Capture and Transport of Laser Accelerated Protons by Pulsed Magnetic Fields: Advancements Toward Laser-Based Proton Therapy
AuthorBurris-Mog, Trevor John
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The interaction of intense laser light (I > 10^18 W/cm2) with a thin target foil leads to the Target Normal Sheath Acceleration mechanism (TNSA). TNSA is responsible for the generation of high current, ultra-low emittance proton beams, which may allow for the development of a compact and cost effective proton therapy system for the treatment of cancer. Before this application can be realized, control is needed over the large divergence and the 100% kinetic energy spread that are characteristic of TNSA proton beams.The work presented here demonstrates control over the divergence and energy spread using strong magnetic fields generated by a pulse power solenoid. The solenoidal field results in a parallel proton beam with a kinetic energy spread of 10%. Assuming that next generation lasers will be able to operate at 10 Hz, the 10% spread in the kinetic energy along with the 23% capture efficiency of the solenoid yield enough protons per laser pulse to, for the first time, consider applications in Radiation Oncology.Current lasers can generate proton beams with kinetic energies up to 67.5 MeV, but for therapy applications, the proton kinetic energy must reach 250 MeV. Since the maximum kinetic energy Emax of the proton scales with laser light intensity as Emax proportional to I^0.5, next generation lasers may very well accelerate 250 MeV protons. As the kinetic energy of the protons is increased, the magnetic field strength of the solenoid will need to increase. The scaling of the magnetic field B with the kinetic energy of the protons follows B proportional to E^1/2. Therefor, the field strength of the solenoid presented in this work will need to be increased by a factor of 2.4 in order to accommodate 250 MeV protons. This scaling factor seems reasonable, even with present technology.This work not only demonstrates control over beam divergence and energy spread, it also allows for us to now perform feasibility studies to further research what a laser- based proton therapy system might look like. A theoretical beam transport system is presented at the end of this dissertation. It shows us that pulse power magnetic optics generating reasonable field strengths can transport a large bandwidth, high kinetic energy proton beam around and into a patient. This gives us insight into the spectrum available per laser pulse at the exit-port of the gantry as well as what types of dose deposition routines and spectral shaping techniques will need to be developed to contour a given dose to a given tumor volume.