Session JO5: Particle Generation and Acceleration

Impact of Distributed Injection on Plasma Wakefield Acceleration at FACET

Impact of Distributed Injection on Plasma Wakefield Acceleration at FACET An electron-beam-driven plasma wakefield accelerator (PWFA) will sustain accelerating gradients of tens of GeV/m in a meter-scale plasma. If the transverse radius of the electron beam is not matched to the plasma, the envelope of this drive beam will execute betatron oscillations in the focusing force of the ion column. At its lowest radius in this oscillation cycle, the electric field of the beam can surpass the ionization threshold of elements, leading to ionization injection of these electrons in to the wake. Electrons from each cycle of this betatron oscillation then accumulate at the back of the wake and decrease the accelerating field. The experiments were carried out at FACET, where the drive electron beam had 3 nC of charge and an energy of 20.35 GeV. Two different plasma sources were used: a \textasciitilde 30 cm self-ionized Rubidium (Rb) vapor confined by argon (Ar) gas at room-temperature and a partially pre-ionized hydrogen gas. The experimental and simulation evidence for the distributed injection of electrons and their impact on the PWFA at FACET will be presented in this talk.   

Status and future plans for open source QuickPIC

QuickPIC is a three dimensional (3D) quasi-static particle-in-cell (PIC) code developed based on the UPIC framework. It can be used for efficiently modeling plasma based accelerator (PBA) problems. With quasi-static approximation, QuickPIC can use different time scales for calculating the beam (or laser) evolution and the plasma response, and a 3D plasma wake field can be simulated using a two-dimensional (2D) PIC code where the time variable is $\xi=ct-z$ and z is the beam propagation direction. QuickPIC can be thousand times faster than the normal PIC code when simulating the PBA. It uses an MPI/OpenMP hybrid parallel algorithm, which can be run on either a laptop or the largest supercomputer. The open source QuickPIC is an object-oriented program with high level classes written in Fortran 2003. It can be found at https://github.com/UCLA-Plasma-Simulation-Group/QuickPIC-OpenSource.git   

Studies of hosing of a witness beam in plasma based acceleration

A major challenge for the next generation of plasma wakefield acceleration is the preservation of emittance of the witness beam. The hosing instability is one source of emittance of the witness beam that occurs when the witness beam has a transverse offset. A general theory has been developed to describe hosing in the blow-out regime and has shown smaller growth than standard theories. However, these theories have not been rigorously tested with witness beams in the relativistic, non-adiabatic regime of interest to plasma wakefield acceleration. A modified theory using an expansion in azimuthal modes is discussed. This theory alongside 3D QuickPIC simulations are used to study perturbations to the wake structure from point charges with transverse displacements as well as witness beams optimized for beam loading.   

Plasma-optical spatiotemporal diagnostics and alignment for electron and laser beams

The steadily increasing demand for compact accelerator-driven light sources imposes new challenges for generating compact, high-quality electron beams and concomitant $\mu$m-scale, fs-scale diagnostics. During the E210 experimental campaign at FACET (SLAC), we have amended state-of-the-art electro-optical sampling timing diagnostics and optical transition radiation spatial diagnostics with novel plasma-based techniques. By harnessing the ultrasensitive plasma response to intersecting laser and electron beams, we have developed novel diagnostic techniques which potentially enable spatiotemporal alignment with sub-fs and sub-$\mu$m accuracy. Furthermore, the diagnostics can be realized in a simple and robust layout; they are based on measuring the time-integrated plasma recombination light from tunnel ionization as well as electron impact ionization. They thus map ultrashort and small dynamics onto much longer and larger scales, such that the main diagnostic element is a simple imaging device. These techniques, the underlying physics and their potentially far-reaching impact will be presented and discussed.   

Abstract Withdrawn

The interaction of a high intensity ($\geq 10^{18}$ W/cm$^2$), high contrast ($\geq 10^{9}$), ultra-short ($30 $fs) laser with solid targets generates a highly dense hot plasma. The quasi-static electric fields in such plasmas are well known for ion acceleration via the target normal sheath acceleration process. Under such conditions charge reduction to generate fast neutral atoms is almost inhibited. Improvised Thomson parabola spectrometry with improved signal to noise ratio has enabled us to measure the signals of fast neutral atoms and negative ions having energies in excess of tens of keV. A study on the neutralization of accelerated protons in plasma shows that the neutral atom to all particle ratio rises sharply from a few percent at the highest detectable energy to $\approx 50 \%$ at $15$ keV. Using usual charge transfer reactions the generation of neutral atoms can not be explained, thus we conjecture that the neutralization of the accelerated ions is not from the hot dense region of the plasma but neutral atom formation takes place by co-propagating ions with low energy electrons enhancing the effective neutral ratio.   

