20th Advanced Accelerator Concepts Workshop

6 Nov 2022, 15:00 → 11 Nov 2022, 17:10 America/New_York

Hyatt Regency Long Island

Mark Palmer (Brookhaven National Laboratory) , Navid Vafaei-Najafabadi (Stony Brook University)

The AAC’22 workshop is the 20th in a series of by-invitation biennial fora for intensive discussions on long-term research in advanced accelerator physics and technology. This research supports the development of capabilities for the basic sciences, from photon science to high energy physics, as well as the development of compact accelerators for industrial, medical and security applications.

AAC'22 will be organized into eight working groups covering the following topical areas:

1. Laser-Plasma Wakefield Acceleration 
2. Computation for Accelerator Physics
3. Laser and High-Gradient Structure-Based Acceleration
4. Beam-Driven Acceleration
5. Beam Sources, Monitoring, and Control
6. Laser-Plasma Acceleration of Ions
7. Radiation Generation and Advanced Concepts
8. Advanced Laser and Beam Technology and Facilities

Conference Home:  https://www.aac2022.org/

 

Sunday, 6 November

18:00 → 19:30 Welcome Reception

Monday, 7 November

10:20 → 10:40 Coffee Break

12:10 → 13:30 Lunch

13:30 → 15:00 WG1: Laser-Plasma Wakefield Acceleration: Session 1

Conveners: Irina Petrushina (Stony Brook University) , Marlene Turner (LBNL) , Yong Ma (University of Michigan)

13:30 Nearly collinear optical injection of electrons into wakefield accelerators

We show the recent results of electron injection into the laser wakefield accelerators by interfering two intense, nearly colinear laser pulses in underdense plasma [1, 2]. In the experiment, electrons could be injected into either laser wakefields, or both, depending on the relative delay between two laser pulses’ arrival time to the interference point. Particle-in-cell simulations revealed that the interference ponderomotively drives a relativistic
plasma grating and triggers the delay-dependent injection. Such injection occurs in later acceleration buckets other than the leading ones and can potentially be combined with optimal plasma tapering, and the dephasing limit of such unprecedented electron beams could be potentially increased by an order of magnitude. Other injection phenomenon like electron beam splitting and ring electrons are also discussed.

[1] Q. Chen, D. Maslarova, J. Wang, S. X. Lee, V. Horný, and D. Umstadter, Transient Relativistic Plasma Grating to Tailor High-Power Laser Fields, Wakefield Plasma Waves, and Electron Injection, Phys. Rev. Lett. 128, 164801 (2022).
[2] Q. Chen, D. Maslarova, J. Wang, S. X. Lee, and D. Umstadter, Injection of electron beams into two laser wakefields and generation of electron rings, Phys. Rev. E. (Accepted).

Speaker: Qiang Chen (Lawrence Berkeley National Laboratory)

Copy of AAC 2022 - to pdf.pptx (2).pdf

13:50 Generating pre-bunched electron beams using modulated density downramp injection

One of the two long-term applications of plasma-based accelerators is to develop the fifth-generation light source such as a compact free electron laser (FEL), which requires the generation of ultrahigh brightness electron bunches [1]. Recently, self-amplified spontaneous emission (SASE) by bunches from both laser- and beam-driven plasma accelerators have been observed [2, 3]. If the drive electron bunch from a plasma accelerator is pre-bunched on the scale of the radiated wavelength, it is then possible to substantially enhance the longitudinal coherence of XFELs by superradiant amplified spontaneous emission. A possible way of generating pre-bunched electron beams is using modulated density downramp injection as recently proposed by Xu et al [4]. Here we show progress on a proof-of-concept experimental realization of this idea, with emphases on a practical method of generating a modulated density downramp by superimposing an ionization induced plasma grating [5] onto a shock front in a supersonic gas flow and the potential detection of bunched electrons using coherent transition radiation.

