Fundamental physics with intense laser fields

Periodic Reporting for period 1 - SF-QFT (Fundamental physics with intense laser fields)

Período documentado: 2016-10-01 hasta 2018-09-30

The intense light sources available at current and next generation high-intensity laser facilities offer exciting prospects for a broad spectrum of applications in the areas of energy, industry and medicine. Furthermore, intense laser light offers new methods to explore questions in basic science, in particular fundamental quantum physics.

While traditionally the realm of accelerator-based experiments (such as those at the Large Hadron Collider (LHC) at Cern), elementary particle physics can also be investigated using intense lasers. Indeed laser light offers us access to a wealth of effects (on the so-called `intensity frontier' of particle physics) which are difficult to probe using traditional accelerator experiments.

Experiments require predictions to test, and theoretical predictions must be tested with experiment. Since the invention of the laser experimental progress has come in fits and spurts, but the development of theory has been slower. Almost immediately after the invention of the laser a basic theory was established which allowed scientists to ask questions on the physics of laser-matter interactions, and to perform the calculations required to answer them. This theory was the `gold standard' for many years. In the last decade, however, researchers have begun to question the assumptions behind this old theory, and the experimental predictions based on it.

It has been realised that improvements to our theoretical tools are needed in order to make accurate predictions for, and analyses of, forthcoming laser experiments. Addressing this need was the objective of the project. The aims of the proposed research were to first improve and refine our current theoretical models of laser-matter interactions, and then to begin the task of applying these to the study of relevant physical process which will be studied in future experiments.

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A great deal was accomplished during the first year of the project.

*Challenges and results achieved*

1. One of the main challenges the project sought to address was that of going beyond the `plane wave' approximation. This is the assumption that the fields of an intense laser may be modelled as a plane wave; this is a simplistic field model which does not encompass the tight spatial focussing of high intensity laser pulses and the effects on particle interactions due to it. It has been recognised recently that there are many situations in which this model fails to capture the relevant physics.

A new framework for the investigation of laser-matter interactions beyond the plane wave model was established early in the project. This was an interdisciplinary work which brought results from the mathematical community (on `superintegrability') to bear on the physics problem. The paper was published in Physical Review Letters and selected as an ""Editors' choice"" for its interdisciplinary nature and potential impact. A paper presenting related results to the mathematical physics community was published in J.Phys.A.

2. In pursuit of experimental signatures of quantum effects we identified a new, observable manifestation of quantum physics in laser-matter collisions. We dubbed this effect ``quenching'' in analogy to fluorescence quenching in chemistry; it is a suppression of radiative losses due to quantum effects which can could be observed in the interaction of energetic particles with short laser pulses. These results were published in Physical Review Letters.

3. Around six months into the project our collaboration completed the analysis of an experiment performed on the Gemini laser in the UK. We have made the first experimental observation of radiation reaction in laser-particle collisions. This paper, essentially the first of its kind, signals the start of a new era of laser experiments and is expected to generate quite some interest in the community. The paper is available on the arXiv preprint server and is under review in Physical Review X.

4. Combining analytic and numerical results, we were able to show how to create an ultra-bright GeV photon source at next-generation laser facilities. The source concept represents the state of the art, and was published in Physical Review X.

5. Toward the end of the first year, a second new framework was developed for investigations of laser-matter interactions beyond the background field approximation. This represents a second significant step forward, allowing theorists to include the hitherto-neglected effects of depletion in back reaction. Importantly, we were able to show how to include these effects within an already well-established theoretical formalism, making our methods and results immediately accessible to other researchers int the field. The paper has been accepted for publication in Physical Review D.

*Dissemination*

1. A twitter account attached to the project was set up to advertise results and make other researchers aware of the project.

2. University press releases were organised to announce publications in high-impact journals such as Physical Review Letters and Physical Review X, and to promote the action at the Faculty and University level. These press releases were also recirculated on twitter in order to reach the research community.

3. The results of the project have been presented at several leading conferences:

ILNPC 2017 in Yokohama, Japan
The UK High Power Laser annual meeting, Oxford, UK
ExHILP 2017 Lisbon, Portugal

4. The researcher was invited to visit ELI-NP, a next-generation EU infrastructure-roadmap-funded laser facility, in order to discuss possible future experiments. Positive interactions between the researcher and ELI-NP led to a standing invitation to lecture at the ELI-NP summer school.

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*Beyond the state of the art*

We have established two new frameworks for tackling the major problems of the research field of high-intensity laser-matter interactions. These frameworks address, specifically, the challenges of going beyond the plane wave model and including back-reaction and beam depletion effects. These results represent significant steps on the road to overhauling our theories of laser-matter interactions in the high-intensity regime.

In deriving these results we have used novel approaches; adapting `superintegrability' methods from the mathematical physics literature has shown us how to construct exactly solvable models of laser-matter interactions within the background field approach. By exploiting the quantum nature of coherent states we have then been able to go beyond the background field limit, in order to include back-reaction on the laser fields.

*Expected results and future impact*

The further investigation, development and application of the two new frameworks which we have established is open ended and will be actively pursued over the coming years. The initial focus will be on expanding our portfolio of examples and identifying areas in which they can be applied.

In an effort to bring our results to more of the community, in particular in order to connect with numericists and experimentalists, we are working toward the inclusion of depletion/back-reaction effects in extended PIC codes. This is work-in-progress in collaboration with researchers in Berkeley, Gothenberg and Lancaster, and we expect to produce a first paper toward the beginning of 2018.

The classical trajectory and quantum wavepacket spread of an electron in a laser donut mode

3D Extended PIC simulations of an electron-positron cascade in a focussed dipole laser pulse

Periodic Reporting for period 1 - SF-QFT (Fundamental physics with intense laser fields)

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