We present X-ray absorption measurements near the K-edge of laser heated aluminum in a pump-probe configuration using X-rays generated in a laser wakefield accelerator. A 30 fs duration laser pulse from the HERCULESlaser system was split into two beamlines, with one used to heat a 4 mu m thickness Al foil and the second to drive a laser wakefield accelerator. The laser-heated plasma was probed at various pump-probe delays using the femtosecond duration X-rays generated by betatron oscillations of the electrons in the wakefield. We observe an apparent blue-shift of the K-edge occurring on a sub-picosecond timescale in the transmission spectra.

Received 25 July 2019Accepted 8 April 2020DOI:https://doi.org/10.1103/PhysRevLett.124.179502© 2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHigh intensity laser-plasma interactionsLaser driven ion accelerationPhotonuclear reactionsPhysical SystemsAstrophysical jetsLasersAccelerators & BeamsPlasma Physics

Laser-plasma interaction experiments using a 70 TW short-pulse laser were conducted to investigate the properties of electron beams produced from laser wakefield acceleration. In addition to narrow divergence electron beams in excess of 200 MeV, lower energy components of the electron beam were observed to have an annular emission pattern with much larger divergence. This ring-shaped component of the beam is up to several MeV in energy and gives rise to a corresponding increase in the divergence of bremsstrahlung x-rays while producing more background radiation. Scaling measurements of the emission angle of the beam with respect to plasma density were also made. From 3D numerical simulations of the interaction, it is shown that this phenomenon results from energetic electrons in the laser-driven wakefield which are not trapped by the plasma wave, but which can still obtain some longitudinal momentum from the interaction.

X-rays generated by the betatron oscillations of electrons in a nonlinear plasma wakefield have been proposed to be useful for a range of applications. In this study, the scaling of the x-ray fluence and energy with electron beam properties and laser power are studied and the application of the x-rays for imaging additive manufactured (3D printed) test objects is investigated. The scalings confirm a strong increase in x-ray fluence with laser power and the imaging shows strong fringe contrast due to the micron scale source size.

The electric and magnetic fields responsible for electron acceleration in a Laser Wakefield Accelerator (LWFA) also cause electrons to radiate x-ray photons. Such x-ray pulses have several desirable properties including short duration and being well collimated with tunable high energy. We measure the scaling of this x-ray source experimentally up to laser powers greater than 100 TW. An increase in laser power allows electron trapping at a lower density as well as with an increased trapped charge. These effects resulted in an x-ray fluence that was measured to increase non-linearly with laser power. The fluence of x-rays was also compared with that produced from K-α emission resulting from a solid target interaction for the same energy laser pulse. The flux was shown to be comparable, but the LWFA x-rays had a significantly smaller source size. This indicates that such a source may be useful as a backlighter for probing high energy density plasmas with ultrafast temporal resolution.

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 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 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 simulations using a Monte Carlo synchrotron x-ray emission algorithm elucidate the time-dependent variations in the radiation emission processes.

Experiments were performed using the HERCULES laser system at the University of Michigan to study backward stimulated Raman scattering (BSRS) from a laser wakefield accelerator driven with a 30 fs pulse. 'The spectrum of backscattered light was found to be significantly broadened and red-shifted in cases where electrons were accelerated. BSRS broadening (red-shifting) was found to increase with respect to both plasma density and accelerated electron charge for laser powers exceeding 100 TW. Two-dimensional Particle-in-Cell simulations reveal temporal dynamics for the BSRS emission, which ceases as the wakefield bubble is evacuated of plasma electrons because of relativistic self-focusing. The intensity and duration of the BSRS signal was found to vary with plasma density and laser intensity. Both experimental and simulation results indicate that backward SRS is associated with plasma electron density within the wakefield bubble. This measurement can serve as a diagnostic of bubble dynamics, and is correlated with trapped electron charge in this regime.