References
[1] Joshi, C. et al. "Perspectives on the generation of electron beams from plasma-based accelerators and their near and long term applications." Physics of Plasmas 27, no. 7 (2020): 070602.
[2] Wang, W. T. et al. "Free-electron lasing at 27 nanometres based on a laser wakefield accelerator." Nature 595, no. 7868 (2021): 516-520.
[3] Pompili, R. et al. "Free-electron lasing with compact beam-driven plasma wakefield accelerator." Nature 605, no. 7911 (2022): 659-662.
[4] Xu, X. L. et al. "Generation of ultrahigh-brightness pre-bunched beams from a plasma cathode for X-ray free-electron lasers." Nature communications 13, no. 1 (2022): 1-8.
[5] Zhang, C. et al. "Ionization induced plasma grating and its applications in strong-field ionization measurements." Plasma Physics and Controlled Fusion 63, no. 9 (2021): 095011.

Speaker: Chaojie Zhang (UCLA)

14:10 External injection of electrons into a laser-driven plasma wakefield at the CLARA facility

We report on the injection of 35MeV electron bunches into a laser-driven plasma wakefield at the CLARA linear accelerator, Daresbury Laboratory, UK. In this initial proof-of-principle experiment, we observed the broadening of the energy spectrum of 6ps electron bunches injected into a plasma, demonstrating successful acceleration/deceleration of electrons within the wakefield. We discuss further planned experiments on external injection at the upgraded CLARA/FEBE facility.

Speaker: Laura Corner (Cockcroft Institute, University of Liverpool)

Corner_AAC_Nov_22_v2.pptx

14:30 First results of the two-color LWFA experiments at ATF

Two-color ionization injection is a promising method for realizing an all-optical, plasma photocathode. In this method, a nonlinear plasma wakefield is driven by a long-wavelength laser, and the ionization injection occurs using a second, high-intensity laser pulse with a short wavelength. Recent upgrades at the Accelerator Test Facility (ATF) of the Brookhaven National Laboratory has provided an ideal opportunity for this experiment by integrating a long-wave infrared (LWIR) CO$_2$ laser pulse (λ~9.2 μm) with a Ti:Sapphire (λ~0.8 μm) laser pulse at the interaction point. Previous simulations have shown the potential for this combination of lasers to produce bright electron beams with normalized emittance of tens of nm [1,2]. In this talk, we present the first results on the impact of a transverse Ti:Sapphire laser pulse on the electrons generated in the CO$_2$-driven LWFA in the self-modulated regime using a ~2.5 TW, 2 ps CO$_2$ laser and a ~ 5 mJ Ti:Sapphire laser. This work lays the foundation towards the realization of the all-optical plasma photocathode experiment as the facility plans upgrades towards >10 TW, sub-ps CO$_2$ pulses and terawatt class Ti:Sapphire lasers.

References:
[1] Schroeder, et al., arXiv:1505.05846 [physics.plasm-ph] (2015)
[2] Xu, et al., Phys. Rev. ST Accel. Beams 17, 061301 (2014)

Speaker: Navid Vafaei-Najafabadi (Stony Brook University, Brookhaven National Laboratory)

AAC_2022_Vafaei v2.pptx

15:00 → 15:30 Coffee Break

Tuesday, 8 November

10:00 → 10:30 Coffee Break

12:00 → 13:20 Lunch

13:30 → 15:00 WG1: Laser-Plasma Wakefield Acceleration: Session 3

Conveners: Irina Petrushina (Stony Brook University) , Marlene Turner (LBNL) , Yong Ma (University of Michigan)

13:30 Optical mode filtering and electron injection in multi-GeV laser wakefield acceleration

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.

[1] B. Miao et al., "Multi-GeV electron bunches from an all-optical laser wakefield accelerator", arXiv:2112.03489 (2021).

[2] L. Feder et al., "Self-waveguiding of relativistic laser pulses in neutral gas channels", Phys. Rev. Res. 2, 043173 (2020).

