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Shock Wave Acceleration of Monoenergetic Protons using a Multi-Terawatt CO2 Laser


Compact and affordable ion accelerators based on laser-produced plasmas have potential applications in many fields of science and medicine. However, the requirement of producing focusable, narrow-energy-spread, energetic beams has proved to be challenging. In this thesis, an experimental demonstration of the generation of monoenergetic ions via collisionless electrostatic shock waves driven in a laser produced plasma is presented.

The physical processes behind shock wave formation, propagation, and their potential to produce monoenergetic ion beams is explored using 1D OSIRIS simulations. It is observed that a shock wave can be formed from the interpenetration of two plasmas via the expansion from a density discontinuity or an initial relative drift velocity. These processes can be instigated in the laser driven case where the laser can produce a moving sharp density spike and strong electron heating as it bores a hole through an overdense plasma. Under appropriate conditions, an electrostatic shock is formed that detaches from he laser and propagates in the plasma even after the laser is turned off. This shock can then overtake and reflect ions from the upstream plasma and accelerate them to yield a relatively narrow energy spread.

A multi-terawatt CO2 laser system was developed at the UCLA Neptune Laboratory for exploring laser-driven shock wave acceleration of ions. The theory behind CO2 amplification of picosecond pulses where the bandwidth is provided by the pressure of the CO2 gas and the field of the laser pulse itself is developed. These ideas are applied for the production of 3 ps CO2 laser pulses with peak powers of up to 15 TW, currently the most powerful CO2 laser pulses ever produced.

These laser pulses were used for laser-driven shock wave acceleration of protons in a hydrogen gas jet where the peak plasma density is in the range of 3-5 ncr, where ncr is the critical plasma density for 10 μm radiation. Interferometry using a picosecond 532 nm laser pulse showed that the plasma has a dynamically formed profile with a sharp (10λ) rise to overcritical densities and a long exponential fall (30&\lambda;). Protons accelerated from this interaction reach energies of 22 MeV, are contained within a narrow energy spread of ~1%, and have geometrical emittances as low as a mm mrad. 2D OSIRIS simulations show that the laser-driven shock overtakes and reflects the protons in the slowly expanding hydrogen plasma resulting in a narrow energy spectrum. Scaling of this mechanism to higher laser powers through simulations predict the production of ~200 MeV protons needed for radiotherapy by using current laser technology.

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