We present a novel method for measurement of ultrashort electron-bunch duration, in principle, as short as zeptosecond (${10}^{-21}\mathrm{s}$). The method employs nonlinear Thomson scattering of relativistically intense laser light, and takes advantage of the nonlinear dependence and coherence of scattered light on electron bunch length. We validate the method and test its range of applicability via simulations by using realistic (nonideal) electron beams. Due to the wide flexibility in choice of interaction geometry and scattering laser pulse properties enabled by the method, it is shown to be applicable over a wide range of electron beam parameters, including energy, energy spread, and divergence angle.

A prototype quasiparasitic thermal neutron beam monitor based on isotropic neutron scattering from a thin natural vanadium foil and standard ${}^3\mathrm{He}$ proportional counters is conceptualized, designed, simulated, calibrated, and commissioned. The European Spallation Source designed to deliver the highest integrated neutron flux originating from a pulsed source is currently under construction in Lund, Sweden. The effort to investigate a vanadium-based neutron beam monitor was triggered by a list of requirements for beam monitors permanently placed in the ESS neutron beams in order to provide reliable monitoring at complex beam lines: low attenuation, linear response over a wide range of neutron fluxes, near to constant efficiency for neutron wavelengths in a range of 0.6–10 Å, calibration stability and the possibility to place the system in vacuum are all desirable characteristics. The scattering-based prototype, employing a natural vanadium foil and standard ${}^3\mathrm{He}$ proportional counters, was investigated at the V17 and V20 neutron beam lines of the Helmholtz-Zentrum in Berlin, Germany, in several different geometrical configurations of the ${}^3\mathrm{He}$ proportional counters around the foil. Response linearity is successfully demonstrated for foil thicknesses ranging from 0.04 mm to 3.15 mm. Attenuation lower than 1% for thermal neutrons is demonstrated for the 0.04 mm and 0.125 mm foils. The geometries used for the experiment were simulated allowing for absolute flux calibration and establishing the possible range of efficiencies for various designs of the prototype. The operational flux limits for the beam monitor prototype were established as a dependency of the background radiation and prototype geometry. The herein demonstrated prototype monitors can be employed for neutron intensities ranging from ${10}^3–{10}^{10}\mathrm{n}/\mathrm{s}$.

The beam-ion interaction is one of the potential limitations of beam performance in ultralow-emittance electron light sources. Conventionally, the beam-ion instability is attributed to two effects: the ion-trapping effect and fast ion effect, which are usually studied individually emphasizing the beam performance in different timescales. However, in accelerators, the electron beam suffers from both effects, requiring a self-consistent treatment. In this study, we do not regularly distinguish the ion-trapping effect and the fast ion effect but treat the beam-ion interaction in a general sense. To clarify the beam-ion dynamics process and evaluate the beam characteristics, a numerical simulation code based on the “quasi-strong-strong” model has been developed, including modules for ionization, beam-ion interaction, synchrotron radiation damping, quantum excitation, bunch-by-bunch feedback, etc. The lattice of the High Energy Photon Source is used as an example to show the beam-ion instability and its mitigation. It is found that the beam-ion instability seriously occurs in a certain low beam current region. When the beam-ion instability is significantly driven and cannot be damped by the synchrotron radiation damping, a bunch-by-bunch feedback system based on a finite impulse response filter can be adopted to mitigate it effectively.

Ionization cooling is the preferred method for producing bright muon beams. This cooling technique requires the operation of normal conducting, radio-frequency (rf) accelerating cavities within the multi-tesla fields of dc solenoid magnets. Under these conditions, cavities exhibit increased susceptibility to rf breakdown, which can damage cooling channel components and imposes limits on channel length and transmission efficiency. We report, for the first time, stable high-vacuum, normal-conducting cavity operation at gradients of $50\mathrm{MV}/\mathrm{m}$ in an external magnetic field of three tesla, through the use of beryllium cavity elements. This eliminates a significant technical risk that has previously been inherent in ionization cooling channel designs.

