A plasma window (PW) is a device for separating two areas of different pressures while letting particle beams pass with little to no loss. It has been introduced by A. Hershcovitch [J. Appl. Phys. 78, 5283 (1995)] In the course of this publication, the properties of a PW with apertures of 3.3 mm and 5.0 mm are presented. Especially the link between the pressure properties relevant for applications in accelerator systems and the underlying plasma properties depending on external parameters are presented. As working gas, a 98%Ar-2%${\mathrm{H}}_2$ mixture has been used due to the intense Stark broadening of the ${\mathrm{H}}_{\beta }$-line and the well-described Ar characteristics, enabling an accurate electron density and temperature analysis. At the low pressure side around some mbar, high-pressure values reached up to 750 mbar while operating with volume flows between 1 slm and 4 slm (standard liter per minute) and discharge currents ranging from 45 A to 60 A. The achieved ratios between high and low pressure with an active discharge range from 40 to 150. This is an improvement of a factor up to 12 over the performance of an ordinary differential pumping stage of the same geometry. Unique features of the presented PW include simultaneous plasma parameter determination and the absence of ceramic insulators between the cooling plates. Optical analysis reveals no significant damage or wear to the components after an operation time well over 10 h, whereas the cathode needle needs replacement after 5 h.

We study the dynamics of an isolated spike of radiation in superradiant regime. We show that conditions exist where the pulse moves with a group velocity larger than the velocity of light in vacuum and is followed by a pedestal resulting from a complex postsaturation dynamics. The tail is constituted by a train of subpulses with both transverse and longitudinal coherence and decaying amplitudes. We analyze the dynamical conditions leading to the formation of the main pulse and the tail. We study the correlation of the tail structure with the longitudinal phase space of the electrons, and provide a recipe to partially suppress this tail.

We present semianalytical methods for calculating the electromagnetic field in dipole and quadrupole stripline kickers with curved plates of infinitesimal thickness. Two different methods are used to solve Laplace’s equation by reducing it either to a single or to two coupled matrix equations; they are shown to yield equivalent results. Approximate analytic solutions for the lowest order fields (dipole or quadrupole) are presented and their useful range of validity are shown. The kickers plates define a set of coupled transmission lines and the characteristic impedances of modes relevant to each configuration are calculated. The solutions are compared with those obtained from a finite element solver and found to be in good agreement. Mode matching to an external impedance determines the kicker geometry and this is discussed for both kicker types. We show that a heuristic scaling law can be used to determine the dependence of the characteristic impedance on plate thickness. The solutions found by semianalytical methods can be used as a starting point for a more detailed kicker design.

The Panofsky-Wenzel theorem connects the transverse deflecting force in an rf structure with the existence of a longitudinal electric field component. In this paper it is shown that a transverse deflecting force is accompanied by an additional longitudinal magnetic field component which leads to an emittance growth in the direction perpendicular to the transverse force. The general description of transverse deflecting waves requires a linear combination of basis modes to cope with the boundary conditions and to support the simultaneous existence of electric and magnetic longitudinal field components. The mode description is preferably performed in the hybrid HM–HE basis to avoid convergence problems, which are fundamental for the transverse TM–TE basis.

We develop a three-dimensional theory of coherent synchrotron radiation in steady-state. Starting from the Hamiltonian in the Courant-Synder theory in a curvilinear coordinate system, we find that the transverse force is essentially the Lorentz force with an additional term: $-e\varphi /\rho$ in the bending plane, where $\varphi$ is the potential and $\rho$ the bending radius. The potential term partially cancels the centrifugal force, resulting in a residual wakefield: $-\mathrm{\Lambda }\delta (z)/\rho$, where, $2\le \mathrm{\Lambda }\le 4$ and $z$ is the longitudinal coordinate. The measurement in the bunch compressor supports the sign predicted by the theory but with a smaller value.

The microbunching instability seeded by small initial density modulation and driven by collective effects can cause significant electron beam quality degradation in next generation x-ray free electron lasers. A method exploiting longitudinal mixing derived from the natural transverse spread of the electron beam through a dispersive bending magnet was proposed to suppress this instability several years ago [Phys. Rev. Lett. 111, 054801 (2013)]. Instead of using bending magnets to introduce the transverse-to-longitudinal coupling, which will lead to an inconvenient deflection of the downstream beam line, in this paper, we propose a scheme to mitigate the microbunching instability by inserting a quadrupole magnet inside a bunch compressor of the accelerator. This results in transverse-to-longitudinal phase space mixing and large slice energy spread that can efficiently mitigate the growth of the microbunching instability through the major accelerator section. Finally, at the exit of the accelerator, a dogleg section is used to restore the emittance and slice energy spread before entering the undulator radiation section. Multiparticle simulations show that the transverse space charge, structure wakefield, and the coherent synchrotron radiation effects will have a relatively small impact on this scheme.