Magnetic reconnection is a fundamental process occurring in many plasma systems. Magnetic field lines break and reconfigure into a lower energy state, converting released magnetic field energy into plasma kinetic energy. Around some of the universe's most energetic objects, such as y -ray burst or active galactic nuclei, where the magnetic field energy exceeds the plasma rest mass energy, the most extreme magnetic reconnection in the relativistic regime is theorized. The presented experiments and three-dimensional particle-in-cell modeling recreate in the laboratory the scaled plasma conditions necessary to access the relativistic electron regime and therefore approach conditions around these distant, inaccessible objects. High-power, ultrashort laser pulses focused to high intensity (I > 2.5 x 10(18) Wcm(-2)) on solid targets produces relativistic temperature electrons within the focal volume. The hot electrons are largely confined to the target surface and form a radial surface current that generates a huge, expanding azimuthal magnetic field. Focusing two laser pulses in close proximity on the target surface leads to oppositely directed magnetic fields being driven together. The fast electron motion due to the magnetic reconnection is inferred using an experimental x-ray imaging technique. The x-ray images enable the measurement of the reconnection layer dimensions and temporal duration. The reconnection rates implied from the aspect ratio of the reconnection layer, delta/L approximate to 0.3, was found to be consistent over a range of experimental pulse durations (40 fs-20 ps) and agreed with the modeling. Further experimental evidence for magnetic reconnection is the formation of a nonthermal electron population shown by the modeling to be accelerated in the reconnection layer.

The development of laser wakefield accelerators (LWFA) over the past several years has led to an interest in very compact sources of X-ray radiation – such as “table-top” free electron lasers. However, the use of conventional undulators using permanent magnets also implies system sizes which are large. In this work, we assess the possibilities for the use of novel mini-undulators in conjunction with a LWFA so that the dimensions of the undulator become comparable with the acceleration distances for LWFA experiments (i.e., centimeters). The use of a prototype undulator using laser machining of permanent magnets for this application is described and the emission characteristics and limitations of such a system are determined. Preliminary electron propagation and X-ray emission measurements are taken with a LWFA electron beam at the University of Michigan.

The reflectivity of a short-pulse laser at intensities of 2 × 10 21 Wcm − 2 with ultra-high contrast ( 10 − 15 ) on sub-micrometer silicon nitride foils was studied experimentally using varying polarizations and target thicknesses. The reflected intensity and beam quality were found to be relatively constant with respect to intensity for bulk targets. For submicron targets, the measured reflectivity drops substantially without a corresponding increase in transmission, indicating increased conversion of fundamental to other wavelengths and particle heating. Experimental results and trends observed in 3D particle-in-cell simulations emphasize the critical role of ion motion due to radiation pressure on the absorption process. Ion motion during ultra-short pulses enhances the electron heating, which subsequently transfers more energy to the ions.

We have studied laser acceleration of ions from Si3N4 and Al foils ranging in thickness from 1800 to 8 nm with particular interest in acceleration of ions from the bulk of the target. The study includes results of experiments conducted with the HERCULES laser with pulse duration 40 fs and intensity 3 x 10(20) Wcm(-2) and corresponding two-dimensional particle-in-cell simulations. When the target thickness was reduced the distribution of ion species heavier than protons transitioned from being dominated by carbon contaminant ions of low ionization states to being dominated by high ionization states of bulk ions (such as Si (12+)) and carbon. Targets in the range 50-150 nm yielded dramatically greater particle number and higher ion maximum energy for these high ionization states compared to thicker targets typifying the Target Normal Sheath Acceleration (TNSA) regime. The high charge states persisted for the thinnest targets, but the accelerated particle numbers decreased for targets 35 nm and thinner. This transition to an enhanced ion TNSA regime, which more efficiently generates ion beams from the bulk target material, is also seen in the simulations.