Speaker: Jaron Shrock (University of Maryland)

13:50 Observation of resonant wakefield excitation by pulse trains guided in long plasma channels

The multi-pulse laser wakefield acceleration (MP-LWFA) scheme [1] provides a route for GeV-scale accelerators operating at kilohertz-repetition-rates driven by picosecond-duration laser pulses such as those available from thin-disk lasers. We recently published theoretical work proposing a new scheme of GeV accelerator based on MP-LWFA, which we call the Plasma-Modulated, Plasma Accelerator (P-MoPA) [2]. In this scheme, trains of pulses are generated from a long, high-energy drive pulse via the spectral modulation caused by a low amplitude wakefield driven by a leading short, low-energy seed pulse. Our simulations show that temporal compression of the modulated drive pulse yields a pulse train that can resonantly drive a wakefield, allowing for acceleration of a test electron bunch to 0.65 GeV in a 100 mm long plasma channel [2].
In earlier work we demonstrated resonant excitation of a plasma wakefield in a 4 mm long gas cell by a train of N ~ 7 laser pulses [3]. In the present work we investigate resonant excitation of plasma waves by trains of N ~ 10 pulses guided in a long hydrodynamic optical-field-ionized (HOFI) plasma channel [4-6].
We present the results of recent experiments with the Astra-Gemini TA3 laser at the Central Laser Facility for parameters relevant to the accelerator stage of the P-MoPA scheme. We demonstrate guiding of 2.5 J pulse trains in a 100 mm long plasma channel. Measurements of the spectrum of the transmitted laser pulse train show that a wakefield was resonantly excited in the plasma channel. We compare these experimental results with numerical simulations, which allows us to deduce the acceleration gradient of the plasma wakefield driven by the guided pulse train.
To our knowledge, these results are the first demonstration of resonant excitation of a plasma wakefield in a plasma channel.

[1] S.M. Hooker et al., J. Phys. B, 47, 234003 (2014)
[2] O. Jakobsson et al., PRL, 127, 184801 (2021)
[3] J. Cowley et al., PRL, 119, 044802 (2017)
[4] R.J. Shalloo et al., PRAB, 22, 041302 (2019)
[5] A. Picksley et al., PRAB, 23, 081303 (2020)
[6] A. Picksley et al., PRE, 102, 053201 (2020)

Speaker: Aimee Ross (University of Oxford)

14:10 GeV-scale accelerators driven by plasma-modulated pulses from kilohertz lasers.

The energy required to drive a large-amplitude plasma wave can be delivered over many plasma periods, rather than in a single period, if the driving pulse is modulated. This approach opens up plasma accelerators to novel laser technologies which can provide the required energy at high pulse repetition rates, and with high wall-plug efficiency. We recently proposed [PRL 127, 184801 (2021)] that the required modulation can be achieved in a two-step process: (i) spectral modulation of the long drive pulse by co-propagation with a low-amplitude plasma wave driven by a short, low-energy seed pulse; (ii) conversion of the spectral modulation to temporal modulation by a dispersive optical system to generate a train of short pulses suitable for resonantly driving a plasma accelerator. We demonstrate the physics of this Plasma-Modulated Plasma Accelerator (P-MoPA) with numerical simulations, and show that the spectral modulation is well described by a 1D analytic model. We find that existing, efficient thin-disk lasers could be used to accelerate electrons to GeV level energies at kHz-repetition-rate. For example, particle-in-cell show that the pulse 1.7 J, 1 ps drive pulse, modulated by a 140 mJ, 40 fs seed pulse in a 120 mm long plasma channel, can generate a pulse train capable of accelerating electrons to an energy of 0.65 GeV in a 100 mm long accelerator stage.

Speaker: Roman Walczak (University of Oxford)

RW AAC22.pdf

14:30 Arbitrarily Structured Laser Pulses

Spatiotemporal control refers to a class of optical techniques for structuring a laser pulse with space-time dependent properties, including moving focal points, dynamic spot sizes, and evolving orbital angular momentum. These structured pulses have the potential to enhance a number of laser-plasma applications, including laser wakefield acceleration (LWFA) [1,2]. Here we introduce the concept of arbitrarily structured laser (ASTRL) pulses which generalizes techniques for spatiotemporal control [3]. The ASTRL formalism employs a superposition of prescribed pulses to create a desired electromagnetic field structure. Explicit ASTRL solutions of Maxwell’s equations in vacuum simplify field initialization in simulations of laser-plasma interactions with structured light, expediting study of novel concepts such as dephasingless LWFA with flying focus pulses. The ASTRL framework also enables design of new classes of laser pulses which may enable novel techniques in laser wakefield acceleration and laser-driven ion acceleration.

