Losses and component activation are limiting performance factors for slow extraction with high-power applications, and new techniques of loss-reduction, such as bent crystals, require a stable and narrow separatrix angular spread. Conventional tune-sweep slow extraction results in an optics change and an accompanying separatrix rotation through the spill. This can be compensated by a dynamic closed-orbit bump, but requires a high level of complexity for setting up and monitoring. For the Super Proton Synchrotron (SPS), a simpler and powerful new extraction technique has been developed and deployed, providing a mechanism to fix the machine optics and hence separatrix completely through the spill. The technique with the name constant optics slow extraction (COSE) relies on high chromaticity and scaling all machine settings with beam rigidity following the momentum distribution of the beam. In this paper we describe the new COSE concept and its successful operational deployment in the SPS during the 2018 run.

The National User Facility for Advanced Accelerator Experimental Tests II (FACET-II) at SLAC National Accelerator Laboratory expands upon the experiments conducted at FACET. Its purpose is to build upon the decades-long experience developed conducting accelerator R&D at SLAC in the areas of advanced acceleration and coherent radiation techniques with high-energy electron and positron beams. This paper summarizes the motivations for the design and resulting capabilities of the FACET-II facility.

Taking advantage of the 0° synchronous phase, the kombinierte null grad struktur (KONUS) beam dynamics strategy enables long accelerating sections with lens-free slim drift tubes in the low and medium energy regime. It has successfully realized worldwide many normal-conducting H-type linacs with compact layouts and good beam performance. In this paper, a further development of this solution, i.e., to combine the KONUS dynamics with the recently developed superconducting CH structure for accelerating very high intensity beams, is presented. The efficiency of the new solution has been shown by systematic design studies performed for a 150 mA, 6 MW deuteron linac.

This paper presents the design of a compact gantry that uses superconducting bending magnets (BMs), for use in carbon-ion beam therapy. The size of the gantry is comparable to those of existing gantries that are used for proton-beam therapy. The designed gantry provides point-to-parallel scanning over an area of $20\mathrm{cm}\times 20\mathrm{cm}$ at the isocenter, and has rotationally invariant optics, which are enabled by quadrupole and dipole magnets together with a 90° combined-function magnet with 18.6-cm bore radius. A 90° BM accommodates large scanning angles; it also provides equal focusing in horizontal and vertical planes, and zero-integrated nonlinear fields to minimize beam distortion at the isocenter. Three-dimensional field analysis of the magnet, and particle-tracking simulation, validate the beam optics of the gantry and point-to-parallel scanning. The Taylor map and the Lie map are shown to be useful in the analysis of magnetic fields and in optimizing the coil windings.

Electropolishing (EP) is a standard process for the treatment of the interior surface of niobium (Nb) superconducting radio-frequency (SRF) cavities used in particle accelerators. We previously conducted a study on vertical electropolishing (VEP), in which the cavity was set vertically during EP of a 1.3 GHz Nb single-cell cavity. In that study, we showed that the major cause of asymmetric removal in VEP is the accumulation of hydrogen gas bubbles, generated during the EP process, on the upper half cell of the cavity. Three types of unique rotating cathodes were tested with the aim to resolve the issues of asymmetric removal and rough surface. Appropriate cathode design and parameters reduced the asymmetric removal in the cell and yielded a smooth surface of the cavity. In this study, we have performed an extensive investigation on VEP of both single- and nine-cell Nb 1.3 GHz cavities with a modified cathode. The VEP parameters with single- and nine-cell coupon cavities were investigated, which facilitate an in situ study of the coupon currents at various positions of the cavities and observation from the viewports located near the iris and equator positions of the cavities. The modified cathode and optimized VEP process for the single-cell cavity yielded uniform removal in the cavity cell and a smooth surface of the interior of the cavity. A novel acid flow method that allows the separation of the acid flow in the cavity and cathode housing was employed in the nine-cell cavity to efficiently remove the gas bubbles from the cavity. The optimized acid flow rates in the cavity and cathode housing, along with other adequate parameters including temperature, cathode rotation speed, and applied voltage, made the surface smooth and significantly reduced the removal nonuniformity in the nine-cell cavity. The single- and nine-cell cavities after an average removal of 46 and $130\mu \mathrm{m}$, respectively, were tested in a vertical cryostat. The single- and nine-cell cavities achieved $31\mathrm{MV}/\mathrm{m}$ at a $Q_0$ value of $9\times {10}^9$ and $28.3\mathrm{MV}/\mathrm{m}$ at a $Q_0$ value of $6.7\times {10}^9$, respectively, in the rf tests at the temperature of 2 K. The achieved SRF performances were as good as the baseline performances achieved after the horizontal electropolishing process of the same cavities.

