Laser-Plasma Interactions at the Intensity Frontier: the Transition to the QED-Plasma Regime

Current high-power lasers focus light to intensities up to 10^23 times higher than the intensity of sunlight at the surface of the Earth. At these extreme intensities the electrons are quickly stripped from the atoms in any matter in the laser focus, generating a plasma. However, as intensities increase from the peak reached today (2x10^22W/cm^2) to those expected to be reached on next-generation facilities such as the Extreme Light Infrastructure (>10^23W/cm^2), due to become operational by 2017, the behaviour of this plasma dramatically alters. At intensities >5x10^22W/cm^-2 the electromagnetic fields in the laser focus are predicted to accelerate the electrons in the plasma so violently that they prolifically radiate gamma-ray photons. These photons can carry away so much energy that the electron's motion is affected by the resulting energy loss and the radiation reaction force (the force the particle exerts on itself as it radiates) becomes significant in determining the plasma's macroscopic dynamics. The laser's electromagnetic fields are so strong that quantum electrodynamics effects also become important. In this case the radiation reaction force no longer behaves deterministically, i.e. instead of knowing the electron's trajectory exactly as in the classical picture, we now can only know the probability that the electron has a given trajectory. In addition, the gamma-ray photons can be converted into electron-positron pairs, these pairs can emit further photons which emit more pairs and an avalanche of antimatter production can ensue with strong consequences for the behaviour of the plasma as a whole. The interplay of radiation reaction, QED effects and ultra-relativistic plasma processes will define the physics of laser-matter interactions in this new 'QED-plasma' regime, but is currently poorly understood. We will elucidate the basic theory of laser propagation and absorption in QED-plasmas. This will provide the foundational theory describing laser matter interactions moving beyond today's intensity frontier and into the foreseeable future. This theory will be underpinned by experiments measuring the rates of the important QED processes for the first time. The new theory will then be used to design the first experiments to generate a QED plasma in the laboratory. This project will culminate in the first generation of a QED-plasma, usually only seen in extreme astrophysical environments such as pulsar magnetospheres, in the laboratory.

The focus of this project is on the development of the new field of QED-plasma physics. Laser-plasma interactions in this new regime could act as novel sources of energetic particles & radiation. The novelty of these sources lies in their extremely high flux (100J-1kJ in a square micron with a duration of 10's of femtoseconds) and concomitant high brightness. The energetic electrons, positrons, ions and gamma-ray photons which are prodigiously produced could have many uses with substantial economic and societal importance. The gamma-ray photons could be useful for radiography of dense materials such as the cores of inertial confinement fusion plasmas or in border security (tuning the sources to nuclear excitations could enable the detection of nuclear materials). The ions could be used in 'hadron therapy' for cancer treatment, however the effect of the QED processes on laser driven ion-acceleration is poorly understood and will be elucidated in the work proposed here.

An important aspect of the development of laser technology over the past three decades has been the exponential increase in laser intensity driven by technological advances (the application of Chirped Pulse Amplification being the prime example) and the subsequent proliferation of high-power laser systems driven by this technology. With further innovations, such as diode-pumped lasers, we envisage multi-petawatt laser systems being commonplace in the next 10-15 years in mid-scale laboratories and perhaps in industry. Our development of the basic principles underpinning the interaction of these laser pulses with matter will be essential to the experimental programmes at these future facilities and the commercial impact they may have.

Another important consideration for impact is the training of staff. The project will involve substantial amounts of computer simulation on high performance computing resources. The PDRA and PhD student at York will be trained in efficient parallel programming techniques, large scale simulation and on how to mine large data sets. These skills are important in a knowledge-based economy such as the UK and will ensure that the staff make a positive contribution to the UK economy beyond the scope of this grant. In addition, the access to internationally leading laser facilities, secured as part of this grant, will provide the ideal training platform for the experimental PDRAs. Finally, the possibility of replicating extreme astrophysical environments, such as those around pulsars, in the laboratory will be of general interest to the public and therefore has a general cultural benefit.