Polarized photon beams provide a unique experimental tool for the study of various polarization-dependent physical processes. Here, we report the experimental demonstration of full polarization control of an oscillator free-electron laser (FEL) using helical undulators of opposite helicities. Using two helical undulator magnets of opposite helicities and a buncher magnet in between, we have generated a linearly polarized FEL beam with any desirable polarization direction. With the development of a high-precision FEL polarimeter, we are able to optimize the highly polarized FEL beams in visible wavelengths and measure the polarization with high accuracy, demonstrating linear polarization $P_{\mathrm{lin}}>0.99$ on the routine basis and with the maximum polarization reaching $P_{\mathrm{lin}}=0.998$. In this paper, we describe the FEL configuration, experimental setup, and related beam diagnostics, including the newly developed high-precision FEL polarimeter. We report our experimental approaches to generate, tune up, and characterize the polarization controllable FEL beams and share a new insight into how high-degree polarization is realized based upon our investigation of the temporal structure of the FEL beam. This FEL polarization control technique has been used successfully to generate a polarization controllable Compton $\gamma$-ray beam for nuclear physics experiments.

We show that the properties of the electron beam and bright x rays produced by a laser wakefield accelerator can be predicted if the distance over which the laser self-focuses and compresses prior to self-injection is taken into account. A model based on oscillations of the beam inside a plasma bubble shows that performance is optimized when the plasma length is matched to the laser depletion length. With a 200 TW laser pulse, this results in an x-ray beam with a median photon energy of 20 keV, $>6\times {10}^8\mathrm{photons}$ above 1 keV per shot, and a peak brightness of $3\times {10}^{22}\text{photons}{\mathrm{s}}^{-1}\mathrm{mra}{\mathrm{d}}^{-2}{\mathrm{mm}}^{-2}$ $(0.1\%\mathrm{BW}{)}^{-1}$.

The Large Hadron Collider (LHC) Schottky monitors have been designed to measure various parameters of relevance to beam quality, namely tune, momentum spread, and chromaticity. In this work, we present how this instrument can be used to estimate longitudinal bunch characteristics, such as longitudinal bunch profile or synchrotron frequency distribution. Under the assumption of bunched beams with no intrabunch coherent motion, we start by deriving the relation between the distribution of synchrotron amplitudes within the bunch population and the longitudinal bunch profile from probabilistic principles. Subsequently, we fit the cumulative power density of acquired Schottky spectra with the underlying distribution of synchrotron amplitudes. Finally, the result of this fit is used to reconstruct the bunch profile using the derived model. The results obtained with this method are verified by comparison with longitudinal profile measurements from the LHC wall current monitors.

As hadron accelerators achieve increasing beam power, damage to targets is becoming increasingly severe. To mitigate this damage, nonlinear beam optics based on octupole magnets is attractive. Nonlinear optics can decrease the beam-focusing hazard due to failure of the rastering magnet. As a side effect of nonlinear optics, the beam size at the tail is known to expand drastically compared with linear optics, owing to defocusing in the nonlinear case; this may cause severe beam loss downstream of the octupole magnets. Beam shape and beam loss as a side effect of nonlinear optics have thus far been studied via a simplified filament model that ignores beam-divergence spread at the octupole magnet, so that the problem may be treated by a simplified transfer matrix. Several beam-tracking studies have been performed for the specified case given by the specified emittance and Twiss parameters, whereas a simplified model is required for application to the pragmatic beam tuning. In this study, a new generalized model is proposed for application to an octupole magnet, regardless of the filament-model approximation. It is found that the transverse distribution obtained by beam tracking can be specified by the introduction of only two parameters, namely the normalized octupole strength of $K_8^{*}$ and the $\cot \varphi$ of the phase advance from the octupole to the arbitrary position. With the new generalized model, the distribution with allowable beam loss is studied in detail. The best uniform shape is found for specific parameters of $K_8^{*}$ and $\cot \varphi$ that cannot be deduced from the filament model. However, to achieve the flat distribution, a considerable $K_8^{*}$ is required, incurring a large beam loss at the position having $\cot \varphi \sim -0.5$ downstream of the octupole. To achieve the two antagonistic requirements of reduction of the beam-peak density and minimization of the beam loss, the transverse distribution is surveyed for a large range of beam position. It is found that a bell-shaped distribution with $K_8^{*}\sim 1$ and $\cot \varphi \sim 3$ can satisfy requirements. This result is applied to beam transport in the spallation neutron source at J-PARC. The beam profile calculated using nonlinear optics is compared with the experimental result. It is proven that the transverse beam distribution with nonlinear optics and an octupole magnet can be simplified by the present generalized model, which can be applied to other facilities using octupole magnets. The calculation result shows good agreement with the experimental data, and the peak current density is reduced by 50% compared with the linear-optics case with allowable beamwidth and beam loss.

