Session T08: Advanced Acceleration, Lasers & Diagnostics

Live

We present our recent experimental efforts toward producing one gigawatt of power at 11.7 GHz with a metallic metamaterial-based power extractor for use in structure-based wakefield acceleration (SWFA). SWFA is a novel acceleration scheme in which high-charge electron bunches are passed through a power extractor structure to produce a high-intensity wakefield. The resulting wakefield can either be used to accelerate a witness bunch in the same beamline or passed through a waveguide to a secondary acceleration beamline. Our approach uses a specifically-tailored metamaterial for the power extractor structure. The properties of the metamaterial allow us to overcome some of the difficulties encountered by other SWFA techniques. Here, we present the Stage 3 experimental design. The Stages 1 and 2 experiments generated 80 MW and 380 MW RF pulses, respectively, with several-nanosecond duration using the 65 MeV beam at the Argonne Wakefield Accelerator. The Stage 3 design includes significant design improvements, including an all-copper structure, fully-symmetric coupler design, and treatment to reduce breakdown risk. With these improvements, simulations predict over 1.1 GW of output power. The experimental run is scheduled for February 2021.   

Live

A future plasma based linear collider has the potential to reach unprecedented energies and transform our understanding of high energy physics. The extremely high brightness beams in such a device would cause the plasma ions to collapse into the beam volume forming a beam-ion quasi-equilibrium. This quasi-equilibrium is characterized by a thin dense ion column inside of the beam. Using a combination of Particle-in-Cell (PIC) simulations and analytical work we investigated the rich physics of the beam-ion interaction. We derived analytical expressions for the equilibrium beam and ion density profiles. We studied beam matching in order to mitigate emittance growth due to the strong nonlinear focusing fields. We developed a 2D symplectic tracking code with Monte Carlo scattering based on Moliere's theory of small angle multiple scattering in order to quantify emittance growth due to scattering and chaotic diffusion. The planned E-314 experiment at the FACET-II facility at SLAC National Accelerator Laboratory aims to demonstrate ion motion experimentally. Work is ongoing at UCLA to develop beam radiation diagnostics and focusing optics for E-314.   

Live

Periodic electron bunch trains can be used to resonantly excite plasmas in the quasi-nonlinear~(QNL) regime. This excitation can produce plasma blowout conditions using very low emittance beams despite having a small charge per bunch. The resulting~plasma density perturbation is extremely nonlinear locally but preserves the resonant response of the plasma electrons at the plasma frequency. Such a resonant~bunch train can be produced~via inverse free electron laser (IFEL) bunching, creating microbunches spaced at the laser wavelength. To match the resonance condition of a laser with a period of a few microns, a high plasma density is employed, resulting in extremely large wakefield amplitudes, near 1 TV/m. The plasma response, beam evolution including density modulation, and various instabilities resulting from such an interaction ~ have been investigated using particle-in-cell (PIC) simulations. This scenario corresponds to~a planned experiment, E-317, at SLAC's FACET-II facility.   

Live

Nonlinear nanoplasmonic modes in nanostructures are modeled to open a new extreme field frontier with access to tens of TeraVolts per meter electromagnetic fields [1,2]. This offers novel pathways in physics of extreme fields particularly with applications in particle acceleration, light sources and nonlinear QED [3,4]. Particle beams interacting with nanomaterials with vacuum-like core regions experience minimal disruptive effects such as filamentation and collisions, whereas the nonlinear surface crunch-in plasmonic modes driven by these beams sustain tens of TV/m electromagnetic fields. Using our recently proposed SLAC experiment [5] which uses charged particle beams to excite unprecedentedly high-amplitude nanoplasmonic modes our $\rm nano^2WA$ collaboration seeks to prototype this transformative possibility. Experimental verification will open new possibilities far beyond the tried and tested gaseous plasma mode techniques which is currently considered to be the frontier in high-field physics. [1] arxiv.org/2004.09452 [2] doi.org/10.1142/S0217751X19430097 [3] https://indico.fnal.gov/event/19478/contributions/52561/ [4] https://indico.cern.ch/event/867535/contribution/3716404/ [5] https://facet.slac.stanford.edu/proposals/pac2020-agenda   

Live

Spatial and temporal resolution of electron diffraction and microscopy techniques can typically not exceed the quality of the electron source. For this reason, ultra-high brightness photocathodes have been actively sought, and found, over the past decades. This however poses a new challenge: Maintaining the increased phase-space density throughout the entire device. Most beam dynamics simulation codes approximate detrimental Coulomb interaction with a mean-field space charge approach. While this approximation is sufficient in traditional beams with large temperature, it fails to capture the more subtle stochastic effects that arise due to the point-like nature of electrons. In this contribution we introduce two numerical methods implemented in the General Particle Tracer (GPT) code to calculate the effects of the photocathode image charge when using a point-to-point interaction model. With these methods we simulate the effects of stochastic Coulomb interactions on two high brightness photoemission beamlines designed for single-shot ultrafast electron diffraction: one using a 200 keV gun, and the other using a 5 MeV gun.   

