arXiv subjects

E. Rockafellow

Publications and source records attributed to E. Rockafellow.

8 recordsLinked to original sources

arXiv2025

Plasma waveguides for high-intensity laser pulses

Fundamental to many applications of laser pulses in science and technology is an extended interaction length with matter that significantly exceeds the distance over which the pulse would normally diffract and transversely spread. At low intensity, the interaction could simply be the linear refraction provided by a glass optical fiber to keep the pulse from spreading. At increased pulse intensity, more than diffraction-free pulse transport is of interest: an extended interaction length of high intensity light can give rise to bright secondary sources of photons, and at relativistic intensities, beams of high energy charged particles. As generation of these secondary sources requires laser intensities well above the threshold for ionization of atoms, new methods for defeating pulse diffraction in a plasma have been developed. Chief among them are plasma waveguides: optical fibers composed of plasma that have characteristic mode structure. This article reviews the methods and theory of plasma waveguides, highlighting the recent development of meter-scale plasma waveguides that have been instrumental to the laser acceleration of high charge electron beams to ~10 GeV.

physics.plasm-ph↗

arXiv2025

Excitation of Giant Surface Waves During Laser Wake Field Acceleration

We have detected the presence of very high intensity surface waves that are excited during plasma waveguided laser wakefield acceleration. Wakefield acceleration can be enchanced by the introduction of an ``all optical" plasma waveguide that confines and guides a laser pulse at the optimal intensity over long distances, producing quasimonoenergetic multi-GeV electron bunches. However strong pulses of radio frequency radiation (RF) are also produced, and particle in cell simulations show why: a continuous stream of multi-MeV electrons are also ejected radially from the plasma due to nonlinear wave breaking, and these excite and copropagate coherently with a giant cylindrical Sommerfeld surface wave. Laboratory measurements, simulations, and analytic approximations all converge on a 20 J laser pulse exciting a 1 Joule, 400 GW broadband THz surface wave, with a peak electric field strength of 35 GV/m.

physics.plasm-ph↗

arXiv2025

Longitudinal shaping of plasma waveguides using diffractive axicons for laser wakefield acceleration

New techniques for the optical generation of plasma waveguides -- optical fibres for ultra-intense light pulses -- have become vital to the advancement of multi-GeV laser wakefield acceleration. Here, we demonstrate the fabrication and characterization of a transmissive eight-level logarithmic diffractive axicon (LDA) for the generation of meter-scale plasma waveguides. These LDAs enable the formation of a Bessel-like beam with controllable start and end locations of the focal line and near-constant intensity on axis. We present measurements of the Bessel-like focal profile produced by the LDA, and of the leading end of the plasma column generated by it. One important feature is the formation of a funnel-mouthed plasma channel entrance that can act as waveguide coupler. We also compare the diffraction efficiency of our 8-level LDA to 4-level and binary versions, with measurements comparing well to theory.

physics.plasm-ph↗

arXiv2024

Meter-scale supersonic gas jets for multi-GeV laser-plasma accelerators

Pushing the high energy frontier of laser wakefield electron acceleration (LWFA) to 10 GeV and beyond requires extending the propagation of relativistic intensity pulses to ~1 m in a low density ($N_e\sim 10^{17} cm^{-3}$) plasma waveguide. We present the development and characterization of two types of supersonic gas jet for meter-scale multi-GeV laser wakefield accelerators. The first type is a 30-cm long single-module gas jet, which demonstrates good axial uniformity using hydrogen, the preferred working gas for LWFA. The second type is a modular jet composed of multiple 11-cm-long modules. Longitudinal density profile control is demonstrated with a 2-module (22 cm long) hydrogen jet using gas valve trigger timing. A 1.0-m-long jet is then assembled from 9 modules, and generation of 1.0-m long hydrogen plasma is demonstrated using a femtosecond Bessel beam. To our knowledge, this is the longest gas jet laser plasma yet generated.

physics.acc-ph↗

arXiv2024

Matched Guiding and Controlled Injection in Dark-Current-Free, 10-GeV-Class, Channel-Guided Laser Plasma Accelerators

We measure the high intensity laser propagation throughout meter-scale, channel-guided LPAs by adjusting the length of the plasma channel on a shot-by-shot basis, showing high quality guiding of 500 TW laser pulses over 30 cm in a hydrogen plasma of density $n_0 \approx 1 \times 10^{17} \, \mathrm{cm^{-3}}$. We observed transverse energy transport of higher-order modes in the first $\approx 12 \, \mathrm{cm}$ of the plasma channel, followed by quasi-matched propagation, and the gradual, dark-current-free depletion of laser energy to the wakefield. We quantify the laser-to-wake transfer efficiency limitations of currently available PW-class laser systems, and demonstrate via simulation how control over the laser mode can significantly improve accelerated beam parameters. Using just 21.3 J of laser energy, and triggering localized electron injection into the accelerator, we observed electron bunches with single, quasimonoenergetic peaks, relative energy spreads as low as 3 % and energy up to 9.2 GeV with charge extending beyond 10 GeV.

physics.plasm-ph↗

arXiv2024

Benchmarking of hydrodynamic plasma waveguides for multi-GeV laser-driven electron acceleration

Hydrodynamic plasma waveguides initiated by optical field ionization (OFI) have recently become a key component of multi-GeV laser wakefield accelerators. Here, we present the most complete and accurate experimental and simulation-based characterization to date, applicable both to current multi-GeV experiments and future 100 GeV-scale laser plasma accelerators. Crucial to the simulations is the correct modeling of intense Bessel beam interaction with meter-scale gas targets, the results of which are used as initial conditions for hydrodynamic simulations. The simulations are in good agreement with our experiments measuring evolving plasma and neutral hydrogen density profiles using two-color short pulse interferometry, enabling realistic determination of the guided mode structure for application to laser-driven plasma accelerator design.

physics.plasm-ph↗

arXiv2023

Guided mode evolution and ionization injection in meter-scale multi-GeV laser wakefield accelerators

We show that laser wakefield electron accelerators in meter-scale, low density hydrodynamic plasma waveguides operate in a new nonlinear propagation regime where sustained beating of lowest order modes of the ponderomotively modified channel plays a significant role, whether or not the injected pulse is linearly matched to the guide. For a continuously doped gas jet, this mode beating effect leads to ionization injection and a striated multi-GeV energy spectrum of multiple quasi-monoenergetic peaks; the same process in a locally doped jet produces single multi-GeV peaks with <10% energy spread. A 3-stage model of drive laser pulse evolution and ionization injection characterizes the beating effect and explains our experimental results.

physics.plasm-ph↗

arXiv2019

Absolute measurement of laser ionization yield in atmospheric pressure range gases over 14 decades

Strong-field ionization is central to intense laser-matter interactions. However, standard ionization measurements have been limited to extremely low density gas samples, ignoring potential high density effects. Here, we measure strong-field ionization in atmospheric pressure range air, N2 and Ar over 14 decades of absolute yield, using mid-IR picosecond avalanche multiplication of single electrons. Our results are consistent with theoretical rates for isolated atoms and molecules and quantify the ubiquitous presence of ultra-low concentration gas contaminants that can significantly affect laser-gas interactions.

physics.plasm-ph↗