Emittance is a key figure-of-merit for high brightness electron beams that enable the success of a broad range of modern accelerator-based instruments. With the successful implementation of the emittance compensation scheme and beam-shaping techniques to align each temporal beam slice in phase space, slice emittance starts to play a more dominating role in determining the beam quality. In this paper, we develop an approach, combining simulations and analytical calculations, to systematically study the evolution of the slice emittance in a very high frequency gun photoinjector, which is under intense R&D worldwide for driving the next generation continuous-wave accelerators. Our approach is capable of tracing various sources that introduce curvature in the transverse phase space, corresponding to the growth of the slice emittance. It is shown that nonlinear transverse space charge forces in the vicinity of cathodes and the spherical aberration of the focusing solenoid are the main sources of nonlinear forces, while in a long drift section space charge forces can actually compensate for nonlinear transverse position-momentum correlations and hence recover the emittance. We further demonstrate that the sources of nonlinearities can be controlled by tailoring the transverse beam density profile and the design of solenoid lenses.

To date, beam dynamics studies and design of combined zero degree drift tube linac (DTL) structures (Kombinierte Null Grad Struktur; KONUS) have only been carried out in multiparticle codes. Quantities such as the beam envelopes are obtained by averaging over particles, whose tracking is computationally intensive for large bunch populations. Tune computations, which depend on this average, are burdensome to obtain. This has motivated the implementation of a simulation of KONUS DTLs in the code transoptr, whose Hamiltonian treatment of beam dynamics enables the integration of energy gain from the longitudinal electric field on axis while simultaneously elaborating the field in transverse directions to obtain the linear optics. The code also features an in-built space charge capability. The evolution of the beam matrix including longitudinal optics is computed in a reference Frenet-Serret frame through the time-dependent DTL cavity fields. This enables fast envelope simulations for DTLs, resulting in a variable energy sequential tune optimization capability. The implementation methodology and optimization techniques, applicable for any combination of DTL tanks and bunchers, is outlined. Comparisons with the code lorasr, in addition to beam-based $E/A$ measurements of a DTL are presented.

The Advanced Light Source Upgrade will implement on axis single-train swap-out injection employing an accumulator between the booster and storage rings. The accumulator ring (AR) design is a twelve period triple-bend achromat that will be installed along the inner circumference of the storage-ring tunnel. A nonconventional injection scheme will be utilized for top-off off axis injection from the booster into the AR meant to accommodate a large $\sim 300\mathrm{nm}$ emittance beam into a vacuum-chamber with a limiting horizontal aperture radius as small as 8 mm. The scheme incorporates three dipole kickers distributed over three sectors, with two kickers perturbing the stored beam and the third affecting both the stored and the injected beam trajectories. This paper describes this “3DK” injection scheme and how it fits the AR’s particular requirements. We describe the design and optimization process, and how we evaluated its fitness as a solution for booster-to-accumulator ring injection.

High-current bright sources are needed to power the next generation of compact rf and microwave systems. A major requirement is that such sources could be sustainably operated at high frequencies, well above 1 GHz, and high gradients, well above $100\mathrm{MV}/\mathrm{m}$. Field emission sources offer simplicity and scalability in a high-frequency era of the injector design, but the output rf cycle charge and high-gradient operation remain a great and largely unaddressed challenge. Here, a field emission cathode based on ultra-nano-crystalline diamond, an efficient planar field emission material, was tested at $100\mathrm{MV}/\mathrm{m}$ in an $L$-band injector. A very high charge of 38 pC per rf cycle was demonstrated (300 nC per rf pulse corresponding to an rf pulse current of 120 mA). This operating condition revealed a two-dimensional space charge limited emission where the one-dimensional Child-Langmuir limit was surpassed. An injector brightness of ${10}^{14}\mathrm{A}/(\mathrm{rad}\mathrm{m}{)}^2$ was estimated for the given operating conditions.

We investigated coherent betatron oscillations of a deuteron beam in the storage ring cooler synchrotron and storage ring, excited by a detuned rf Wien filter (WF). The beam oscillations were detected by conventional beam position monitors. With the currently available apparatus, we show that oscillation amplitudes down to $1\mu \mathrm{m}$ can be detected. The interpretation of the response of the stored beam to the detuned rf WF is based on simulations of the beam evolution in the lattice of the ring and realistic time-dependent 3D field maps of the WF. Future measurements of the electric dipole moment of protons will, however, require control of the relative position of counter-propagating beams in the sub-picometer range. Since here the stored beam can be considered as a rarefied gas of uncorrelated particles, we moreover demonstrate that the amplitudes of the zero-point (ground state) betatron oscillations of individual particles are only a factor of about 10 larger than the Heisenberg uncertainty limit. As a consequence of this, we conclude that quantum mechanics does not preclude the control of the beam centroids to sub-picometer accuracy. The smallest Lorentz force exerted on a single particle that we have been able to determine is 10 aN.

