Session JM10: Mini-Conference: The High Repetition Rate Frontier in High Energy Density Physics

Accelerating the rate of discovery: Toward high-repetition-rate HED science

As high-intensity short-pulse lasers that can operate at high-repetition-rate (HRR) (>10 Hz) come online around the world, the high-energy-density (HED) science they enable will experience a radical paradigm shift. The >1000x increase in shot rate over today’s shot-per-hour drivers translates into dramatically faster data acquisition and more experiments, and thus the potential to significantly accelerate the advancement of HED science. 

 

Current energetic driver facilities depend on the ability to manually tune the lasers, the targets, the diagnostics settings, and more, between single shots or sets of shots through a manual feedback loop of data collection, data analysis, and optimization largely driven by experience and intuition. At 10 Hz, this paradigm is no longer sustainable as more complex data is collected more quickly than is possible to analyze manually.

 

Fully realizing the potential benefits of HRR facilities requires a fundamental shift in the design and execution of experiments done on them, the development of supporting technologies such as high-throughput targetry and diagnostics, and the evolution of machine learning techniques to couple traditional scientific computing with advanced data analytics. On-the-fly optimization of experiments will become ever more crucial as higher repetition rates will lead to more deliberate inter-shot variations and the improved operational range to allow exploration over larger regions of phase space. 

 

We will present the vision and ongoing work to realize a HRR framework for rapidly delivered optimized experiments coupled to cognitive simulation to provide new insights in HED science.   

Towards high-repetition rate HED science at the Matter in Extreme Conditions (MEC) instrument at the Linac Coherent Light Source (LCLS)

The collocation of high-power lasers with an X-ray free electron laser has recently enabled ultrafast pump-probe measurements of material structural responses to ionizing radiation. To recreate extreme radiation environments, such as those in fusion reactors, higher-flux sources of ions and neutrons are required. This is one of many science cases driving a project to construct an all-new MEC facility, which will consist of a new underground cavern housing two experimental areas and three optical laser drivers: a high-power short pulse (150J, 150fs 10Hz); a 100J-class long pulse (10Hz); and a kJ long pulse. After giving an update on the upgrade, we will present preparatory single-shot experiments on laser-driven ion acceleration from cryogenic jet targets. The results show that these targets can reliably produce bright, high-energy proton and deuteron beams using the Texas Petawatt laser (135J, 135fs, 1/hour). The high-repetition rate compatibility of this target will allow us to fully optimize the ion beams using machine learning techniques and active laser–target–diagnostic feedback loops. To this end, we will briefly outline expansions of the LCLS data environment (handling, storage, processing) for experiments operating at up to 1 MHz in the context of MEC.   

Automated control and optimisation of high-intensity laser-solid interactions at up to 5 Hz.

The exploitation of new multi-Hz, relativistically-intense laser facilities for laser-driven acceleration of MeV protons holds numerous technical challenges. In recent years, there has been a large community effort to develop refreshable targets with suitable parameters and positional accuracy, together with online diagnostics robust to EMP so that the potential of these new facilities can be fully exploited. This has led to multiple experiments in which increased data rate has enabled higher resolution 1D parameters scans and repeat measurements to support improved quantification of accelerator stability [1].   Here, we present results from a recent experiment in which automated laser-control enabled parameter scanning with > 80,000 laser-shots at up to 5 Hz.  As well as 2D [MS1] mapping of the parameter space, online feedback between diagnostics and control of laser parameters, including spatial and temporal shaping, allowed for optimization of the interaction for desirable particle beam characteristics.  We will also discuss concerns that are often raised for high repetition rate overcritical target experiments, including diagnostic feedback, targetry, optic degradation and debris.   

 

[1] Noaman-ul-Haq et al., PRSTAB (2017); Gauthier et al., APL (2017); Morrison et al., NJP (2018); Dover et al., HEDP (2020).   

