Generation of electron-positron pairs via the multiphoton Breit-Wheeler process in an all-optical scheme will be made possible on forthcoming high-power laser facilities through the collision of wakefield-accelerated GeV electrons with a counter-propagating laser pulse of $10^{22}–10^{23}\mathrm{W}{\mathrm{cm}}^{-2}$ peak intensity. By means of integrated 3D particle-in-cell simulations, we show that the production of high-density sources of ultrarelativistic electron-positron pairs is within the reach of soon-to-be-available laser systems. Under physical conditions accessible to the dual-beam CILEX-Apollon facility, we find that the generated positrons can carry a total charge of 0.05–1 nC, with a mean energy of 100–400 MeV and an angular divergence of 0.01–0.1 rad. The variations of the positron source’s properties with respect to the laser parameters are also examined.

A new muon beam line, the muon science innovative channel, was set up at the Research Center for Nuclear Physics, Osaka University, in Osaka, Japan, using the 392 MeV proton beam impinging on a target. The production of an intense muon beam relies on the efficient capture of pions, which subsequently decay to muons, using a novel superconducting solenoid magnet system. After the pion-capture solenoid, the first 36° of the curved muon transport line was commissioned and the muon flux was measured. In order to detect muons, a target of either copper or magnesium was placed to stop muons at the end of the muon beam line. Two stations of plastic scintillators located upstream and downstream from the muon target were used to reconstruct the decay spectrum of muons. In a complementary method to detect negatively charged muons, the x-ray spectrum yielded by muonic atoms in the target was measured in a germanium detector. Measurements, at a proton beam current of 6 pA, yielded $(10.4\pm 2.7)\times 10^5\mathrm{muons}$ per watt of proton beam power (${\mu }^{+}$ and ${\mu }^{-}$), far in excess of other facilities. At full beam power (400 W), this implies a rate of muons of $(4.2\pm 1.1)\times 10^8\mathrm{muons}{\mathrm{s}}^{-1}$, among the highest in the world. The number of ${\mu }^{-}$ measured was about a factor of 10 lower, again by far the most efficient muon beam produced. The setup is a prototype for future experiments requiring a high-intensity muon beam, such as a muon collider or neutrino factory, or the search for rare muon decays which would be a signature for phenomena beyond the Standard Model of particle physics. Such a muon beam can also be used in other branches of physics, nuclear and condensed matter, as well as other areas of scientific research.

The use of the “negative mass” regime provides stabilization of longitudinal size of dense photoinjector electron bunches moving through a long undulator. This allows one to increase significantly the power capabilities of a terahertz source based on coherent spontaneous emission from a short bunch. However, such type of emission is produced if the bunch length is comparable with the radiation wavelength. This work discusses the use of the negative mass regime to provide effective compression of dense bunches down to “terahertz” lengths.

Harmonic lasing is a possible mode of operation of X-ray FEL user facilities that allows us to provide brilliant beams of higher energy photons for user experiments. Another useful application of harmonic lasing is the so-called harmonic lasing self-seeded free electron laser (HLSS FEL) that allows the improvement of spectral brightness of these facilities. In the past, harmonic lasing has been demonstrated in the FEL oscillators in infrared and visible wavelength ranges, but not in high-gain FELs and not at short wavelengths. In this paper we report on the first evidence of the harmonic lasing and the first operation of the HLSS FEL at the soft X-ray FEL user facility FLASH in the wavelength range between 4.5 nm and 15 nm. Spectral brightness was improved in comparison with self-amplified spontaneous emission (SASE) FEL by a factor of six in the exponential gain regime. A better performance of HLSS FEL with respect to SASE FEL in the post-saturation regime with a tapered undulator was observed as well. The first demonstration of harmonic lasing in a high-gain FEL and at short wavelengths paves the way for a variety of applications of this new operation mode in X-ray FELs.

The $Z$ pulsed-power generator at Sandia National Laboratories drives high energy density physics experiments with load currents of up to 26 MA. $Z$ utilizes a double post-hole convolute to combine the current from four parallel magnetically insulated transmission lines into a single transmission line just upstream of the load. Current loss is observed in most experiments and is traditionally attributed to inefficient convolute performance. The apparent loss current varies substantially for z-pinch loads with different inductance histories; however, a similar convolute impedance history is observed for all load types. This paper details direct spectroscopic measurements of plasma density, temperature, and apparent and actual plasma closure velocities within the convolute. Spectral measurements indicate a correlation between impedance collapse and plasma formation in the convolute. Absorption features in the spectra show the convolute plasma consists primarily of hydrogen, which likely forms from desorbed electrode contaminant species such as ${\mathrm{H}}_2\mathrm{O}$, ${\mathrm{H}}_2$, and hydrocarbons. Plasma densities increase from $1\times {10}^{16}{\mathrm{cm}}^{-3}$ (level of detectability) just before peak current to over $1\times {10}^{17}{\mathrm{cm}}^{-3}$ at stagnation (tens of ns later). The density seems to be highest near the cathode surface, with an apparent cathode to anode plasma velocity in the range of $35–50\mathrm{cm}/\mu \mathrm{s}$. Similar plasma conditions and convolute impedance histories are observed in experiments with high and low losses, suggesting that losses are driven largely by load dynamics, which determine the voltage on the convolute.

