Harmonic lasing provides an opportunity to extend the photon energy range of existing and planned x-ray free electron laser (FEL) user facilities. Contrary to nonlinear harmonic generation, harmonic lasing can generate a much more intense, stable, and narrow-band FEL beam. Another interesting application is harmonic lasing self-seeding that allows to improve the longitudinal coherence and spectral power of a self-amplified spontaneous emission FEL. This concept was tested at the soft x-ray FEL user facility FLASH in the range of 4.5–15 nm and at Pohang accelerator laboratory X-ray Free Electron Laser (XFEL) at 1 nm. In this paper we present recent results from the European XFEL where we successfully demonstrated harmonic lasing at 5.9 Angstrom and 2.8 Angstrom. In the latter case we obtained both third and fifth harmonic lasing and, for the first time, operated a harmonic lasing cascade (fifth-third-first harmonics of the undulator). These results pave the way for reaching very high photon energies, up to 100 keV.

A novel method has been developed that allows extracting the loaded quality factor of particle accelerating radio-frequency cavities at any time during beam operation from acquired control system process parameters without the need of additional measurement equipment. Concurrently it assesses the attenuation in a cavities’ power transmission line for any rf generator to rf cavity mismatch condition, and thus can correct calibration inaccuracies of measured forward and reflected power levels. The mathematical formalism is described. It is specifically derived for cryogenically cooled superconducting cavities for which the loaded quality factor can be equated with the external quality factor of the input power coupler. The described method has been applied for 418 superconducting cavities in operation at the Continuous Electron Beam Accelerator Facility at the Thomas Jefferson National Accelerator Facility. The validation of the method as well as its beneficial use as a diagnostic tool is demonstrated for a finite number of examples utilizing process variables archived during a period in the past when the linac beam current was ramped up multiple times. The method is in the process of being implemented in the machines’ control system for an in situ and in operando evaluation of the power coupler’s external quality factor.

Beam loss detection is essential for the machine protection and the fine-tuning of the accelerator to reduce the induced radioactivity. Monte Carlo simulations are also vital for the choice of beam loss monitor (BLM) type, for predicting and understanding the BLM response to beam losses. At the China Spallation Neutron Source (CSNS), the cylindrical ionization chamber (IC) filled with $\mathrm{Ar}/{\mathrm{N}}_2$ gas mixture is the main type of the BLM to detect the beam losses. This paper presents the detailed beam loss experiments and fluka simulations for the CSNS BLM system. It includes the response functions of the BLM detectors by taking into account the contributions of different secondary particles to the energy deposition in the sensitive volume of the BLM. Dedicated experiments were compared with fluka simulations. It was found that in the low energy section of the linac the usual ion chamber based BLMs are not sensitive enough. A BLM based on ${\mathrm{BF}}_3$ enclosed by a high-density polyethylene (PE) moderator is effective to detect the beam losses through detecting thermal neutrons. Its signal is about 3 orders of magnitude higher than that of $\mathrm{Ar}/{\mathrm{N}}_2$ BLM for a beam energy of 15 MeV. The simulations provide the results of the intrinsic delay time for the ${\mathrm{BF}}_3$ monitor and the thickness optimization for PE to reduce the delay. Finally, the simulated spatial resolution of loss location by detection of neutrons is also evaluated for a beam energy of 15 MeV, which presents a resolution of $\sim 2\mathrm{m}$ in our experimental configuration.

Bunch-by-bunch beam parameters monitor is a very useful tool for accelerator operation and optimization. An in situ bunch-by-bunch three-dimensional position and charge measurement system has been developed at the Shanghai Synchrotron Radiation Facility. Signals from beam position monitors carry almost full beam information, a broadband digital receiver can retain most information. Therefore, a four-channel wideband oscilloscope is employed to capture the signals from all electrodes of the beam position monitor. Correlation coefficient-based methods are used to extract these bunch-by-bunch parameters synchronously. We evaluated the performance of this bunch-by-bunch information extraction method, the measurement uncertainty of longitudinal phase is less than 0.2 ps, the uncertainty of transverse position is less than $10\mu \mathrm{m}$ and the charge uncertainty is 0.3% under the condition that the bunch charge is 600 pC. Thanks to the great performance of the new system, it is accurate enough to study the transient state during injection. In addition, an in situ wakefield measurement system is planned to be established based on this system.

