arXiv subjects

J. Nees

Publications and source records attributed to J. Nees.

6 recordsLinked to original sources

arXiv2026

Electron Injection and Beam Dynamics in a Laser Wakefield Acceleration Driven by Laser Pulses Carrying Orbital Angular Momentum

Laser pulses carrying orbital angular momentum (OAM) provide a new degree of freedom for controlling plasma-based accelerators. Here, we demonstrate experimentally laser wakefield acceler- ation driven by OAM laser pulses, producing electron beams with distinct broad-band energy spectra and angularly structured phase space. The measured spectra exhibit a two-beamlet structure with correlated angular dispersion, indicating injection occurring at multiple azimuthal phases of the plasma wake. Particle-in-cell simulations reproduced the observed spectral features and revealed helical electron trajectories driven by the OAM-induced wakefields. These results show that the phase structure of the laser driver can shape electron injection and acceleration dynamics, opening a route toward optimal control of beam structure in compact laser-driven accelerators.

physics.plasm-ph↗

arXiv2019

High repetition rate (>= kHz) targets and optics from liquid microjets for the study and application of high intensity laser-plasma interactions

High intensity laser-plasma interactions produce a wide array of energetic particles and beams with promising applications. Unfortunately, high repetition rate and high average power requirements for many applications are not satisfied by the lasers, optics, targets, and diagnostics currently employed. Here, we address the need for high repetition rate targets and optics through the use of liquids. A novel nozzle assembly is used to generate high-velocity, laminar-flowing liquid microjets which are compatible with a low-vacuum environment, generate little to no debris, and exhibit precise positional and dimensional tolerances. Jets, droplets, submicron thick sheets, and other configurations are characterized with pump-probe shadowgraphy to evaluate their use as targets. To demonstrate a high repetition rate, consumable liquid optical element, we present a plasma mirror created by a submicron thick liquid sheet. This plasma mirror provides etalon-like anti-reflection properties in the low-field of 0.1% and high reflectivity as a plasma, 69%, at a repetition rate of 1 kHz. Practical considerations of fluid compatibility, in-vacuum operation, and estimates of maximum repetition rate, in excess of 10 kHz, are addressed. The targets and optics presented here enable the use of relativistically intense lasers at high average power and make possible many long proposed applications.

physics.plasm-ph↗

arXiv2017

High Flux Femtosecond X-ray Emission from the Electron-Hose Instability in Laser Wakefield Accelerators

Bright and ultrashort duration X-ray pulses can be produced by through betatron oscillations of electrons during Laser Wakefield Acceleration (LWFA). Our experimental measurements using the \textsc{Hercules} laser system demonstrate a dramatic increase in X-ray flux for interaction distances beyond the depletion/dephasing lengths, where the initial electron bunch injected into the first wake bucket catches up with the laser pulse front and the laser pulse depletes. A transition from an LWFA regime to a beam-driven plasma wakefield acceleration (PWFA) regime consequently occurs. The drive electron bunch is susceptible to the electron-hose instability and rapidly develops large amplitude oscillations in its tail, which leads to greatly enhanced X-ray radiation emission. We measure the X-ray flux as a function of acceleration length using a variable length gas cell. 3D particle-in-cell (PIC) simulations using a Monte Carlo synchrotron X-ray emission algorithm elucidate the time-dependent variations in the radiation emission processes.

physics.acc-ph↗

arXiv2016

Relativistic Magnetic Reconnection in the Laboratory

Magnetic reconnection is a fundamental plasma process involving an exchange of magnetic energy to plasma kinetic energy through changes in the magnetic field topology. In many astrophysical plasmas magnetic reconnection plays a key role in the release of large amounts of energy \cite{hoshino1}, although making direct measurements is challenging in the case of high-energy astrophysical systems such as pulsar wind emissions \cite{lyubarsky1}, gamma-ray bursts \cite{thompson1}, and jets from active galactic nuclei \cite{liu1}. Therefore, laboratory studies of magnetic reconnection provide an important platform for testing theories and characterising different regimes. Here we present experimental measurements as well as numerical modeling of relativistic magnetic reconnection driven by short-pulse, high-intensity lasers that produce relativistic plasma along with extremely strong magnetic fields. Evidence of magnetic reconnection was identified by the plasma's X-ray emission patterns, changes to the electron energy spectrum, and by measuring the time over which reconnection occurs. Accessing these relativistic conditions in the laboratory allows for further investigation that may provide insight into unresolved areas in space and astro-physics.

physics.plasm-ph↗

arXiv2010

Multiple colliding electromagnetic pulses: a way to lower the threshold of $e^+e^-$ pair production from vacuum

The scheme of simultaneous multiple pulse focusing on one spot naturally arises from the structural features of projected new laser systems, such as ELI and HiPER. It is shown that the multiple pulse configuration is beneficial for observing $e^+e^-$ pair production from vacuum under the action of sufficiently strong electromagnetic fields. The field of the focused pulses is described using a realistic three-dimensional model based on an exact solution of the Maxwell equations. The $e^+e^-$ pair production threshold in terms of electromagnetic field energy can be substantially lowered if, instead of one or even two colliding pulses, multiple pulses focused on one spot are used. The multiple pulse interaction geometry gives rise to subwavelength field features in the focal region. These features result in the production of extremely short $e^+e^-$ bunches.

hep-ph↗

arXiv2007

Electromagnetic pulse reflection at self-generated plasma mirrors: laser pulse shaping and high order harmonic generation

A thin layer of overdense plasma is created when an electromagnetic pulse interacts with a rapidly ionizing thin foil. This layer will reflect the incoming pulse, forming a so-called plasma mirror. A simple realistic model based on paired kinetic and wave equations is used to describe analytically the process of mirror formation and the reflection and transmission of the incident pulse. The model incorporates the exact description of the ionization process in the foil and the polarization and conduction currents that follow. The analytical description of the reflected and transmitted pulses as well as their dependence on foil parameters, and initial pulse amplitude and form are presented. Possible application and effectiveness of this process to improve laser pulse contrast are discussed. In the case of the linearly polarized incident pulse, there harmonic generation occurs, that is absent in the case of the circular polarization. The spectra of the reflected pulses for different initial forms and amplitudes are studied.

physics.plasm-ph↗