Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session GO08: Laser-Plasma Wakefield and Direct Laser AcceleratorsLive Streamed
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Chair: Frank Tsung, University of California, Los Angeles Room: 402 ABC |
Tuesday, October 18, 2022 9:30AM - 9:42AM |
GO08.00001: Active transverse stabilization of laser plasma accelerator electron beam source Samuel Barber, Curtis Berger, Kyle Jensen, Fumika Isono, Joseph Natal, Carl B Schroeder, Anthony J Gonsalves, Tobias Ostermayr, Eric H Esarey, Jeroen van Tilborg Laser plasma accelerators (LPAs) have several unique features, including compactness and the ability to produce high brightness beams, which make them an attractive alternative for a wide range of accelerator-based applications like light sources and future colliders. However, using them as alternatives to conventional linacs presents several challenges that must be addressed. In particular, applications like an XFEL or collider require transverse stability of the electron beam source to be a small fraction of a beam size, which can be a substantial challenge given typical LPA beams are micron scale in size. We report substantial improvement in the transverse stability of LPA generated beams achieved by the integration of a non-destructive fast active-feedback system to stabilize a high power wakefield driver. Nearly a factor of four improvement is made in the positional jitter of the electron beam source from 12.5 to 3 microns. Slow drifts (<1 Hz) as well as shot-to-shot jitter at frequencies up to ~20Hz are nearly eliminated. This is an important step in improving electron beam source stability in LPAs which will result in improved accelerator performance and help bridge the gap from proof-of-concept experiments to applications. |
Tuesday, October 18, 2022 9:42AM - 9:54AM |
GO08.00002: Electron acceleration by ultra-intense helical laser beams injected during transmission through relativistically transparent targets David R Blackman, Yin Shi, Sallee R Klein, Mihail O Cernaianu, Domenico Doria, Petru Virgil Ghenuche, Alexey Arefiev The concept of electron acceleration by a laser beam in vacuum is attractive due to its seeming simplicity, but its implementation has been elusive, as it requires efficient electron injection into the beam and a mechanism for counteracting transverse expulsion. We show how a specific configuration of a helical laser beam, such that the transverse field profiles are hollow while the longitudinal fields are peaked on central axis, can accelerate dense bunches of electrons to high energies. Electrons accelerated within these fields have an extremely high acceleration gradient, achieving their maximum energy within a few Rayleigh lengths (~100 microns). These electrons are collected into dense bunches, with short (~100 as) bunch duration, by the longitudinal magnetic fields at early times. Electrons injection into the central accelerating region of the helical beam is demonstrated as the helical beam passes through a strongly transparent low-density target. This mechanism is explored using three-dimensional particle-in-cell simulations. Using these simulations we demonstrate that a 3 PW helical laser can generate a 50 pC low-divergence electron beam with a maximum energy of 1.5 GeV. |
Tuesday, October 18, 2022 9:54AM - 10:06AM |
GO08.00003: Effects of plasma density fluctuations in laser wakefield accelerators Claudia C Cobo, Matthew Streeter, Eva E Los, Christopher Arran, Gianluca Sarri, Christopher P Ridgers, Stuart P.D. Mangles, Chris D Murphy Laser wakefield accelerators are promising candidates for compact sources of relativistic electron beams and bright x-rays. Highly stable accelerator performance is required for applications of these electrons, but this is difficult to achieve due to the sensitivity of the injection and acceleration dynamics to initial conditions, resulting from the non-linear underlying physics. A key parameter in determining the quality of the accelerated electrons is the plasma density, often taken as a constant and controlled by the backing pressure of the gas target. By tailoring the density profile, such as introducing a sharp longitudinal density transition in the target, it may be possible to improve the shot-to-shot stability of the accelerator. |
Tuesday, October 18, 2022 10:06AM - 10:18AM |
GO08.00004: Direct Laser Acceleration of Electrons using a Shaped Tilted Ponderomotive Mirror Patrick Hunt, Alex Wilhelm, Daniel Adams, Charles G Durfee While there has been success in Wakefield acceleration of electrons, there are a number of applications that could benefit from acceleration to modest energy (~MeV) by the laser field, for example, ultrafast electron diffraction and injection into higher-energy laser-driven accelerators. Here we outline our scheme for ponderomotive acceleration of electrons (and in principle, positrons) in which we control the group velocity of ultrafast pulses through pulse front tilt. Provided the intensity is above the threshold for capture of electrons, the leading part of the pulse front effectively acts like a moving mirror whose shape is controlled by the spatio-temporal topology of the intensity profile. Our analytic models of the propagation of spatially-chirped beams, simple relativistic single-particle models of the laser-electron interaction and our implementation of these beams in particle-in-cell simulations help to predict the output electron energy and direction. |
