Laser plasma protons and applications in cancer therapy and proton radiography

Recent developments in high power, ultrashort pulse laser systems enable laser intensities beyond 10^21 W/cm^2 to be achieved. When focused onto thin foil targets, plasmas with extremely high electrostatic fields (>10^12V/m) are produced, resulting in the acceleration of protons/ions to very high energies (~60MeV). During my PhD, I have worked on experimental investigations into proton acceleration driven by high power laser pulses. Key to successful deployment of laser proton sources one one side is getting higher proton energies through to achieve the ultimate goal of realising table top machines for the treatment of cancer and on the other side, optimising the beam quality, an objective that was of the main motivation for my PhD work. My two main achievements were: 1. The production of bright, ultrashort and radially smooth pulsed proton beams using laser heating of pre-plasmas formed with long (nanosecond) pulses with ultrahigh intensity picosecond pulses. 2. Use of these beams to study the ultrafast dynamics of target implosion under intense laser irradiation The experiments on proton acceleration with the specific goal of controlling the proton beam quality by optical tool design, were performed at RAL. This scheme involves the use of multiple laser pulses to enhance and control the properties of beams of protons accelerated in ultra-intense laser irradiation of planar foil targets. Specifically, one laser pulse produces and controls the expansion of the target to enhance the energy coupling to the main (delayed) laser and/or drives shock deformation of the target to change the direction of the proton beam. The preplasma formed by this low intensity nanosecond beam (~ 0.5-5x10^12 W/cm^2) was used to enhance the laser absorption of the main (delayed) CPA (Chirped pulse amplified). The main CPA picosecond beam was used at high intensity (~ 4x 10^20 W /cm^2) to produce intense proton beams from the hydrogen rich target. The optimum intensity of the nanosecond beam was investigated and optimised to yield a very smooth and circular distribution of the proton beam achieved using a second long pulse laser at 5x10^12w/cm^2. The second achievement concerns an experiment also performed at RAL on proton radiography. As the laser based protons are characterised by small source size, high degree of collimation and short duration, they can be used in point projection backlighting schemes to perform radiography. In particular, I used this idea to perform radiography of a cylindrical target ~ 200µm long imploding under irradiation by long laser pulses of nanosecond duration. This allows measuring the degree of compression of the target as well as the stagnation time in the dynamic regime. The experiment took place in the framework of the HiPER project (the European High Power laser Energy Research facility Project). The final goal of the experiment was to study the transport of fast electron in cylindrical compressed target a subject of interest for fast ignition. In parallel to proton radiography x-ray radiography was used to compare the results. One of the specific advantages of using laser generated protons is that their spectrum is continuous upto a high energy cutoff. Because of their different time of flights protons proved to be very effective in revealing the implosion history of the target. In principle, the obtained implosion can be followed in time with a single shot sensitivity. Instead x-ray radiograph gives one image per laser shot at one fixed time and one has to make several shots in order to reveal the complete history of implosion. Another advantage of using proton radiography is a simpler experimental setup keeping imploding cylinder between proton target and proton detector on the same axis. Simulations of formation of proton images were made with the Monte Carlo MCNPX Code using the density profiles of the imploded cylinder obtained with the 2D-hydro CHIC code. A detailed study of Multiple Coulomb Scattering and Stopping Powers of the protons in low energy regimes for cold and warm matter was done to interpret the experimental results. Finally, I’m taking part in the analysis of experimental results obtained at the University of Rochester (USA) on the Omega-EP laser, and concerning magnetic field effect on the proton radiographs of a wired cone.