Ultrafast laser-driven ion interactions in matter: Evolving dose distribution at the nanoscale and nonlinear response

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In physics, scaling laws provide a dual function. First, they can reveal the underlying physical mechanisms that govern a system by establishing how the system responds to changes or perturbations. This is particularly true of nonlinear scaling laws where small changes in an input perturbation can lead to dramatic changes in the response of the system. Secondly scaling laws provide researchers with a tool that they can use to predict how a system will evolve for a given set of input parameters. This is a crucial step towards providing highly-targeted, cutting-edge applications. It is within this framework that we propose to study the ultrafast dynamics that result from ion interactions in matter to determine how the characteristic response of the medium scales with the incident ion flux. To study any ultrafast process directly it is critical that the perturbation causing the system to change is significantly shorter than the natural recovery time of the system. If the perturbation is significantly longer that this recovery there will be repeated cycles of excitation and relaxation within a single interaction. This inhibits the ability to extract fundamental information about the system without complicated approximations and assumptions. Unfortunately, to date, this has been the overriding problem for the study of ion interactions in matter. The ion pulses that have been available from large accelerator facilities have been 100's of picoseconds in duration which is significantly longer than the femtosecond and few picosecond characteristic recovery times of matter in response to irradiation. Accordingly, existing experimental results relating to the earliest accessible stages of ion matter interactions have prohibitively large associated uncertainties. Our approach overcomes this issue by generating ultrafast pulses of ions using laser driven ion accelerators. This performance will allow the stopping of energetic ions (> 1 MeV/nucleon) in matter to be studied on femtosecond and picosecond timescales. We will use this capability to understand how the resulting pathways to equilibrium can be controlled by varying the incident flux of ions and investigate the new possibilities this offers for advanced applications in both radiation chemistry and hadrontherapy. The Centre for Plasma Physics in Queen's University Belfast is currently constructing the world's highest energy few-optical-cycle laser system, TARANIS-X, due to come online in late 2016. This unique environment will allow us to generate the shortest pulses of ions produced in the laboratory to date. With this state of the art facility it will be possible to test, in real time, the fundamental limits of ion interactions in matter. Understanding this behaviour is a key goal of this research. In particular extending these experiments to ion interactions in water will allow us to investigate the potential for new modalities of dose delivery during hadron (or ion beam) therapy. This is because water makes up over >70% of human cells and so it makes for an ideal system in which to study the effects of ionising radiation in the human body. Finally, one of the key motivators for this proposal is the indication of nonlinear response with respect to ion flux in low temporal resolution experiments performed to support the scientific case for this work. Together with our international partners in Germany (Munich) and the U.S. (Texas) we will investigate multiple different interaction regimes to determine the scaling of this nonlinear response and, in partnership with the GEANT4 DNA collaboration, we will develop numerical approaches to form a clear understanding of the scaling law (or laws) that governs it.