The characterization of a high current, relativistic electron beam, designated the Super Pinch electron beam, has been performed using compact pulsed-power accelerators of various architectures with a novel diode geometry. The Thunderbird accelerator has an initial $1.5\text{−}\mu \mathrm{s}$ rise time and 150-kA peak current. The drive voltage is compressed to produce a 12-ns 500-kV voltage pulse generating a $\sim 40\text{−}\mathrm{kA}$ electron beam, which apparently exceeds the Alvén current limit although the useful current at small radius is an order of magnitude less. Using an insulated hollow cathode with a 0.5-cm anode-cathode gap, the large current is enabled by the evolution of plasma from the dielectric sleeve enveloping the cathode and a 0.5-mm wire anode. Post-shot recovery of the anode target and measurements of its deformation and damage allow an evaluation of the enhanced electron-beam focusing whereby significant beam energy is delivered to the record small, microscopic volume inside the anode target. The electron beam is seen to have conditions favorable to those needed to ignite compressed fuel in inertial confinement fusion. These results motivated a deeper insight using theoretical modeling with hybrid particle-in-cell codes. The modeling presented in this paper shows an electron beam radius of $<10\mu \mathrm{m}$ containing a current of 3–6 kA or $>500\mathrm{MA}/{\mathrm{cm}}^2$ current density. Scaling up the accelerator, such preferred focusing, target penetration, and affordability of the pulsed power generated electron beams open a new opportunity for the application of the mainstream pulsed power devices in the research and development of fusion energy.

Many high-gain free electron lasers worldwide are planning to incorporate seeding setups into their day-to-day operation. These techniques provide both longitudinal and transverse coherence and extended control of the output free electron laser radiation spectral properties. However, the output wavelength and repetition rate strongly depend on the properties of the seed laser system. With the laser peak power required for successful seeded operation, it is currently not possible to increase their repetition rate to an extent that it matches the electron bunch repetition rate of superconducting accelerators. Here, we investigate the advantages of a modification of standard seeding setups, by combining the seeding with the so-called optical klystron. With this new seeding setup, it is possible to decrease the seed laser power requirements and therefore, seed laser systems can increase their repetition rate at the same wavelength. We show simulation results in a high-gain harmonic generation (HGHG) setup for a range of harmonics (8th to 15th) and we verify the reduction of seed laser power required with an Optical Klystron HGHG scheme. Finally, we comment on the stability of the proposed setup to jitter sources and to shot-to-shot fluctuations and compare to the standard HGHG scheme.

The Japan Atomic Energy Agency (JAEA) is working on the research and development of a 30-MW continuous wave (CW) proton linear accelerator (linac) for the JAEA accelerator-driven subcritical system (ADS) proposal. The linac will accelerate a 20 mA proton beam to 1.5 GeV, using mainly superconducting cavities. The main challenge for an ADS accelerator is the high reliability required to prevent thermal stress in the subcritical reactor; thus, we pursue a robust lattice to achieve stable operation. To this end, the beam optics design reduces the emittance growth and the beam halo through the superconducting part of the linac. First, we simulated an ideal machine without any errors to establish the operation conditions of the beam. Second, we applied element errors and input beam errors to estimate the tolerance of the linac design. Finally, we implemented a correction scheme to increase the lattice tolerance by reducing the beam centroid offset on the transverse plane. Massive multiparticle simulations and a cumulative statistic of $1\times {10}^8$ macroparticles have shown that the JAEA-ADS linac can operate with less than $1\mathrm{W}/\mathrm{m}$ beam losses in error scenarios.

An active plasma lens (APL) focuses the beam in both the horizontal and vertical planes simultaneously using a magnetic field generated by a discharge current through the plasma. A beam size of $5–10\mu \mathrm{m}$ can be achieved within a short distance using a focusing gradient on the order of $100\mathrm{T}/\mathrm{m}$. The APL is therefore an attractive element for plasma wakefield acceleration, because an ultrasmall size of the witness electron beam is required for injection into the plasma wakefield to minimize emittance growth and to enhance the capturing efficiency. When the drive beam and witness electron beam copropagate through the APL, interactions between the drive and witness beams, and the plasma must be considered. In this paper, through particle-in-cell simulations, we discuss the possibility of using an APL for the final focusing of the electron beam for the AWAKE RUN 2 experiments. It is confirmed that the amplitude of the plasma wakefield excited by proton bunches remains the same even after propagation through the APL. The emittance of the witness electron beam increases rapidly in the plasma density ramp regions of the lens. Nevertheless, when the witness electron beam has a charge of 100 pC, emittance of 10 mm mrad, and bunch length of $60\mu \mathrm{m}$, its emittance growth is not significant along the active plasma lens. For small emittance, such as 2 mm mrad, the emittance growth is found to be strongly dependent on the rms beam size, plasma density, and multiple Coulomb scattering.