The Large Hadron Collider has a complex collimation system installed to protect its sensitive equipment from normal and abnormal beam losses. The collimators are set around the beam following a multistage transverse setting hierarchy. The insertion position of each collimator is established using beam-based alignment techniques to determine the local beam position and rms beam size at each collimator. During previous years, collimator alignments were performed semiautomatically, with collimation experts present to oversee and control the alignment. During run II, a new fully automatic alignment tool was developed and used for collimator alignments throughout 2018. This paper discusses the improvements on the alignment software to automate it using machine learning, whilst focusing on the operational results obtained when testing the new software in the LHC. The alignment tests were conducted with both proton and ion beams, and angular alignments were performed with proton beams. This upgraded software successfully decreased the alignment time by a factor of 3 and made the results more reproducible, which is particularly important when performing angular alignments.

The investigation of two different photocathodes (PCs) based on nanodiamond (ND) layers, irradiated by a KrF nanosecond excimer laser (wavelength, $\lambda =248\mathrm{nm}$; photon energy, $E_{\mathrm{Ph}}=5\mathrm{eV}$) is reported. The ND layers were deposited by means of a pulsed spray technique. Specifically, the active layer of each PC consisted of untreated (as-received) and hydrogenated ND particles, 250 nm in size, sprayed on a $p$-doped silicon substrate. The ND-based photocathodes were tested in a vacuum chamber at ${10}^{-6}\mathrm{mbar}$ and compared to a Cu-based one, used as reference. All the photocathodes were irradiated at normal incidence. The quantum efficiency (QE) of the photocathodes was assessed. QE values of the ND-based photocathodes were higher than that of the reference one. In particular, the hydrogenated ND-based PC exhibited the highest QE due to the negative electron affinity that results from the surface terminated by hydrogen. Additionally, the photocathode surface/local temperature and the multiphoton process contribution to the electron emission were studied.

The flux of high-energy protons slow-extracted from the CERN Super Proton Synchrotron (SPS) is limited by the induced radioactivity caused by the beam loss intrinsic to the extraction process. Methods to substantially increase the efficiency of the extraction process are of great interest to fulfill requests for an increasing flux of 400 GeV protons to the present experiments, located in the North Area of the SPS, and also for potential future experiments with very high demanded protons on target. A crystal shadowing technique to significantly reduce the beam scattered and lost on the electrostatic extraction septum during the third-integer resonant slow extraction process has been developed and a prototype system tested with beam. The technique is based on the use of a thin, bent silicon crystal to coherently channel or volume reflect the portion of beam that would otherwise impinge the wire array of the electrostatic septum and instead eject it into the transfer line toward the production targets of the experiments. In this paper, the concept is described and applied to the SPS machine in order to specify the requirements of the prototype crystal shadowing system. Beam dynamics simulations of the prototype system are compared and benchmarked to the results obtained through beam tests, before being exploited to understand the characteristics of the present system and the potential performance reach of an optimized, future operational configuration. The remaining challenges faced to bring the system into operation, the optimization possibilities and other potential applications are discussed.

A high peak current, flat longitudinal phase space electron beam is desirable for efficient x-ray free electron laser (FEL) radiation in next generation light sources. To attain such a beam requires the extensive design of the linear accelerator (linac) including both linear and nonlinear effects. In this paper, we propose a lumped longitudinal beam dynamics model for fast optimization of the electron beam longitudinal phase space through the accelerator. This model is much faster than available tracking programs and also shows good agreement with the fully three-dimensional element-by-element multiparticle simulations. We applied this model in a parallel multiobjective differential evolution optimization program to an existing LCLS-II superconducting linac design and obtained an optimal solution with significantly higher core peak current than the original design.