Tuesday, October 18, 2022 10:18AM - 10:30AM |
GO08.00005: Efficient modeling of ion-motion in LWFA using electromagnetic Particle-in-Cell code with mesh refinement Prabhat Kumar, Remi Lehe, Axel Huebl, Andrew Myers, Olga Shapoval, Weiqun Zhang, Edoardo Zoni, Jean-Luc Vay
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Tuesday, October 18, 2022 10:30AM - 10:42AM |
GO08.00006: Bayesian multi-task optimization of laser-plasma accelerators using Particle-In-Cell codes with different fidelities Angel Ferran Pousa, Soeren Jalas, Manuel Kirchen, Alberto Martinez de la Ossa, Maxence Thevenet, Jeffrey Larson, Stephen Hudson, Axel Huebl, Jean-Luc Vay, Remi Lehe When designing a laser-plasma acceleration setup, it is common to explore the parameter space (plasma density, laser intensity, focal position, etc.) with Particle-In-Cell (PIC) simulations in order to find an optimal configuration that, for example, minimizes the energy spread or emittance of the accelerated beam. However, laser-plasma acceleration is typically modeled with full PIC codes, which can be computationally expensive. Various reduced models can approximate beam behavior at a much lower computational cost. Although such models do not capture the full physics, they could still suggest promising sets of parameters to be simulated with a full PIC code and thereby speed up the overall design optimization. |
Tuesday, October 18, 2022 10:42AM - 10:54AM |
GO08.00007: Laser electron acceleration using relativistic transparency injection Feiyu Li, Prashant Singh, Chengkun Huang, Adam Moreau, Reed C Hollinger, Ann Junghans, Andrea Favalli, Chase Calvi, Shoujun Wang, Huanyu Song, Jorge J Rocca, Robert E Reinovsky, Sasi Palaniyappan Accelerating charged particles using intense lasers has been actively pursued over the past few decades. Such laser-based compact accelerators have several potential applications including fast ignition, high-energy physics and radiography. Among the several schemes of laser-plasma based electron acceleration, the vacuum laser acceleration (VLA) features great simplicity where electrons are directly accelerated by the intense laser field. However, a grand challenge of VLA lies in how to efficiently load free electrons into the fast-oscillating laser field such that the injected electrons remain within given half cycles of the laser wave and see a unipolar field for continuous acceleration. This requirement necessitates the injected electrons to be pre-accelerated close to the speed of laser light before they are captured and accelerated by the laser field. Recently, we have experimentally demonstrated electron acceleration up to 20 MeV by driving a thin solid plasma in the transparency regime. Numerical simulations and theoretical analyses suggest a new electron injection scheme for VLA based on the relativistic transparency effect, where an initial opaque plasma becomes transparent to the driving laser due to relativistic electron mass increase. When a high-contrast intense laser drives a thin solid foil, the electrons from the dense plasma are first accelerated to near-light speed by the standing laser wave formed in front of the opaque foil and they are subsequently injected into the transmitted laser field as the dense plasma becomes relativistically transparent. This work not only solves an outstanding problem in VLA by demonstrating a viable injection method, but also provides insight into the electron acceleration in relativistically transparent plasmas, which acts as the primary driver for laser-foil based ion accelerators, x-ray sources and relativistic optics. |
Tuesday, October 18, 2022 10:54AM - 11:06AM |
GO08.00008: Direct laser acceleration of positrons in a plasma channel Bertrand Martinez, Bernardo Barbosa, Marija Vranic Sources of relativistic positrons are relevant for a wide range of applied and fundamental research domains. However, the energy achieved by current accelerator facilities is limited by their size of several kilometers, and potential alternatives are rarely discussed. Here, we show that positrons can be created, injected, and accelerated during the propagation of an ultra-intense laser in a plasma channel over a distance of 400 micrometers. Positrons are generated when the laser interacts with a relativistic electron beam propagating at 90 degrees of incidence via Quantum Electrodynamics processes [1]. We derive a semi-analytical estimate to determine the number of positions created within the plasma channel. We also demonstrate that a few percent of them are deflected by the laser along its propagation direction. Direct acceleration of the positrons in the laser field is maintained over hundreds of microns in the plasma channel. We prove that positrons are guided along the channel main axis thanks to a high-charge self-injected electron beam driven by the high laser intensity. Our proposal opens a path toward a new kind of compact and relativistic positron source, based on future-generation laser systems. |