The concept of using a single nonlinear kicker (NLK) to inject electron beams into a storage ring has been proposed and tested in several synchrotron radiation light source facilities. Different from pulsed dipole kicker magnets used in a conventional local-bump injection, the single nonlinear kicker provides a nonlinear distribution of magnetic fields which has a maximum value off axis where the injected beam arrives and a zero or near-zero value at the center where the stored beam passes by. Therefore, the injected beam will receive a kick from the NLK and lose its transverse momentum, and will be eventually captured by the storage ring. In the meantime the stored beam at the center will receive no kick or less kick, which significantly reduces the injection perturbations on the stored beam. In addition, the NLK injection requires less space for the kicker and removes the complications of synchronizing four pulsed kicker magnets. Because of these advantages, several light source facilities are either proposing or already using this NLK injection as a replacement of the conventional local-bump injection scheme. In this paper, we will discuss the working principal of this NLK injection, and use both Advanced Light Source and Advanced Light source Upgrade as examples to optimize the NLK injections. By optimizing the NLK design and injection conditions, we could achieve maximum injection efficiencies for both facilities with a large injected beam from the existing ALS booster.

Dielectric-loaded structures are promising candidates for use in structure wakefield acceleration. They could be useful for both the collinear wakefield and the two-beam acceleration (TBA) approaches, due to their low fabrication cost, low radio-frequency (rf) losses, and potential to withstand high gradients. A short rf pulse ($\le 20\mathrm{ns}$) TBA program is currently being developed at the Argonne Wakefield Accelerator (AWA) facility, where dielectric-loaded structures are being used for both the power extraction and transfer structure (PETS) and the accelerator. In this study, an $X$-band 11.7 GHz dielectric PETS was developed and tested at AWA to demonstrate high-power, short-pulse wakefield generation. The PETS was driven by a 1.3 GHz train of eight electron bunches separated by 769.2 ps (9 times the $X$-band rf period) in order to achieve coherent wakefield superposition. A total train charge of 360 nC was passed through the PETS structure to generate $\sim 200\mathrm{MW}$, $\sim 3\mathrm{ns}$ flat-top rf pulses without rf breakdown. A future experiment is being planned to increase the generated rf power to approximately $\sim 1\mathrm{GW}$ by optimizing the structure design and improving the drive beam quality.

In plasma wakefield acceleration, the witness beam’s emittance needs to be preserved when it propagates through a plasma stage. The plasma includes density ramps at both the entrance and the exit. Using the Wentzel-Kramers-Brillouin solution of a single particle’s motion, analytical expressions for the evolution of the beam emittance and the Twiss parameters in an arbitrary adiabatic plasma profile are provided neglecting the acceleration of the beam inside the plasma. It is shown that the beam emittance can be preserved under the matching condition even when the beam has an initial energy spread. It is also shown that the emittance growth for an unmatched beam is minimized when it is focused to the same vacuum plane for a matched beam. The emittance evolution from 3D QuickPIC simulation results agree well with the theoretical results. In the some of the proposed experiments on nearly completed FACET II facility, the matching condition may not be perfectly satisfied and the wake may not be perfectly symmetric. It is shown that for a given set of beam parameters that are consistent with FACET II capabilities, even when the assumptions of the theory are not satisfied, the emittance growth can still be minimized by choosing the optimal focal plane. Last, another issue that may cause emittance growth in realistic plasmas is also examined. When using a lithium plasma source in FACET II experiments a helium buffer gas is used. The plasma is formed from field ionization which can lead to a nonlinear focusing force when there are nonuniform helium ions due to its high ionization potential. For an initial beam emittance of $20\mu \mathrm{m}$, the helium ionization is found to be small and the witness beam’s emittance can be preserved.

The advancement of the techniques of producing electromagnetic radiation in the terahertz range (1–20 THz) attracts a wide range of potential applications and the quest for powerful THz sources is ever-increasing. The means to generate THz is an active research field and the energy per pulse is limited due to breakdown of the medium used. In this paper, we propose a compact yet powerful narrowband THz source based on two pillars of active research in the field of novel accelerators: laser plasma wakefield acceleration and high gradient dielectric lined structures. The laser plasma wakefield (LPW) injector is used to provide the relativistic electron bunches to an interaction tube, a dielectric lined waveguide (DLW), in order to generate THz. Due to the high gradient nature of both devices, the combination ends up with a very compact solution for mJ scale THz pulse generation. Potentially these pulses can be used as drivers for nonlinear phenomena in condensed matter, which can be probed using ultrashort x-ray pulses at the same free electron laser facility.

We study the impact of a spectral noise pedestal on the statistical properties of a self-seeded free electron laser (FEL). The broad pedestal is assumed to come from self amplified spontaneous emission (SASE) in the second stage of the self-seeding system and is uncorrelated with the narrowband amplified seed. An analytic description is developed based on the statistical theory of a one-dimensional FEL in the high gain linear regime. The theory shows good agreement with experimental data, which show that the presence of a SASE background can have a strong impact on the statistical behavior of self-seeding systems.

