Session BO6: AB: Laser Wakefield and Direct Laser Acceleration

Dephasingless laser wakefield acceleration

The energy gain in conventional laser wakefield acceleration (LWFA) is ultimately limited by dephasing, occurring when trapped electrons outrun the accelerating phase of the wakefield. Here we apply spatiotemporal pulse shaping to overcome this limitation. The ponderomotive force of spatiotemporally shaped pulses can drive a wakefield with a phase velocity equal to the speed of light in vacuum, preventing trapped electrons from outrunning the wake. Analytic scalings in the linear and bubble regimes illustrate the distinct parameter regimes required to optimize traditional and dephasingless LWFAs.   

Wakefields in a Cluster Plasma

We report the first comprehensive study of large amplitude Langmuir waves in a plasma of nanometer-scale clusters. The shape of these wakefields was captured by a single-shot frequency-domain holography diagnostic at an oblique angle of incidence for the first time. The wavefronts are observed to curve backwards, in contrast to the forwards curvature of wakefields in uniform plasma. The first wakefield period is longer than those trainling it. The features of the data are well described by fully relativistic two-dimensional particle-in-cell simulations and a one-dimensional model solving a coupled system consisiting of the equation of motion, Ampere's law and the Poisson equation for strong density perturbations.   

Enhanced Self-Injection by Optical Field Ionization Heating in a Laser Wakefield Accelerator

Laser-wakefield acceleration (LWFA) experiments performed using the Hercules laser show a decrease of the self-injection threshold by using circularly polarized (CP) laser pulses compared with linearly polarized (LP) pulses, similar to the threshold lowering in LWFA driven in a warm plasma. In addition to the lower injection threshold, a significantly higher electron beam charge was observed for CP compared to LP over a wide range of parameters. Theoretical analyses and quasi-3D Particle-in-Cell simulations agree with the observed experimental findings and indicate a modified injection path for CP laser pulses, which enables electrons from a much broader range of parameters to be injected.   

Development of a laser wakefield acceleration platform at the National Ignition Facility

We present the development of a laser-wakefield electron acceleration experimental capability by focusing one beamlet (1 ps, 250 J) of the Advanced Radiographic Capability (ARC, LLNL) onto a gas tube target filled with helium. When a picosecond, 10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}}$ intensity laser pulse is focused on a gas target with a plasma electron density of about 10$^{\mathrm{19}}$ cm$^{\mathrm{-3}}$, electrons are accelerated to multi-100 MeV energies by the interplay of self-modulated laser wakefield and direct laser acceleration. Applications include hard x-ray sources using betatron, Compton scattering and bremsstrahlung mechanisms. We performed experiments with the OMEGA-EP short pulse focused at intensities around 10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}}$ onto a 3 mm plastic gas tube filled with helium at atmospheric pressure, as well as with the ARC beam at LLNL in similar conditions. The gas tubes are closed with 1 \textmu m thick mylar windows that are blown off with long pulses 5-10 ns before the short pulse. We measured the plasma density at the entrance of the gas tube with the 4-omega probe diagnostic at OMEGA-EP and Optical Thomson Scattering at NIF. EPPS (electron proton positron spectrometer), measured accelerated electron energies in the 10-150 MeV range.   

Optimization of laser-heated capillary discharge waveguides for laser wakefield acceleration

Laser-heated discharge capillary waveguides provide low plasma density guiding structures to guide laser pulses over many diffraction lengths and have been recently employed in laser-plasma acceleration experiments to achieve 7.8 GeV. Optimizing accelerator performance requires control of waveguide plasma density and matched spot size, which experiments show can be tuned via initial discharge and laser parameters. Characterization of the matched spot size and plasma density in laser-heated capillary discharges is presented. Measurements are compared to modeling using the MHD code MARPLE. Trends in waveguide properties with respect to initial plasma density and temperature, as well as heating laser parameters, have been identified. Strategies for optimizing accelerator performance are described.   

Shock injection producing narrow energy spread, GeV electron beams from a laser wakefield accelerator

The parameters of electron beams produced by a laser wakefield accelerator are in large part determined by the dominant injection mechanism. In shock injection the driving laser pulse crosses abruptly from a region of high plasma density to one of lower density. The sudden change in the plasma wavelength leads to injection of electrons. Shock injection has been successfully employed on lower power ($< 100$ TW) systems where it produces tunable narrow energy spread electron beams (Buck et al PRL 2013). Here we present an investigation of shock injection on the higher power Gemini laser system ($>$ 200 TW). In this case shock injection can produce high energy ($>$ 1 GeV), narrow energy spread ($< 5\%$) electron beams. The injection here is found to be sensitive to the position of shock front within the accelerator, in contrast to previous results at lower power.   