Live

Current works, devoted to changing of group velocity of laser pulses by spatial structuring laser beams, allow us to solve only direct problem -- to define group velocity using information about wave structure. We propose a math model of wave propagation based on using real functions of spatial distributions of amplitude, phase and group velocity, which allows to solve as direct as inverse problem -- to define spatial structure of beam using data about necessary group velocity [1]. Applying new model to experimental data from, for example, [2,3] shows high quality results. Practical applicability of our results are also can be in future confirmed in experiments with spatial light modulator. [1] M. K. Galinsky, V. V. Rumyantsev, Problems of Artificial Intelligence (ISSN: 2413-7383) 10, 14 (2018). [2] D. Giovannini et al., Science. 347, 857 (2015). [3] N. D. Bareza, N. Hermose, Sci. Rep. 6, 26842 (2016).   

Live

The theoretical framework for Raman spectroscopy using a UV probe laser pulse train consisting of multi femtosecond pulses is developed. We show selective excitation of a single Raman mode by tuning the pulse parameters. The use of UV radiation for the probe has a number of advantages for this application. The pulse train consists of multiple pulses of the form, $I_{1} (\tau )=I_{0} \sin^{2}(\pi \tau /\tau_{L} )\left( {\Theta \left( \tau \right)-\Theta \left( {\tau -\tau_{L} } \right)} \right)$, where $\tau_{L} $ is the duration of a single pulse, $\tau_{FWHM} ={\tau_{L} } \mathord{\left/ {\vphantom {{\tau_{L} } 2}} \right. \kern-\nulldelimiterspace} 2,\mbox{\thinspace \thinspace }I_{0} =n_{0} c\varepsilon_{0} {E_{peak}^{2} } \mathord{\left/ {\vphantom {{E_{peak}^{2} } 2}} \right. \kern-\nulldelimiterspace} 2$, is the peak intensity and $\Theta \left( \tau \right)$ is the Heaviside function. The analysis is performed in the group velocity frame, where $\tau =t-z/\mbox{v}_{G} \,\,\mbox{and}\,\,\eta =z$. The reduced propagation equation for the probe pulse field is ${\partial E_{P} (\eta ,\tau )} \mathord{\left/ {\vphantom {{\partial E_{P} (\eta ,\tau )} {\partial \eta }}} \right. \kern-\nulldelimiterspace} {\partial \eta }=i(\mu_{0} \omega_{0}^{2} /2k_{0} )P_{NL} (\eta ,\tau )$ where $P_{NL} $ is the non-linear polarization field. The probe intensity is modulated and grows approximately linearly with the interaction distance. We simulate the detection of the COVID-19 pathogen with a laser pulse train consisting of 10 micro-pulses, each with a duration of $\approx $ 32 fs, peak intensity of 10$^{\mathrm{10\thinspace }}$W/cm$^{\mathrm{2}}$ and central wavelength of 250 nm (frequency tripled Ti:Sapphire). The micro-pulse duration is chosen to match the vibrational period of the smallest Raman shift resonance of the pathogen, $\omega_{V} /(2\pi c)=1032\mbox{cm}^{\mbox{-1}}$. This simulation showed the selective excitation of a single Raman mode.   

Live

With its shorter wavelength compared to microwave radiation and recent advances in available intensities, THz radiation offers an interesting prospect to develop compact and powerful particle accelerators. To this aim, reaching high accelerating gradients at THz frequencies is paramount and requires advances in THz characterization techniques. The electrooptic sampling method presented here allows measurement of the 3D field profile of the THz radiation. THz pulses are generated by optical rectification in organic crystals pumped by a Ti:Sapphire-pumped OPA. The electric field of the THz pulse induces a local polarization rotation in an ultrashort and transversely oversized probe pulse in zinc blende crystals. This polarization rotation manifests as an intensity response on a CCD after passing through a polarizer, which is analyzed to recover the instantaneous 2D transverse profile of the THz electric field. The full 3D distribution of the THz pulse is finally obtained by varying the delay of the ultrashort probe pulse.