Superconducting cavities made from niobium allow accelerating gradients of about $50\mathrm{MV}/\mathrm{m}$ close to the theoretical limit. Quite often, however, the rf losses increase with the gradient faster than quadratic. This observation is equivalent with a decrease of the quality factor $Q$ with the gradient, called “$Q$ slope” for intermediate gradients, and “$Q$ drop” for larger ones. The paper provides an explanation by an elementary model based on the two-fluid theory of rf superconductivity and applies it to experimental data for a large variety of cavity tests.

A method for generation of quasimonoenergetic, low emittance fast ion/nuclei bunches of solid densities from nanotargets by two counterpropagating laser pulses of ultrarelativistic intensities is proposed, based on the threshold phenomenon of particles reflection due to induced nonlinear Compton scattering. Particularly, a setup is considered which provides generation of ion bunches with parameters that are required in hadron therapy.

We apply the Venturini-Reiser (V-R) envelope-dispersion equations [M. Venturini and M. Reiser, Phys. Rev. Lett. 81, 96 (1998)] to a continuous beam in a uniform focusing/bending lattice to study the combined effects of linear dispersion and space charge. Within this simple model we investigate the scaling of average dispersion and the effects on beam dimensions and show that the V-R equations lead to the correct zero-current limits. We also introduce a generalization of the space charge intensity parameter and apply it to the University of Maryland Electron Ring and other machines. In addition, we present results of calculations to test the smooth approximation by solving the V-R original equations and also through simulations with the matrix code ELEGANT.

During the past decade, intense experimental studies on the heavy-ion induced molecular desorption were performed in several particle accelerator laboratories worldwide in order to understand and overcome large dynamic pressure rises caused by lost beam ions. Different target materials and various coatings were studied for desorption and mitigation techniques were applied to heavy-ion accelerators. For the upgrade of the CERN injector complex, a coating of the Super Proton Synchrotron (SPS) vacuum system with a thin film of amorphous carbon is under study to mitigate the electron cloud effect observed during SPS operation with the nominal proton beam for the Large Hadron Collider (LHC). Since the SPS is also part of the heavy-ion injector chain for LHC, dynamic vacuum studies of amorphous carbon films are important to determine their ion induced desorption yields. At the CERN Heavy Ion Accelerator (LINAC 3), carbon-coated accelerator-type stainless steel vacuum chambers were tested for desorption using $4.2\mathrm{MeV}/\mathrm{u}$ ${\mathrm{Pb}}^{54+}$ ions. We describe the experimental setup and method, present the results for unbaked and baked films, and summarize surface characterizations such as secondary electron yield measurements, x-ray photoemission spectroscopy, and scanning electron microscopy studies. Finally, we present a high-energy scaling of lead-ion induced desorption yields from the $\mathrm{MeV}/\mathrm{u}$ to $\mathrm{GeV}/\mathrm{u}$ range.

Neutral atoms entering an electric field experience a defocusing force in the dipole field direction, which is proportional to the field gradient. If an experiment, such as the search for a permanent electron electric dipole moment (eEDM), requires a very strong electric field ($13.5\mathrm{MV}/\mathrm{m}$), then this end-field defocusing results in beam blowup and much reduced phase-space acceptance. In this paper we discuss how these defocusing fields arise from the longitudinal changes in the electric dipole field and their dependence on the electrode shape and spacing between lenses. We find that the end-field defocusing comes from strong impulse forces, whose defocusing power was calculated for simple electrodes with rounded ends. To compensate for this end-field defocusing, a triplet of transverse-focusing lenses was added to the pure dipole field plates in the generic eEDM cesium fountain experiment used to study the neutral beam optics. Envelope equations, which calculated the beam sizes of the atom bunch for the linear forces, are used to obtain a set of lens parameters that give a well focused beam in the fountain. Atom trajectory equations allow us to calculate the phase-space acceptance of the lens system with the nonlinear force terms included.

An investigation on the plasma-induced field emission (PFE) properties of a large area carbon nanotube (CNT) cathode on a 2 MeV linear induction accelerator injector is presented. Experimental results show that the cathode is able to emit intense electron beams. Intense electron beams of $14.9–127.8\mathrm{A}/{\mathrm{cm}}^2$ are obtained from the cathode. The CNT cathode desorbs gases from the CNTs during the PFE process. The fast cathode plasma expansion affects the diode perveance. The amount of outgassing is estimated to be $0.06–0.49\mathrm{Pa}·\mathrm{L}$, and the ratio of outgassing and electron are roughly calculated to be within the range of 170–350 atoms per electron. The effect of the outgassing is analyzed, and the outgassing mass spectrum of the CNT cathode has been studied during the PFE. There is a significant desorption of ${\mathrm{CO}}_2$, ${\mathrm{N}}_2(\mathrm{CO})$, and ${\mathrm{H}}_2$ gases, which plays an important role during the PFE process. All the experiments demonstrate that the outgassing plays an important role in the formation of the cathode plasma. Moreover, the characteristic turn-on time of the CNT cathode was measured to be 39 ns.