Neutron generation in deuterated nanowire arrays irradiated by femtosecond pulses of relativistic intensity

Nuclear fusion is regularly created in spherical plasma compressions driven with multi-kilojoule lasers. Driving fusion reactions with compact lasers that can be fired at much higher repetition rates is also of interest. We have demonstrated a new dense fusion environment created by irradiating arrays of deuterated nanostructures with Joule--level pulses from a compact Ti:Sa laser. The irradiation of ordered deuterated polyethylene nanowires arrays with femtosecond pulses of relativistic intensity is shown to create ultra-high energy density plasmas in which deuterons are accelerated to MeV energies, efficiently driving D-D fusion reactions and ultrafast neutron pulses. We have measured up to 2 x 10$^{\mathrm{6}}$ fusion neutrons/Joule, a 500 times increase respect to flat solid targets, a record yield for Joule-level lasers, and have also observed a rapid increase in neutron yield with laser pulse energy. We present results of a first experiments conducted at intensities \textgreater 1 x 10$^{\mathrm{21}}$ W cm$^{\mathrm{-2}}$ that generated \textgreater 1 x 10$^{\mathrm{7}}$ fusion neutrons per shot.   

Neutron beams driven by the Texas Petawatt laser

Intense laser-driven ion beams produced in the relativistically-induced transparency regime have been used to generate intense $\gamma $-ray and neutron beams [1]. For neutrons, a laser-driven deuteron beam is directed at a Be disk ``converter'', where deuterons split producing mainly forward-directed neutrons. The aforementioned experiments have been done at the Trident laser using a 0.5 ps laser pulse of 1 $\mu $m wavelength focused up to 10$^{\mathrm{21}}$ W/cm$^{\mathrm{2}}$ onto nanofoils of deuterated-plastic (CD$_{\mathrm{x}}$ where x$=$1$-$2), making 1x10$^{\mathrm{10}}$ neutrons/sr at $\sim $ MeV average energies [2]. Here we report on the first experiments to explore the same regime at the Texas Petawatt (TPW) laser facility. With one plasma mirror, TPW delivers high-contrast laser pulses as short as 0.15 ps at intensities up to 2x10$^{\mathrm{21}}$ W/cm$^{\mathrm{2}}$. CD and Al/CD multilayer targets of thickness in the range of 50 -- 750 nm have been used. This setup has delivered up to 5x10$^{\mathrm{9}}$ neutrons/sr. The dependence of neutron yield on target composition and thickness, and on laser pulse length is presented and discussed. . . . . . . . . . . . . . [1] J.C. Fern\'{a}ndez et al$.$, \textit{Laser-plasmas in the relativistic-transparency regime: Science and applications, }Phys. of Plasmas \textbf{24}, 056702 (2017) [2] M. Roth et al., \textit{Bright Laser-Driven Neutron Source Based on the Relativistic Transparency of Solids,} Phys.Rev. Lett. \textbf{110, }044802 (2013)   

Proton probing of ultra-thin foil dynamics in high intensity regime

The field of laser driven ion acceleration has been enriched significantly over the past decade, thanks to the advanced laser technologies. Already, from 100s TW class systems, laser driven sources of particles and radiations are being considered in number of potential applications in science and medicine due to their unique properties. New physical effects unearthed at these systems may help understand and conduct successful experiments at several PW class multi-beam facilities with high rep rate systems, e.g. ELI. Here we present the first experimental results on ultra-thin foil dynamics irradiated by an ultra-high intensity (10$^{\mathrm{20}}$ W/cm$^{\mathrm{2}})$, ultra-high contrast (10$^{\mathrm{-12}})$ laser pulse at ARCTURUS laser facility at HHU Duesseldorf. By employing the elegant proton probing technique it is observed that for the circular polarization of laser light, a 100nm thin target is pushed forward as a compressed layer due to the radiation pressure of light. Whereas, the linear polarization seems to decompress the target drastically. 2D particle-in-cell simulations corroborate the experimental findings. Our results confirm the previous simulation studies investigating the fundamental role played by light polarization, finite focus spot size effect and eventually electron heating including the oblique incidence at the target edges.   

