Abstract Laser-driven plasma accelerators provide tabletop sources of relativistic electron bunches and femtosecond x-ray pulses, but usually require petawatt-class solid-state-laser pulses of wavelength λ L ~ 1 μm. Longer-λ L lasers can potentially accelerate higher-quality bunches, since they require less power to drive larger wakes in less dense plasma. Here, we report on a self-injecting plasma accelerator driven by a long-wave-infrared laser: a chirped-pulse-amplified CO2 laser (λ L ≈ 10 μm). Through optical scattering experiments, we observed wakes that 4-ps CO2 pulses with < 1/2 terawatt (TW) peak power drove in hydrogen plasma of electron density down to 4 × 1017 cm−3 (1/100 atmospheric density) via a self-modulation (SM) instability. Shorter, more powerful CO2 pulses drove wakes in plasma down to 3 × 1016 cm−3 that captured and accelerated plasma electrons to relativistic energy. Collimated quasi-monoenergetic features in the electron output marked the onset of a transition from SM to bubble-regime acceleration, portending future higher-quality accelerators driven by yet shorter, more powerful pulses.

The fabrication process is presented for a typical transition section located between each cylindrical corrugated waveguide structure comprising the wakefield accelerator module. The transition section includes couplers for extracting the 180 GHz TM01 accelerating mode and separate couplers for extracting the 190 GHz HE11 dipole mode, both modes induced by the electron bunch traversing the cylindrical corrugated waveguide structure. Extraction of the high-power accelerating mode reduces the heat load due to the subterahertz wave power dissipation within the corrugated accelerating structure. Extraction of the low-power dipole mode serves the purpose of detecting the electron bunch transverse oscillations within the wakefield accelerator and identifying the onset of beam breakup instability. Comprehensive testing of the fully functional transition section with an electron beam was done at the Accelerator Test Facility in Brookhaven National Laboratory which verified the functionality of the transition section.

A class of novel electron accelerator, electron Cyclotron Resonance Accelerator (eCRA) [1], is under the development to be remarkably-compact and highly efficient to produce high power electron beams and x-ray beams for medical, research, sterilization, and national security applications, so as to replace radioactive materials. The eCRA has several attractive features including: a compact robust room-temperature single-cell RF cavity as the accelerator structure; and a high-current self-scanning accelerated energetic e-beam, obviating need for a separate beam scanner. Numerical simulation and on-going experimental efforts will be reported here.

We report on an experimental realization of a two-color ionization seeded laser wakefield accelerator with a long wave infrared drive laser and a transverse, tightly focused, near infrared injector laser.

A timing method for experiments on the interaction of a near-infrared laser and an ultra-relativistic electron beam via a semiconductor plasma switch is experimentally validated. As an intermediate medium, a thin Si plate is excited by the energetic, intense electron beam to produce a semiconductor plasma, which in turn deflects counter-colliding laser light having 1 μm wavelength. An electron beam of sub-nC charge sufficiently induces the needed electron number density gradient of 1 × 1020 cm−3 per tens of μm length at the interaction point. Demonstration during an inverse Compton scattering experiment by a counter-colliding electron beam of 300 pC and 70 MeV with an Nd: YAG laser at a wavelength of 1 μm is reported.

Significant progress has been made at Argonne National Laboratory in the development of a compact wakefield accelerator based on a cylindrical corrugated waveguide with a 2-mm ID and fine corrugations on the wall. The fabrication process of 10-cm-long corrugated waveguide structures has been established and a high quality of the final product has been confirmed by precision metrology. Several samples have been tested using the electron beam at Brookhaven National Laboratory's Accelerator Test Facility. The frequency and group velocity of the fundamental monopole TM01 mode of sub-terahertz Cerenkov radiation produced by a 1.5-ps-long electron bunch propagating in the corrugated waveguide, as well as the frequencies of the dipole HEM11 and the quadrupole HEM21 modes have been measured and found to be in good agreement with the design values calculated using CST Microwave Studio. The energy modulation of a 5-ps-long electron bunch caused by the self-induced wakefield has also been measured and found to be in good agreement with the calculated values.

We report picosecond bunch length measurements using an interferometric method for a 3 MeV electron beam having bunch charge ranging from 1 to 14 pC. The method senses the single-cycle sub-terahertz (THz) pulse emitted by each electron bunch as coherent transition radiation which, in turn, is analyzed using a Michelson-type interferometer, forming an interferogram that is then processed to yield the nominal electron bunch length. This sub-THz coherent radiation intensity was measured using a quasi-optical detector (QOD) operated at room temperature. This experiment was quite challenging since the divergence angle of the sub-THz pulse emitted by the low-energy electron bunch exceeds ±10°, and its pulse energy at the entrance to the detector was as low as 100 pJ. When compared to a conventional helium-cooled silicon composite bolometer designed for frequencies above 0.5 THz, the QOD provided much better signal-to-noise ratio in the ∼80 GHz frequency range, which was critical for the successful measurement of the bunch length.

