Session TO08: Beam and Relativistic Plasma Diagnostics

High-Areal Density (>100g/cm 2 ) Radiography using MeV X-rays Performed at the NIF-ARC Laser System

Laser-driven X-ray radiography is a potential radiographic source for static and dynamic imaging

and could soon compete with conventional sources. In particular, bremsstrahlung X-rays from

high-intensity laser-solid target interactions (>1 x 10^18 W/cm^2) can produce a broadband of

high-energy/MeV X-rays with a source size in the hundreds of microns. At the NIF-ARC laser

facility, we have conducted numerous studies on the production and scaling of MeV X-rays. We

have measured the X-ray spectrum, angular distribution, and performed radiography of high-

areal density objects [1]. The NIF-ARC laser can deliver up to 2.4 kJ in 10 ps or 4 kJ in 38 ps.

Here, we present recent experimental data obtained at the NIF-ARC from several shots at each

pulse duration to better understand the scaling of MeV X-ray production and maximize X-ray

dose. The best performance is achieved at 2.4 kJ in 10 ps, and this was used to perform

radiography on the line-of-sight of the ARC laser, providing a radiograph through >100 g/cm^2

of tungsten. Scaling of the X-ray flux and radiography performance is complemented with

analytical and simulation modeling that agrees with the experimental results.   

Field mapping of CO2-laser-driven LWFA at low density using electron beam probing

Laser wakefield accelerators (LWFAs) have been experimentally shown to produce sustained gradients of tens of GeV/m over tens of centimetres. While the strength of these fields has been demonstrated, a direct measurement of the field configurations inside an LWFA represents an emerging research area. 

A recently published study(1) suggests that the unique combination of parameters used during the experiment, where the laser pulse length spans only a few plasma wake periods, places laser plasma interaction in a novel regime, as an intermediary stage between self modulated and blowout regime. OSIRIS simulations support the presence of an elongated cylindrical cavity with an almost flat accelerating field along the length of the ion channel, making this regime a potential candidate for laser plasma lens. 

Here we report on the transverse probing of the fields inside an LWFA at such a regime in some of the lowest densities ever observed, i.e. in the range of 1015 — 1017 cm-3. The LWFA is driven by BNL Accelerator Test Facility’s long-wave-infrared CO2 laser (9.2 μm) pulse, which currently generates 2 ps long pulses at 2-3 TW. The linac-produced electron beam has an energy of 50-60 MeV and about a 200 fs long bunch length. Electrons traversing the wakefield are detected using YAG:Ce scintillator screens, with one being stationary at 50 cm and the other placed on a translation stage imaging the electron beam density profile at distances of up to 10 cm from the plasma. Particle-In-Cell Simulations using OSIRIS are used to corroborate the results of the experiment.   

Simultaneous Transverse Radiography Using Short-Pulse Generated Probes

Radiography is a primary tool for determining the position of shocks and interfaces in both heed and larger scale dynamic objects.  These objects are frequently of high atomic number and have large areal density.  More information about an expensive experiment could be gained by using probe beams of different species, e.g. x ray and protons, to measure the same phenomenon simultaneously.  We have succeeded in making simultaneous measurements of a static object using short-pulls laser generated x-ray and proton beams.  The two probes were at right angles to one another.  An impediment to this test was the presence of a large electron background emitted by the proton-generating target.  These electrons were blocked by a simple shield that protected the x-ray detector placed at right angles to the proton beam.   

Laser-driven high flux, high-repetition rate ion and neutron beams for radiography applications

Laser-driven ion and neutron sources offer an attractive approach for the generation of short, intense bursts of ions and neutrons with MeV energies. Such beams are desirable to generate high energy density (HED) matter states of fusion-relevant materials and serve as probes for high-resolution static or in-situ HED radiography applications. Short pulse (few 10s fs) lasers offer advantages over conventional accelerators with shorter pulse duration, higher spatial resolution and the ability to scale to higher average fluxes. The next generation of high repetition rate (>1 Hz) petawatt-class short pulse lasers promises to deliver high flux particle beams capable of static radiography applications using accumulation in seconds-to-minute time scales while also achieving higher contrast on a single shot exposure.