The compensation of correlated emittance contributions is of primary importance to optimize the performance of high brightness photoinjectors. While only extended numerical simulations can capture the complex beam dynamics of space-charge-dominated beams in sufficient detail to optimize a specific injector layout, simplified models are required to gain a deeper understanding of the involved dynamics, to guide the optimization procedure, and to interpret experimental results. In this paper, a slice envelope model for the emittance compensation process in a split photoinjector is presented. The emittance term is included in the analytical solution of the beam envelope in a drift, which is essential to take the emittance contribution due to a beam size mismatch into account. The appearance of two emittance minima in the drift is explained, and the matching into the booster cavity is discussed. A comparison with simulation results points out effects which are not treated in the envelope model, such as overfocusing and field nonlinearities.

With the progress made in 2015, the beams produced by the CERN Proton Synchrotron using multiturn extraction (MTE) have been delivered to the Super Proton Synchrotron (SPS) for the fixed-target physics run. Operation successfully started in the second half of September 2015 and continued until the end of the proton physics program by mid November. In this paper the overall performance and beam quality is discussed in detail considering the complete chain of accelerators, from the PS-Booster to the SPS. Moreover, a thorough comparison of the global performance of the MTE scheme against the previously used technique, the so-called continuous transfer (CT), is also carried out.

Quarter wavelength resonator (QWR) based deflecting cavities with the capability of supporting multiple odd-harmonic modes have been developed for an ultrafast periodic kicker system in the proposed Jefferson Lab Electron Ion Collider (JLEIC, formerly MEIC). Previous work on the kicking pulse synthesis and the transverse beam dynamics tracking simulations show that a flat-top kicking pulse can be generated with minimal emittance growth during injection and circulation of the cooling electron bunches. This flat-top kicking pulse can be obtained when a DC component and 10 harmonic modes with appropriate amplitude and phase are combined together. To support 10 such harmonic modes, four QWR cavities are used with 5, 3, 1, and 1 modes, respectively. In the multiple-mode cavities, several slightly tapered segments of the inner conductor are introduced to tune the higher order deflecting modes to be harmonic, and stub tuners are used to fine tune each frequency to compensate for potential errors. In this paper, we summarize the electromagnetic design of the five-mode cavity, including the geometry optimization to get high transverse shunt impedance, the frequency tuning and sensitivity analysis, and the single loop coupler design for coupling to all of the harmonic modes. In particular we report on the design and fabrication of a half-scale copper prototype of this proof-of-principle five-odd-mode cavity, as well as the rf bench measurements. Finally, we demonstrate mode superposition in this cavity experimentally, which illustrates the kicking pulse generation concept.

The transverse decoherence of injected bunches is an important phenomenon in synchrotrons and storage rings. The initial stage of this process determines the transverse emittance blowup, which should be taken into account for the design of feedback systems, for example. The interplay of different high-intensity effects can strongly affect the initial decoherence stage. We present a model that explains decoherence and emittance growth with chromaticity, space charge, and image charges within the first synchrotron period. We compare the model for different combinations of parameters with self-consistent particle tracking simulations and measurements in the SIS18 synchrotron at GSI Darmstadt. Generally, space charge slows down the decoherence process and can cause the loss of decoherence. Chromaticity and image charges can partly compensate this loss and restore the decoherence. We also analyze the single-particle excitation driven by space charge during the decoherence process. Particles gain large amplitudes from the coherent beam oscillation, which leads to halo buildup and losses.

Dedicated optics with extremely short electron bunches enable synchrotron light sources to generate intense coherent THz radiation. The high degree of spatial compression in this so-called low-${\alpha }_c$ optics entails a complex longitudinal dynamics of the electron bunches, which can be probed studying the fluctuations in the emitted terahertz radiation caused by the microbunching instability (“bursting”). This article presents a “quasi-instantaneous” method for measuring the bursting characteristics by simultaneously collecting and evaluating the information from all bunches in a multibunch fill, reducing the measurement time from hours to seconds. This speed-up allows systematic studies of the bursting characteristics for various accelerator settings within a single fill of the machine, enabling a comprehensive comparison of the measured bursting thresholds with theoretical predictions by the bunched-beam theory. This paper introduces the method and presents first results obtained at the ANKA synchrotron radiation facility.