[1] Palastro et al., PRL 124, 134802 (2020)
[2] Palastro et al., Phys. Plasmas 28, 013109 (2021)
[3] Pierce et al., arXiv:2207.13849, (2022)

Speaker: Jacob Pierce (UCLA)

2022.10-AAC-astrl-upload.key

15:00 → 15:30 Coffee Break/Exhibits

15:30 → 17:00 WG1: Laser-Plasma Wakefield Acceleration: Session 4

Conveners: Irina Petrushina (Stony Brook University) , Marlene Turner (LBNL) , Yong Ma (University of Michigan)

15:30 kHz Laser-Driven Electron Beams up to 50 MeV at ELI-Beamlines.

The extremely high electric fields sustainable by a plasma make the Laser Wakefield Acceleration (LWFA) the most compact technique to generate very highly relativistic electron beams up to the GeV regime. The limited repetition rate and low efficiency of this technology has, to date, prevented to unleash its full potential as a unique source for basic research, biomedical applications and high flux sources of secondary radiations as hard x-rays and gamma-rays. In recent years a new direction emerged showing the possibility to accelerate electron beams at 1 kHz repetition rate. All these works are based on commercial lasers, requiring laser pulse compression to single-cycle by fiber technology, having limits in terms of maximum available laser pulse energy and achievable electron beam energy.

Here I will show the generation of very collimated (2 mrad) relativistic quasi-monoenergetic (< 30% energy spread) electron beams accelerated to the highest energy (up to 50 MeV) ever reached up to date with a kHz laser. Said innovative results have been achieved in the new Laser Wakefield ALFA platform for user experiments, that has been fully integrated to the in-house developed L1-Allegra 1 kHz multi-cycle (15 fs FWHM) laser system. The acceleration was driven by 1,7 TW pulses but, thanks to its modular OPCPA (Optical Parametric Chirped Pulse Amplification) design, the system is scalable to above 5 TW. I will introduce both the laser and the ALFA platform with related diagnostics.

The electron beams reported in this work are a step forward towards the development of in-demand high brilliance X-ray sources for medical imaging, high dose rate machines for radiotherapy based on high energy electrons, and to the future realization of a kHz 1 GeV electron beamline.

Speaker: Carlo Maria Lazzarini (ELI Beamlines)

15:50 CO2-laser-driven wakefield acceleration

To date only solid-state laser pulses of wavelength 𝜆 ~ 1 micron have been powerful enough to drive laser wakefield accelerators (LWFAs). Chirped-pulse-amplified multi-terawatt, ~1 ps laser pulses of 𝜆 ~ 10 µm are now emerging from mixed-isotope, high-pressure CO$_2$ laser technology [1]. Such pulses open new opportunities to drive large ($R_b \sim 300$ µm) bubbles in low-density ($n_e < 10^{17}$ cm$^{-3}$) plasma more efficiently, and to preserve energy spread and emittance of accelerated electrons better, than is possible using conventional ~1 µm drive pulses [2]. At the previous AAC we reported observations of wakes driven by sub-terawatt (sub-TW) CO$_2$ laser pulses in plasma of density down to $5 \times 10^{17}$ cm$^{-3}$ via collective Thomson scatter of a probe pulse. However, no electrons were accelerated in those experiments. Here we report new experiments in which copious relativistic electrons emerge from high-amplitude, self-modulated wakes driven in plasma of density down to $n_e < 10^{17}$ cm$^{-3}$ driven by 5- to 10-TW, 2 ps CO$_2$ laser pulses. Measurements and simulations of wake structure and e-beam properties as conditions change detail the physics of long-wavelength-infrared self-modulated wakefield acceleration. Peaked electron spectra observed on many shots indicate that we are close to generating strongly nonlinear wakes, portending future higher-quality accelerators driven in the bubble regime [2] by yet shorter (0.5 ps), more powerful (≳ 20 TW) CO$_2$ laser pulses [3]. Experiments are carried out at Brookhaven National Laboratory's Accelerator Test Facility.
[1] M. N. Polyanskyi, I. V. Pogorelsky, M. Babzien, and M. A. Palmer, OSA Continuum 3, 459-472 (2020).
[2] P. Kumar, K. Yu, R. Zgadzaj, M. C. Downer, I. Petrushina, R. Samulyak, V. N. Litvinenko and N. Vafaei-Najafabadi, Phys. Plasmas 28, 013102 (2021).
[3] M. N. Polyanskyi, I. V. Pogorelsky, M. Babzien, R. Kupfer, K. L. Vodpyanov, and M. A. Palmer, Opt. Express 29, 31714 (2021).