It was observed at the Relativistic Heavy Ion Collider that the polarization of the stored beam decays by 0.17 to 3% per hour. This effect could not be simulated up to now since the simulation time for an exact calculation exceeded the computer capacity. To solve this problem the action of the whole arc was considered as one element acting on the spin and not, as before, each optical element in the arc individually (called lattice independent integration). In addition to former simulations the coupling of synchrotron and betatron motions in the beam interaction regions and their action on the polarization were taken into account (called addition of longitudinal dynamics). It is shown that with these two changes the action on the polarization life time can be explained.

An efficient way to enhance laser-driven proton acceleration is by increasing the laser-to-target energy transfer, which can be obtained using nanostructured target surfaces. In this paper, we show that inexpensive and easily producible solid target nanostructuration using ultrasmall nanoparticles having 10 nm in diameter exhibits a nearly twofold maximum proton energy and proton number enhancement. Results are confirmed by particle-in-cell simulations, for several laser pulse lengths. A parameter scan analyzing the effect of the nanoparticle diameter and space gap between the nanospheres shows that the gap has a stronger influence on the enhancement mechanism than the sphere diameter.

Beam loading compensation in the rf cavities is a key for acceleration of high-intensity beams in the 3 GeV rapid cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC). Since we employ wideband magnetic alloy rf cavities for the J-PARC RCS and the wake voltage contains several harmonics, multiharmonic beam loading compensation is required. The multiharmonic rf feedforward for the six most important harmonics is implemented in the existing low-level rf (LLRF) control system, which has been working fairly well for acceleration of high-intensity beams of up to 1 MW. However, we found degradation of the performance for compensation of the feedforward with very-high-intensity beams. The gain variation with output current of the tube amplifier affects the performance of the feedforward, since it has an open loop configuration. Also, there is a voltage waveform distortion caused by the distortion of the output current, which essentially cannot be compensated by the feedforward. Therefore, we consider employing the vector rf voltage feedback control for beam loading compensation, where the feedback is expected to work with the gain variation within the stability margin. A multiharmonic vector rf voltage control has been developed as the core part of the next-generation LLRF control system for RCS. Prior to the completion of the new system, the multiharmonic vector rf voltage control was tested with one of the 12 cavities. The details of the system configuration, commissioning methodology, and beam test results using very-high-intensity beams are described. Beam loading by the 1 MW equivalent beam in the cavity is successfully compensated.

Undulators and wigglers based on Halbach arrays have been in widespread use since their description in the 1980s, due to the high fields they generate from permanent magnets. The most common implementation of a pure permanent magnet Halbach undulator consists of four magnetic segment widths per undulator period. In typical undulators with periods of order $\sim \mathrm{cm}$ it is straightforward to use sufficiently small magnetic segments to achieve the desired undulator period. However, in the context of new microundulators with periods of order $\sim \mathrm{mm}$ or shorter, there is a lower bound on the feature sizes achievable by existing fabrication techniques. This motivates the use of modified Halbach arrays comprised of right triangle magnet segments with only two magnet segment widths per undulator period. These configurations are demonstrated to have superior field strengths to up-down arrays with no increase to the undulator period. Additional considerations arising from the broken symmetries in such configurations, such as the presence of previously forbidden harmonic components, are also considered. Such right triangle Halbach arrays may be fabricated using MEMS techniques for use in the next generation of compact light sources.

We report on the optical commissioning of the high power laser beam circulator for the high brightness Compton $\gamma$-ray source Extreme Light Infrastructure for Nuclear Physics. Tests aiming at demonstrating the optical performances of the laser beam circulator have been realized with a low-power pulsed laser-beam system and without electron beam. We show that, with the developed alignment and synchronization methods coming from the laser beam circulator design study presented in the Dupraz et al. paper [Phys. Rev. Accel. Beams 17, 033501 (2014)], the laser beam circulator enhances the laser-beam power available at the interaction point by a factor in excess of 25. This corresponds to a potential of bringing the average laser-beam power in excess of 1 kW when the laser beam circulator is injected with the interaction point laser-beam pulse energy of 400 mJ at 100 Hz.