We developed an electron gun system capable of delivering high brightness beams to drive soft x-ray free-electron lasers (SXFEL). We modified the conventional radio-frequency (rf) gun concept by using a 50-kV gun with a gridded thermionic cathode connected with a 238-MHz acceleration cavity. We achieved a low target emittance by applying the following two methods: (1) to obtain a parallel beam after the control grid, we optimized the grid voltage to compensate for the electric potential distortion near the grid (i.e., the lens effect); and (2) to avoid emittance growth due to the space charge effect at low energy regions, the 50-kV gun was connected to a 238-MHz rf cavity with a minimum distance, which immediately accelerated the beam energy to 500 keV. We built a gun test stand to verify the high beam performance, i.e., a normalized emittance of less than 4 mm mrad with a bunch charge of 1 nC and a pulse length shorter than 0.7 ns, as predicted by cst and parmela codes. Our proof-of-performance experiments demonstrated that a projection emittance of 1.7 mm mrad was obtained as a core part, representing 60% of all extracted electrons and the measured emittance agreed well with the simulation value of 2.0 mm mrad. This paper presents an overview of the electron gun system and our proof-of-performance experimental results.

Recently, laser processing of copper samples has been demonstrated to produce rough surfaces whose nanostructuring ensures unquestionable advantages for electron cloud mitigation in future particle accelerators. The actual application of laser treated surfaces in accelerators implies that this new material is compliant with many issues, going from impedance vacuum properties to many others. A significant experimental effort is therefore ongoing to study and optimize their various properties of interest. Here we analyze their vacuum behavior versus temperature. To this end, we studied thermal programmed desorption from CO, ${\mathrm{CH}}_4$ and ${\mathrm{H}}_2$ once cryosorbed on laser treated copper substrate and on its flat counterpart. These molecules are typically present in the residual vacuum of any accelerator. The results show that the desorption of such gases from the laser treated substrates occurs in a much broader and higher temperature range with respect to what is observed from the flat substrate. We also show that, at equal doses, treated samples adsorb/desorb significantly more gas than their flat counterpart. These findings can be ascribed to their nanostructured porous morphology. A quantitative analysis is given, allowing to properly estimate fluctuations of the number of molecules during unavoidable temperature variations of the cryogenic vacuum system.

Transverse emittance growth due to space-charge-induced resonance was studied for high-power beam operation in the J-PARC main ring synchrotron. The turn-by-turn beam sizes were measured near the structure resonance at ${\nu }_x=21$ and near the nonstructure resonance at ${\nu }_x=22$. A clear difference in the emittances was observed. The emittance was observed to grow by 5 times at the operating tune ${\nu }_x=21.10$, and by 1.8 times at ${\nu }_x=22.05$ in the first 100 turns. The results show the advantage of tuning in the region of ${\nu }_x=22$ for high intensity beam operation. The turn-by-turn emittance varied depending on the horizontal tune in the region of ${\nu }_x=21$. This indicates that the mechanism of emittance growth is related to the tune spread distribution. The results were satisfactorily reproduced by the simulations of the beam dynamics, including the space-charge effect incorporated via the particle-in-cell algorithm. To reveal the mechanism of the emittance growth, the space-charge effects on the resonances were evaluated by simulations as well as analytical calculations.

We present a systematic comparison of two complementary methods for determining the longitudinal charge density profile of the compressed electron bunches in the soft x-ray free-electron laser FLASH: a frequency-domain technique—coherent transition radiation (CTR) spectroscopy—and a time-domain technique—streaking of the electron beam with a transversely deflecting microwave structure (TDS). While the direct time-profile measurement with a TDS is a well-established method invented at SLAC, our group has pioneered high-resolution bunch shape analysis based on coherent radiation spectroscopy. We have developed a broadband spectrometer covering the wavelength range from $5\mu \mathrm{m}$ to $433\mu \mathrm{m}$ with two sets of remotely interchangeable staged reflection gratings. The measured spectral intensity allows to compute the absolute magnitude of the bunch form factor but not its phase which, however, is needed to retrieve the bunch profile. Two phase retrieval methods are investigated in detail: analytic phase computation by means of the Kramers-Kronig dispersion relation, and iterative phase retrieval. Several computational techniques are compared and evaluated in view of their applicability and efficiency. For a large variety of bunch shapes, the time profiles derived from the spectroscopic data are compared with the TDS profiles, and generally excellent agreement is observed down to the 10 fs level. For part of the measurements, two independent CTR spectrometer systems have been available, yielding almost identical bunch shapes. In summary, we demonstrate that using well calibrated and broadband spectroscopy data, a fast and reliable phase reconstruction algorithm leads to bunch profiles competitive to high resolution TDS measurements.