 

Here we present results of reliable and controllable proton generation using high-repetition rate laser systems coupled to compatible target systems. Neutrons were generated in a pitcher-catcher setup through nuclear reactions between impinging protons and catcher target atoms. Experiments were conducted at the HAPLS laser at ELI (800 nm, 30fs, 8J, 0.2 Hz) and the ALEPH laser at Colorado State University (800 nm, 30 fs, 22 J, 0.5 Hz). We studied the effect of target thickness, laser energy and laser pulse shape on the ion acceleration and neutron production, which will inform the development of a flexible radiography setup for future facilities such as MEC-U.   

Techniques and Results for a Study of Relativistic Birefringence using Particle-in-Cell Simulations

Unique plasma behaviors emerge when electrons in a plasma are driven to relativistic velocities by an intense laser pulse (>10¹⁸ W/cm²). Two effects in this regime, Relativistic Transparency and Relativistic Birefringence, have been proposed for possible use in plasma-based optics. Experimental efforts at the Scarlet Laser Facility explored these effects along with a complementary simulation campaign using a novel particle-in-cell pump-probe (two laser) methodology. In this work, we present techniques developed to analyze Relativistic Birefringence using particle-in-cell simulations as well as preliminary results from the simulation campaign.   

Development of a double-grating differential interferometer for laser-plasma diagnostics

In laser-plasma acceleration experiments, the characteristics of the accelerated particle beams are strongly influenced by the plasma density, making precise diagnostics of this parameter critically important. Generally, interferometry can be employed to measure the density of such plasmas. However, the reliability of this method can be limited in certain regions due to the noise amplification of Abel inversion, a common issue encountered with this method. Therefore, differential interferometry would be better for diagnostics of a plasma with a high gradient in density as it measures the phase shift derivative directly. For this purpose, a special shearing interferometer with two gratings was developed to apply differential interferometry in laser-produced plasma diagnostics. Compared with other types of interferometers, our design provides a significant advantage in that the shear distance, shear direction, and fringe width can be controlled independently, enabling precise adjustment of these parameters. This double-grating-based differential interferometer was demonstrated for diagnostics of the laser-produced plasma by focusing a 1 TW/35 fs Ti:sapphire laser pulse in a gas jet with a 100 μm orifice diameter. It was verified that our differential interferometer can give more reliable and precise plasma densities, particularly for plasmas having a steep spatial density gradient. In this presentation, detailed results will be given.   

Nuclear physics using ultrafast high-power laser ion acceleration

Ultrafast high-power lasers provide a new tool for the study of nuclear science, producing large numbers of energetic ions in a single burst, allowing activation measurements that would be difficult using traditional accelerator techniques.  The Short-Lived Isotope Counting System (SLICS) has been developed to detect the 20 ms to 10 s half life beta decays of reaction products formed as a result of light-ion reactions initiated by laser-ion acceleration or inertial confinement fusion (ICF).  Results of a recent SLICS test using the Multi-Terawatt Laser (MTW) at the Laboratory for Laser Energetics (LLE) will be presented.  In the experiment, target normal sheath acceleration (TNSA) was used to produce a pulse of roughly 0.1-10 MeV deuterons which struck a thin natural Li target film, causing the⁷Li(d,p)⁸Li reaction.  SLICS counted beta decays of the ⁸Li, beginning a few milliseconds after the laser shot, allowing a fit to the 840 ms half-life decay to be used to determine the ⁸Li yield, which was compared to the yield predicted from previously published cross section measurements.  Work is currently underway to test and field SLICS as a TIM-based diagnostic.   

Investigation of Power Flow in a Tapered Transmission Line Modification to HERMES -III using an Array of Dose- and Dose-rate Detectors in the Near- and Far-field*

HERMES-III (18 MV peak, 650 kA peak, 40 ns radiation pulse) delivers power down constant-diameter MITLs to an electron beam-converter combination to generate intense bremsstrahlung pulses. Electron dynamics in this configuration has been extensively characterized and found to be quite stable. An array of P-I-N diodes in the far-field with accompanying TLDs mounted within tungsten collimators has been developed to estimate variations in electron angle(s) on the converter.