The radiation reaction (RR) is expected to play a critical role in light-matter interactions at extreme intensity. Utilizing the theoretical analyses and three-dimensional (3D) numerical simulations, we demonstrate that electron reflection, induced by the RR in a head-on collision with an intense laser pulse, can provide pronounced signatures to discern the classical and quantum RR. In the classical regime, there is a precipitous threshold of laser intensity to achieve the whole electron bunch rebound. However, this threshold becomes a gradual transition in the quantum regime, where the electron bunch is quasi-isotropically scattered by the laser pulse and this process resembles a water splash. Leveraged on the derived dependence of classical radiation rebound on the parameters of laser pulses and electron bunches, a practical detecting method is proposed to distinguish the quantum discrete recoil and classical continuous RR force.

We propose a short period undulator which is based on the alternating electromagnetic field pattern of THz-driven surface plasmons in a thin conductive layer on a dielectric grating. An approximate analytical model allows to assess the key performance parameters of the undulator and to estimate the emitted radiation spectrum. The specific example of a graphene based undulator is simulated in detail. For a moderate electron beam energy of 100 MeV and a bunch charge of 0.5 pC the 40 mm long undulator is shown to emit narrow band 1 keV x-ray pulses with a peak brightness of approximately $10^{16}\mathrm{photons}/(\mathrm{s}{\mathrm{mrad}}^2{\mathrm{mm}}^{2}0.1\%\mathrm{BW})$. It therefore has potential for a compact and low cost x-ray source.

The parallel operation of multiple undulator lines with a wide spectral range is an important way to increase the usability of x-ray free electron laser (XFEL) facilities, especially for machines with high-repetition rate. In this paper, a delay system based on four double bend achromats is proposed to delay electron beams, thereby changing the arrival time of those delayed electron beams in the accelerating structure behind the system. Combined with kickers, the delay system can be used to generate bunch-to-bunch energy changed electron beams in a continuous wave XFEL facility. Start-to-end simulations based on the parameters of the Shanghai high-repetition-rate XFEL and extreme light facility are performed to demonstrate that the delay system can flexibly control electron beam energy from 1.48 to 8.74 GeV at the end of the linac.

A continuously tunable narrow-band terahertz source based on Cherenkov wakefield generation in a dielectric lined waveguide driven by short sub-picosecond electron bunches has been experimentally demonstrated. A large tunability range of 0.55–0.95 THz was achieved by implementing the waveguide design as a planar structure with variable 0.15–1.10 mm gap and thin $25\mu \mathrm{m}$ dielectric layers. The source operates essentially in a single mode regime with higher order modes of negligible intensity. THz pulse energy was measured across the tunable range and compared with theory.

The extraction of accelerated positrons from the electron-beam-driven nonlinear plasma wakefield is investigated. If the trailing positron beam traverses a plasma density down-ramp at the exit of the plasma, it can become ring-shaped due to the defocusing force of the elongating wake bubble produced by the electron drive bunch. Ionization-induced head erosion process can be used to mitigate this effect because the drive electron beam leaves a hollow plasma channel with zero focusing force along the propagation axis. Particle-in-cell simulations are performed for the case where a drive-trailing electron bunch configuration traverses through a high-$Z$ tantalum target placed at the plasma entrance, during which each bunch generates electron-positron pairs. If the plasma density upstream of this converter foil is correctly chosen such that the bubble length is equal to the inter-bunch spacing then the secondary positrons experience both a focusing and accelerating force and can gain energy from the wake. The simulation results demonstrate that when the uniform plasma length is longer than the beam head erosion distance for the drive electron bunch, the positrons can be extracted with a far smaller divergence than when they are extracted by the usual plasma density down-ramp. The transverse density distribution and the positron spectra of the trailing positrons for various acceptance angles of the spectrometer are presented to discuss the detectability of the accelerated positrons under experimental conditions at FACET-II.