We report on a direct search for elastic photon-photon scattering using x-ray and γ photons from a laser-plasma based experiment. A γ photon beam produced by a laser wakefield accelerator provided a broadband γ spectrum extending to above Image 1. These were collided with a dense x-ray field produced by the emission from a laser heated germanium foil at Image 2, corresponding to an invariant mass of Image 3. In these asymmetric collisions elastic scattering removes one x-ray and one high-energy γ photon and outputs two lower energy γ photons. No changes in the γ photon spectrum were observed as a result of the collisions allowing us to place a 95% upper bound on the cross section of Image 4. Although far from the QED prediction, this represents the lowest upper limit obtained so far for Image 5.

We describe a laser-plasma platform for photon-photon collision experiments to measure fundamental quantum electrodynamic processes such as the linear Breit-Wheeler process with real photons. The platform has been developed using the Gemini laser facility at the Rutherford Appleton Laboratory. A laser wakefield accelerator and a bremsstrahlung convertor are used to generate a collimated beam of photons with energies of hundreds of MeV, that collide with keV x-ray photons generated by a laser heated plasma target. To detect the pairs generated by the photon-photon collisions, a magnetic transport system has been developed which directs the pairs onto scintillation-based and hybrid silicon pixel single particle detectors. We present commissioning results from an experimental campaign using this laser-plasma platform for photon-photon physics, demonstrating successful generation of both photon sources, characterisation of the magnetic transport system and calibration of the single particle detectors, and discuss the feasibility of this platform for the observation of the Breit-Wheeler process. The design of the platform will also serve as the basis for the investigation of strong-field quantum electrodynamic processes such as the nonlinear Breit-Wheeler and the Trident process, or eventually, photon-photon scattering.

We describe a laser-plasma platform for photon-photon collision experiments to measure fundamental quantum electrodynamic processes. As an example we describe using this platform to attempt to observe the linear Breit-Wheeler process. The platform has been developed using the Gemini laser facility at the Rutherford Appleton Laboratory. A laser Wakefield accelerator and a bremsstrahlung convertor are used to generate a collimated beam of photons with energies of hundreds of MeV, that collide with keV x-ray photons generated by a laser heated plasma target. To detect the pairs generated by the photon-photon collisions, a magnetic transport system has been developed which directs the pairs onto scintillation-based and hybrid silicon pixel single particle detectors (SPDs). We present commissioning results from an experimental campaign using this laser-plasma platform for photon-photon physics, demonstrating successful generation of both photon sources, characterisation of the magnetic transport system and calibration of the SPDs, and discuss the feasibility of this platform for the observation of the Breit-Wheeler process. The design of the platform will also serve as the basis for the investigation of strong-field quantum electrodynamic processes such as the nonlinear Breit-Wheeler and the Trident process, or eventually, photon-photon scattering.

Optimisation of betatron X-rays and their applications, SPIE Optics + Optoelectronics, 2021

We report on the characterisation of an x-ray source, generated by a laser-driven plasma wakefield accelerator. The spectrum of the optimised source was consistent with an on-axis synchrotron spectrum with a critical energy of 13.8−1.9+2.2 keV and the number of photons per pulse generated above 1 keV was calculated to be 6−0.9+1.2×109 . The x-ray beam was used to image a resolution grid placed 37 cm from the source, which gave a measured spatial resolution of 4 µm ×  5 µm. The inferred emission region had a radius and length of 0.5 ± 0.2 µm and 3.2 ± 0.9 mm respectively. It was also observed that laser damage to the exit aperture of the gas cell led to a reduction in the accelerated electron beam charge and a corresponding reduction in x-ray flux due to the change in the plasma density profile.

A new type of particle accelerator that uses lasers to accelerate particles, called Laser Wakefield Acceleration (LWFA), is becoming a wide spread tool in the scientific community. These compact accelerators are capable of delivering very high energy electron beams (100s MeV) over very short distances (mm). With such high energy electron beams it is possible to produce very hard X-rays (100s keV to MeV) for use in imaging. This poster will explore the basic principles of this technique and how it is being applied to industrial samples for X-ray computed tomography (CT) inspection.