Automated laser plasma accelerators

Laser-plasma wakefield accelerators are often cited for their potential to provide compact MeV-GeV particle and photon beams, and nearly as often for their shot-to-shot variation due to the highly non-linear nature of the underlying physics. They are highly sensitive to many parameters, and so precise tuning and maintaining output stability is an ongoing challenge for the application of plasma accelerators. To address this issue, many research groups are striving to develop on-line control and analysis of their experiments, allowing the opportunity to incorporate active feedback. Machine learning techniques, such as Bayesian optimization, can then provide efficient search algorithms for optimizing the plasma accelerator, as well as providing statistical models of the multi-dimensional parameter space to provide physical insight. Here, we will discuss results from recent work utilizing machine learning within plasma accelerators and the data-driven future of these machines.   

Development of Deep-Learning-Based Automated Analysis for Diagnostics for High-Repetition Rate Laser Driven Acceleration Experiments

 

Advancements in high-repetition rate (HRR) laser systems, that is lasers that can fire at rates greater than ~1 Hz, as well as machine learning tools provide novel opportunities to accelerate the rate of discovery on short-pulse laser systems relevant for laser driven particle acceleration and high-energy density science more broadly. However, to realize the full potential of these HRR systems, HRR diagnostics and diagnostic analysis must be developed to measure and analyze the resulting particle sources at a rate that matches the laser. Towards this goal, we present a deep-learning based framework for real-time analysis of a differential filter-based x-ray spectrometer that is common on short-pulse laser experiments. The analysis framework was trained with a large repository of synthetic data to retrieve key experimental metrics, such as slope temperature. With traditional analysis methods, these quantities would have to be extracted from data using a time-intensive and manual analysis. This framework was developed for this specific diagnostic but is applicable to a wide variety of diagnostics common to laser experiments and thus will be especially important for the development of high-repetition rate diagnostics for HRR laser systems that are coming online.   

Towards stabilizing laser-plasma accelerators through non-perturbative focus diagnostics of 100-TW-class laser pulses

High power laser systems are attracting great interest due to their compact footprint compared to alternative technologies. However, the non-linear physics at play in the high-power laser plasma interaction makes applications highly sensitive to laser alignment. We demonstrate an accurate non-perturbative high-power laser diagnostic, allowing shot-tagged laser delivery information on position and angle by the installation of a specialty final steering mirror (the last optic delivering the converging laser beam from the parabola towards focus), where its rear surface reflection of the main beam is used to provide online non perturbative monitoring of the high-power laser focus position and angle at a two-camera setup. We refer to the beam reflected from the rear surface as the "witness beam". It is a fully correlated copy of the high-power beam since it shares the same mirror reflections from the oscillator all the way to the final steering mirror and final focus, limiting the source of the uncorrelated factors. The demonstrated correlation of the high-power laser pulse train to the non-amplified background pulse train highlights the option for feedback integration. If high-power laser systems were to become available at kHz repetition rates and the focus position and pointing angle fluctuation dominated by frequency are well below 500 Hz, fast feedback mechanisms using the witness beam technique demonstrated here could be able to correct. This would allow significant improvements in stability, quality, and applicability of high-power laser systems.   

A quasi-monoenergetic compact proton source for probing high energy density states of matter at high repetition rate

Development of monochromatic laser-driven ion sources is of large interest for many applications such as probing extreme states of matter, proton radiography and medical applications. We report [1] on the generation of narrow energy band, short time duration proton beam operating at high repetition rate at CLPU laser facility (Spain). The protons are generated with an ultrashort-pulse laser interacting with a solid target and converted to a pencil-like narrow-band beam using a compact magnet-based energy selector. We experimentally demonstrate the production of a proton beam with an energy of 500 keV, energy spread well below 10%. The calculated pulse duration downstream the selector is 260 ps. To demonstrate the feasibility of the platform for stopping power measurements in warm dense matter [2], we measure the energy loss of this beam in a solid target and find a good agreement with the theoretical predictions to within a few percent.  Further optimization and improvement of performance of the energy selector platform will be performed at CSU ALEPH laser facility.   