Staged acceleration, driven by terahertz (THz) frequency radiation pulses in a lattice with alternating orientation dielectric-lined waveguides and intervening matching optics, is shown to mitigate transverse emittance and energy spread growth, opening a route to multistage THz linacs. Decomposition of the longitudinal THz field into the multipolar components reveals a quadrupole field component with strong radial dependence. As such, it induces a transverse energy correlation in the beam during acceleration due to the large variation in the electric field with radius and azimuthal position of the electrons. An alternating orientation of stages separated by a matching section provides a compensation of transverse energy spread correlation induced in the beam during its interaction with the THz field. Furthermore, the monopolar component of the accelerating ${\mathrm{LSM}}_{11}$ mode was found to be constant with respect to transverse position, entailing zero monopolar transverse voltage and preventing emittance growth, unlike conventional radio-frequency structures. We demonstrate in a rectangular dielectric-lined waveguide structure that, when used for the acceleration of relativistic electrons, the slice transverse emittance is conserved and the growth in the slice energy spread is reduced by 70%–80% simultaneously over a system of two stages, each providing an interaction length of 4 mm and an energy gain of up to 2 MeV.

In an effort to develop and design next generation high power target materials for particle physics research, the possibility of fabricating nonwoven metallic or ceramic nanofibers by electrospinning process is explored. A low-cost electrospinning unit is set up for in-house production of various ceramic nanofibers. Yttria-stabilized zirconia nanofibers are successfully fabricated by electrospinning a mixture of zirconium carbonate with high-molecular weight polyvinylpyrrolidone polymer solution. Some of the inherent weaknesses of electrospinning process like thickness of nanofiber mat and slow production rate are overcome by modifying certain parts of electrospinning system and their arrangements to get thicker nanofiber mats of millimeter order at a faster rate. Continuous long nanofibers of about hundred nanometers in diameter are produced and subsequently heat treated to get rid of polymer and allow crystallize zirconia. Specimens were prepared to meet certain minimum physical properties such as thickness, structural integrity, thermal stability, and flexibility. An easy innovative technique based on atomic force microscopy was employed for evaluating mechanical properties of single nanofiber, which were found to be comparable to bulk zirconia. Nanofibers were tested for their high-temperature resistance using an electron beam. It showed resistance to radiation damage when irradiated with 1 MeV ${\mathrm{Kr}}^{2++}$ ion. Some zirconia nanofibers were also tested under high-intensity pulsed proton beam and maintained their structural integrity. This study shows for the first time that a ceramic nanofiber has been tested under different beams and irradiation condition to qualify their physical properties for practical use as accelerator targets. Advantages and challenges of such nanofibers as potential future targets over bulk material targets are discussed.

We examine through numerical calculation the collision of counter-propagating trains of optically spaced electron or positron microbunches in a 1 TeV collider scenario for a dielectric laser accelerator. A time-dependent envelope equation is derived for arbitrary number of bunches in the classical limit, with inclusion of the radiation reaction force. Example parameters are examined based on a constrained luminosity relation that takes into account the bunch charge for optimal efficiency, material damage limits, and power constraints. We find that for initially identical counter-propagating Gaussian bunch trains, the periodic temporal structure leads to a peak in luminosity with number of bunches. For longer bunch trains, the enhancement then decreases inversely with number of bunches. The corresponding fractional energy loss of the beam is found to be of order 1.75%, which is reduced to 0.35% when the nonlinear radial dependence of the transverse force is included, with an average beamstrahlung parameter of 0.075, an important result considering that beamstrahlung losses are a critical concern for future TeV colliders.