Speaker: Dr Rafal Zgadzaj (The University of Texas at Austin)

16:10 Self-injection process in laser‑wakefield accelerator driven by CO$_2$ laser pulses

The study of laser wakefield acceleration (LWFA) using long wavelength infrared laser drivers is a promising path for future laser-driven electron accelerators when compared to traditional near-infrared laser drivers operating at $0.8-1$ $\mu\rm{m}$ central wavelength [1,2]. For a fixed laser intensity I, lasers with longer wavelengths $\lambda$ have larger ponderomotive potential ($\propto$ I $\lambda^2$). Stronger wakes can be generated at relatively low laser intensities by using a long wavelength laser driver (i.e. $\lambda=9.2$ $\mu\rm{m}$ CO$_2$ laser) due to its very large ponderomotive potential. LWFA driven by CO$_2$ laser may have significant advantages to applications requiring compact and industrially robust accelerators and radiation sources.

In this work, we use particle-in-cell (PIC) simulations to investigate the self-injection process in CO$_2$ laser-driven wakefield acceleration for various laser and plasma parameters in the blowout regime. PIC code FBPIC [3] is used to extend the results obtained in [1] to model the interaction of a sub-picosecond CO$_2$ laser pulse with wavelength $\lambda=9.2$ $\mu\rm{m}$ and pre-ionized uniform plasma with $a_0$ ranging between 2 and 5. We have explored a wide range of parameters like pulse durations, laser amplitudes, spot size, and plasma densities to determine the self-injection mechanisms through bubble evolution. The accelerating bubble structure of LWFA is dynamic and highly sensitive to the local laser and plasma properties. It can expand and contract as it responds to the evolution of the laser and plasma fields. We report a parameter range that suppresses self-injection in fully blown-out bubbles which is an essential requirement in the experiments of controlled injection in LWFA.

References

[1] Prabhat Kumar, Kwangmin Yu, Rafal Zgadzaj, Michael Downer, Irina Petrushina, Roman Samulyak, Vladimir Litvinenko, and Navid Vafaei-Najafabadi, “Evolution of the self-injection process in long wavelength infrared laser driven LWFA,” Phys. Plasmas 28, 013102 (2021).

[2] Enrico Brunetti1, R. Neil Campbell, Jack Lovell, and Dino A. Jaroszynski, “High-charge electron beams from a laser‑wakefield accelerator driven by a CO$_2$ laser,” Scientific Reports 12, 6703 (2022).

[3] Rémi Lehe, Manuel Kirchen, Igor A. Andriyash, Brendan B. Godfrey, and Jean-Luc Vay, “A spectral, quasi-cylindrical and dispersion-free Particle-In-Cell algorithm,” Computer Physics Communications 203, 66–82 (2016)

Speaker: Arohi Jain (Stony Brook University)

AAC_2022_F.pdf

16:30 High-efficiency compact laser-plasma electron accelerator

Laser-plasma accelerators (LPAs) operating in the bubble regime require driver lasers with relativistic intensities and pulse durations that are significantly shorter than the plasma wavelengths. This severely limits the laser technology that can be used to drive LPAs and with that their wide spread and the currently achievable LPA parameters, such as repetition rate. Here, we report a widely unexplored regime of laser-plasma electron acceleration that is based on the direct parametric excitation of plasma waves. This method markedly relaxes the driver laser requirements in terms of peak power and pulse duration. We show preliminary experimental data that demonstrates the generation of high-charge mildly relativistic electron bunches with laser-to-electron conversion efficiency of nearly 10% which is unprecedented in gas-phase targets. The electron beams were generated using a gas target that can reach near the critical plasma density using a driver laser with moderate intensity. The experimental results demonstrate a novel regime that opens LPA electron acceleration for a wide range of driver laser technologies and holds the promise for a path to high-repetition rate LPAs for future compact particle accelerators and secondary sources.