An x-ray beam position monitor (XBPM) prototype that can be operated in pulse mode has been designed and demonstrated. The monitor has a microstripline structure for signal transmission lines and a titanium electrode sputtered on a diamond heat sink as a photocathode. The detection elements of this monitor generate unipolar single pulses with a full width at half maximum of less than 1 ns, allowing a pulse-by-pulse measurement of the synchrotron radiation beam. By synthesizing these pulses, a directly readable position sensitive signal is provided from pulse to pulse. This monitor can be used in the direct current mode in which it exhibits a stability and resolution as good as those of conventional XBPMs. Results of the performance tests of the monitor at the SPring-8 synchrotron radiation beam line are also presented.

We propose a method to use traveling-wave Thomson scattering for spatiotemporally-resolved electron spectroscopy. This can enable ultrafast time-resolved measurements of the dynamics of relativistic electrons in the presence of extremely intense light fields, either in vacuum or in plasma, such as in laser wakefield accelerators. We demonstrate, with test-particle simulation and analysis, the capability of this technique for measurements of various high field phenomena: radiation reaction of electrons due to scattering, dephasing of a laser wakefield accelerator, and acceleration of electrons in multiple buckets by a laser wakefield.

Laser-assisted charge exchange (LACE) is a novel scheme for injecting $H^{-}$ ions into proton rings. Lorentz stripping of $H^{-}$ ions into bare protons in magnetic dipoles is enabled by laser excitation that reduces the electron binding energy. We show that such atomic transitions impose very precise requirements on ion and laser parameters which can be utilized to measure the ion velocity. At the LACE experimental station at the Spallation Neutron Source (SNS), proton beam energy has been measured via LACE to a precision of $<1\mathrm{MeV}$ for a $\approx 1\mathrm{GeV}$ beam. To verify the results against the existing energy measurement method at the SNS which calculates revolution frequency in the accumulator ring, we discuss how knowledge of the beam energy can be employed to synchronize phase probes. Such beam-based calibration using energy measurements via both LACE and the SNS ring show that the two methods produce consistent results.

The multi-pass recirculating proton linac can significantly improve the usage efficiency of rf superconducting cavities by accelerating the proton beam through the same cavity multiple times. However, in order to achieve acceleration for different energy proton beams, longitudinal phase synchronous conditions among these beams have to be satisfied. In this paper, we propose a fixed field phase shifter system based on two superconducting bending magnets with an optimized field distribution to meet the synchronous conditions. Such a phase shifter provides both longitudinal phase delay and transverse focusing of multiple energy beams. Self-consistent macroparticle simulations including space-charge effects through the system show that the proton beam emittances can be well preserved after the phase shifter for multiple energy beams.

Active plasma lenses (APLs) have been proved as an effective device for focusing charged particles. In this paper, we have theoretically studied the feasibility of using a discharged-capillary based high gradient APL to focus laser accelerated pulsed proton beams. We specify equations of motion of proton beams in both straight and curved discharge-capillary APLs, on which the regulation law of a specific APL focusing and tuning protons can be calculated. Then, we summarize several constraints of APLs. Confirmed by a 2D PIC simulation, we find that with a proper plasma state, the proton energy loss, the transverse current distribution, and the Z-pinch effect do not significantly affect the beam transport, while the passive plasma lens (PPL) effect can promote the APL focusing within an acceptable extent. The regime of working parameters, including the plasma density $n_0$, the discharge current $I$, the length $L$ and the radius $R$ of APLs are given for different aimed proton energy. The results show that APL, whose parameters can be adjusted flexibly, is suitable for focusing and transporting proton beams with energy of 1–100 MeV. It is expected to build a beam-line based on plasma with centimeter length scale for pulsed proton beams. Such compact beam-line can have significant impact on various applications.