The electron injection process into a plasma-based laser wakefield accelerator can be influenced by modifying the parameters of the driver pulse. We present an experimental study on the combined effect of the laser pulse duration, pulse shape, and frequency chirp on the electron injection and acceleration process and the associated radiation emission for two different gas types-a 97.5% He and 2.5% N-2 mixture and pure He. In general, the shortest pulse duration with minimal frequency chirp produced the highest energy electrons and the most charge. Pulses on the positive chirp side sustained electron injection and produced higher charge, but lower peak energy electrons, compared with negatively chirped pulses. A similar trend was observed for the radiant energy. The relationship between the radiant energy and the electron charge remained linear over a threefold change in the electron density and was independent of the drive pulse characteristics. X-ray spectra showed that ionization injection of electrons into the wakefield generally produced more photons than self-injection for all pulse durations/frequency chirp and had less of a spread in the number of photons around the peak x-ray energy.

Reduced surface area targets were studied using an ultra-high intensity femtosecond laser in order to determine the effect of electron sheath field confinement on electron dynamics. X-ray emission due to energetic electrons was imaged using a Ka imaging crystal. Electrons were observed to travel along the surface of wire targets, and were slowed mainly by the induced fields. Targets with reduced surface areas were correlated with increased hot electron densities and proton energies. Hybrid Vlasov-Fokker-Planck simulations demonstrated increased electric sheath field strength in reduced surface area targets.

We present an analytical formalism elucidating how information is stored in chirped optical probes by describing the effects of sinusoidal temporal modulations on the electric field. We show that the modulations produce spectral sidebands which can be interpreted as temporal sidebands due to the time-wavelength mapping, an effect we call temporally encoded spectral shifting (TESS). A derivation is presented for the case of chirped-pulse spectral interferometry showing how to recover both the amplitude and the periodicity of the modulation from a Fourier transform of the interferogram. The TESS effect, which provides an intuitive picture for interpreting pump-probe experiments with chirped pulses, is illustrated for probing wakefields from a laser-plasma accelerator.

Annular quasimonoenergetic electron beams with a mean energy in the range 200-400 MeV and charge on the order of several picocoulombs were generated in a laser wakefield accelerator and subsequently accelerated using a plasma afterburner in a two-stage gas cell. Generation of these beams is associated with injection occurring on the density down ramp between the stages. This well-localized injection produces a bunch of electrons performing coherent betatron oscillations in the wakefield, resulting in a significant increase in the x-ray yield. Annular electron distributions are detected in 40% of shots under optimal conditions. Simultaneous control of the pulse duration and frequency chirp enables optimization of both the energy and the energy spread of the annular beam and boosts the radiant energy per unit charge by almost an order of magnitude. These well-defined annular distributions of electrons are a promising source of high-brightness laser plasma-based x rays.

Single photon counting techniques were used with an x-ray CCD camera to measure features of synchrotron-like x-ray spectra generated by betatron oscillations of electrons in a laser wakefield accelerator (LWFA) with different injection techniques. Measurements were made using the Hercules laser system at the University of Michigan. With a single stage gas cell, we demonstrate that pure helium gas in our wakefield accelerator will produce spectra with higher critical energies than when helium mixed with nitrogen is used. This result was not evident when a two stage gas cell was used.

Evidence of magnetic reconnection (MR) events driven by relativistic electrons is observed between two high-intensity laser/plasma interaction sites. The two laser foci were on average 20um FWHM containing 50TW of power each, delivered with a split f/3 paraboloid onto copper foil targets at a focused intensity of 10(19) W/cm(2) with the HERCULES laser system. Cu K-alpha emissions from the interactions were imaged with a spherically bent Quartz crystal, and by motorizing one half of the paraboloid vertically the focal separation was varied between 0-400um.Splitting the beam halves revealed an enhanced region between the foci with the highest a maximized K-alpha signal intensity at one inter-beam separation, evidencing inflow from relativistic electron driven MR. A filtered LANEX screen was imaged to search for outflow/jet electrons along the plane of the target surface and normal to the axis defined by the two spots, to calculate the electron temperature and to search for spatial profile nonuniformities potentially directly originating from reconnection events. Ongoing 2D and 3D PIC simulations are being conducted to better understand and model the measured electron outflow dynamics.