UCLA, Los Angeles, USA
SLAC, Menlo Park, California, USA
RadiaBeam, Santa Monica, California, USA
The University of Liverpool, Liverpool, United Kingdom
The extremely intense beam generated at FACET-II provides the unique opportunity to investigate the effects of beam-driven GV/m fields in dielectrics exceeding meter-long interaction lengths. The diverse range of phenomena to be explored, such as material response in the terahertz regime, suppression of high-field pulse damping effects, advanced geometry structures, and methods for beam break up (BBU) mitigation, all within a single UHV vacuum vessel, requires flexibility and precision in the experimental layout. We present here details of the experimental design for the dielectric program at FACET-II. Specifically, consideration is given to the alignment of the dielectric structures due to the extreme fields associated with the electron beam, as well as implementation of electron beam and Cherenkov radiation-based diagnostics.
RadiaBeam, Santa Monica, California, USA
RadiaSoft LLC, Boulder, Colorado, USA
ANL, Lemont, Illinois, USA
UCLA, Los Angeles, California, USA
The plasma photocathode concept, whereby a two-species gas mixture is used to generate a beam -driven accelerating wakefield and a laser-ionized generation of a witness beam, was recently experimentally demonstrated. In a variation of this concept, a beam-driven dielectric wakefield accelerator is employed, filled with a neutral gas for laser-ionization and creation of a witness beam. The dielectric wakefields, in the terahertz regime, provide comparatively modest timing requirements for the injection phase of the witness beam. In this paper, we provide an update on the progress of the experimental realization of the hybrid dielectric wakefield accelerator with laser injected witness beam at the Argonne Wakefield Accelerator (AWA), including engineering considerations for gas delivery, and preliminary simulations.
UCLA, Los Angeles, California, USA
RadiaBeam, Santa Monica, California, USA
The University of Liverpool, Liverpool, United Kingdom
Cockcroft Institute, Warrington, Cheshire, United Kingdom
Wakefields in dielectric structures are a useful tool for beam diagnostics and manipulation with applications including acceleration, shaping, chirping, and THz radiation generation. It is possible to use the produced THz radiation to diagnose the fields produced during the DWA interaction but, to do so, it is necessary to effectively out-couple this radiation to free space for transport to diagnostics such as a bolometer or interferometer. To this end, simulations have been conducted using CST Studio for a 10 GeV beam with FACET-II parameters in a slab-symmetric, dielectric waveguide. Various termination geometries were studied including flat cuts, metal horns, and the "Vlasov antenna". Simulations indicate that the Vlasov antenna geometry is optimal and detailed studies were conducted on a variety of dielectrics including quartz, diamond, and silicon. Multiple modes were excited and coherent Cherenkov radiation (CCR) was computationally generated for both symmetric and asymmetric beams. Finally, we include witness beams to study transport and acceleration dynamics as well as the achievable field gradients.
TUXB06
Enrico Fermi Institute, University of Chicago, Chicago, Illinois, USA
UCLA, Los Angeles, USA
Northern Illinois University, DeKalb, Illinois, USA
ANL, Lemont, Illinois, USA
To overcome limits on total acceleration achievable in plasma wakefield accelerators, specially shaped drive beams can be used to increase the transformer ratio, implying that the drive beam deceleration is minimized in comparison with acceleration obtained in the wake. We report the results of a nonlinear PWFA, high transformer ratio experiment using high-charge, longitudinally asymmetric drive beams in a plasma cell. An emittance exchange process is used to generate variable drive current profiles, in conjunction with a long (multiple plasma wavelength) witness beam. The witness beam is energy-modulated by the wakefield, yielding a response that contains detailed spectral information in a single-shot measurement. Using these methods, we generate a variety of beam profiles and characterize the wakefields, directly observing beam-loaded transformer ratios up to 7.8. Further, a spectrally-based current reconstruction technique, validated by 3D particle-in-cell simulations, is introduced to obtain the drive beam profile from the decelerating wakefield data.
UCLA, Los Angeles, California, USA
SLAC, Menlo Park, California, USA
The ultra-compact x-ray free-electron laser (UC-XFEL), described in [J. B. Rosenzweig, et al. 2020 New J. Phys. 22 093067], combines several cutting edge beam physics techniques and technologies to realize an x-ray free electron laser at a fraction of the cost and footprint of existing XFEL installations. These elements include cryogenic, normally conducting RF structures for both the gun and linac, IFEL bunch compression, and short-period undulators. In this work, several stepping-stone, demonstrator scenarios under discussion for the UCLA SAMURAI Laboratory are detailed and simulated, employing different subsets of these elements. The cost, footprint, and technology risk for these scenarios are considered in addition to the anticipated engineering and physics experience gained.
UCLA, Los Angeles, California, USA
SLAC, Menlo Park, California, USA
PBPL, Los Angeles, USA
The University of Liverpool, Liverpool, United Kingdom
UCLA has recently constructed SAMURAI, a new radiation bunker and laser infrastructure for advanced accelerator research. In its first phase, we will build a 30 MeV photoinjector with an S-band hybrid gun. The beam dynamics simulation for this beamline showed the generation of the beam with the emittance 2.4 um and the peak current 270 A. FIR-FEL experiments are planned in this beamline. The saturation peak power was expected at 170 MW.
UCLA, Los Angeles, California, USA
Michigan State University, East Lansing, Michigan, USA
Northern Illinois University, DeKalb, Illinois, USA
SLAC, Menlo Park, California, USA
The dielectric wakefield acceleration (DWA) program at FACET produced a multitude of new physics results that range from GeV/m acceleration to the discovery of high field-induced conductivity in THz waves, and beyond, to a demonstration of positron-driven wakes. Here we review the rich program now developing in the DWA experiments at FACET-II. With increases in beam quality, a key feature of this program is extended interaction lengths, near 0.5 m, permitting GeV-class acceleration. Detailed physics studies in this context include beam breakup and its control through the exploitation of DWA structure symmetry. The next step in understanding DWA limits requires the exploration of new materials with low loss tangent, large bandgap, and improved thermal characteristics. Advanced structures with photonic features for mode confinement and exclusion of the field from the dielectric, as well as quasi-optical handling of coherent Cerenkov signals is discussed. Use of DWA for laser-based injection and advanced temporal diagnostics is examined.
UCLA, Los Angeles, California, USA
We propose here to exploit the quadrupole wakefields in an alternating symmetry slab-based dielectric wakefield accelerator (DWA) to produce second-order focusing. The resultant focusing is found to be strongly dependent on longitudinal position in the bunch. We analyze this effect with analytical estimates and electromagnetic PIC simulations. We examine the use of this scenario to induce beam stability in very high gradient DWA, with positive implications for applications in linear colliders and free-electron lasers.
UCLA, Los Angeles, USA
We present the design of a Compton spectrometer for use at FACET-II. A sextupole is used for magnetic spectral analysis, giving a broad dynamic range (180 keV through 28 MeV) and the capability to capture an energy-angular double-differential spectrum in a single shot. At low gamma energies, below 1 MeV, Compton spectroscopy becomes increasingly challenging as the scattering cross-section becomes more isotropic. To extend the range of the spectrometer down to around 180 keV, we use a 3D-printed tungsten collimator at the detector plane to preferentially select forward-scattered electrons at the Compton edge.