In the scope of the Facility for Antiproton and Ion Research (FAIR) project, several new synchrotrons and storage rings will be built. The existing heavy-ion synchrotron SIS18 has to be upgraded to serve as an injector for the FAIR accelerators. All this imposes new requirements on the low-level rf (LLRF) systems. These requirements include fast ramping modes, arbitrary ion species, and complex beam manipulations such as dual-harmonic operation, bunch merging/splitting, barrier bucket operation, or bunch compression. In order to fulfill these tasks, a completely new and unique system architecture has been developed since 2002, and the system is now used in SIS18 operation. The presentation of this novel system architecture is the purpose of this paper. We first describe the requirements and the design of the LLRF system. Afterwards, some key components and key interfaces of the system are summarized followed by a discussion of technological aspects. Finally, we present some beam experiment results that were obtained using the new LLRF system.

Thermonuclear reaction cross sections of astrophysical interest decrease exponentially with energy, approaching the level of femtobarn or less at the Gamow window. Experimental investigations of such small reaction rates in laboratories at the earth’s surface are hampered by the cosmic-ray background into detectors. For such studies, Dakota Ion Accelerator for Nuclear Astrophysics, a deep underground, high detector efficiency, high target density, high beam intensity accelerator facility is being designed. We report on a 100 mA, 400 kV accelerator design. To take into account the beam space-charge effects, advanced three-dimensional transportation calculations have been performed. These highly realistic beam calculations demonstrate that high beam currents can be transported to a gas-jet target with a diameter of few millimeters.

In this paper the tomographic diagnostic developed to characterize the 2D density distribution of a particle beam from a negative ion source is described. In particular, the reliability of this diagnostic has been tested by considering the geometry of the source for the production of ions of deuterium extracted from an rf plasma (SPIDER). SPIDER is a low energy prototype negative ion source for the international thermonuclear experimental reactor (ITER) neutral beam injector, aimed at demonstrating the capability to create and extract a current of $D^{-}$ ($H^{-}$) ions up to 50 A (60 A) accelerated at 100 kV. The ions are extracted over a wide surface ($1.52\times 0.56{\mathrm{m}}^2$) with a uniform plasma density which is prescribed to remain within 10% of the mean value. The main target of the tomographic diagnostic is the measurement of the beam uniformity with sufficient spatial resolution and of its evolution throughout the pulse duration. To reach this target, a tomographic algorithm based on the simultaneous algebraic reconstruction technique is developed and the geometry of the lines of sight is optimized so as to cover the whole area of the beam. Phantoms that reproduce different experimental beam configurations are simulated and reconstructed, and the role of the noise in the signals is studied. The simulated phantoms are correctly reconstructed and their two-dimensional spatial nonuniformity is correctly estimated, up to a noise level of 10% with respect to the signal.

Beam loading in traveling-wave linear accelerating structures leads to unacceptable spread of particle energies across an extended train of bunched particles due to beam-induced field and dispersion. Methods for modulating the rf power driving linacs are effective at reducing energy spread, but for general linacs do not have a clear analytic foundation. We report here methods for calculating how to modulate the rf drive in arbitrarily nonuniform traveling-wave linacs within the convective-transport (power-diffusion) model that results in no additional energy spread due to beam loading (but not dispersion). Varying group velocity, loss factor, and cell quality factor within a structure, and nonzero particle velocity, are handled.

Consideration of laser-driven plasma-based electron/positron accelerators with a 2 TeV center-of-mass energy is presented, employing a multistaging scheme consisting of successive multi-GeV laser wakefield accelerators operated at the plasma density range of ${10}^{15}–{10}^{18}{\mathrm{cm}}^{-3}$ in the quasilinear regime. A total accelerator length is determined by an operating plasma density and a coupling distance allowed for both laser and beam focusing systems. We investigate beam dynamics and synchrotron radiation due to the betatron oscillation of the beam in laser-plasma acceleration, characterizing the beam qualities such as energy spread and transverse emittance. According to the criteria on the beam qualities for applications and available laser sources, the operating plasma density will be optimized. We note that in the low density operation the required wall-plug power for the laser driver will be much reduced in comparison with the high-density options.

In $\mathrm{DA}\Phi \mathrm{NE}$, the Frascati $e^{+}/e^{-}$ collider, the crab waist collision scheme has been successfully implemented and tested during the years 2008 and 2009. During operations for the Siddharta experiment an unusual synchrotron damping effect induced by beam-beam collisions has been observed. Indeed, the positron beam becomes unstable above currents in the order of 200–300 mA when the longitudinal feedback is off. The longitudinal instability is damped by colliding the positron beam with a high current electron beam ($\sim 2\mathrm{A}$) and a shift of $\approx -600\mathrm{Hz}$ in the residual synchrotron sidebands is observed. Precise measurements have been performed by using both a commercial spectrum analyzer and the diagnostic capabilities of the $\mathrm{DA}\Phi \mathrm{NE}$ longitudinal bunch-by-bunch feedback. This damping effect has been observed in $\mathrm{DA}\Phi \mathrm{NE}$ for the first time during collisions with the crab waist scheme. Our explanation is that beam collisions with a large crossing angle produce longitudinal tune shift and spread, providing Landau damping of synchrotron oscillations.

