Since the introduction of the radio-frequency (rf) photoinjector electron source over thirty years ago, peak performance demands have dictated the use of high accelerating electric fields. With recent strong advances in obtainable field values, attendant increases in beam brightness are expected to be dramatic. In this article, we examine the implementation of very high gradient acceleration in a high frequency, cryogenic rf photoinjector. We discuss in detail the effects of introducing, through an optimized rf cavity shape, rich spatial harmonic content in the accelerating modes in this device. Higher spatial harmonics give useful, enhanced linear focusing effects, as well as potentially deleterious nonlinear transverse forces. They also serve to strongly increase the ratio of average accelerating field to peak surface field, thus aiding in managing power and dark current-related challenges. We investigate two scenarios which are aimed at unique exploitation of the capabilities of this source. First, we investigate the obtaining of extremely high six-dimensional brightness for advanced free-electron laser applications. We also examine the use of a magnetized photocathode in the device for producing unprecedented low asymmetric emittance, high-current electron beams that reach linear collider-compatible performance. As both of the scenarios demand an advanced, compact solenoid design, we describe a novel cryogenic solenoid system. With the high field rf and magnetostatic structures introduced, we analyze the collective beam dynamics in these systems through theory and multiparticle simulations, including a particular emphasis on granularity effects associated with microscopic Coulomb interactions.

In this Letter, we report on the experimental generation of high energy (10 GeV), ultrashort (femtosecond-duration), ultrahigh current (∼0.1 MA), petawatt peak power electron beams in a particle accelerator. These extreme beams enable the exploration of a new frontier of high-intensity beam-light and beam-matter interactions broadly relevant across fields ranging from laboratory astrophysics to strong field quantum electrodynamics and ultrafast quantum chemistry. We demonstrate our ability to generate and control the properties of these electron beams by means of a laser-electron beam shaping technique. This experimental demonstration opens the door to on-the-fly customization of extreme beam current profiles for desired experiments and is poised to benefit a broad swath of cross-cutting applications of relativistic electron beams.

An approach to fabricating Halbach array undulators using “combs” machined from single magnets is introduced. This technique is especially relevant to the fabrication of short period micro-undulators with period lengths considerably less than the few-centimeter-scale typical of current undulators. Manual, magnet-by-magnet assembly of micro-undulators would require the manipulation and alignment of thousands of magnets smaller than a grain of rice: comb fabrication dramatically increases the size of the basic unit cell of assembly with no increase in undulator period by creating many periods from a single piece, in a single machining modality. Further, as these comb teeth are intrinsically indexed to each other, tolerances are dictated by a single manufacturing step rather than accumulating errors by assembling many tiny magnets relative to each other. Different Halbach geometries, including $M^{\prime }=2$ , $M^{\prime }=4$ , isosceles triangle, and hybrid, are examined both from a theoretical perspective and with 3D magnetostatic simulations.

The use of a periodic electron beam bunch train to resonantly excite plasma wakefields in the quasinonlinear (QNL) regime has distinct advantages over employing a single, higher charge bunch. Resonant QNL excitation can produce plasma electron blowout using a small charge per pulse if the beam emittance beams are very low. The local density perturbation in such a case is extremely nonlinear, achieving total rarefaction, yet the resonant response of the plasma electrons at the plasma frequency is preserved. The needed electron beam pulse train with interbunch spacing equal to the plasma period can be produced via inverse free-electron laser bunching. As such, in achieving resonance with a laser wavelength of a few microns, a high plasma density is employed, with the attendant possibility of obtaining extremely large wakefield amplitudes, near $1\mathrm{TV}/\mathrm{m}$ for FACET-II parameters. In this article, we use particle-in-cell (PIC) simulations to study the plasma response, the beam evolution including density modulation, and the instabilities encountered when using a bunched-beam scheme to resonantly excite waves in a dense plasma.

Undulators and wigglers based on Halbach arrays have been in widespread use since their description in the 1980s, due to the high fields they generate from permanent magnets. The most common implementation of a pure permanent magnet Halbach undulator consists of four magnetic segment widths per undulator period. In typical undulators with periods of order $\sim \mathrm{cm}$ it is straightforward to use sufficiently small magnetic segments to achieve the desired undulator period. However, in the context of new microundulators with periods of order $\sim \mathrm{mm}$ or shorter, there is a lower bound on the feature sizes achievable by existing fabrication techniques. This motivates the use of modified Halbach arrays comprised of right triangle magnet segments with only two magnet segment widths per undulator period. These configurations are demonstrated to have superior field strengths to up-down arrays with no increase to the undulator period. Additional considerations arising from the broken symmetries in such configurations, such as the presence of previously forbidden harmonic components, are also considered. Such right triangle Halbach arrays may be fabricated using MEMS techniques for use in the next generation of compact light sources.

We report the development of a multileaf collimator (MLC) for charged particle beams, based on independently actuated tungsten strips that can selectively scatter unwanted particles. The MLC is used in conjunction with an emittance exchange beamline to rapidly generate highly variable longitudinal bunch profiles. The developed MLC consists of 40 independent leaves that are 2 mm wide and can move up to 10 mm and operates in an ultrahigh vacuum environment, enabled by novel features such as magnetically coupled actuation. An experiment at the Argonne Wakefield Accelerator, which previously used inflexible, laser-cut masks for beam shaping before an emittance exchange beamline, was conducted to test functionality. The experiment demonstrated myriad transverse mask silhouettes, as measured on a scintillator downstream of the MLC, and the corresponding longitudinal profiles after emittance exchange, as measured using a transverse-deflecting cavity. Rapidly changing between mask shapes enables expeditious execution of various experiments without the downtime associated with traditional methods. The many degrees of freedom of the MLC can enable the optimization of experimental figures of merit using feed-forward control and advanced machine learning methods.