Beam-Matter Interaction

The interaction of intense charged particle beams with condensed matter is encountered in a variety of scenarios in our research. For example, the Cerenkov interaction is the basis of many time-dependent, picosecond-resolution diagnostics, and its inverse process gives rise to dielectric-based laser-powered accelerator schemes. Scintillating materials can be directly excited by relativistic beams, but their resolution can be limited by the presence of very large collective beam fields. The interaction of a beam with a metal boundary produces transition radiation; optical transition radiation enables micron-resolution beam profile monitors, while coherent transition radiation is used in our experiments to diagnose beam bunching at the 100 micron level, as well as microbunching at the sub-micron level.

Dielectric Wakefield Acceleration (DWA) at FACET-II

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.

Ultrafast Electron Diffraction (UED)

Real time resolution of atomic motion is one of the great open challenges in science today and is manifested in many fields of research. Any significant progress in this direction will undoubtedly leave a profound impact on how we view and understand the most basic processes in the study of molecules, materials, and biological systems. This broad research field is particularly new and active, as only in the last twenty years the progress in laser technology has enabled the generation of optical pulses of time duration comparable to the time-scale of atomic and molecular motion (less than 100 fs).

So far most of the structural changes on atomic length scales have only been inferred indirectly from the analysis of optical spectra (pump-and-probe spectroscopy). In order to directly observe atomic and molecular structures it is necessary to use waves of sufficiently short wavelength or, from an equivalent point of view, particles (either photons or electrons) of sufficiently high energy as probe projectiles. With the relevant spatial lengths of the order of the atomic dimensions, only x-rays or high energy electrons can provide the sufficient spatial resolution to directly resolve structural changes.

The time resolution of such investigations is set by the length of the electromagnetic or particle wave packet �i.e. the x-ray or electron pulse length--, and the challenge becomes then to produce suitable very short probe beams. In the last ten years, various ways of generating ultrashort x-ray pulses have been demonstrated and thus time resolved x-ray diffraction has been the primary choice in ultrafast diffraction studies. Due to the different nature of how the charged particles couple to matter, the use of electrons to directly probe matter is in many ways complementary to x-ray diffraction.

Dielectric Laser Acceleration (DLA)

The Accelerator on a Chip International Program (ACHIP) seeks to use dielectric laser accelerators (DLAs) to shrink accelerators to the sub-millimeter scale. The nonrelativistic regime has had success in obtaining net acceleration and attosecond bunching using nanofabricated pillar based structures made from B:Si. Fused silica has a high damage threshold for femtosecond-class laser pulses, allowing for GV/m gradients as demonstrated in the relativistic regime. These relativistic structures are dual grating structures, meaning they are simple to fabricate in addition to having built-in filtering – electrons that do not get accelerated simply hit the glass, and do not obfuscate a modulation signal.