Ion acceleration driven by ultra-short, ultra-intense pulses

Lead Research Organisation: Queen's University of Belfast
Department Name: Sch of Mathematics and Physics


The project aims to investigate an important process taking place during the interaction of very high power laser radiation with matter, namely the acceleration of ions to very high energies. This process, capable to reach over micormetric distances the energies that are otherwise obtained in large scale accelerators, has been made possible very recently due to the development of a new class of laser systems based on a principle called Chirped Pulse Amplification. The ions are accelerated by ultralarge electric fields set at thesurface of laser irradiated foils by escaping high energy electrons which have acquired energy directly from the laser. The energy of the ion scales with the intensity of the laser radiation used to accelerate them, and so called scaling laws relationships can be used to predict energies reachable in the future and to compare experimental results with relevant theory in order to test the understanding of the accelerating mechanisms.A novel laser facility, accessing unprecedented intensity regimes, called GEMINI will be opened up to experiments in the second half of 2007. This facility will achieve enormous fluxes of laser energies (larger than 10^21 W/cm^2) by concentrating light in ultrashort pulses, of duration 30 femtoseconds (1 fs =10^-15 s), in spots with radius of the order of a micrometer. Theoretical predictions suggest that at this intensities it will be possible to accelerate protons up to energies close to the ones required for groundbreaking applications such as cancer radiotherapy. This projects aims to exploit the unique opportunities offered by the GEMINI development to investigate ion acceleration in this novel ultraintense, ultrashort regime, and understand the details of the mechanisms taking place. We also aim to investigate some unexplored aspects of laser-matter interactions in the ultraintense regime by applying a technique (proton probing) developed by our group to diagnose the interaction of the GEMINI laser pulses with solid and gaseous targets.Another important aim of the project is to train a PhD student in this exciting area of research, and provide him/her with the skills necessary to continue a research career and become a skilled user of future scientific installations.
Description This project funded a PhD studentship for research closely linked to the larger consortium grant EP/E035728/1 (LIBRA). It therefore shares several of the objectives and outcomes of the LIBRA grant, although, for simplicity, we have listed here only those outcomes specifically related to the activities of the student (Prasad) funded under this grant.

The project has investigated the acceleration of ions employing high power lasers, with particular emphasis on the use of ultrashort (tens of femtoseconds, 1 femtosecond = 10^-15 seconds), high-repetition laser systems, such as the ASTRA GEMINI laser at the RCUK Rutherford Appleton Laboratory near Oxford, or the 100 TW system at the Max Born Institute (MBI) in Berlin.

On the ASTRA experiments, we have highlighted features relating to two main acceleration mechanisms :

1) Sheath Acceleration (SA), in which the large electric fields accelerating the ions are produced at the surfaces of a laser-irradiated foil by laser-energized high-energy electrons. This is a well known process, but we have extended its study to novel regimes of laser intensities and laser pulse durations.

2)Radiation Pressure Acceleration (RPA), in which ion acceleration results from the application of the enormous pressure carried by state-of-the-art laser pulses (10s of Gigabar, which is of the order of the pressure reached in the core of large planets). This is a regime mainly explored theoretically up to now, but currently emerging in experiments.

By irradiating with the GEMINI laser pulses ultrathin foils (down to 10 nanometer thickness), we have accelerated beams of proton and carbon ions with energies extending to 20 Megaelectron volts - these are the highest ion energies obtained so far with ultrashort laser pulses. At the same time we have demonstrated high conversion efficiencies from laser to ion energies (up to 10%).

For the thinnest targets (10-25 nm thick Carbon foils), we have highlighted the emergency of RPA, characterized by a sudden increase of the energy of Carbon ions and by the appearance of monoenergetic peaks of protons (from surface contaminants). From the comparison of experimental and computational results we can infer a large amount of information on the complex acceleration dynamics in this regime, which will assist greatly further development of the field.