Speaker: Matthias Fuchs (University of Nebraska–Lincoln)

Wednesday, 9 November

10:00 → 10:30 Coffee Break/Exhibits

12:00 → 13:20 Lunch

Thursday, 10 November

08:30 → 10:00 WG1: Laser-Plasma Wakefield Acceleration: Session 6

Conveners: Irina Petrushina (Stony Brook University) , Marlene Turner (LBNL) , Yong Ma (University of Michigan)

08:30 Experimental demonstration of Hydrodynamic Optical-Field-Ionized plasma channels at kHz repetition rate

Many potential applications of plasma accelerators - such as light sources and future particle colliders - require the stable generation of multi-GeV electron bunches at high (>kHz) repetition rate. A consequent goal for current research into laser-driven plasma accelerators involves the development of waveguides capable of operating at densities of ~10¹⁷ cm^-3, over lengths of several centimetres or more, guiding laser pulses at kHz repetition rate. Whilst guiding structures such as capillaries are not well suited to this repetition rate due to laser damage and heating, plasma waveguides formed from hydrodynamic optical-field-ionized (HOFI) channels can potentially meet the requirements. Guiding of high intensity laser pulses in HOFI channels has been demonstrated previously at on-axis densities of 1 $\times$ 10¹⁷ cm^-3 over lengths of >10 cm, whilst in this work we demonstrate experimentally that HOFI channels can be generated at kHz-scale repetition rates for an extended period of time. Using a pump-probe arrangement, we show via transverse interferometry that the properties of two HOFI channels generated 1 ms apart are essentially the same, and that HOFI channels can be generated at a mean repetition rate of 0.4 kHz for a period of 6.5 hours without degradation of the channel properties. The results suggest that HOFI channels are ideal for future high-repetition rate, multi-GeV laser-plasma accelerator stages.

Speaker: James Cowley (John Adams Institute for Accelerator Science and Department of Physics, University of Oxford)

AAC_kHz_HOFI.key

AAC_kHz_HOFI.pdf

08:50 Laser Wakefield Acceleration to Electron Energies in the GeV Regime

For the creation of matter-antimatter pairs from the quantum vacuum via the Breit-Wheeler effect, an intense laser and energetic γ-rays need to interact with each other. At the Stanford Linear Accelerator Center the Breit-Wheeler experiment in the perturbative regime has been accomplished in 1997 but was not yet implemented in the non-perturbative regime, where the laser strength parameter a0>>1 and pair production occurs when an electron from the negative energy Dirac-sea tunnels to positive energy levels. This experiment is at the moment in preparation in a fully laser-driven set-up with the ATLAS3000 laser at the Centre for Advanced Laser Applications in Munich. Laser Wakefield Acceleration (LWFA) will be used to accelerate electrons to high energies. This high energy electron beam will be sent onto a Bremsstrahlung converter to generate γ-rays that will interact with the intense laser. An electron beam with multi-GeV energies is needed for this. LWFA has been improved to reach multi-GeV electron energies in the recent years. However, building a reliable and stable source with low divergence and low pointing jitter with quasi-monoenergetic bunches over 2 GeV, as is needed for the Breit-Wheeler experiment, still holds challenges. Essential is the careful design of gas targets. These have to provide homogeneous gas densities over a distance of a few centimeters. In preparation for the Breit-Wheeler experiment, Computational Fluid Dynamic simulations were conducted to design centimeter-long gas nozzles. First LWFA results can be shown with electron energies reaching over 1.5 GeV using these nozzles and energies reaching over 2 GeV with a gas cell as target. Moreover, different injection techniques using these nozzles, or the gas cell were tested with the goal to obtain quasi-monoenergetic electron bunches in the GeV regime.

