Session UO5: Radiation Generation

Forward directed x-ray from source produced by relativistic electrons from a Self-Modulated Laser Wakefield Accelerator

Plasma-based particle accelerators are now able to provide the scientific community with novel light sources. Their applications span many disciplines, including high-energy density sciences, where they can be used as probes to explore the physics of dense plasmas and warm dense matter. A recent advance is in the experimental and theoretical characterization of x-ray emission from electrons in the self-modulated laser wakefield regime (SMLWFA) where little is known about the x-ray properties. A series of experiments at the LLNL Jupiter Laser Facility, using the 1 ps 150 J Titan laser, have demonstrated low divergence electron beams with energies up to 300 MeV and 6 nCs of charge, and betatron x-rays with critical energies up to 20 keV. This work identifies two other mechanisms which produce high energy broadband x-rays and gamma-rays from the SMLWFA: Bremsstrahlung and inverse Compton scattering. We demonstrate the use of Compton scattering and bremsstrahlung to generate x/Gamma-rays from 3 keV up to 1.5 MeV with a source size of 50um and a divergence of 100 mrad. This work is an important step towards developing this x-ray light source on large-scale international laser facilities, and also opens up the prospect of using them for applications.~   

Simulation of a brilliant betatron gamma-ray source from a two-stage wakefield accelerator

Thanks to the recent progress in laser-driven plasma acceleration of electrons, the ultra-short, compact and spatially coherent X-ray betatron sources generated in a wakefield accelerator have been successfully applied to high-resolution imaging or ultra-fast probing of matter evolution in the last few years. Here, based on three-dimensional particle-in-cell simulations, we propose an original hybrid scheme in which an electron beam produced in a first stage of laser-driven wakefield, interacts in a second stage with ahigher plasma density to generate a beam-driven wakefield and undergo strong betatron oscillation.This second stage acts as an efficient plasma radiator: we show that this scheme greatly improves the energy efficiency of the source, with about 1{\%} of the laser energy transferred to the radiation, and that the gamma-ray photon energy exceeds the MeV range when using a 15 J laser pulse. This new scheme opens the way to a wide range of applications requiring high-brilliance MeV photon source, such as photo-nuclear reaction study, radiography of dense objects, probing in nuclear physics or electron-positron pair production.   

Plasma channel undulator excited by high-order laser modes.

The possibility of utilizing plasma undulators and plasma accelerators to produce compact and economical ultraviolet and X-ray radiation sources has attracted considerable interest for a few decades. This interest has been driven by the great potential to decrease the threshold for accessing such sources, which are now mainly provided by a very few dedicated large-scale synchrotron or free-electron laser (FEL) facilities. However, the typically broad radiation bandwidth of such plasma devices limits the source brightness and makes it difficult for the FEL instability to develop. Here, using multi-dimensional electromagnetic particle-in-cell simulations, we demonstrate that a plasma undulator generated by the beating of a mixture of high-order laser modes propagating inside a plasma channel, leads to a few percent radiation bandwidth. The strength of the undulator can reach unity, the period can be less than a millimeter, and the total number of undulator periods can be significantly increased by a phase locking technique based on the longitudinal density modulation. According to analytical estimates and simulations, in the fully beam loaded regime, the electron current in the undulator can reach 0.3 kA, making such an undulator a potential candidate towards a table-top FEL.   

Intense single attosecond pulse generation through coherent synchrotron emission from laser interaction with capacitor-nanofoil target

In the relativistic laser-plasma interaction process, coherent synchrotron emission (CSE) has been identified as one of the most efficient mechanisms to produce attosecond pulse. However, the electron nanobunch, which is the key character of CSE, is highly sensitive to the interaction condition and is hard to be formed. Here we show that through irradiating on a capacitor-nanofoil target, which is composed of two separated nanofoils, this difficulty can be overcome. Both one-dimensional and two-dimensional particle-in-cell simulations reveal that the strong electrostatic field developed between two foils is responsible for the formation and the acceleration of the ultradense electron nanobunch. This nanobunch reaches both high density and energy in only half laser cycle and smears out in others, resulting in a single attosecond pulse with intensity up to $10^{21}W/cm^{2}$ and duration of 8as when the intensity of the driving laser of $7.7\times 10^{21}W/cm^{2}$.   