Tuesday, October 18, 2022 11:06AM - 11:18AM |
GO08.00009: Resolving the Transition between Direct Laser Acceleration and Laser Wakefield Acceleration in Near-Critical Plasmas Talia Meir, Itamar Cohen, Kavin Tangtartharakul, Lior perelmutter, Michal Elkind, Jonathan Gershuni, Asaf Levanon, Alexey Arefiev, Ishay Pomerantz Irradiation of thin foil targets by low-contrast relativistic laser pulses results in the emission of collimated jets of multi-MeV electrons. This two-decade-old observation forms the most efficient way to convert optical eV photons to MeV particles. We examine the acceleration of electrons due to the combined effect of Laser Wakefield Acceleration (LWFA) and Direct Laser Acceleration (DLA). Our new insights were obtained from Particle-In-Cell (PIC) simulations done in order to explain experimental results. The experiments were performed using the high-contrast 20 TW laser system at Tel-Aviv University. In those experiments an Au plasma is generated from a sub-µm foil. The plasma plume density profile was set using controlled µJ-mJ pre-pulses arriving 0-90 ns prior to the main pulse. The plume’s density profile was characterized by in-situ interferometric measurements. The analysis follows the plasma plume expansion model we developed, which serves as input density to PIC simulations. I will present the insights from those simulations in order to reveal the DLA and LWFA effects on the energetic electrons in different plasma conditions. We can observe highly efficient acceleration of electrons from a specific range of ionization levels. |
Tuesday, October 18, 2022 11:18AM - 11:30AM |
GO08.00010: Improving electron dephasing of an all-optical multi-GeV laser wakefield accelerator Bo Miao, Jaron E Shrock, Ela M Rockafellow, Alexander Picksley, Reed C Hollinger, Shoujun Wang, Jorge J Rocca, Howard M Milchberg Laser plasma accelerators can generate acceleration gradients of 10~100 GeV/m and have delivered multi-GeV electron beams. In a recent experiment, we demonstrated electron acceleration up to 5 GeV in a 20-cm plasma waveguide, formed via self-waveguiding pulses in a low density hydrogen gas jet [1,2]. The long optical guiding of multi-100 TW pulses causes complex evolution of the laser driver envelope and spectrum, which entails further study to improve the accelerated electron bunch quality. In this work we investigate both numerically and experimentally the additional dephasing due to pump depletion during laser propagation in a plasma waveguide. We propose and present two potential ways to alleviate this effect by either controlling the laser driver chirp or using shorter wavelength driver, such as its second harmonic. Finally, we present numerical simulations and electron energy spectra validating the proposed methods. |
Tuesday, October 18, 2022 11:30AM - 11:42AM |
GO08.00011: Optical mode filtering and electron injection in multi-GeV laser wakefield acceleration Jaron E Shrock, Bo Miao, Ela M Rockafellow, Alex Picksley, Reed C Hollinger, Shoujun Wang, Jorge J Rocca, Howard M Milchberg Recent experiments [1] have demonstrated acceleration of electron bunches up to 5 GeV in long (20 cm) low density (~10^17 cm^-3) ionization-injected plasma waveguides [2]. The spectra of the recorded electron bunches showed multiple quasi-monoenergetic peaks with resolution limited energy spreads ~15%. For eventual development of a 10 GeV laser wakefield acceleration (LWFA) module for a staged electron accelerator, it is essential that the lower energy peaks in the spectra be eliminated. Analysis of the results in [1] suggests that the multiple peaks correspond to localized injection enhancement (or suppression), exacerbated by fluctuations in the drive laser pointing and longitudinal waveguide variations, both of which strongly affect the guided mode evolution. Here, we present experimental results and particle-in-cell simulations detailing the linear and non-linear effects contributing to guided mode evolution and electron injection. We discuss how the early part of a meter-scale plasma waveguide can be used as a ‘mode filter’ to ensure controllable electron injection in multi-GeV LWFAs. |
Tuesday, October 18, 2022 11:42AM - 11:54AM |
GO08.00012: Relativistic electron acceleration at non-relativistic intensities using sub-lambda targets Ratul Sabui, Rakesh Y Kumar, M Krishnamurthy, Vandana Sharma Intense laser plasma interactions have traditionally been seen as a source of accelerated charged particles and radiation and involves a transfer of energy from a laser pulse to particles. This transfer of energy from EM wave to the plasma and subsequently to individual particles have been attributed to various mechanisms and their scaling laws are well documented. At intensities of 1016W/cm2, one can ideally expect electron temperatures of 50keV. Recent studies conducted at our lab have shown that at similar intensities, with certain structural modifications of the target, one can get a temperature enhancement of 20 times, with maximum electron energies reaching up to 6MeV. The structural modification is brought about by carefully designing the low intensity