Speaker: Katinka von Grafenstein (LMU Munich)

20221110_GeVElectrons_v0.pptx

Summary_Slide_20221110_GeVElectrons.pptx

09:10 GeV electron bunches in low-density plasma channels by all-optical density transition injection

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. Because they are freestanding, they can be operated at high repetition-rate [5]. 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.

[1] Shalloo, RJ, et al, (2018). PRE, 97(5)
[2] Shalloo, RJ, et al, (2019). PRAB, 22(4)
[3] Picksley, A, et al, (2020). PRE, 102(5)
[4] Feder, L, et al, (2020). PRR, 2(4)
[5] Alejo, A, et al, (2022), PRAB, 25(1)

Speaker: Alex Picksley (University of Oxford)

09:30 Spatiotemporal optical vortices (STOVs) and relativistic optical guiding

We study the generation of spatiotemporal optical vortices (STOVs) from self-focusing processes in plasma and their role in mediating intrapulse energy transport in intense, self-guided laser pulses using fully three-dimensional, particle-in-cell simulations.
In previous work, STOVs were observed both in experiment and in simulation to emerge from self-focusing collapse arrest from filamentation in air using lower-intensity femtosecond pulses (~10^13-10^14 W/cm^2), and they have been generated linearly using a pulse shaper [1-2]. In the case of atmospheric filamentation, n2 of air induces an intensity-dependent refractive index within the laser pulse that leads to runaway self-focusing until collapse arrest via plasma generation [3]; phase shear between the higher-intensity, self-focusing core and the lower-intensity periphery of the laser was found to spawn these vortices [1]. These STOVs then proceed to mediate intrapulse electromagnetic energy flow during filamentation [1].
In plasmas, analogous though distinct mechanisms such as from relativistic and ponderomotive effects that are relevant for high-intensity, self-guided pulses (>10^18 W/cm^2) impose a similar intensity-dependent refractive index and induce self-focusing. Understanding the interaction of relativistically intense laser pulses with plasma would be of use for laser wakefield acceleration experiments, which tend to operate in this regime. In this work, we simulate relativistic self-guiding of an intense laser pulse and show, for the first time, the generation of STOVs in plasma by nonlinear phase shear from relativistic and ponderomotive effects. We observe, in three dimensions, the nucleation of STOVs at points around the pulse, their expansion, reconnection, and evolution into a pair of vortex rings. Finally, we find that these vortices mediate the flow of electromagnetic energy within the pulse and illustrate how this influences the broader propagation of the laser.

[1] N. Jhajj et al. Spatiotemporal optical vortices Phys. Rev. X 6, 031037 (2016)
[2] S. W. Hancock et al. Free-space propagation of spatiotemporal optical vortices, Optica 6, 1547 (2019)
[3] A. Couairon and A. Mysyrowicz, Femtosecond Filamentation in Transparent Media, Phys. Rep. 441, 47 (2007).

Speaker: Manh Le (University of Maryland)

AAC2022-Matthew.pptx

10:00 → 10:30 Coffee Break/Exhibits

12:00 → 13:20 Lunch

13:30 → 15:00 WG1: Laser-Plasma Wakefield Acceleration: Session 8

Conveners: Irina Petrushina (Stony Brook University) , Marlene Turner (LBNL) , Yong Ma (University of Michigan)

13:30 Single-Shot Reconstruction of Electron Beam Phase-Space in a Laser Wakefield Accelerator

We report on a single-shot longitudinal phase-space reconstruction diagnostic for an electron beam in a laser wakefield accelerator via the experimental observation of distinct periodic modulations in the angularly resolved spectrum. Such modulated angular spectra arise as a result of the direct interaction between the ultra-relativistic electron beam and the laser driver in the presence of the plasma wakefield. A constrained theoretical model for the coupled oscillator, assisted by a machine learning algorithm, can recreate the experimental electron spectra, and thus fully reconstructs the phase-space distribution of the electron beam. In particular, it reveals the slice energy-spread of the electron beam, which is important to measure for applications such as XFELs. In our experiment, the root-mean-square slice energy spread retrieved is bounded at 17 MeV, corresponding to 1.7-4.2% relative spread, despite the overall GeV energy beam having ~100% relative energy spread.