 

We have recently investigated power flow where the constant diameter MITLs have been narrowed through a short tapered section to a reduced-diameter cathode and converter. A set of near-field radially arrayed PIN diodes was placed perpendicular to the axis, behind a set of TLDs mounted outside the vacuum. In addition, an x-ray pinhole camera (PHC) was mounted on the machine axis, several meters away. In contrast to normal HERMES operation, the resulting beam as imaged by the PHC indicated a relatively small pinch ~ 3 cm in diameter. Waveforms from the radial PIN diode array clearly show a photon sweep across the radiation axis, peaking first further away from the axis, then moving towards the axis. From this we can infer an electron beam sweep. Simulations using the CHICAGO PIC code indicate that such a tight beam pinch can only be explained by the presence of ions produced by electron strikes to the anode. Investigations are continuing, and latest results will be presented.   

Investigating the Dynamics of Short-Pulse Laser Beam Filamentation in Underdense Plasmas

The filamentation instability results from the ponderomotive and thermal ejection of electrons from the high-intensity regions of a laser beam within a plasma. This process creates modulations in the plasma density and the refractive index that can lead to self-focusing and filamentation of the beam. The resulting intensified beam and modulated plasma can inhibit the propagation of the beam and limit the efficacy of laser-plasma interactions such as Thomson scattering, Raman amplification, and laser guiding structures. We present experimental and simulation results investigating the growth rate of the filamentation instability. The experiment utilizes the joint operation of the OMEGA 60 and OMEGA EP laser systems at the University of Rochester’s Laboratory for Laser Energetics. In our experiment, a 1ω short-pulse (1—100-ps) laser beam from OMEGA EP is coupled into a preheated plasma on the OMEGA 60 laser-plasma interaction platform. The resulting beam spray profile of the filamented short-pulse beam is recorded, while plasma parameters are determined via spatially-resolved Thomson scattering. We compare experimental results with 2D, axisymmetric simulations utilizing a paraxial electromagnetic wave solver coupled to a single-fluid nonlinear hydrodynamic solver. The simulations model the experiment based on the plasma condition retrieved from the Thomson scattering data and the transmitted nearfield measurements of a vacuum-propagated beam. By modifying the incident short-pulse beam pulse duration in the experiment and simulations, we limit the growth of the instability allowing us to connect the recorded beam spray to the underlying physics of filamentation at discrete steps in its temporal evolution.

 

A Reduced Lagrangian for Photon-Photon Interactions in Vacuum

Electromagnetic waves travelling through vacuum excite virtual electron-positron pairs that can modify their propagation. Nonlinear wave equations describing this propagation can be derived from the Euler-Heisenberg Lagrangian density, which captures vacuum polarization effects up to the one-loop contribution. Here, we introduce a reduced action integral approach that facilitates modeling of processes arising from the Euler-Heisenberg Lagrangian, including photon-photon scattering and vacuum birefringence between two spectrally distinct light beams. The reduced Lagrangian derived from this action describes the evolution of familiar light-beam parameters, such as the centroid, spot size, phase, polarization, and phase-front curvature. To demonstrate the approach, the reduced action is applied to the scattering of an X-ray beam from a counter-propagating ultrahigh-intensity optical beam.   

Reflected light and harmonic generation from thin foils as a diagnostic for laser-driven ion acceleration experiments

Recent developments in laser driven ion acceleration show its potential as an attractive alternative to conventional accelerators for promising applications. In order to further develop and achieve better control over these sources, clear understanding of the physics behind the extreme laser-matter interaction is fundamental.  

Our group has recently reported an increase of proton energies when manipulating the spectral phase of the driving laser pulse [1]. While the effect has been observed repeatedly over several experiments, further studies and experimental observables are required to benchmark simulations and study changes in plasma dynamics that lead to higher particle energies when using certain phase conditions.

Reflected laser light and harmonics coming from the critical density surface are ideal diagnostics to study electron dynamics and energy coupling between the laser and plasma [2]. In this work, we focus on spectral modulations on reflected light and harmonics when manipulating the spectral phase of the pulse during laser driven ion acceleration experiments with the DRACO laser (30 fs, 5×10²¹ W/cm²). We study possible reasons for these changes, comparing with previous experiments and simulations [3], and explore the potential of the effect to study the impact of varying laser compression on particle acceleration.   

   

[1] Ziegler, T. et al. Sci Rep 11 (2021)

[2] Chopineau, L. et al. Phys.Rev. X 9 (2019)

[3] Porat. E. et al. Phys. Rev. Res. 4 (2022)