Experiments at MBI have investigated the acceleration of ions from innovative water spray jets, containing nanoscale water droplets surrounded by low density water vapor. High intensity laser irradiation of these targets results in the acceleration not only of positive ions, but also of bright beams of high energy negative ions and neutrals, which is a totally novel observation in the field of laser ion acceleration, and add novel capabilities to the general area of ion sources. The experiments clarify that this takes place through a two-step process, consisting of positive ion acceleration via laser-initiated explosion of the nanodroplets, followed by charge-exchange processes involving the positive ions and taking place in the water vapour medium.
Exploitation Route there are potential applications of laser-driven ion beams in a number of non-academic contexts:

1) Healthcare. Laser-driven ions show potential for future use in cancer therapy and diagnosis. Ion beams from conventional accelerators are already used in this context, but the large size and cost of the installations is preventing their widespread use (e.g. use of ions for cancer therapy in the UK is severely limited at the moment, despite the proven clinical advantages of this approach in a number of cases). If laser acceleration could match in the future the requirements in terms of energy and stability, it could become a competitor technology with potential benefit in terms of cost and installation constraints.Also in the healthcare context, there have been world-wide interruptions in the production of medical isotopes in recent years. Intense proton sources such as now available from high energy lasers may provide an alternative route of production which could, in due course, be very compact and cost-effective.

2) Industrial applications. Laser-driven sources are highly versatile. By changing the irradiated sample, it is possible to change the accelerated species, and we have demonstrated that it is possible to accelerate both negative and positive ions, and even neutrals. This versatility, as the ability of accelerating easily ions to multi-MeV energies, may be of relevance to a range of manufacturing/industrial activities, such as ion lithography and ion implantation. Laser-ion sources offer also unique capabilities for testing the resilience of materials under intense ion bombardment, and for investigating the fundamental processes taking place during the ion interaction with substrates of relevance to manufacturing.

3) Energy. Application in this context mainly relates to use in Thermonuclear Fusion. Laser-driven ions have been proposed as the trigger to start fusion ignition in a compressed fusion fuel, in the so-called Fast Ignition approach, but they can also be used to diagnose the compression process, or to achieve information on several aspects of the laser-driven, Inertial Confinement Fusion approach. As mentioned earlier, there may be also a role to play for laser-driven ions in diagnosing plasmas inside the tokamaks used in magnetic confinement fusion (the approach brought forward by the ITER project) and for investigating the resilience of material to be used as vessels to contain the fuel of future fusion plants. Thanks to the ultralarge accelerating fields that can be obtained, techniques based on high power lasers can in future lead to a reduction in size and cost of large-scale accelerators, and therefore facilitate broad use of high energy accelerators in science, medicine and technology. In particular, there is scope for exploring the potential of laser-driven ion sources for medical applications such as cancer therapy and diagnosis. Proposed designs based on laser-driven sources may lead to advantages also in terms of reduction of shielding and ion beam transport requirements, reducing for example the size of the size of the gantries, enormous magnetic systems used to stir the beam around the patient.

Bright beams of multi-Mev ions may find application in industrial context, as sources for ion implantation and lithography. Thanks to the large numbers of ions produced in a laser irradiation, laser-driven sources can allow testing proton damage of semiconductors, simulating conditions encountered in satellites exposed to protons from solar events. Ion damage of materials at high flux is also of relevance to damage of materials used in future nuclear plants based on tyhermonuclear fusion.

In a fusion energy context, beams of high energy negative ions and neutrals are also of potential use for diagnosing conditions in the interior of the large vessels (tokamak) containing the fusion plasmas in the magnetic confinement approach.

Use of the ion beams in radiography is aready exploiting the unique properties of the beams (particularly ultrashort emission and excellent spatial emission quality) for obtaining novel information of the dynamics of large electric and magnetic fields associated to plasma phenomena - providing in this way data of great importance to activities aimed to achieve thermonuclear fusion, or to investigate phenomena of astrophysical relevance in the laboratory.

The work carried out within this project was mainly aimed to the development and characterization of the ion sources, which can in principle contribute to all the application areas listed above.
Sectors Energy,Healthcare,Manufacturing, including Industrial Biotechology

Description Ion acceleration driven by ultra-¬≠intense, high contrast laser pulses, invited talk by M.Borghesi at International Workshop on Laser-Matter Interaction 2012, Porquerolles (France), June 2012 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Scientific discussions

Year(s) Of Engagement Activity 2012