Session CO08: Coherent radiation and secondary particle generation

Generation of TW-Class THz Sources from Picosecond Laser-Solid Interactions

High intensity (>10¹⁸ W/cm²) laser–matter interactions have been found to be efficient (>0.1%) sources of terahertz (THz) radiation using a wide variety of solid targets irradiated by currently available lasers with picosecond-scale pulses. We present the results of several years of study using foil, wire and microchannel targets irradiated with the joule-class Multi-Terawatt laser and kilojoule-class OMEGA EP laser. THz radiation measurements were performed with a custom-designed, EMP-hardened pyrometer. The experiments were observed to generate THz sources in the range of 10’s of millijoules to greater than 1 joule of pulse energy, which correspond to a terawatts in peak power. The physics of THz generation from each target type is examined analytically and compared to the experimental results. Future uses of these powerful THz sources are discussed, with an emphasis on extreme light–matter interactions using powerful THz sources to drive matter into new states as well as a probe of carrier and molecular dynamics in HED materials.    

Robust relativistic high harmonic generation from 400 nm laser matter interactions

Relativistically intense laser solid density plasma interactions are capable of upshifting the laser frequency into the soft and hard x-ray wavelengths via high order harmonic generation. Experiments were performed at the ALEPH laser system at 10²¹ Wcm^-2 and at a wavelength of 400 nm. Results from conductive and dielectric targets at a variety of thicknesses will be shown. The second harmonic generation process that produces the 400 nm laser pulses greatly increases laser contrast and enables highly consistent harmonic production. Mechanisms of attosecond pulse generation were identified, including Relativistic Oscillating Mirror (ROM) and Coherent Wake Emission (CWE). Numerical modeling was performed as well.    

Microwave emission and conductivity of two-color short pulse laser produced plasmas

Laser plasmas in gases produced by intense, short pulses are sources of microwave and terahertz waves. The amplitude and spectrum of the low-frequency radiation may contain information about the plasma that is inaccessible by existing diagnostic techniques, however work is ongoing to understand the precise generation mechanisms that may connect the characteristics of the radiation to those of the plasma. To better understand the relationship, we present simultaneous measurements of the conductivity and microwave fields radiated by a plasma created using a two-color laser pulse, where the laser fundamental and second harmonic frequencies are coherently superimposed and their relative phase precisely controlled. The electric field waveform determined by the two-color relative phase changes the electron trajectories immediately following ionization, but it is unclear if that change of initial conditions influences the subsequent plasma density, size, or electron momentum transfer collision rate. Our findings enable assessment of the viability of the microwave radiation for use as a non-invasive diagnostic for laser plasmas that are compatible with few existing methods.   

Laser-wakefield accelerator source of positron beams suitable for plasma accelerator injection

As laser wakefield acceleration matures as a technology, the use of these accelerators for applications previously only accessible through conventional accelerator facilities is being increasingly explored by international research efforts. One of these applications is the use of a plasma wakefields to accelerate positrons to energies sufficient to use in high-energy collider experiments. Beyond the linear wakefield regime, positron injection and acceleration in plasma wakefields presents a particular challenge that requires relatively narrow bandwidth and low-emittance positron beams. These requirements that have thus limited results of positron acceleration via wakefield acceleration. In this work, we report an experimental demonstration of a laser-driven electron converter source of GeV-scale positron beams with spectral and emittance quality suitable to be used as an injection seed in a wakefield accelerator. Numerical simulations agree with the measured beams and show transport of beams containing N_e+ ≥ 10⁵ positrons in a 5% bandwidth around 600 MeV with femtosecond-scale duration and micron-scale normalized emittance. Particle-in-cell simulations show that the positrons as measured in experiment may be efficiently injected into a laser wakefield accelerator for subsequent acceleration after production.   

Efficient backward x-ray emission in a plasma irradiated by a ps laser pulse

Motivated by experiments employing ps-long laser pulses, we examined x-ray emission in a plasma irradiated by $10^{20}$ W $ m{cm^{-2}}$ ps-long pulse using particle-in-cell simulations. We found that, in addition to the expected forward emission, the plasma also efficiently emits in the backward direction. The backward emission occurs when the laser exits the plasma. The longitudinal plasma electric field that is generated by the laser at the density down-ramp turns around some of the electrons and accelerates them in the backward direction. As the electrons collide with the laser, they emit hundred keV photons. The ps-duration is required for an overlap between the laser and backward energetic electrons. The conversion efficiency is comparable to the forward emission, but the effective source size is much smaller.   