pre-pulse that precedes the main pulse. Studies were conducted both experimentally and through simulations to reveal the exact mechanism leading to this enhancement. Parametric Instabilities triggered by the modifications were ascertained to be the chief cause of this energy enhancement. The emissions were temporally and spatially compact, thus making this technique a promising contender for various applications. The emission ranges that were only possible with low repetition rate multi-terawatt laser systems could now be realized using a high rep-rate sub-terawatt university class laser. The above experiments were conducted using particles that were several multiples of the laser input wavelength in size, thus ensuring the occurrence of the concerned structural modification. The change in the density profile was largely expected to have a stringent dependence on the initial target structure, but experiments have proved the contrary. In later studies it was observed that even with smaller targets (some of them smaller than the wavelength of light) similar temperature enhancements could be seen in the electron emission spectra, thus offering an incentive for further exploration of such systems. |
Tuesday, October 18, 2022 11:54AM - 12:06PM |
GO08.00013: Polarization dependent beam pointing jitter in laser wake field accelerators Daniel Seipt, Andreas Seidel, Bifeng Lei, Carola Zepter, Alex Sävert, Malte C Kaluza, Matt Zepf Laser wakefield accelerators (LWFA) have rapidly evolved from proof-of-principle experiments to advanced concepts focusing on improving their beam properties, stability and reliability. With various techniques LWFAs have achieved high energy, low energy spread, high charge and low emittance beams with high spectral stability, essential for a high quality accelerator. The stability of beam pointing and source position however, is equally critical. Here we present experimental results, which show a laser polarization dependent contribution to electron beam pointing jitter in laser wakefield accelerators (LWFA). We develop a theoretical model for the polarization dependence in terms of the transverse dynamics of trapped electrons, resonantly driven by bubble centroid oscillations. The latter are generated by the carrier wave phase evolution at the self-steepened laser pulse front. In the model, the polarization dependent jitter originates from shot-to-shot fluctuations of the laser carrier envelope phase. This suggests that for non-CEP stabilized systems the polarization dependent jitter may form an ultimate limit to beam pointing stability in LWFAs. |
Tuesday, October 18, 2022 12:06PM - 12:18PM |
GO08.00014: GeV electron bunches in low-density plasma channels by all-optical density transition injection Alex Picksley, James Chappell, Emily Archer, Nicolas Bourgeois, James Cowley, Linus Feder, Oscar Jakobsson, Aimee J Ross, Wei-Ting (Warren) Wang, Roman Walczak, Simon M Hooker Hydrodynamic [1,2] and conditioned hydrodynamic [3,4] optical-field-ionised plasma channels are promising candidates to support low-density, high repetition-rate multi-GeV laser wakefield accelerator (LWFA) stages. They are generated by focusing an ultrashort pulse into neutral gas, forming a hot column of plasma via optical field ionization, which expands hydrodynamically to form a plasma channel. An advantage of optically generated channels is the potential to sculpt the plasma density along the LWFA stage, for example to promote injection. Here we explore the use of a density down-ramp generated between neutral gas immediately prior to the channel and the channel itself to trap electrons. We present results of a recent experiment at the Gemini TA3 laser (RAL) in which ~ 1 GeV bunches, with percent-level energy spread, were generated by sub-100 TW laser pulses. The effect of the longitudinal and transverse position of the drive pulse focus on the generated electron bunches was investigated. These results, and particle-in-cell simulations, demonstrate that the channel entrance down-ramp is responsible for electron injection. |
Tuesday, October 18, 2022 12:18PM - 12:30PM |
GO08.00015: The Effect of Laser Focusing Geometry on the Direct Laser Acceleration of Electrons Hongmei Tang, Paul T Campbell, Brandon K Russell, Yong Ma, I-Lin Yeh, Kavin Tangtartharakul, Alex V Arefiev, Hui Chen, Felicie Albert, Jessica L Shaw, Philip M Nilson, Louise Willingale Direct laser acceleration (DLA) is capable of generating super-ponderomotive energy electrons to hundreds of MeV, as well as secondary particles and radiation from high-intensity picosecond laser pulses interacting with underdense plasma. The dynamic and complex process of DLA is strongly dependent on a combination of plasma and laser parameters. Experiments performed on the OMEGA EP facility using apodized beams and 2D particle-in-cell simulations study the effect of laser focusing geometry on DLA. Simulations reveal the laser channel creation, channel fields evolution, as well as the laser fields' contributions to the corresponding electron dynamics. Our results show an optimal laser focusing geometry and a path towards optimizing DLA conditions. |
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