Since the beginning of the Large Hadron Collider (LHC) commissioning, spectral components at harmonics of the mains frequency (50 Hz) have been observed in the transverse beam spectrum. This paper presents an overview of the most important observations, collected during the latest physics operation of the LHC in 2018, which clearly indicates that the harmonics are the result of a real beam excitation rather than an instrumental feature. Based on these findings, potential sources of the perturbation are discussed and a correlation with power supply ripple originating from the magnets’ power supplies is presented.

Harmonics of the mains frequency (50 Hz) have been systematically observed in the transverse beam spectrum of the Large Hadron Collider (LHC) since the start of its operation in the form of dipolar excitations. In the presence of strong nonlinearities such as beam-beam interactions, as many of these 50 Hz harmonics reside in the vicinity of the betatron tune they can increase the tune diffusion of the particles in the distribution, leading to proton losses and eventually to a significant reduction of the beam lifetime. The aim of this paper is to determine whether the 50 Hz harmonics have an impact on the beam performance of the LHC. A quantitative characterization of the 50 Hz ripple spectrum present in the operation of the accelerator, together with an understanding of its source is an essential ingredient to also evaluate the impact of the 50 Hz harmonics on the future upgrade of the LHC, the High Luminosity LHC (HL-LHC). To this end, simulations with the single-particle tracking code, sixtrack, are employed including a realistic 50 Hz ripple spectrum as extracted from experimental observations to quantify the impact of such effects in terms of tune diffusion, dynamic aperture, and beam lifetime. The methods and results of the tracking studies are reported and discussed in this paper.

This paper presents the design and testing of several different configurations of spiral generator, designed to trigger high current switches in the next generation of pulsed power devices. In particular, it details the development of spiral generators that utilize new ultrafast thyristor technology as an input switch, along with a polarity dependent output gap to improve the efficiency of the spiral generator design. The generator produced 50 kV from a 3.6 kV charging voltage, with a rise time of only 50 ns and a jitter of 1.3 ns—directly comparable, if not better than, a generator employing a triggered spark gap as the input switch. The output gap was constructed in house from commonly available components and a 3D printed case, and showed remarkable repeatability and stability—simple alterations to the output gap could further reduce the rise time. The entire spiral generator, along with control and charging electronics, fitted into a case only $210\times 145\times 33\mathrm{mm}$.

A method for using betatron coupling to suppress partial snake driven resonances in a synchrotron is described. While the partial snakes permit avoidance of the strong resonances associated with the vertical betatron motion of the particles, they also excite numerous weak resonances associated with the horizontal betatron motion. These resonances occur whenever the fractional spin tune equals the fractional horizontal betatron tune. Since these are the same frequencies at which depolarizing resonances from betatron coupling occur, coupling resonances can be intentionally driven to exactly cancel the resonance driving terms of the partial snakes, thereby avoiding polarization loss due to these resonance crossings. Presented here is an explicit derivation of the partial snake driven resonances as betatron sidebands of strong imperfection resonances and a description of a practical scheme for their correction using the Brookhaven AGS as an example. Tracking results that verify the suppression via coupling are presented.

The free-electron laser (FEL) community is interested in taking full advantage of the high-repetition-rates of FELs run by superconducting machines while maintaining the spectral properties achieved with external seeding techniques. Since the feasibility of seed lasers operating at a repetition-rate of MHz and with sufficient energy in a useful wavelength range, such as the ultraviolet (UV) range is challenging, a seeded oscillator-amplifier scheme is proposed instead for generation of fully coherent and high-repetition-rate radiation. The process is triggered by an external seed laser while an optical feedback system feeds the radiation back to the entrance of the modulator where it overlaps with the next electron bunch. Downstream from the feedback system, the electron bunches are then used for harmonic generation. We discuss the optimization of dedicated simulations and we investigate the stability of this scheme with numerical simulations. As a result, we address the control of the reflectivity of the resonator as a key parameter to achieve a stable HGHG seeded radiation. Finally, we show the impact of the power fluctuations in the oscillator on the bunching amplitude with analytical and simulated results. The output FEL radiation wavelengths considered are 4.167 nm and 60 nm.