Narrow-bandwidth photon beams in the x-ray and $\gamma$-ray energy ranges are expected to be applied in various fields. An energy recovery linac (ERL)-based laser Compton scattering (LCS) source employing a laser enhancement cavity can produce a high-flux and narrow-bandwidth photon beam. We conducted the first experiment of an ERL-based LCS source in combination with a laser enhancement cavity. We obtained LCS photons with an energy of $6.95\pm 0.01\mathrm{keV}$ by colliding an electron beam of 20 MeV with a laser of 1064 nm wavelength. The photon flux at the interaction point was evaluated to be $(2.6\pm 0.1)\times 10^7\mathrm{photons}/\mathrm{s}$ with an average beam current of $58\mu \mathrm{A}$ and an average laser power of 10 kW. The energy bandwidth was evaluated to be 0.4% (rms) with an opening angle of 0.14 mrad. The technologies demonstrated in this experiment are applicable for future ERL-based LCS sources.

This paper addresses a detailed experimental study of collective instability bands generated near every half-integer tune per lattice period by coherent dipole and quadrupole resonances. Both instabilities appear side by side or overlap each other but are mostly separable because the dipole resonance often creates a narrower stop band accompanied by more severe particle losses. The separation of these low-order resonance bands becomes greater as the beam intensity increases. In principle, the double stop-band structure can be formed even without machine imperfections when the beam’s initial phase-space profile is deviated from the ideal stationary distribution. The tabletop ion-trap system called “S-POD” is employed to experimentally demonstrate the parameter dependence of the double stop-band structure. Numerical simulations are also performed for comparison with experimental observations.

The SwissFEL Injector Test Facility operated at the Paul Scherrer Institute between 2010 and 2014, serving as a pilot plant and test bed for the development and realization of SwissFEL, the x-ray Free-Electron Laser facility under construction at the same institute. The test facility consisted of a laser-driven rf electron gun followed by an S-band booster linac, a magnetic bunch compression chicane and a diagnostic section including a transverse deflecting rf cavity. It delivered electron bunches of up to 200 pC charge and up to 250 MeV beam energy at a repetition rate of 10 Hz. The measurements performed at the test facility not only demonstrated the beam parameters required to drive the first stage of an FEL facility, but also led to significant advances in instrumentation technologies, beam characterization methods and the generation, transport and compression of ultralow-emittance beams. We give a comprehensive overview of the commissioning experience of the principal subsystems and the beam physics measurements performed during the operation of the test facility, including the results of the test of an in-vacuum undulator prototype generating radiation in the vacuum ultraviolet and optical range.

Bulk GaAs samples with different surface cleave planes were implanted with 100 and 10 000 V hydrogen ions inside an ultrahigh vacuum test apparatus to simulate ion back-bombardment of the photocathode inside a DC high voltage photogun. The photocathode yield, or quantum efficiency, could easily be recovered following implantation with 100 V hydrogen ions but not for 10 000 V ions. Moreover, the implantation damage with 10 000 V hydrogen ions was more pronounced for GaAs photocathode samples with (100) and (111A) cleave planes, compared to the photocathode with (110) cleave plane. This result is consistent with enhanced ion channeling for the (110) cleave plane compared to the other cleave planes, with ions penetrating deeper into the photocathode material beyond the absorption depth of the laser light and beyond the region of the photocathode where the photoemitted electrons originate.

Petawatt, picosecond laser pulses offer rich opportunities in generating synchrotron x-rays. This paper concentrates on the regimes accessible with the PETAL laser, which is a part of the Laser Megajoule (LMJ) facility. We explore two physically distinct scenarios through Particle-in-Cell simulations. The first one realizes in a dense plasma, such that the period of electron Langmuir oscillations is much shorter than the pulse duration. Hallmarks of this regime are longitudinal breakup (“self-modulation”) of the picosecond-scale laser pulse and excitation of a rapidly evolving broken plasma wake. It is found that electron beams with a charge of several tens of nC can be obtained, with a quasi-Maxwellian energy distribution extending to a few-GeV level. In the second scenario, at lower plasma densities, the pulse is shorter than the electron plasma period. The pulse blows out plasma electrons, creating a single accelerating cavity, while injection on the density downramp creates a nC quasi-monoenergetic electron bunch within the cavity. This bunch accelerates without degradation beyond 1 GeV. The x-ray sources in the self-modulated regime offer a high number of photons ($\sim 10^{12}$) with the slowly decaying energy spectra extending beyond 60 keV. In turn, quasimonoenergetic character of the electron beam in the blowout regime results in the synchrotron-like spectra with the critical energy around 10 MeV and a number of photons $>10^9$. Yet, much smaller source duration and transverse size increase the x-ray brilliance by more than an order of magnitude against the self-modulated case, also favoring high spatial and temporal resolution in x-ray imaging. In all explored cases, accelerated electrons emit synchrotron x-rays of high brilliance, $B>10^{20}\mathrm{photons}/\mathrm{s}/{\mathrm{mm}}^2/{\mathrm{mrad}}^2/0.1\%\mathrm{BW}$. Synchrotron sources driven by picosecond kilojoule lasers may thus find an application in x-ray diagnostics on such facilities such as the LMJ or National Ignition Facility (NIF).