Type-II superconductors, like the ${\mathrm{Nb}}_3\mathrm{Sn}$ used in the new HL–LHC quadrupole triplet and 11 T dipoles or FCC dipoles, are known to show an unstable behavior during magnetic-field ramps. This paper presents an analysis of the possible effects on the beam dynamics of this behavior of the magnets, with a focus on emittance blow-up.

The CERN Proton Synchrotron Booster (PSB) has been known to suffer from horizontal instabilities since its early operation in the 1970s. These instabilities appear at specific beam energies and range of working points. The source of the instability and the reason why the instabilities appear at specific energies remained unidentified. However, in routine operation, the instabilities have not been limiting the PSB performance reach thanks to the use of a horizontal feedback system, which can suppress their onset for all beam intensities needed in the PSB. Recently, the interest in these instabilities has been revived by the ongoing LHC Injectors Upgrade (LIU) program, as well as, the Physics Beyond Colliders (PBC) study group. In fact, the PSB is being upgraded to a new energy range and higher beam intensities will be requested in future operation. To ensure that these instabilities will not limit the PSB performance in the future parameter range, a systematic characterization has been carried out through several measurements and models. At fixed energy, the dependence of the instability on the working point has been fully studied experimentally. Macroparticle simulations and analytical modeling have been applied to explain the measurements, suggesting that the main driving term behind these instabilities is the unmatched termination of the PSB extraction kickers. Analytical studies also explained for the first time why the instability appears at specific energies in the PSB cycle and showed that no other instability is expected above 1.4 GeV. Finally, the hypothesis on the main instability driving term has been verified experimentally by performing measurements with the kicker terminations temporarily matched. In this configuration, no sign of instability was observed and the extraction kicker could be unambiguously identified as the source of the instability. Some options to permanently suppress the source of the instability are proposed.

We report the results from the measurements of high purity Nb samples and superconducting radio-frequency (SRF) cavities doped with nitrogen and followed by either electropolishing (EP) or buffered chemical polishing (BCP), in order to understand the role of the postdoping treatment on the performance of SRF cavities. The samples characterization via scanning electron microscope, x-ray photoelectron spectroscopy and secondary ion mass spectroscopy showed topographical differences on the surface of the samples after EP versus BCP treatment, but similar surface composition. Radio-frequency measurements were done on single cell cavities made from fine-grain and large-grain Nb treated by nitrogen doping followed by BCP and showed that improved $Q_0$ in the medium field in both fine-grain and large-grain cavities is possible with BCP postprocessing. However, there are differences between performances of large-grain versus fine-grain cavities after BCP. A cavity made from large-grain Nb showed a larger increase in $Q_0$ and a lower quench field compared to cavities made from fine-grain Nb.

A rotatable-jaw collimator design was conceived as a solution to recover from catastrophic beam impacts which would damage a collimator at the Large Hadron Collider (LHC) or its High-Luminosity upgrade (HL-LHC). One such rotatable collimator prototype was designed and built at SLAC and delivered to CERN for tests with LHC-type circulating beams in the Super Proton Synchrotron (SPS). This was followed by destructive tests at the dedicated High Radiation to Materials (HiRadMat) facility to validate the design and rotation functionality. An overview of the collimator design, together with results from tests without and with beam are presented.

Radio frequency quadrupole linac (RFQ) is the key component which realized modern high-current proton linacs, however, many RFQs are designed based on very conventional design schemes. We developed 3-MeV 50-mA ${\mathrm{H}}^{-}$ (negative hydrogen) RFQ based on a beam space-charge physics concept. The equipartitioning scheme, which is widely used in the high intensity linac design, is implemented into the RFQ design. Design performances of 99.1% transmission, $0.24\pi \mathrm{mm}\mathrm{mrad}$ transverse normalized rms emittance, and $0.11\pi \mathrm{MeV}\mathrm{deg}$ for longitudinal direction are achieved with a vane length of 3 m and the final energy of 3 MeV. An RFQ employing this design concept was fabricated, and the design performance was confirmed by beam commissioning. In this paper, the design and result of the beam commissioning of this fully equipartitioned RFQ are described.

We present the design of a rf undulator at 91.392 GHz that has a period of 1.75 mm, while the minimum beam aperture seen by the beam is 2.375 mm. To confine the fields inside the undulator a corrugated waveguide is connected through a matching section to a linear taper and a mirror. After the mirror, a Bragg reflector and a matching section are used to reflect back all the fields leaking out of the mirror opening. This undulator requires approximately 1.4 MW for submicrosecond pulses to generate an equivalent K value of 0.1. Transferring such amounts of power at mm-wave frequencies requires overmoded corrugated waveguides, and coupling through irises creates excessive pulsed heating. We have designed a coupling scheme that allows coupling power from a highly overmoded corrugated waveguide to the undulator cavity through the beam pipe without disturbing the undulator fields. This system of the rf undulator and its coupling scheme allows for fast dynamic control of the polarization of the emitted light.