We present a theory that can compute the transverse coupled-bunch instability growth rates at any chromaticity and for any longitudinal potential provided only that the long-range wakefield varies slowly over the bunch. The theory is expressed in terms of the usual coupled-bunch eigenvalues at zero chromaticity, and when the longitudinal motion is simple harmonic our solution only requires numerical root-finding that is easy to implement and fast to solve; the more general case requires some additional calculations, but is still relatively fast. The theory predicts that the coupled-bunch growth rates can be significantly reduced when the chromatic betatron tune spread is larger than the coupled-bunch growth rate at zero chromaticity. Our theoretical results are compared favorably with tracking simulations for the long-range resistive wall instability, and we also indicate how damping and diffusion from sychrotron emission can further reduce or even stabilize the dynamics.

Vertical excursion fixed-field alternating gradient accelerators can be designed with tunes that are invariant with respect to momentum and trajectories that are scaled images of each other displaced only in the vertical direction. This is possible using guiding fields that have a vertical exponential increase, with a skew quadrupole component in the magnet body and a solenoid component at the magnet ends. Numerical analysis was used in optimization due to the coupling effects on orbits and optics. In this paper, idealized magnetic fields are calculated from first principles, taking into account end fields. The parameter dependence of the optics and the dynamic aperture of the ring are calculated for the example of a ring with an approximately 25 m circumference that accelerates proton beams from 3 MeV to 12 MeV. The paper reports for the first time the design of such an accelerator lattice using tools specifically devised to analyze transverse coupled optics without the need for approximations.

A higher harmonic cavity (HHC), used to cause bunch lengthening for an increase in the Touschek lifetime, is a feature of several fourth generation synchrotron light sources. The desired bunch lengthening is complicated by the presence of required gaps in the bunch train. In a recent paper the author and Venturini studied the effect of various fill patterns by calculating the charge densities in the equilibrium state, through coupled Haïssinski equations. We assumed that the only collective force was from the beam loading (wakefield) of the harmonic cavity in its lowest mode. The present paper improves the notation and organization of the equations so as to allow an easy inclusion of multiple resonator wakefields. This allows one to study the effects of beam loading of the main accelerating cavity, higher order modes of the cavities, and short range geometric wakes represented by low-$Q$ resonators. As an example these effects are explored for ALS-U. The compensation of the induced voltage in the main cavity, achieved in practice by a feedback system, is modeled by adjustment of the generator voltage through a new iterative scheme. Except in the case of a complete fill, the compensated main cavity beam loading has a substantial effect on the bunch profiles and the Touschek lifetimes. A $Q=6$ resonator, approximating the effect of a realistic short range wake, is also consequential for the bunch forms.

A 4 MV flash x-ray radiographic machine based on induction voltage adders has been developed. The configuration and design of this machine are reviewed. A three-dimensional, fully electromagnetic model and a circuit simulation model are established to compare with the experiments. The simulation results are in overall agreement with the electrical measurements. The pulsed power performances and output fluctuations of this machine over successive shot sequences are demonstrated. Among the 54 shots, the average peak output voltage is $4.4\pm 0.3\mathrm{MV}$ ($1\text{−}\sigma$) and the average diode current is $81.6\pm 4.5\mathrm{kA}$ ($1\text{−}\sigma$). Four typical malfunction modes are identified shot by shot including the diode-impedance collapse, insulator flashover, core saturation, and drive mistiming. Some remarkable features from each fault mode are recognized. The first-to-last time spreads of the four drive pluses, $t_{\mathrm{spread}}$, are chosen to quantify the drive synchronization and the influences of the $t_{\mathrm{spread}}$ on the peak voltages and diode currents are summarized from the almost 100 shots since the machine was commissioned. It is found that, in order to achieve a voltage of up to 4 MV, $t_{\mathrm{spread}}$ should not exceed 25 ns, which is approximately twice the time for electromagnetic wave propagation from the first cavity to the last cavity in vacuum. In addition, the rise time and FWHM duration of output voltages varying with $t_{\mathrm{spread}}$ are given. The results indicate that the rise time changes little at the beginning but increases exponentially once the $t_{\mathrm{spread}}$ exceeds 30 ns. The FWHM duration nearly increases linearly with $t_{\mathrm{spread}}$.