Wakefield acceleration by incoherent radiation

Particle acceleration is central in many physical scenarios, from collider experiments to extreme astrophysical environments, gamma-ray bursts, supernovae and their remnants. Plasma wakefield accelerators have proven to deliver relativistic electrons in the laboratory by using laser drivers (coherent radiation) or charged particle beams. We show that an incoherent pulse of radiation can excite a plasma wake capable of accelerating particles to relativistic energies. We provide estimates for the wake amplitude, the condition for particle trapping and acceleration, and the maximum achievable energy. This mechanism could be exploitable in the laboratory, and it is an additional mechanism that can contribute to particle acceleration in astrophysics. Our results are confirmed by self-consistent particle-in-cell simulations performed with the PIC code OSIRIS where a Compton scattering module has been implemented, following the pioneering numerical work of Frederiksen [ApJ 680, L5 (2008)].   

Advanced Diagnostics for GeV Class Laser Plasma Accelerated Electron Beams Using Active Plasma Lens

As laser plasma accelerators (LPA) capable of generating GeV class electron beams become increasingly ubiquitous, there is an ever evolving need to augment existing electron beam diagnostics and measurement techniques in order to address the unique challenges associated with LPA sources and their environments. At the BELLA PW facility, we developed an active plasma lens (APL) which allows for single shot emittance measurements over large range of electron beam energies. Furthermore, with short focal length capabilities for energies \textgreater 1 GeV, the APL is combined with relatively small dipole assembly to produce a compact, high resolution, GeV class magnetic spectrometer. These diagnostic techniques are facilitated through the use of a \textasciitilde 30 nm thick liquid crystal based plasma mirror used to separate the remnant PW laser pulse from the electron beam.   

Efficient petawatt beam redirection for GeV class laser plasma accelerated electron beams using thin film liquid crystal plasma mirrors

State-of-the-art laser-plasma accelerators (LPA's) can routinely produce GeV electron beams. Further development of this technology requires better control of the petawatt beam near the LPA, including beam redirection into and out of the electron beamline via ultrathin plasma mirrors. Here we report on an experiment done at the BELLA PW facility that achieved 94{\%} attenuation via beam redirection and absorption using \textasciitilde 30 nm thick liquid crystal plasma mirrors. Liquid crystal films were formed in situ on-demand and successfully used to protect an active plasma lens placed about 20 cm downstream. Additionally, emittance degradation arising from well-known Coulomb scattering interactions is intrinsically suppressed due to the nm scale thicknesses of the films. We describe the device capabilities and associated diagnostics developed, film survival tests in the LPA environment, and the transmittance as a function of intensity. We describe a simple model of plasma mirror performance at high intensities.   

Planar Laser-Induced Fluorescence For Tailored Laser Plasma Accelerator Gas Jet Targets

The ability to pre- cisely shape gas jets for controlled injection of electrons in laser plasma accelerators (LPAs) is crucial for developing high quality electron beams. Verifying tailored density profiles has called for more detailed gas density diagnostics than those traditionally used. Most diagnostics give line-of- sight measurements which integrate over and blur sharp or asymmetric features. In this study, planar laser-induced fluorescence (PLIF) has been prototyped for characterizing LPA gas jet targets. PLIF has the distinct advantage of isolating thin slices of the gas plume using a laser sheet, providing more direct density information at regions of interest. This sheet can be scanned across the jet to map out a detailed 3-D gas plume profile. It was shown that PLIF is able to resolve critical fea- tures such as gas density shocks. Blade-in jets under low vacuum were characterized with PLIF. It was found that blade position dramatically alters characteristic flow parameters, affecting plume axis and effective Mach number. These results, together with simulations of gas flow, are being used to understand and design flow and shock regimes for LPA targets for several applications in the BELLA Center.   

Recent Progress with Brookhaven's ATF LWIR Laser and Future Experimental Plans

Recent interest in driving laser wakefield acceleration (LWFA) with mid- and long-wave infrared sources at plasma densities of 10$^{\mathrm{15}}$ to 10$^{\mathrm{17}}$ cm$^{\mathrm{-3}}$ has been motivated by the advantages of high ponderomotive potential, larger critical density bubble volume, and relaxed phasing/staging tolerances. The highest energy drivers in the LWIR regime are large-aperture CO$_{\mathrm{2}}$ lasers capable of producing several joules and few picosecond pulse duration. The Brookhaven ATF LWIR laser has continuously evolved to deliver higher peak powers benefiting a range of experiments, including LWFA. Up to 5 TW in a 2 ps pulse at 9.2 micron is presently available for experiments in combination with synchronized electron bunches and NIR laser pulses. Techniques used to achieve the current operating parameters will be described, compared with laser simulations, and highlighted by comparison with requirements for LWFA experiments underway at the facility. CO$_{\mathrm{2}}$ laser-driven LWFA plasma measurements using both optical and electron probes will be presented and compared with numerical simulations.   