We are currently focusing on the improvement of contrast pedestal (CP) in the compressed laser pulse of PW Ti:Sapphire lasers. In our previous studies, we have identified the stretcher in our laser system as the source of CP. In order to underpin the true origins of CP, we have quantitatively characterised the surface quality of large optics used in the Gemini laser stretcher, where the laser beam is spatially dispersed and the spectral phase noise is induced by the optical surface roughness. We have measured the surface profiles of 2 different gold gratings, the new and old grating, and back mirror to a very high precision (~ a fraction of nm) by using ZYGO Dynafiz, with a spatial resolution of ~50µm over a width up to ~320mm, an unprecedented combination of very high spatial resolution with a very wide field of view. The surface roughness of the large curved mirror was determined experimentally. We have developed a simple physical model to deal with the influence of the surface roughness on the contrast pedestal. Based on the measured surface profiles and by taking the actual laser beam size into account, we are able to determine the spectral phase noise induced by the optical surface roughness in the stretcher. Consequently, we are able to accurately evaluate the impact of individual large optics in the stretcher and an overall impact of the stretcher on the contrast pedestal. The calculated contrast induced by both stretches with the new and old gratings are in an excellent agreement with the experimental results measured by the Sequoia scan. For the stretcher with the old grating, the grating is the dominant impact factor on the contrast. However, for the stretcher with the new gold grating of higher quality, the impact of the curved mirror on the contrast is comparable to that of grating. This implies that the influence of curved mirror on the contrast pedestal becomes more significant when the surface quality of grating is further improved. It is clearly observed that the impact of back mirror on the contrast is more than one order of magnitude lower than that of gratings and also much lower than that of curved mirror. In conclusion, we have demonstrated a novel method to evaluate the impact of large optics in the stretcher on the contrast pedestal by precisely quantitative characterization of optical surface quality. It is possible to accurately predict the contrast pedestal based on the stretcher configuration and precise characterisation of the optical surface in the stretcher prior to the construction of actual CPA high power laser system.

We describe the use of a genetic algorithm to apply active feedback to a laser wakefield accelerator at a higher power (10 TW) and a lower repetition rate (5 Hz) than previous work. The temporal shape of the drive laser pulse was adjusted automatically to optimize the properties of the electron beam. By changing the software configuration, different properties could be improved. This included the total accelerated charge per bunch, which was doubled, and the average electron energy, which was increased from 22 to 27 MeV. Using experimental measurements directly to provide feedback allows the system to work even when the underlying acceleration mechanisms are not fully understood, and, in fact, studying the optimized pulse shape might reveal new insights into the physical processes responsible. Our work suggests that this technique, which has already been applied with low-power lasers, can be extended to work with petawatt-class laser systems.

We describe how active feedback routines can be applied at a limited repetition rate (5 Hz) to optimize high-power (>10 TW) laser interactions with clustered gases. Optimization of x-ray production from an argon cluster jet, using a genetic algorithm, approximately doubled the measured energy through temporal modification of the 150 mJ driving laser pulse. This approach achieved an increased radiation yield through exploration of a multi-dimensional parameter space, without requiring detailed a priori knowledge of the complex cluster dynamics. The optimized laser pulses exhibited a slow rising edge to the intensity profile, which enhanced the laser energy coupling into the cluster medium, compared to the optimally compressed FWHM pulse (40 fs). Our work suggests that this technique can be more widely utilized for control of intense pulsed secondary radiation from petawatt-class laser systems.