In a so-called waveguide free electron laser (FEL) for THz radiations, an extremely small aperture ($\sim \mathrm{mm}$) waveguide is used to confine angularly wide spread radiation fields from a low energy electron beam into a small area. This confinement increases the interaction between the electron beam and the radiation fields to achieve a much higher FEL gain. The radiation fields propagate inside the waveguide as waveguide modes, not like a light flux in a free space FEL. This characteristic behavior of the radiation fields makes intuitive understanding of the waveguide FEL difficult. We developed a three-dimensional waveguide FEL theory to calculate a gain of THz waveguide FEL including the effects of the energy spread, the beam size and the betatron oscillations of an electron beam, and effects of a rectangular waveguide. The FEL gain can be calculated as a function of frequency by solving the dispersion relation. Theoretical gains are compared with simulation results for a waveguide FEL with a planar undulator similar to the KAERI one. Good agreements are obtained.

Plasma wake field accelerators driven with solid-state near-IR lasers have been considered as an alternative to conventional rf accelerators for next-generation TeV-class lepton colliders. Here, we extend this study to the mid-IR spectral domain covered by ${\mathrm{CO}}_2$ lasers. We conclude that the increase in the laser driver wavelength favors the regime of laser wake field acceleration with a low plasma density and high electric charge. This regime is the most beneficial for gamma colliders to be converted from lepton colliders via inverse Compton scattering. Selecting a laser wavelength to drive a Compton gamma source is essential for the design of such a machine. The revealed benefits from spectral diversification of laser drivers for future colliders and off-spring applications validate ongoing efforts in advancing the ultrafast ${\mathrm{CO}}_2$ laser technology.

The theory of a plasma undulator excited by a short intense laser pulse in a parabolic plasma channel is presented. The undulator fields are generated either by the laser pulse incident off-axis and/or under the angle with respect to the channel axis. Linear plasma theory is used to derive the wakefield structure. It is shown that the electrons injected into the plasma wakefields experience betatron motion and undulator oscillations. Optimal electron beam injection conditions are derived for minimizing the amplitude of the betatron motion, producing narrow-bandwidth undulator radiation. Polarization control is readily achieved by varying the laser pulse injection conditions.

The production of over 800 1.3-GHz superconducting (SC) cavities for the European X-ray Free Electron Laser (EXFEL), the largest in the history of cavity fabrication, has now been successfully completed. In the past, manufacturing of SC resonators was only partly industrialized; the main challenge for the EXFEL production was transferring the high-performance surface treatment to industry. The production was shared by the two companies RI Research Instruments GmbH (RI) and Ettore Zanon S.p.A. (EZ) on the principle of “build to print”. DESY provided the high-purity niobium and NbTi for the resonators. Conformity with the European Pressure Equipment Directive (PED) was developed together with the contracted notified body TUEV NORD. New or upgraded infrastructure has been established at both companies. Series production and delivery of fully-equipped cavities ready for cold rf testing was started in December 2012, and finished in December 2015. More than half the cavities delivered to DESY as specified (referred to “as received”) fulfilled the EXFEL specification. Further improvement of low-performing cavities was achieved by supplementary surface treatment at DESY or at the companies. The final achieved average gradient exceeded the EXFEL specification by approximately 25%. In the following paper, experience with the 1.3-GHz cavity production for EXFEL is reported and the main lessons learned are discussed.

We study the effects of laser pulse focusing on the spectral properties of Thomson scattered radiation. Modeling the laser as a paraxial beam we find that, in all but the most extreme cases of focusing, the temporal envelope has a much bigger effect on the spectrum than the focusing itself. For the case of ultrashort pulses, where the paraxial model is no longer valid, we adopt a subcycle vector beam description of the field. It is found that the emission harmonics are blue shifted and broaden out in frequency space as the pulse becomes shorter. Additionally the carrier envelope phase becomes important, resulting in an angular asymmetry in the spectrum. We then use the same model to study the effects of focusing beyond the limit where the paraxial expansion is valid. It is found that fields focussed to subwavelength spot sizes produce spectra that are qualitatively similar to those from subcycle pulses due to the shortening of the pulse with focusing. Finally, we study high-intensity fields and find that, in general, the focusing makes negligible difference to the spectra in the regime of radiation reaction.