Session TO03: Beams: Laser Wakefield Acceleration (LWFA)

Live

We report the development and initial guiding results of a variable-radius, cryogenically-formed, gas-filled capillary discharge waveguide. The channel was created by freezing nitrous oxide gas onto the inner walls of a sapphire capillary such that the channel radius could be adjusted in situ by controlling the freezing process. We demonstrate guiding of low-power laser pulses through a 6 cm-long waveguide with varying channel diameters. Through measurements of the pulse energy transmission, the ability to control the matched spot size with the ice layer thickness was shown with experiments and magnetohydrodynamic simulations. This work was supported by the Director, Office of Science, Office of High Energy Physics, of the U.S. Department of Energy under Contract Nos. DE-AC02-05CH11231 and DE-FG02- 12ER41798,~ the NSF, and the project High Field Initiative (Grant No. CZ.02.1.01/0.0/0.0/15 003/0000449) from the European Regional Development Fund.   

Live

Laser plasma accelerators are capable of generating multi-GeV electron beams. Increasing the electron beam energy further requires optical guiding in a plasma channel with low plasma density ($N_{e}\simeq 10^{17}cm^{-3}$) on axis [1,2]. Here we report a new highly tunable technique for generating meter-scale low density plasma waveguides [3]. Plasma waveguides are imprinted in hydrogen gas by optical field ionization induced by two time-separated Bessel beam pulses: The first pulse, a $J_{0}$ beam, generates the core of the waveguide, while the delayed second pulse, here a $J_{8}$ or $J_{16}$ beam, generates the waveguide cladding. We present plasma density profiles characterized by interferometry, and demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths with on axis plasma densities as low as $N_{e}\approx 5\times 10^{16}cm^{-3}$. [1] Gonsalves, A. J., et al. Physical review letters 122.8 (2019): 084801. [2] Shalloo, R. J., et al. Physical Review Accelerators and Beams 22.4 (2019): 041302. [3] Miao, B., et al. arXiv preprint arXiv:2005.14389 (2020).   

Live

Laser driven plasma wakefield accelerators can accelerate charged particles with GeV/cm gradients. To sustain these gradients over cm-scale distances, laser pulse guiding is typically required. Control and tunability of the laser pulse guiding process are key ingredients to enable charged particle acceleration that results in high bunch quality with GeV energies. In this contribution we study the stability, reproducibility, quality and limits of optical guiding in 9 and 20 cm-long plasma channels. We show that a discharge in a hydrogen-filled capillary can provide a plasma channel that is controllable and reproducible. We also show that when guiding low power laser pulses, the waveguide preserves laser pulse transverse phase space, pointing and spot size fluctuations. This work demonstrates an important technological basis for reproducible and repeatable acceleration of electron beams in laser plasma wakefield accelerators that use capillary discharge waveguides as guiding structures.   

Live

High-power laser systems are now routinely employed at labs all over the world, ranging from peak powers of 10s of TW (teraWatt) to multi-PW (petaWatt) and beyond. While attracting great interest due to their compact footprint compared to alternative technologies, the non-linear physics at play in the high-power laser-plasma interactions makes the applications highly sensitive to shot-to-shot fluctuations. Here, we present an on-line non-perturbative monitoring of the high-power laser focus position using a weaker back-surface-reflected fully-correlated copy of the high-power beam (the "witness beam"). This system works both for the 5 Hz amplified pulses as well as for the kHz background pulse train. . The kHz diagnostic revealed a pointing jitter spectrum dominated by environmental fluctuations below 100 Hz, which thus has the potential to be corrected by fast piezo feedback. We also found that the centroids between the 1-Hz amplified pulses and the temporally-adjacent 1-kHz background pulses were well correlated, which suggests that the pointing of the 100-TW-class laser can be actively controlled through stabilization of the kHz beam.   

Live

A new generation of particle accelerator facilities based on laser-plasma sources are being established globally. Controlling the highly nonlinear physics of laser-plasma accelerators is challenging, as small changes to the input parameters can create a fundamental shift in behaviour. Here, we demonstrate the application of machine learning techniques to automate the control and optimisation of the electron and X-ray beams generated by a plasma accelerator. The algorithm, based on Bayesian optimisation, simultaneously varied up to 6 parameters controlling the spectral and spatial phase of the laser as well as the plasma density and length. This enables efficient optimisation and tailoring of electron and X-ray source properties for different potential applications. It is also shown that interrogation of the models generated by the machine learning algorithm can be used to provide physical insight into the systems under study.   