X-Ray generation by the laser-plasma interaction in the regime of relativistic electronic spring

Inducing and controlling relativistic motion of surface electrons in overdense plasmas with high-intensity lasers is a promising way to produce X-rays with unique properties, including high brightness, ultra-short duration and tunable polarization. Although the well-studied {\it relativistic oscillating mirror} (ROM) regime provides robust generation of high harmonics, the amplitude of the outgoing light in this regime is always equal to that of the incident radiation because the conversion takes place continuously without energy accumulation. This restriction can be overcome by increasing the laser intensity and/or decreasing the plasma density such that $n/a<10$. In this case the plasma acts as a spring, first accumulating up to 60\% of the energy of one laser cycle, then re-emitting it in the form of a burst of high harmonics. Under optimal conditions this burst can be both 100 times shorter in duration and 100 times higher in intensity. The theory of {\it relativistic electronic spring} (RES) [Gonoskov {\it et al.} PRE {\bf 84}, 046403 (2011)] describes a wide variety of interaction scenarios in this regime and provides insight into the underlying physics. The talk will concern the prospects of creating and controlling XUV bursts of exceptional brightness in the RES regime.   

Narrowband Compton Scattering Yield Enhancement

Compton Scattering (CS) of laser light off high-energy electrons is a well-established source of X- and gamma-rays for applications in medicine, biology, nuclear and material sciences. Main advantage of CS photon sources is the possibility to generate narrow spectra as opposed to a broad continuum obtained when utilizing Bremsstrahlung. However, due to the low cross-section of the linear process, the total photon yield is quite low. The most straightforward way to increase the number of photon-electron beam scattering events is to increase the laser pulse intensity at the interaction point by harder focusing. This leads to an unfortunate consequence. Increase in the laser pulse normalized amplitude $a_0$, leads to additional ponderomotive spectrum broadening of the scattered radiation. The ponderomotive broadening is caused by the $\mathbf{v}\times\mathbf{B}$ force, which slows the electron down near the peak of the laser pulse where the intensity is high, and can be neglected near the wings of the pulse, where the intensity is low. We show that laser pulse chirping, both nonlinear (laser pulse frequency "following" the envelope of the pulse) and linear, leads to compensation of the ponderomotive broadening and considerably enhances the yield of the nonlinear Compton sources.   

Electron Beam Diagnosis Using K-edge Absorption of Laser-Compton Photons

The mean energy, energy spread and divergence of the electron beam can be deduced from laser-Compton scattered X-rays filtered by a material whose K-edge is near the energy of the X-rays. This technique, combined with a spot size measurement of the beam, can be used to measure the emittance of electron bunches, and can be especially useful in LWFA experiments where conventional methods are unavailable. The effects of the electron beam parameters on X-ray absorption images are discussed, along with experimental demonstrations of the technique using the Compact Laser-Compton X-ray Source at LLNL.   

Compact gain saturated plasma based X-ray lasers down to 6.9nm

Plasma based soft x-ray amplifiers allow many experiments requiring bright, high energy soft x-ray laser pulses to be conducted in compact facilities. We have extended the wavelength of compact gain saturated x-ray lasers to 6.89 nm in a Ni-like Gd plasma generated by a Ti:Sa laser. Gain saturated laser operation was also obtained at 7.36 nm in Ni-like Sm. Isolectronic scaling and optimization of laser pre-pulse duration allowed us to also observe strong lasing at 6.6nm and 6.1 nm in Ni-like Tb, and amplification at 6.4nm and 5.89 nm in Ni-like Dy. The results were obtained by transient laser heating of solid targets with traveling wave excitation at progressively increased gracing incidence angles. We show that the optimum pump angle of incidence for collisional Ni-like lasers increases linearly with atomic number from Z$=$42 to Z$=$66, reaching 43 degrees for Ni-like Dy, in good agreement with hydrodynamic/atomic physics simulations. These results will enable single-shot nano-scale imaging and other application of sub-7 nm lasers to be performed at compact facilities.   