The Ion Channel Laser: Advances in Theory and Simulation

The ion channel laser (ICL) is an alternative to the free electron laser (FEL) that uses the electric fields in an ion channel rather than the magnetic fields in an undulator to transversely oscillate a relativistic electron beam and produce coherent radiation. The strong focusing force of the ion channel leads to a Pierce parameter more than an order of magnitude larger than the typical values associated with FELs. This allows the ICL to lase in an extremely short distance while using electron beams with an energy spread of up to a few percent. The ICL may thus be able to accommodate beams that can be produced by laser wakefield accelerators today. ICLs have several practical challenges, however, including stringent constraints on the beam's transverse phase space and unique physics in the high K regime. We discuss recent advances in the physics of the ion channel laser as well as experimental plans at SLAC's FACET-II facility and the potential for future plasma plasma based light sources.   

Generalized Superradiance in the Ion Channel Laser

Radiation emission plasmas is often a result of collective effects associated with the dynamics of large numbers of relativistic charged particles. The Particle-In-Cell scheme is commonly used to model their motion but may fail to capture the corresponding ultra-high radiation frequencies. The Radiation Diagnostic for OSIRIS (RaDiO) can retrieve the full EM field structure of the emitted radiation in OSIRIS PIC simulations. These codes can run reliably with a high level of efficiency in the largest CPU-based supercomputers, but the radiation algorithm has been recently adapted to the GPU architecture.

In this work, we take advantage of the latest developments in RaDiO to generalize the Ion Channel Laser concept towards superradiant Betatron emission through Generalized Superradiance. This scheme allows arbitrarily long particle beams to radiate coherently at wavelengths much smaller than the particle beam size, exploiting optical shocks coming from superluminal beam structures directed at the Cherenkov angle of structures' the phase speed. We show that by resonantly combining betatron oscillations with the effect of a low frequency laser pulse, an electron beam acquires a modulation with a superluminal phase-like velocity, and explore the necessary conditions to obtain such a modulation. The generalized ion channel laser concept can then be seeded by traditional infra-red laser pulses, and lead to temporally coherent radiation that can extend all the way up to x-ray frequencies.    

Optical Free-Electron Lasing Driven by a Flying Focus

Optical free-electron lasers (OFELs) replace magnetic undulators with the electromagnetic field of a laser pulse. By decreasing the undulator period to half an optical wavelength, OFELs lower the electron energy needed to produce the same radiation frequency and shorten the interaction length needed for the same gain. Appreciable amplification in an OFEL requires sustaining a highly uniform, high-intensity laser field over the interaction length. Here we evaluate the application of flying focus pulses to OFELs. Flying-focus pulses feature an intensity peak that travels at a tunable velocity over many Rayleigh ranges while maintaining a near-constant profile. An intensity peak that travels backward with respect to the phase fronts of the pulse enables an OFEL configuration in which an electron beam collides head-on with the phase fronts and experiences a near-constant undulator strength over the entire interaction length.   

Fast Neutron Generation from Ultrafast Spatially and Temporally Coherently Combined Fiber Laser Driver

To date the architecture of most laser driven neutron sources are designed around high pulse energy (>100mJ) laser systems; however, the neutron flux of these systems is limited by their repetition rate of 1-10Hz. We are developing a new ultrashort fiber laser based driver architecture which delivers high pulse energies at kHz repetition rates, suitable for driving both high flux particle sources and allowing for real time feedback control to achieve optimized system performance. Our demonstration system currently achieves pulses with energies of tens of millijoules at a 2kHz repetition rate by simultaneously incorporating spatial and temporal coherent combining techniques. As a practical demonstration of this unique architecture for scientific applications, we show coincident isotropic fast neutron generation via D(d,n)³He fusion reactions in a free-flowing microscale deuterated liquid jet target. To our knowledge, this is the first fast neutron source driven by a fiber laser system. This proof-of-principle experimental demonstration with near-relativistic pulses highlights the potential for increased scaling of the fluxes of particle accelerators and secondary radiation sources as this fiber technology matures towards multiple joule pulses at several kilohertz. This work is funded under DOE Advanced Accelerator Stewardship Grant FP00013287 and DOE Grant DE-SC0016804   