Ultrafast electron microscopy techniques have demonstrated the potential to reach very high combined spatiotemporal resolution. In order to achieve high resolution, strong focusing magnets must be used as the objective and projector lenses. In this paper, we discuss the design and development of a high-resolution objective lens for use in the BNL UEM. The objective lens is a quintuplet array of permanent magnet quadrupoles, which in sum, provide symmetric focusing, high magnification, and control of higher order aberration terms. The application and design for a proof-of-concept experiment using a calibrated slit for imaging are presented. The image resolution is monitored as a function of beam parameters (energy, energy spread, charge, bunch length, spot size), and quintuplet lens parameters (drifts between lenses).

Next Generation of Colliders N. Vafaei-Najafabadi*1,2, V. Litvinenko1,2, Y. Jing1,2, I. Pogorelsky2, M. Polyanskyi2, M. Fedurin2, M. Babzien2, R. Kupfer2, M. A. Palmer2, A. Sahai3, A. G. R. Thomas4, C. Joshi5, G. Andonian5, Y. Sakai5, J. Rosenzweig5, R. Zgadzaj6, M. C. Downer6, S. Mirov7, C. G. R. Geddes8, C. Schroeder8, E. Esarey8 1Stony Brook University, 2Brookhaven National Laboratory, 3University of Colorado, Denver, 4University of Michigan, 5University of California, Los Angeles, 6University of Texas, Austin, 7University of Alabama, 8Lawrence Berkeley National Laboratory

The capability of accelerating electron bunches at high repetition rate is one of the key performance criteria for all high average power particle accelerator applications. High gradient laser-driven acceleration holds the potential for greatly reducing size and costs of future machines, but typically requires very high peak laser powers. On the other hand, MHz pulse trains of TW-class laser beams are much beyond the state of the art, so that laser recycling and recirculation is a necessary step to bridge that gap. In this experiment we demonstrate for the first time an inverse free electron laser accelerator (IFEL) operating within an active optical cavity showing the ability to laser-accelerate electron bunch trains in burst mode at > 20 MHz repetition rate. The experimental setup, synchronization challenges and acceleration results are presented. It is found that careful control of the dispersive properties of the cavity is required in order to sustain high accelerating gradients over many passes in the laser pulse train.

The generation of X-rays and γ-rays based on synchrotron radiation from free electrons, emitted in magnet arrays such as undulators, forms the basis of much of modern X-ray science. This approach has the drawback of requiring very high energy, up to the multi-GeV-scale, electron beams, to obtain the required photon energy. Due to the limit in accelerating gradients in conventional particle accelerators, reaching high energy typically demands use of instruments exceeding 100’s of meters in length. Compact, less costly, monochromatic X-ray sources based on very high field acceleration and very short period undulators, however, may enable diverse, paradigm-changing X-ray applications ranging from novel X-ray therapy techniques to active interrogation of sensitive materials, by making them accessible in energy reach, cost and size. Such compactness and enhanced energy reach may be obtained by an all-optical approach, which employs a laser-driven high gradient accelerator based on inverse free electron laser (IFEL), followed by a collision point for inverse Compton scattering (ICS), a scheme where a laser is used to provide undulator fields. We present an experimental proof-of-principle of this approach, where a TW-class CO 2 laser pulse is split in two, with half used to accelerate a high quality electron beam up to 84 MeV through the IFEL interaction, and the other half acts as an electromagnetic undulator to generate up to 13 keV X-rays via ICS. These results demonstrate the feasibility of this scheme, which can be joined with other techniques such as laser recirculation to yield very compact photon sources, with both high peak and average brilliance, and with energies extending from the keV to MeV scale. Further, use of the IFEL acceleration with the ICS interaction produces a train of high intensity X-ray pulses, thus enabling a unique tool synchronized with a laser pulse for ultra-fast strobe, pump-probe experimental scenarios.

We present results of an experiment showing the first successful demonstration of a cascaded microbunching scheme. Two modulator-chicane prebunchers arranged in series and a high power mid-IR laser seed are used to modulate a 52 MeV electron beam into a train of sharp microbunches phase locked to the external drive laser. This configuration is shown to greatly improve matching of the beam into the small longitudinal phase space acceptance of short-wavelength accelerators. We demonstrate trapping of nearly all (96%) of the electrons in a strongly tapered inverse free-electron laser accelerator, with an order-of-magnitude reduction in injection losses compared to the classical single-buncher scheme. These results represent a critical advance in laser-based longitudinal phase space manipulations and find application in high gradient advanced acceleration as well as in high peak and average power coherent radiation sources.