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Details of Grant 

EPSRC Reference: EP/X009971/1
Title: Support for photoionisation experiments at the OMEGA EP high-power laser facility
Principal Investigator: Keenan, Professor FP
Other Investigators:
White, Dr S Sarri, Professor G Riley, Professor D
Researcher Co-Investigators:
Project Partners:
Department: Sch of Mathematics and Physics
Organisation: Queen's University of Belfast
Scheme: Standard Research
Starts: 01 May 2023 Ends: 31 July 2024 Value (£): 35,532
EPSRC Research Topic Classifications:
Plasmas - Laser & Fusion
EPSRC Industrial Sector Classifications:
No relevance to Underpinning Sectors
Related Grants:
Panel History:  
Summary on Grant Application Form
Imaging and spectroscopy are of major importance in many areas of plasma physics, and the analyses of such observations using modelling codes provide vital information on a range of plasma properties, including (but not limited to) temperature, density and chemical composition. Modelling codes such as GALAXY, ALICE and FLYCHK are widely employed by the laser plasma communities in the UK and elsewhere. However a major limitation to such codes is that we do not know a priori if they are producing accurate theoretical results, and hence if we can perform a reliable analysis of experimental data. In principle, one can ensure that such codes are producing accurate results if the model predictions can be directly benchmarked against experimental data obtained via well-diagnosed laboratory experiments under known plasma conditions.

However, although the predictions of modelling codes have been extensively tested against experiment for collisionally-dominated plasmas, such as tokamaks, this is not the case for laser-produced plasmas in which the excitation and ionisation are dominated by the radiation field, particularly for plasmas in a steady-state. When the radiation field dominates the excitation and ionisation processes, the plasma is termed photoionisation-dominated. The degree of photoionisation in such a plasma is characterised by the photoionisation parameter xi = (4 x pi x F)/Ne, where F is the flux incident on the plasma and Ne the electron density.

Creating a well-diagnosed, high photoionisation parameter, near steady-state plasma in the laboratory would be a truly outstanding achievement, allowing plasma modelling codes to be rigorously tested under the most extreme conditions. Given its importance, there have been several attempts in recent years to create such a plasma in the laboratory, which is extremely challenging and hence has met with very limited success. As the electron density in the plasma is large, so too must be the X-ray flux to generate high values of photoionisation parameter, and this flux must be maintained to allow the plasma to achieve near steady-state conditions. Unfortunately, radiation temperatures in large photoionisation parameter plasmas are often very high (~ 1 keV), too large for a Planckian source at that temperature to be produced in the laboratory for the required duration of greater than ~1 ns to truly achieve steady-state.

However, we have implemented a novel experimental technique that allows us to generate such plasmas in the laboratory. We replace the quasi-blackbody X-ray source normally used to ionise a plasma with an intense line or line-group radiation source. This provides much more efficient coupling between the radiation source and the plasma to be photoionised, allowing keV radiation to dominate over softer X-rays and hence mimic the effect of a greater radiation temperature on the photoionisation of the target gas. Consequently, we can generate a larger radiation temperature than achievable with a quasi-blackbody source of similar flux.

We have already undertaken successful proof-of-concept experiments using the VULCAN laser in 2016, with follow-up VULCAN experiments in 2019 as well as on the Chinese SG-II laser in 2017. These allowed us to improve our experimental design, and our team has now been awarded time on the OMEGA EP high-power laser facility at the US Laboratory for Laser Energetics, scheduled for September 2022. In this proposal we seek funding for the team members to support these experiments, which should generate plasmas with values of photoionisation parameter up to 1000 ergcm/s, matched only by the most intense astronomical accretion-powered X-ray sources, namely active galaxies.
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