In the present report, we discussed our recent results concerning the investigations of high energy density matter. By using of high-resolution X-ray spectroscopic measurements and kinetic simulations, we demonstrate that the energy of femtosecond laser pulses is efficiently converted to X-ray radiation and produces exotic states in solid density plasma regions. It is shown that exotic hollow multicharged ions states can be obtained and studied not only by ultra-intense X-ray radiation of XFEL lasers, but also upon using optical laser technology. We have concluded that observable features of the hollow-ion spectra are sensitive to such plasma parameters as density, temperature, hot-electron fraction, and intensity of the X-ray pumping radiation and could be used for effective diagnostics of warm dense matter parameters.

The first two amplifiers of the Gemini laser system (pulse energy up to the Joule-level) operate at 10 Hz before being split into two alternating 5 Hz beams, which separately feed into TA2 and TA3. While the repetition rate of the Quantel pump lasers in the final Gemini amplifier limits TA3 operations to 0.05 Hz, TA2 can be operated at 5 Hz. Until now TA2 has reduced the repetition rate to a maximum of 1 Hz because of the practical challenges of increasing to the full capability. In this report we address these issues and describe the first experimental campaign conducted with the full 5 Hz repetition rate. Furthermore, this allowed us to employ active feedback routines [1] to directly optimize experimental parameters using controllable elements in the laser system.

We present experimental data showing the beneficial effects of performing parameter scans with a high repetition rate laser. The transmitted laser and electron beam generated in a laser-driven plasma wakefield accelerator are clearly observed to depend on the gas jet backing pressure. The spectral properties of these beams are seen to be smooth functions of the input parameters, when averaging over a set 49 shots for each value and employing gradual changes in pressure. This allows for detailed examination of the interaction physics, and can better reveal threshold behaviour and highly localised optima. Extending high-repetition rate operation to higher power laser systems is expected to yield great benefits in performance, and will enable a range of new applications.

It is widely understood within the high-power laser community that recent developments in diode pumped and high repetition rate laser systems will give unprecedented access to laser shots. This will provide a challenge for target fabrication in making enough experimental samples. While in the past access to facilities and shot rates during access periods have been the limiting factor for high power laser experiments this will soon not be the case. There has already been a shift in development of the user base from fundamental science experiments to industrial applications using the laser experiment as a reliable source for secondary aims. The Astra Gemini laser system has been operating at a high repetition rate for high intensity (0.5PW) experiments for a number of years and the Central Laser Facility has developed a target methodology to deliver to the user community the maximum number of solid targets and to fully utilise the available time on the laser. Targets for the High Accuracy Microtarget Supply (HAMS) system have been tested and have been proven to survive in a manner to allow shot rates comparable with the available laser repetition rate (0.1Hz). Investigations into target geometry have been carried out and debris production has been studied by high frame rate camera imaging. The study of the relationship between target geometry and debris production has allowed the design of optimal target support infrastructure, such as aperture size and structure, for high rep rate experiments on the Gemini system.

Many of the new large European facilities that are in the process of coming online will be operating at high power and high repetition rates. The ability to operate at high repetition rates is important for studies including secondary source generation and inertial confinement fusion research. In these interaction conditions, with solid targets, debris mitigation for the protection of beamline and diagnostic equipment becomes of the upmost importance. These facilities have the potential to take hundreds, if not thousands, of shots every day, creating massive volumes of debris and shot materials. In recent testing of the Central Laser Facility's High Accuracy Microtargetry Supply (HAMS) system on the mid-repetition rate Gemini facility (15 J, 40 fs, 1 shot every 20 seconds), diagnostics were deployed in order to specifically look at the debris emitted from targets designed for high repetition rate experiments. By using a high frame rate camera, it has been possible to observe and characterize some of the debris production, whilst also looking at target fratricide. Alongside these results from Gemini, we also present results of static debris measurements undertaken on the Vulcan Petawatt high energy, high power facility, where the cumulative effects of debris produced by high power laser experiments have been observed.