Plasma instability control toward high fluence, high energy x-ray continuum source

X-ray source development at Omega and NIF seeks to produce powerful radiation with high conversion efficiency for material effects studies in extreme fluence environments. While current K-shell emission sources can achieve tens of kJ on NIF up to 22 keV, the conversion efficiency drops rapidly for higher Z K-alpha energies. Pulsed power devices are efficient generators of MeV bremsstrahlung x-rays but are unable to produce lower energy photons in isolation, and so a capability gap exists for high fluence x-rays in the 30 -- 100 keV range. A continuum source under development utilizes instabilities like Stimulated Raman Scattering (SRS) to generate plasma waves that accelerate electrons into high-Z converter walls. Optimizing instabilities using existing knowledge on their elimination will allow sufficiently hot and high yield electron distributions to create a superior bremsstrahlung x-ray source. An Omega experiment has been performed to investigate the optimization of SRS and high energy x-rays using Au hohlraums with parylene inner lining and foam fills, producing \textasciitilde 10x greater x-ray yield at 50 keV than conventional direct drive experiments on the facility. Experiment and simulation details on this campaign will be presented. This work was performed under the auspices of the US DoE by LLNL under Contract No. DE-AC52-07NA27344.   

Focusing and up-shift of laser light by relativistic flying mirrors in the high power and large wavelength difference regime

Frequency up-shift and compression of electromagnetic waves by relativistic flying mirrors (RFM) have been demonstrated theoretically, numerically and experimentally (see review [1]). RFM are generated with ultra-high power laser pulses (driver pulses) propagating in plasma from breaking plasma waves. Lasers counter-propagating to the breaking plasma waves (source pulses) are reflected, up-shifted and compressed. Here, we investigate the focusing and reflectivity where source pulses with varying intensity have a much longer wavelength than the driver pulse and where both pulses are the same intensity using 2D particle-in-cell simulations. We show that the source pulse can significantly modify the RFM at high intensity and show the generation of harmonics when both pulses are the same intensity. [1] S. V. Bulanov,el al., Phys. Usp. 56, 429 (2013).   

Recent progress in simulation and theory towards using nonlinear plasma wakefields to drive a compact X-FEL

Plasma-based wakefield accelerators can generate and accelerate electrons with 10~100 GV/m acceleration gradient. Compared with conventional radio frequency based accelerators, plasma accelerators can much shrink the size and reduce the cost of X-ray Free-electron-lasers which require high quality and high energy electrons. However there are many challenges needed to be overcome before plasma wakefields can generate electron beams with the required beam quality (brightnesses and low energy spreads) inside the plasma and before these beams can be transported from the plasma to the undulator without beam quality degradation. In this talk, we will present our recent progress from PIC simulations and theory on this topic, including concepts for producing beams with unprecedented normalized brightnesses using density down ramp injection in the nonlinear blowout regime, matching the beam out of the plasma using longitudinally tailored plasma profiles, and start-to-end simulations of such plasma wakefied accelerators driven X-FELs.   

Ultrahigh 6D-brightness electron beams for the light sources of the next generation

The plasma photocathode mechanism (aka Trojan Horse) enables a path towards electron beams with nm-level normalized emittance and kA range peak currents, hence ultrahigh 5D-brightness. This ultrahigh 5D-brightness beams hold great prospects to realize laboratory scale free-electron-lasers. However, the GV/m-accelerating gradient in plasma accelerators leads to substantial energy chirp and spread. The large energy spread is a major show-stopper towards key application such as the free-electron-laser. Here we present a novel method for energy chirp compensation which takes advantage of tailored beam loading due to a second ``escort'' bunch released via plasma photocathode. The escort bunch reverses the accelerating field locally at the trapping position of the ultrahigh 5D-brightness beam. This induces a counter-clockwise rotation within the longitudinal phase space and allows to compensate the chirp completely. Analytical scaling predicts energy spread values below 0.01 percentage level. Ultrahigh 5D-brightness combined with minimized energy spread opens a path towards witness beams with unprecedented ultrahigh 6D-brightness [1]. [1] Manahan*, G.G. and Habib*, A.F. et al. Nat. Commun.8,15705 (2017) (* equal contribution)