results on transverse compression of the charge bunch in which the amplitude of the applied oscillatory focusing voltage is changed instantaneously, and adiabatically. The experimental data are also compared with analytical estimates and 2D WARP particle-in-cell simulations.

Bunched beam stochastic cooling in the Relativistic Heavy Ion Collider (RHIC) at 100 GeV has been achieved. The longitudinal cooling system is designed for heavy ion operation but was tested using protons. A very low intensity bunch with $\sim {10}^9$ protons was prepared so that cooling times and voltage requirements would be comparable to the heavy ion case. With this bunch a cooling time of the order of an hour was observed through shortening of the bunch length and narrowing of the Schottky lines.

Optical transition radiation (OTR) has been widely used in electron beam profile imaging. Optical diffraction radiation (ODR) has recently been used to measure the electron beam’s transverse size with the angular distribution. Because of the close relationship between OTR and ODR, it is natural to ask whether ODR could be used to image the beam profile as is done with OTR. In this paper, the image formation process is investigated as a two-dimensional (2D) convolution. The image formed with ODR as a single electron passes through a circular aperture, through a rectangular slit, and beneath a semi-infinite plane is studied from first principle and taken to be the point spread function (PSF) of the imaging system. It is found that, unlike the OTR case, the PSF of ODR is space variant and largely depends on the shape of the ODR target. With this characteristic, the beam image formed with ODR differs greatly from the real beam profile, and the deconvolution process is generally needed in order to retrieve the real beam distribution from the ODR image. The possibility of using an image formed with ODR from a rectangular slit and a semi-infinite plane to determine beam profile in the direction parallel to the edge of the slit or plane and monitoring the beam’s position are estimated. The theoretical prediction is compared to recently reported experimental results and a qualitative agreement is achieved.

Next-generation high-intensity large acceptance fragment separators require a careful design due to the large high order aberrations induced by the large aperture superconducting magnets needed to collect rare isotopes obtained from a high energy primary heavy-ion beam hitting a target. In this paper we propose a fragment separator layout based on various symmetries that satisfies the baseline requirements. Analytical calculations based on symmetry theories simplify the design to numerical optimization of a basic cell with only a few magnetic elements. The insight provided by these calculations resulted in the specification of a simple layout with large acceptance, transmission, and resolution. The design method may be easily adapted to project-specific needs. The important effects of energy degraders necessary for full fragment separator design will be addressed in a future publication.

A physical optics approach to wavefront tracking is presented based on the stationary phase approximation. It allows for modeling transmission of radiation through a system of optical elements including coherence properties with an acceptable numerical effort. The code PHASE incorporating these ideas assumes small angles as it is the case for most synchrotron radiation and the free electron laser beam lines.

We explore a practical approach for designing ionization cooling channels with periodic solenoidal focusing. We examine the lattice characteristics in terms of the properties of the coils and the cell geometry. The peak magnetic field in the coils is an important engineering constraint in lattice design. We examine the dependence of the peak field, momentum passband locations, and the beta function on the coil parameters. We make a systematic examination of all allowed lattice configurations taking into account the symmetry properties of the current densities and the beta function. We introduce a unique classification for comparing cooling lattice configurations. While solutions with a single coil per cell illustrate most of the effects that are important for cooling channel design, the introduction of additional coils allows more flexibility in selecting the lattice properties. We look at example solutions for the problem of the initial transverse cooling stage of a neutrino factory or muon collider and compare our results with the properties of some published cooling lattice designs. Scaling laws are used to compare solutions from different symmetry classes.