Live

High-repetition-rate laser systems have been widely used with evolutionary algorithms to solve optimization problems in the field of relativistic laser-plasma interactions. However, these algorithms usually provide little information besides the optimized result, which can be hard to interpret. Machine learning methods can generate predictive models to reveal more information in the dataset and help understand the physics relations. In this work, we measured the electron beam charge from a laser-wakefield accelerator upon changing the laser wavefront using a deformable mirror. Through model training with weight learning, we predict the electron beam charge given the wavefront using four supervised learning methods: random forest, neural networks, deep joint-informed neural networks, and Gaussian process. We show that generating higher beam charge favors specific wavefront by ranking the feature importance. We show that machine learning can help understand the measured data quality as well as recognize irreproducible data and outliers. We also include virtual measurement errors in the dataset to exam the model performance. This work demonstrates how machine learning methods can benefit the data analysis and physics interpretation in a nonlinear LPI problem.   

Live

We report on the transfer efficiency of laser energy to the accelerated electron bunch in a laser wakefield accelerator. This was explored experimentally through simultaneous measurement of the deceleration of laser photons and the acceleration of the trapped electrons as a function of the accelerator length. The efficiency, expressed as the ratio of total electron energy gain to total laser loss, was maximised at $>20$\% by tuning of the plasma density and pulse compression. Using ionisation injection allowed for 160~pC bunches to be accelerated to a maximum energy of 1.5 GeV over 25 mm of plasma at a density of $1.25\times10^{18}$~cm$^{-3}$. At higher densities, the laser was observed to redshift over a full octave, from 800~nm to 1600~nm. Simulations show that at the optimal conditions, the evolution of the driving laser pulse enables the electrons to maintain phase with the peak accelerating field, and so the accelerator becomes effectively dephasingless.   

Live

Our prior experiments using 30fs, mJ-scale, kHz repetition rate laser pulses and near-critical density gas jet targets [1] lead to electron acceleration to \textasciitilde MeV levels in an exponential energy distribution [2]. Our more recent experiments, using \textasciitilde 5 fs, \textless 3 mJ pulses, lead to operation in the so-called bubble regime and the generation of quasi mono-energetic electron bunches up to 15 MeV [3]. Here, we describe how axially (z) scanning the focused laser beam waist with respect to the gas jet affects the transverse shape of the accelerated electron bunch. When the laser beam waist is located at the exit side of the jet in the density down-ramp, accelerated electron bunches have a Lorentzian transverse shape of the form$\sigma_{q} \propto [1+((x-x_{0} )/w_{x} )^{2}+((y-y_{0} )/w_{y} )^{2}]^{-3/2}$ \begin{figure}[htbp] \centerline{\includegraphics[width=3.50in,height=0.23in]{300620201.eps}} \label{fig1} \end{figure} . Our measured profiles are compatible with the so-called Kappa distribution [4], a non-thermal distribution well-known in space plasmas, but heretofore unobserved in laser plasma acceleration experiments. \begin{enumerate} \item F. Salehi, \textit{et al}., Rev. Sci. Instrum. \textbf{90}, 103001 (2019). \item F. Salehi, \textit{et al}., Opt. Lett. \textbf{42}, 215--218 (2017). \item F. Salehi, \textit{et al}., FiO$+$LS, JW4A.116, 2019 \item G. Livadiotis, J. Geophys. Res. Sp. Phys. \textbf{120}, 880 (2015). \end{enumerate}   

Live

Recent advances in CO$_2$ laser technologies have renewed interest in long wave infrared (LWIR) laser driven wakefield accelerators in low density ($10^{16} - 10^{17}$ $cm^{-3}$) plasmas$^{1,2}$. Evolution of the self-injection process in the transition of a LWIR laser driven LWFA from self-modulation to blowout regime has been investigated using 3D Particle-in-Cell simulations. The simulation results show that in SM-LWFA regime, self-injection arises with wave breaking, whereas in the blowout regime, self-injection is not observed under the simulation conditions. The wave breaking process in SM-LWFA regime occurs at a field strength that is significantly below the 1D wave-breaking threshold. This process intensifies at higher laser power and plasma density and is suppressed at low plasma densities ($\leq 1\times10^{17}$ $cm^{-3}$ here). The produced electrons show spatial modulations with a period matching that of the laser wavelength, which is a clear signature of direct laser acceleration (DLA). Optimal parameters for transition into the blowout regime have been presented. [1] P. Kumar, et al., Physics of Plasmas, vol. 26, no. 8, 2019. [2] P. Kumar, et al.,Journal of Physics: Conf Ser, vol. 1067, no. 4, p. 42008, 2018.   