Demonstration of a high repetition rate directional MeV laser-driven neutron source

Neutron beams generated by laser-driven ion sources exhibit promising characteristics including high peak flux and short pulse duration. However, low repetition rates have previously limited these sources' development towards maturity for applications. We present the first demonstration of a high repetition rate (HRR, 0.5 Hz) laser-driven neutron source operating in the pitcher-catcher geometry, producing up to 8x10⁵ neutrons/sr/shot within a 20-degree cone centered on the target normal direction. We implemented an ambient-temperature microjet target¹ producing planar liquid H₂O or D₂O sheets 1 micron thick at the ALEPH laser facility (400 nm, 5 J, 45 fs). We placed a 1.5 mm-thick beryllium converter with an on-axis pinhole behind the liquid target to enable simultaneous ion beam characterization and neutron beam production. In the target normal direction we observed a 17-fold increase in the neutron yield for D₂O targets compared to H₂O targets, confirming a significant contribution from deuteron breakup reactions². This suggests that increased laser energies and intensities, additionally assisted by laser pulse shaping on the fs timescale, will facilitate the production of high-flux, HRR beams of 14 MeV neutrons for studies of neutron-induced damage in fusion materials.

1. F. Treffert and G. D. Glenn et al., Phys. Plasmas 2022

2. A. Alejo et al., Plasma Phys. Control. Fusion 2017   

High flux directional laser driven neutron sources for static radiography applications

Laser-driven neutron sources offer an attractive, alternative approach for generation of short, intense bursts of multi-MeV neutrons, especially desirable for neutron radiography applications. High repetition-rate petawatt lasers driving novel neutron sources could outperform existing conventional sources with respect to peak brilliance¹. These high repetition-rate, high flux sources show great promise for applications of radiography of static or slowly evolving systems (seconds-to-minutes) such as nondestructive evaluation of manufactured materials or compounds or water uptake in biological or structural systems.

Here, we present results of high flux directional neutron beam generation at the Texas Petawatt (TPW) Laser at the University of Texas, Austin (1 um, 150 fs, 100 J) on a single shot basis using a pitcher-catcher setup² implementing a cryogenic liquid deuterium jet target as well as subsequent experiments using a similar platform showing reliable neutron generation in a laser-driven pitcher catcher setup for the first time at a repetition rate of 0.5 Hz. These were performed using a converging heavy water sheet target³ with the ALEPH laser at Colorado State University (400 nm, 45 fs, 4.6 J). We observe peak neutron yields of 7.2x10⁹ neutrons/sr/shot within 20 deg from target normal direction at the TPW. At lower laser energy (ALEPH) but higher repetition rate the average measured neutron yield is 2.5x10⁵ neutrons/sr/s. Both results show the potential of achieving a high flux, high repetition-rate laser-driven neutron source suitable for radiography applications when scaled to high repetition-rate Petawatt laser systems such as the MEC-U project at SLAC.

 

References

¹ S. N. Chen et al., Matter and Radiation at Extremes 4, 054402 (2019)

² F. Treffert et al., Instruments 5, 38 (2021)

³ F. Treffert et al., Appl. Phys. Lett. 121, 074104 (2022)

 

This work was supported by the U.S. DOE Office of Science, Fusion Energy Sciences under FWP 100182 and Contract No. DE-SC0021246: the LaserNetUS initiative at Colorado State University. This work was partially performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and was supported by the LLNL-LDRD Program under Project No. 22-ERD-022. LLNL-ABS-851233   

No X-Rays shining through a wall: A search for heavy, axion-like particles with a Free Electron Laser

The axion was posed as a solution to the CP problem of QCD, but axion-like particles (ALPs) also arise in string theory and are a dark matter (DM) candidate. Most laboratory axion searches have concentrated on the 0.001-0.1 meV mass range however there is growing interest in heavier (DFSZ) axions above 10 meV which avoid the cosmological domain wall catastrophe [1].