Tilted Snowplow Ponderomotive Electron Acceleration with Ultrafast Laser Pulses

Using simultaneous spatial and temporal focusing (SSTF), ultrafast laser pulses can be generated which are fully compressed only at the focal plane and which exhibit a tilted spatio-temporal intensity envelope. We propose a novel technique for ponderomotively accelerating electrons in free space using SSTF pulses where the laser effectively acts as a snowplow. The pulse front tilt (PFT) lengthens the interaction time the pulse has with the electrons which allows them to accelerate from rest while staying on the pulse. Non-relativistic and relativistic single particle analyses are presented in the adiabatic ponderomotive approximation for idealized infinitely wide pulses as well as finite width pulses. These analyses show that the acceleration intensity threshold is a function of the PFT angle and that the final electron energy is equal to four times the ponderomotive energy. We confirm the basic scheme using full-field, many particle 2D OSIRIS 4.0 particle-in-cell simulations and show how further tailoring the pulse's spatio-temporal profile enhances the electron bunch characteristics. We are currently working to implement this scheme in our lab to experimentally verify the predictions of our analyses. This ponderomotive snowplow scheme shows promise as a laboratory scale MeV-level accelerator with narrow energy and angular spreads which has applications in ultrafast electron diffraction and accelerator injection.   

Tunable Relativistic Infrared Pulses from Laser-produced Wakes in Tailored Plasma Structures

The development of intense few-cycle mid-infrared (mid-IR, $\lambda $\textless 5 $\mu $m) laser sources has made significant progress during the past decade, which has opened many opportunities for infrared nonlinear optics, high-harmonic generation and pump-probe experiments in the ``molecular fingerprint'' region. However, even longer carrier wavelength (\textasciitilde 10 $\mu $m) are needed in many applications. It is one of the current challenges facing ultrafast laser technology to generate high-energy, ultra-short long-wave IR (LWIR) pulses, beyond the capability of existing methods. Recently, a new scheme that utilizes asymmetric self-phase modulation (SPM) in a tailored plasma density structure to generate multi-millijoule energy, single-cycle LWIR pulses tunable in a wide spectral range was proposed. Here, we experimentally demonstrate this novel scheme for the first time. An intense single-cycle IR pulse with a central wavelength of 9.4 $\mu $m and energy of 3.4 mJ is generated using a \textasciitilde 580 mJ, 36 fs, 810 nm drive laser. Furthermore, the tunability of the IR wavelength in the range of 3-20 $\mu $m is also demonstrated through simple adjustment of the plasma structure.   

Laser-wakefield driven generation of subcycle pulses

A scheme for the generation of intense, isolated, carrier-envelope-phase (CEP)-tunable, subcycle-pulses by laser-driven wakes in plasmas is proposed. It relies on the interaction of a low-intensity, CEP-stable, long-wavelength seed pulse with a wake driven by an intense, not necessarily CEP-stable pump laser pulse. We show through 3D particle-in-cell (PIC) simulations that a seed pulse with wavelength longer than the plasma skin depth, $c/\omega_{pe}$, can extract energy from the leading density spike of the wake. As a result of localized amplification, an intense subcycle pulse is formed. Through a parametric study with 2D PIC simulations we show that the subcycle pulse is CEP-tunable by varying either the CEP of the seed pulse or the delay between the seed and pump pulses. Moreover, we show that we can control the subcycle pulse intensity, mean frequency and spectral range by varying the plasma density and pump laser intensity. In particular, relativistic intensity subcycle pulses can be obtained in the mid-IR regime, which are hard to obtain by conventional methods. Such pulses could be used to probe slow processes (e.g. rovibrational spectra) with subcycle resolution or to excite transient nonlinear effects at non-ionizing wavelengths.   

PIC simulations of mid-infrared radiation from a laser wakefield accelerator.

The formation of a plasma ``bubble'' during Laser Wakefield Acceleration (LWFA) results in a co-propagating refractive index (electron density) gradient that produces time dependent frequency shifts in the driving laser pulse$^{\mathrm{1,2}}$. High-resolution spectral measurements of mid-infrared radiation extending to 2.5 microns during LWFA in the bubble regime were obtained on the HERCULES laser system at the University of Michigan. Particle-in-cell (PIC) simulations with OSIRIS show radiation extending briefly up to 50 microns in this regime. The PIC simulations indicate that the slow-moving long-wavelength radiation, which slips backward relative to the driving laser pulse, can interact the accelerated electron bunch. The interaction blue-shifts and scatters the long-wavelength radiation while decreasing the energy of the electron beam. These results suggest that measurements of side-scattered radiation may serve as a diagnostics of electron dynamics and bunch formation. $^{\mathrm{1}}$J. D. Ludwig, P.-E. Masson-Laborde, S. H\"{u}ller, et al. Phys. Plasmas \textbf{25}, 053108 (2018) $^{\mathrm{2}}$Zan Nie, Chih-Hao Pai, Jianfei Hua, et al. Nature Photonics vol. \textbf{12}, 489--494 (2018)