Solid-density plasma expansion in intense ultra-short laser irradiation measured on nanometer scale and in real time

Small Angle X-ray Scattering (SAXS) is discussed to allow unprecedented direct measurements limited only by the probe X-ray wavelength and duration. Here we present the first direct in-situ measurement of intense short-pulse laser - solid interaction that allows nanometer and high temporal resolution at the same time. A 120 fs laser pulse with energy 1 J was focused on a silicon membrane. The density was probed with an X-ray beam of 49 fs duration by SAXS. Despite prepulses, we can exclude premature bulk expansion. The plasma expansion is triggered only shortly before the main pulse, when an expansion of 10 nm within less than 200 fs was measured. Analysis of scattering patterns allows the first direct verification of numerical simulations.   

Optimum and Controllable Multi-stage Proton Acceleration Manipulated by Double Beam Image Technique

With the development of ultra-intense laser technology, laser intensity can increase up to the order of \textasciitilde 10$^{\mathrm{22}}$ W/cm2 in the laboratory. Ion beams in the MeV range and even the GeV range, driven by terawatt or petawatt lasers, exhibit ultra-short pulse duration, excellent emission, and ultra-high peak current. Thus, they can potentially be applied in fast ignition of inertial confinement fusion, medical therapy, proton imaging, and pre-accelerators for conventional acceleration devices. However, the generation of quasi-monoenergetic proton beams for realistic applications is still an experimental challenge. Here, the optimum and controllable two-stage proton acceleration is realized for the first time by a novel double beam image (DBI) technique in experiment. Two laser pulses are successfully tuned on two separated foils with both spatial collineation and time synchronizing, resulting in spectrum tailoring and an energy increase at the same time. Such a novel DBI technique can help us to realize the optimum two-stage acceleration in a feasible way, which opens the door for the exact manipulation of multi-stage acceleration to further improve the energy and spectra of particle beams.   

Recent Progress on Laser Produced Positron Research At LLN

We report the recent results on laser-produced relativistic electron-positron plasma jets. This includes: the prepulse [1] and material dependence of pair generation [2]; time dependent positron acceleration [3] and maximum achieved pair density [4]. We will highlight the results from recent experiments on the Omega EP laser testing nanostructured target to increase pair yield.~ ~We will also report on a newly commissioned platform using the NIF ARC lasers which was developed for efficient pair creation using 10 ps laser duration at near relativistic laser intensity. [1] Jaebum Park \textit{et a}l., High Power Laser Science and Engineering\textbf{ 4} 26 (2016) [2] G. Jackson Williams \textit{et a}l,, Physics of Plasmas\textbf{ 2}3, 123109 (2016) [3] Shaun Kerr \textit{et al}., this conference. [4] Brandon Edghill \textit{et al}., this conference.   

Sheath field dynamics from time-dependent acceleration of laser-generated positrons

Positrons produced in ultraintense laser-matter interactions are accelerated by the sheath fields established by fast electrons, typically resulting in quasi-monoenergetic beams [1]. Experimental results from OMEGA EP show higher order features developing in the positron spectra when the laser energy exceeds one kilojoule [2]. 2D PIC simulations using the LSP code were performed to give insight into these spectral features. They suggest that for high laser energies multiple, distinct phases of acceleration can occur due to time-dependent sheath field acceleration. The detailed dynamics of positron acceleration will be discussed. [1] Chen et al., PRL 105, 015003 (2010). [2] Chen et al., PoP 22, 056705 (2015).   

Scaling laws for positron production in laser-electron beam collisions

Showers of gamma rays and positrons are produced when a multi-GeV electron beam collides with a super-intense laser pulse. All-optical realisation of this geometry, where the electron beam is generated by laser-wakefield acceleration, is currently attracting much experimental interest as a probe of radiation reaction and QED effects. These interactions may be modelled theoretically in the framework of strong-field QED or numerically by large-scale PIC simulation. To complement these, we present analytical scaling laws for the electron beam energy loss, gamma ray spectrum, and the positron yield and energy that are valid in the radiation-reaction--dominated regime. These indicate that by employing the collision of a 2 GeV electron beam with a laser pulse of intensity $5\times10^{21}\,\mathrm{Wcm}^{-2}$, it is possible to produce 10,000 positrons in a single shot at currently available laser facilities.