We describe a new search for axions at EuXFEL Hamburg, sensitive to a mass range including 1 meV-1 eV, which is largely unconstrained by laboratory experiments. Our experiment exploits the Primakoff effect, via which (in the presence of strong external fields) photons can decay into axions and then reconvert back into photons. This has previously been employed in experiments with lasers and external magnetic fields; however using X-rays it is possible to increase the detection sensitivity by using the electric field present within a crystalline material. X-Ray searches were previously performed at a 3rd generation synchrotron [2]; however limited flux prevented probing down to DM relevant couplings. Our work is a first step in developing a platform with improved sensitivity thanks to an increase in brightness by ~10¹⁰ at EuXFEL. Initial work has confirmed previous bounds on the coupling strength and probed a previously unexplored mass range. In future we hope to probe down to a coupling relevant to QCD axions in the keV mass range.

[1] K. Beyer & S. Sarkar SciPost Phys. 15 (2023) 003

[2] T. Yamaji et al., Phys. Lett. B 782 (2018) 523   

Pulse compression in a gradient-density plasma for a compact exawatt laser

Ultraintense laser pulses are essential tools for experimental study of modern theoretical physics in quantum electrodynamics and lab-astrophysics. It is believed that with exawatt to zettawatt laser pulses, it would be possible to observe the pair-production process in vacuum, Hawking radiation, radiation reaction, and early universe states. However, the conventional chirped pulse amplification (CPA) technique is already close to its technological limitations, mainly due to the vulnerability of compression gratings. Several alternative ideas have been investigated for the past tens of years and particularly, plasma-based schemes for pulse amplification or compression are also emerging as promising methods for the next generation of lasers. Recently we devised a novel idea of compressing a chirped pulse from a critical density plasma with a density gradient exploiting the optical dispersive property of the plasma; in this new scheme, differential reflective paths for different frequency components of the chirped pulse are generated from the density gradient, resulting in the spatial concentration of the photons. From particle-in-cell simulations, compression of a pico-second chirped pulse by more than two hundred times into tens of femtoseconds was observed. Various realistic effects such as collisional heating and density fluctuation were theoretically estimated to reach the conclusion that the compression process will be quite robust in expreiments (under progress). The research has been accepted to a prestigous journal recently [1]. In this presentation, I will talk about the fundamental concept, simulation results, and theoretical estimates relevant to the new idea.   

Capabilities of the GALADRIEL Facility for Addressing Questions in Engineering Science

Present and future lasers used for studying high-energy-density (HED) physics and inertial fusion energy (IFE) will operate at rates of ~0.1-10 Hz, yet most diagnostics and target-fielding strategies presently implemented require breaking vacuum between shots to exchange film-media, replace a stalked target, etc. This experimental paradigm must change as the community moves to rep-rated operation, and the General Atomics (GA) LAboratory for Developing Rep-rated Instrumentation and Experiments with Lasers (GALADRIEL) facility will serve as a platform for this change, providing a test bed to enable rapid development and testing of rep-rated technologies required to address questions in Engineering Science relevant to commercial applications of rep-rated, high-power lasers. The ~1 TW system has been commissioned in single-shot, high-power mode (~25 fs, ~25 mJ, 800 nm) by generating electrons up to >1MeV in a laser-wakefield accelerator experiment using a nitrogen gas jet. The modest size of this facility allows users to focus on outstanding questions in Engineering Science; including targetry, diagnostics, environmental and materials studies, analysis and machine learning algorithm development, as well as feedback control systems. GALADRIEL fills a niche presently missing in the US-based user-facility community by providing a flexible experimental platform to address engineering problems relevant to HED and IFE experiments. Current capabilities and recent progress towards automated operation and data acquisition at ~0.1 Hz will be discussed.   

Withdrawn

The process by which high-intensity lasers accelerate electrons to high energies has been the subject to many experimental, computational, and theoretical studies over the past 3 decades. Many the proposed applications of laser-driven sources from solid targets, principally MeV radiography, depend heavily on optimizing the acceleration of the electrons and in particular the  optimizing the electron temperature. Previously, scaling relations of the electron temperature were solely dependent on the intensity and wavelength of the laser (Iλ²), but more recently models have been developed to incorporate the laser pulse duration and scale length of the pre-plasma.  Through a thorough literature review, we have gathered hundreds of published experimental and simulated measurements of the electron temperature and have attempted, using Bayesian inference and other techniques, developed additional scaling laws. We test and compare these new scaling laws with existing ones and propose laser and plasma parameters that can optimize the production of MeV x-rays.