Peak current represents a key demand for new generation electron beam photoinjectors. Many beam applications, such as free electron laser, inverse Compton scattering, terahertz radiation generation, have efficiencies strongly dependent on the bunch length and current. A method of beam longitudinal compression (called velocity bunching) has been proposed some years ago, based on beam longitudinal phase space rotation in a rf field potential. The control of such rotation can lead to a compression factor in excess of 10, depending on the initial longitudinal emittance. Code simulations have shown the possibility to fully compensate the transverse emittance growth during rf compression, and this regime has been experimentally proven recently at SPARC. The key point is the control of transverse beam plasma oscillations, in order to freeze the emittance at its lowest value at the end of compression. Longitudinal and transverse phase space distortions have been observed during the experiments, leading to asymmetric current profiles and higher final projected emittances. In this paper we discuss in detail the results obtained at SPARC in the regime of velocity bunching, analyzing such nonlinearities and identifying the causes. The beam degradation is discussed, both for slice and projected parameters. Analytical tools are derived to experimentally quantify the effect of such distortions on the projected emittance.

Radio frequency (rf) accelerating system noise can have a detrimental impact on the Large Hadron Collider (LHC) performance through longitudinal motion and longitudinal emittance growth. A theoretical formalism has been developed to relate the beam and rf station dynamics with the bunch length growth [T. Mastorides et al., Phys. Rev. ST Accel. Beams 13, 102801 (2010)]. Measurements were conducted at LHC to determine the performance limiting rf components and validate the formalism through studies of the beam diffusion dependence on rf noise. As a result, a noise threshold was established for acceptable performance which provides the foundation for beam diffusion estimates for higher energies and intensities. Measurements were also conducted to determine the low level rf noise spectrum and its major contributions, as well as to validate models and simulations of this system.

Long-range beam-beam effects are a potential limit to the LHC performance with the nominal design parameters, and certain upgrade scenarios under discussion. To mitigate long-range effects, current carrying wires parallel to the beam were proposed and space is reserved in the LHC for such wires. Two current carrying wires were installed in the Relativistic Heavy Ion Collider to study the effect of strong long-range beam-beam effects in a collider, as well as test the compensation of a single long-range interaction. The experimental data were used to benchmark simulations. We summarize this work.

Analysis of known experimental data for the boundaries of the multipactor discharge in a flat gap shows that these data cannot be explained in the frames of a simple two-point theory. More complicated theories include repeating impacts of the electrons on the same surface before they hit the opposite one. Here an approach is proposed that can illustrate many of these theories: a phase map connecting the start and arrival phases of the electrons coming to the same or opposite plate. This illustrative technique leads to a concept of a generalized phase stability. Introduction of the generalized condition of stability helps to understand the expanding of multipacting zones beyond the limits predicted by the simple stability condition. The phase diagrams illustrate the phase motion when the simple condition of stability is applicable or not applicable.

Transverse instability of a bunch in a ring accelerator is considered with space charge and wakefield taken into account. It is assumed that space charge tune shift significantly exceeds the synchrotron tune. Bunch spectrum, instability growth rate, and effects of chromaticity are studied with different bunch and wake forms. Fast instability caused by coupling of transverse modes is studied in detail. It is shown that, for monotonic wakes, the transverse mode coupling instability is possible only with a certain sign of the wake. Its threshold and growth rate are calculated precisely over a wide range of parameters.

We review the current status of metallic photocathodes based on thin films prepared by pulsed laser deposition (PLD) and we explore ways to improve the performance of these devices. PLD seems to be a very efficient and suitable technique for producing adherent and uniform thin films. Time-resolved mass spectrometric investigations definitively suggest that the deposition of high-purity metallic thin films should be carried out in ultrahigh vacuum systems and after a deep and careful laser cleaning of the target surface. Moreover, the laser cleaning of the target surface is highly recommended not only to remove the first contaminated layers but also to improve the quality of the vacuum by reducing the partial pressure of reactive chemical species as ${\mathrm{H}}_2\mathrm{O}$, ${\mathrm{H}}_2$, and ${\mathrm{O}}_2$ molecules. The challenge to realize high-purity Mg and Y thin films is very interesting for the photocathode R&D due to the good photoemission properties of these metals. Photocathodes based on Mg and Y thin films have been characterized by scanning electron microscopy and x-ray diffraction techniques to derive the morphological and structural features, respectively. They were also tested in a photodiode cell to deduce the photoelectron properties. The quantum efficiency of such photocathodes was systematically improved by in situ laser cleaning treatments of the surface in order to remove the contaminated layers reaching, in this way, the quantum efficiency of the corresponding bulk materials.