Progress towards rep-rated proton imaging for use at next-generation high-energy-density (HED) science facilities

Many high-power, petawatt-class laser systems have the capabilities to run at high (~0.1-10Hz) rep-rates. In order to leverage the rep-rate capabilties of these laser systems at next-generation, much work is needed to develop diagnostics and targetry systems that can also operate at these rates. This talk will cover recent progress towards a rep-rated proton radiography diagnostic utilizing plastic scintillators as a detecting medium for the protons. Proton radiography is one of the most popular methods for detecting and diagnosing electromagnetic fields in HED plasmas. This diagnostic technique is a nonperturbative way to visualize 2D distributions of path-integrated electromagnetic field structures, but quantitative analysis is complex and requires accurate knowledge of the proton source and detecting medium. The fundamentals of scintillator use in proton radiography will be covered and results using novel pixelated scintillators to enhance the spatial resolution will be presented.   

The SAS Gamma-Ray Spectrometer For High Repetition Rate Laser Applications

A new type of compact high-resolution high-sensitivity gamma-ray spectrometer for short-pulse

0.5-50 MeV gamma-rays has been developed by combining the principles of pixelated scintillators

and attenuation spectrometers. The first prototype of this scintillator attenuation

spectrometer (SAS) was tested successfully on Trident laser experiments and later improved

versions have been used extensively in Texas Petawatt laser experiments in Austin TX and 

OMEGA-EP laser experiments at LLE, Rochester NY. The SAS spectrometer is ideally suited

to diagnose gamma-rays from high-repetition-rate laser and HED experiments since it does not require the

use of image plates or radiographic films.  Instead the scintillation light profiles are recorded

by a CCD camera with millisecond time exposure.  Hence the SAS can be used in laser and HED

experiments of up to kHz repetition rates.  Here we provide a concise description of the design principles,

capabilities and preliminary results of the SAS from recent short-pulse laser experiments.   

A gatling-gun target delivery system for high-intensity laser irradiation experiments

Intense laser irradiation experiments of sub-micrometer scale targets are currently performed at slow shot rates. This limitation is the result of the inability to place such targets quickly and accurately in the focus of the laser.

I will present a target setup that enables irradiation at a high rate and sub-wavelength positioning accuracy. Three hundred targets were micro-machined and mounted on a single Si wafer. The system implements a closed feedback loop between a triangulation displacement sensor and a motorized manipulator. Using this setup, we demonstrate repeatable stable proton acceleration from 600 nm thick Au foils at a rate of 0.2 Hz. This system enables studies that require a large overall dose, high statistical significance, or a fine scan of target geometric attributes.This target system may be employeed to serve in HEDP laser experiments.   

High repetition rate diagnostics with integrated machine learning analysis

High repetition rate intense short-pulse and high-energy long-pulse lasers are already online around the world and promise to revolutionize the way HED physics is done by increasing shot rates by more than three orders of magnitude. We will detail progress on our mission to develop high repetition rate short-pulse HED plasma diagnostics, that can run at a commensurate rate as these new lasers.  These diagnostics must be designed to be robust to the unique perils of HED environments such as EMP and high radiation.

 

We will outline progress on x-ray, proton, and fast electron diagnostics, and our attempts to run them simultaneously; some of the challenges we have come across when fielding at current state of the art facilities; and present preliminary results from our first round of testing at US and UK based laser facilities. We describe how such multimodal diagnostic suites can be integrated with automated analysis through the application of machine learning that will enable active feedback loops to directly control driver input and build a fully integrated and automated experimental system for investigating high intensity laser solid interactions.

 

This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported under DOE FES Measurements Innovations grant SCW1720, DOE Early Career grant SCW1651-1, and with funding support from the Laboratory Directed Research and Development Program under tracking codes 20-ERD-048 and 21-ERD-015. The CSU laser facility is supported by DOE LaserNet US (DE-SC0019076).