Speaker: Yong Ma (University of Michigan)

2022_AAC_YongMa.pdf

13:50 Data-driven modelling of laser-plasma experiments enabled by large datasets

Laser Wakefield Acceleration (LWFA) is a process by which high gradient plasma waves are excited by a laser leading to the acceleration of electrons. The process is highly nonlinear leading to difficulties in developing 3 dimensional models for a priori, and/or ab initio prediction.

Recent experiments at the Rutherford Appleton Laboratory’s (RAL) Central Laser Facility (CLF) in the United Kingdom using the 5Hz repetition rate Astra-Gemini laser have produced new results in LWFA research, inviting analysis of data with unprecedented resolution. Additionally, data driven modeling, scaling laws and models can be extended into new ranges or refined with less bias.
We will present results of training deep neural networks to learn latent representations of experimental diagnostic data and validate the latent space by comparing the distribution of beam divergences and other metrics of randomly generated spectra against the distribution in the training data. We will discuss the ability of the model to generalize results to different conditions. This work will use architectures which rely on reparameterization using a small dense network connected to a larger, convolutional neural network.

Speaker: Mr André Antoine (University of Michigan)

14:10 Polarization and CEP Dependence of the Transverse Phase-Space in Laser-Driven Accelerators

We have conducted experiments at the JeTi-200 laser facility ($\lambda_0=800nm$, spotsize $w_0=22\mu m$, pulse length $\tau=23 fs$, $a_0 = 2.4$) to investigate the contribution of laser polarization and carrier envelop phase (CEP) -fluctuations on the electron beam pointing jitter in laser wakefield accelerators(LWFAs). Furthermore, we developed a theory describing the transverse dynamics of the trapped electrons inside the resonantly oscillating wakefield bubble.
LWFAs were studied extensively and many advances were made in important beam parameters such as bunch energy, charge, and emittance. However, with the emergence of QED and beam colliding experiments requiring very precise positioning of the electron beam the beam jitter and its control become very important. Until now, electron beam jitter was generally contributed to imperfections in the plasma target reproducibility. Here we report on a polarization and CEP-dependent
mechanism that is intrinsic to LWFAs. The ponderomotive force of the laser pushes electrons aside and creates the well-known bubble structure. However, with few-cycle laser pulses or self-steepened longer pulses due to etching at the front the electron density in the trailing bubble can become asymmetric. The difference in phase and group velocity of the laser leads to an oscillation of the bubble centroid which couples to the trailing electron bunch. We measured the electron beam pointing in experiments with ionization and self-injection showing an increased beam-pointing jitter in the polarization direction regardless of the injection mechanism. In 2D PIC simulations we found countrolling the CEP phase of the laser within $500mrad$ constrains the polarization-induced jitter to below $50 \mu rad$.

Speaker: Andreas Seidel (Friedrich-Schiller-University Jena)

14:30 Stable injection into a laser plasma accelerator with colliding laser pulses

Colliding pulse injection of electron beams into a laser plasma accelerator (LPA), thus producing compact, stable, and monoenergetic electron beams, has important applications for narrow bandwidth Thomson gamma ray sources and novel x-ray free-electron lasers. The colliding laser pulses are independently optimized in terms of energy, beam size, and pulse compression. The spatiotemporal overlap of the femtosecond-duration colliding pulses in the underdense plasma is ensured with femtosecond shadowgraphy and top imaging of the plasma. High-quality, stable LPA electron beams from colliding pulse injection were demonstrated over consecutive 100’s of shots. The absolute rms energy spread of the injected electron beam could be reduced down to just a few MeV (rms), at peak electron energy of ~150MeV. The peak energy is stably tunable over large respective ranges of the plasma density, gas jet positions, and injector pulse delays. Parameter scan of injection position and plasma density indicate the driver laser pulse depletion and/or diffraction at large plasma density, which causes the decrease of the maximum electron beam energy.

Speaker: Qiang Chen (Lawrence Berkeley National Laboratory)

15:00 → 18:00 Afternoon at Leisure

Friday, 11 November

10:00 → 10:30 Coffee Break

12:00 → 13:00 Lunch

14:40 → 15:00 Coffee Break