Live

Laser wakefield accelerators (LWFAs) can sustain accelerating gradients that greatly surpass those of conventional accelerators. Long ($\sim$ps) and intense ($>$TW) laser pulses have been employed in LWFAs to generate bright, hard X-rays which are of interest for imaging and diagnosing warm-dense matter. The CO$_2$ laser at the ATF facility of the Brookhaven National Laboratory is a unique source, which can generate $~$2 ps-long, multi-TW laser pulses in the mid-IR (9.2 $\mu$m) regime. The properties of the laser-plasma interactions were characterized by imaging the plasma wakefields with the linac-produced short (150-250 fs) relativistic electron beam at ATF. The evolution of a self-modulated laser wake in an underdense plasma has been directly observed and analyzed. Experimental results as well as simulations exploring the properties of this regime will be presented.   

Live

The advent of multi-TW CPA CO2 lasers[1] is opening mid-IR wavelengths for laser wakefield accelerators, favoring low plasma densities ne[2], and large accelerating structures enabling precise external lepton injection and optical and electron probing of wake density and field structure. We report (experiment AE95 BNL/ATF) the demonstration of electron acceleration, in the self-modulated regime, using the ATF CO2 mid-IR laser (2ps, 5J, w0\textasciitilde 30$\mu $m). Electron self-injection and acceleration was observed in a 2mm long H2 gas jet (\textasciitilde 1e17 \textless ne\textless 1e18cm-3) with bunch charge exceeding 10pc and energies exceeding 10MeV. The results agree well with 3D PIC simulations. [1] M. N. Polyanskiy et al., OSA Continuum 3, 459-472 (2020) [2] I. Pogorelsky et al, Plasma Phys. Control. Fusion 56, 084017 (2014)   

Live

Laser heated capillary discharge waveguides have been used to guide PW-scale laser pulses over many diffraction lengths at plasma densities suitable for multi-GeV laser plasma acceleration. These structures recently enabled the acceleration of electrons to 7.8 GeV in a single stage with 850 TW of laser power. Experiments and simulations elucidating the physical processes by which waveguide formation occurs, as well as the effect of laser and plasma parameters on waveguide properties, are presented. The implications of these results for production of high-performance acceleration structures is discussed. Finally, methods for production of quasi-monoenergetic electron beams at the multi-GeV level are discussed.   

Live

The low transverse emittance of electron bunches from laser plasma accelerators (LPAs) makes these advanced accelerators attractive for compact FELs and colliders. To date, direct measurement of this emittance has proven difficult due to the micron-scale beam waist near the accelerator. Here we present single-shot coherent transition radiation (CTR) imaging and interferometry data from electron bunches only \textasciitilde 1 mm after emerging from a 300 MeV LWFA. Using eight cameras with different wavelength bandpass filters, we image CTR emitted from a foil placed directly after the LPA. At each of these wavelengths, we observe radially polarized annular distributions, albeit with detailed shape variations, but sharing a strong central minimum, consistent with CTR. These images help us to characterize the micron-scale transverse beam shape. We employ a multioctave spectrometer to measure the spatially averaged TR spectrum from IR to near-UV wavelengths to characterize longitudinal beam shape. Wavelength-dependent variations in the size and radial distribution of the TR images can be correlated with features in the reconstructed longitudinal profile. Combining the longitudinal information acquired by the multi-octave spectrometer with multi-wavelength images of the foil, we observe features in the 3D beam that are unresolvable using other techniques, and, with the aid of physically reasonable assumptions about the bunch profile, to reconstruct a 3D charge distribution at the foil.   

On Demand

We present a new method for forming long plasma waveguides using modified Bessel beams and an extended gas jet. In recent work [1], we demonstrated a two pulse method for forming plasma waveguides: a $J_0$ Bessel beam ionizes a plasma column via optical field ionization which expands outwards, then a high order $J_q$ Bessel beam follows at a delay, ionizing a ring at the edge of the expanded plasma, forming a plasma guiding structure with on axis densities as low as $5\times10^{16}~ cm^{-3}$ and attenuation lengths up to several meters. High-power guiding was demonstrated in a $5~cm$ long gas jet, but for longer jets the guide quality was significantly limited by obstruction of the focusing $J_q$ beam by the gas nozzle. Here, we demonstrate mitigation of this problem using a modified ‘binary’ $J_q$ formed with a $\pi$-step phase plate [2], which generates a non-axisymmetric Bessel-like focal line with resistance to focal deterioration in the presence of beam obstructions. Using such a beam in our two-pulse waveguide formation technique can generate plasma waveguides in gas jet targets several tens of $cm$ long. [1] Miao, B., et al. arXiv preprint arXiv:2005.14389 (2020). [2] Shutova, M., et al., J. Opt. Soc. Am. B 36, 1313-1319 (2019)