Magnetic reconnections play essential roles in space, astrophysical, and laboratory plasmas, where the anti-parallel magnetic field components re-connect and the magnetic energy is converted to the plasma energy as Alfvénic out flows. Although the electron dynamics is considered to be essential, it is highly challenging to observe electron scale reconnections. Here we show the experimental results on an electron scale reconnection driven by the electron dynamics in laser-produced plasmas. We apply a weak-external magnetic field in the direction perpendicular to the plasma propagation, where the magnetic field is directly coupled with only the electrons but not for the ions. Since the kinetic pressure of plasma is much larger than the magnetic pressure, the magnetic field is distorted and locally anti-parallel. We observe plasma collimations, cusp and plasmoid like features with optical diagnostics. The plasmoid propagates at the electron Alfvén velocity, indicating a reconnection driven by the electron dynamics.

Clustered gases are a very interesting target in laser-plasma physics due to their ability to very efficiently absorb short pulse laser energy; this can be as high as 95% [1]. This makes clustered gases excellent x-ray sources, as large amounts of energy absorbed produces a high temperature plasma leading to bright x-ray emission [2]. This report will describe a recent experiment in Gemini TA2 in which the effect of the temporal shape of a laser pulse on x-ray emission from a clustered gas target was explored, and the optimum pulse shape that yielded the highest x-ray emission was found. A genetic algorithm was used to find this optimum pulse shape, a technique enabled by operation of the beam line at 5Hz, allowing for rapid averaging of large data sets. Genetic algorithms are a useful tool for exploring large and multidimensional parameter spaces. The optimization of a result based on the variation of a single parameter is achievable with a simple parameter scan. However, scanning a multidimensional parameter space would require taking a large data set and be very time consuming. A genetic algorithm averts this problem as it does not require every possible point in the parameter space to be tested. In this experiment a three-dimensional parameter space was explored, these were the second, third and fourth order spectral phase terms of the laser. Changing these properties of the laser pulse allows the direct manipulation of the pulse shape in time.

X-rays generated by betatron oscillations of electrons in a laser-driven plasma accelerator were characterised and applied to imaging industrial samples. With a 125 TW laser, a low divergence beam with 5.2±1.7 × 107 photons mrad−2 per pulse was produced with a synchrotron spectrum with a critical energy of 14.6±1.3 keV. Radiographs were obtained of a metrology test sample, battery electrodes, and a damage site in a composite material. These results demonstrate the suitability of the source for non-destructive evaluation applications. The potential for industrial implementation of plasma accelerators is discussed.

We report a novel method to accurately evaluate the impact of stretcher gratings on the contrast pedestal of high power laser pulses by precise quantitative characterization of the grating surface. The results are in a good agreement with the experimental measurement. We have, for the first time, established a relation between the contrast pedestal and natural property of stretcher gratings.

We present a method for evaluating the properties of electrostatic shock in laser-produced plasmas by using optical diagnostics. A shock is formed by a collimated jet in counter-streaming plasmas in nearly collisionless condition, showing the steepening of the transition width in time. In the present experiment, a streaked optical pyrometry was applied to evaluate the electron density and temperatures in the upstream and downstream regions of the shock so that the shock conditions are satisfied, by assuming thermal bremsstrahlung emission in optically thin plasmas. The derived electron densities are nearly consistent with those estimated from interferometry.

We present X-ray spectroscopic diagnostics in femto-second laser-driven experiments revealing nonlinear phenomena caused by the strong coupling of the laser radiation with the created plasma. Among those nonlinear phenomena, we found the signatures of the Two Plasmon Decay (TPD) instability in a laser-driven CO2 cluster-based plasma by analyzing the Langmuir dips in the profile of the O VIII Lyε line, caused by the Langmuir waves created at the high laser intensity 3 1018Wcm-2. With similar laser intensities, we reveal also the nonlinear phenomenon of the Second Harmonic Generation (SHG) of the laser frequency by analyzing the nonlinear phenomenon of satellites of Lyman δ and ε lines of Ar XVII. In the case of relativistic laser-plasma interaction we discovered the Parametric Decay Instability (PDI)-induced ion acoustic turbulence produced simultaneously with Langmuir waves via irradiation of thin Si foils by laser intensities of 1021Wcm-2.