| Summary: | 碩士 === 國立中央大學 === 物理研究所 === 98 === The laser-accelerated proton beam has attracted particular interest due to its compactness and low cost as compared to the traditional proton beam accelerators. X. Q. Yan et al. proposed the phase-stable acceleration (PSA) mechanism for producing the mono-energetic proton beam with excellent energy scalability by varying the laser intensity and pulse duration. The stable electrostatic field formed by the light pressure accelerates the proton beam with high collimation. The phase-stable acceleration has become the most promising mechanism for the development of the new generation proton accelerator.
In the study, we applied the particle-in-cell (PIC) code on the examination of operational criteria for the phase-stable acceleration proposed by Yan et al. We found that the phase-stable acceleration fails when the plasma density is lower than a certain value, no matter how thick the target is. The density threshold is an essential criterion for the phase stable acceleration. The physical reason for failure of the phase-stable acceleration is the continuous erosion of the surface electrons by the laser pulse at the plasma density lower than the critical value. According to the energy conservation law, we derived the theory to predict the density threshold for different laser parameters. The theoretical results and the simulation result agree very well.
In corporation with parametric studies by varying the laser intensity, plasma density, and target thickness, we explore the marginal conditions for the production of mono-energetic proton beams. The optimal operational range for the phase stable proton accelerators obtained by the theoretical criterion and the parametric studies provide a good reference for the design of a novel proton accelerator.
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