The Integrable Optics Test Accelerator (IOTA) is a novel storage ring at Fermi National Accelerator Laboratory designed (in part) to investigate the dynamics of beams in the presence of highly nonlinear transverse focusing fields that generate integrable single-particle motion with a large spread in the intrinsic betatron tunes. We describe how contemporary geometrical methods from the theory of integrable Hamiltonian systems may be used to locate all critical separatrixlike structures in the 4D transverse phase space, and to construct a complete analysis of the dynamical bifurcations of the system. Application of these techniques results in a global picture of the nominal on-energy transverse dynamics, revealing a rich diversity of accessible dynamical behavior. Similar techniques may be applied to future facilities that exploit the concept of nonlinear integrable optics.

Transient beam loading effects and microwave instability are very important in ion synchrotrons. A simulation code longitudinal particle tracking code (LPTC) based on multiparticle tracking is developed to simulate the beam loading effects and the longitudinal beam instabilities in the booster ring (BRing) of the High Intensity heavy-ion Accelerator Facility (HIAF). A process of beam capture, acceleration, debunching, and the second capture is simulated with a reference ion beam of ${{}^{78}\mathrm{Kr}}^{19+}$. The simulation results show that the beam loading effects have major contribution to the beam behavior during the debunching stage and the second capture stage. A feed-forward system is suggested to compensate the beam loading effects, and the main parameters of the feed-forward system are given. The microwave instability of coasting ${{}^{78}\mathrm{Kr}}^{19+}$ beams is observed from the simulation results. The threshold of the microwave instability is also given. The development of self-bunching and methods to suppress the microwave instability in the BRing are discussed.

A new beam dump has been designed, built, installed and operated to withstand the future proton beam extracted from the proton synchrotron booster (PSB) in the framework of the LHC Injector Upgrade (LIU) Project at CERN. The future proton beam consists of up to $1\times {10}^{14}$ protons per pulse at 2 GeV and is foreseen after the machine upgrades planned for CERN’s Long Shutdown 2 (2019-2020). In order to be able to efficiently dissipate the heat deposited by the primary beam, the new dump was designed as a cylindrical block assembly, made out of a copper alloy and cooled by forced airflow. In order to determine the energy density distribution deposited by the beam in the dump, Monte Carlo simulations were performed using the fluka code, and thermomechanical analyses were carried out by importing the energy density into ANSYS. In addition, computational fluid dynamics (CFD) simulations of the airflow were performed in order to accurately estimate the heat transfer convection coefficient on the surface of the dump. This paper describes the design process, highlights the constraints and challenges of integrating a new dump for increased beam power into the existing facility and provides data on the operation of the dump.

An ultrarelativistic electron beam passing through an intense laser pulse emits radiation around its direction of propagation into a characteristic angular profile. Here, we show that measurement of the variances of this profile in the planes parallel and perpendicular to the laser polarization, and the mean initial and final energies of the electron beam, allows the intensity of the laser pulse to be inferred in a way that is independent of the model of the electron dynamics. The method presented applies whether radiation reaction is important or not, and whether it is classical or quantum in nature, with an accuracy of a few percent across 3 orders of magnitude in intensity. It is tolerant of electron beams with a broad energy spread and finite divergence. In laser-electron-beam collision experiments, where spatiotemporal fluctuations cause the alignment of the beams to vary from shot to shot, this permits inference of the laser intensity at the collision point, thereby facilitating comparisons between theoretical calculations and experimental data.

The acceleration of polarized electrons and protons in strong laser and plasma fields is a very attractive option to obtain polarized beams in the GeV range. We investigate the feasibility of particle acceleration in strong fields without destroying an initial polarization, taking into account all relevant mechanisms that could cause polarization losses, i.e. the spin precession described by the T-BMT equation, the Sokolov-Ternov effect and the Stern-Gerlach force. Scaling laws for the (de-)polarization time caused by these effects reveal that the dominant polarization limiting effect is the rotation of the single particle spins around the local electromagnetic fields. We compare our findings to test-particle simulations for high energetic electrons moving in a homogeneous electric field. For high particle energies the observed depolarization times are in good agreement with the analytically estimated ones.