Laser-plasma accelerators (LPAs) now routinely produce electron beams with GeV energies over acceleration lengths on the order of a few centimeters. This capability and the demonstration of multistage coupling make LPAs promising for numerous applications. However, beam quality preservation in multistage accelerators remains an obstacle because of the need to separate the laser pulse from the electron beam. Plasma mirrors can be used to redirect the laser pulse, but their substrate thickness threatens to substantially degrade the electron beam emittance. Ultrathin ($\sim 20\mathrm{nm}$) liquid crystal (LC) plasma mirrors are an excellent candidate to address this challenge. This work investigates the robustness of thin LC plasma mirrors in the presence of capillary discharge plasma and an auxiliary heater laser. We find they can be operated $\sim 10\mathrm{cm}$ from the capillary exit when a heater laser is used. We then performed a normalized emittance measurement enabled using a 20 nm LC plasma mirror to protect electron beam optics after the LPA. The emittance contribution from scattering through the plasma mirror is calculated to be of order 100 nm, much less than the measured emittance of $4.0\mu \mathrm{m}$. Finally, we develop a model to calculate plasma mirror performance based on the laser polarization and intensity, and plasma mirror thickness.

Active longitudinal beam optics can help free-electron laser facilities achieve cutting edge performance by optimizing the beam to produce multicolor pulses, suppress caustics, or support attosecond lasing. As the next generation of superconducting accelerators comes online, there is a need to find new elements which can both operate at high-beam power and which offer multiplexing capabilities at MHz repetition rate. Laser heater shaping promises to satisfy both criteria by imparting a programmable slice-energy spread on a shot-by-shot basis. We use a simple kinetic analysis to show how control of the slice energy spread translates into control of the bunch current profile, and then we present a collection of start-to-end simulations at LCLS-II in order to illustrate the technique.

In this work, we perform particle-in-cell simulations to investigate the electron bunch quality using a chirped laser pulse in a laser wakefield acceleration mechanism. Particular attention is devoted to the electron beam quality improvement using a chirped laser pulse through group delay dispersion. By adjusting the value of group delay dispersion, the electron beam quality was controlled and improved in the range: charge (31–325 pC), energy (240–485 MeV), energy spread (2.1%–12.5%), and bunch duration (3.3–17 fs). The electron bunch charge highly depends on the chirping of the laser pulse. We anticipate that the ultrashort sub-GeV electron bunch generated in the laser wakefield acceleration mechanism using chirped laser pulses may be crucial in developing free-electron laser-based compact radiation sources.

Breakdowns (BDs) may occur in high-voltage applications even in ultrahigh vacuum conditions. Previously, we showed that it is important to pay attention to the post-BD voltage recovery in order to limit the appearance of secondary BDs associated with the primary ones. This can improve the overall efficiency of the high-voltage device. In this study, we focus on the optimization of the linear post-BD voltage recovery, with the principle aim of alleviating the problem of the secondary BDs. We investigate voltage recovery scenarios with different starting voltages and slopes of linear voltage increase by using a pulsed dc system. We find that a higher number of pulses during the voltage recovery produces fewer secondary BDs and a lower overall BD rate. Lowering the number of pulses led to more dramatic voltage recovery resulting in higher BD rates. A steeper voltage increase rate led to a more localized occurrence of the secondary BDs near the end of the voltage recovery period. It was also found that the peak BD probability is regularly observed around 1 s after the end of the ramping period and that its value decreases exponentially with the amount of energy put into the system during the ramping. The value also decays exponentially with a half-life of ($1.4\pm 0.3$) ms if the voltage only increased between the voltage recovery steps.

It has been known that a high-brightness electron beam emits broadband synchrotron radiation when traversing a curved orbit. The radiation reaction at wavelengths comparable to the bunch length or to the wavelength of a phase space modulated beam may lead to collective instabilities. In this paper, we investigate a potential single-pass instability mechanism of coherent-radiation-induced longitudinal multibunch beam breakup (BBU) instability in the presence of a closely spaced microbunch train in an undulator, particularly when the microbunch spacing is close to the resonant wavelength of the undulator. This problem is formulated based on the macroparticle model together with the slippage constraint on the beam-wave interaction. The set of coupled differential equations for individual microbunches can be solved analytically for the first few microbunches with linearization of the coherent radiation wakefield, and numerically in general nonlinear cases for unequal spacing or nonuniform filling charges. The underlying mechanisms, including the slippage effect, the potential-well effect leading to extra focusing, dependence of microbunch spacing and filling patterns, are discussed. The analysis is then applied to the recently proposed steady-state microbunching (SSMB) mechanism with two examples serving for the high average coherent radiation power sources for extreme ultraviolet (EUV) and infrared wavelength regions. For the specific scenario considered in this paper, it is found that when the microbunch spacing is close to the fundamental resonant wavelength, the coherent radiation can provide extra longitudinal focusing for the individual microbunches, leading to more stable multibunch oscillations. For the preliminary nominal SSMB designs with the average beam current of 1 A, our studies show that the single-pass longitudinal BBU instability should not be a severe issue.