使用高強度雷射與電漿作用產生高能( )質子束是近年來雷射電漿交互作用研究中的一項重要成果。使用圓偏振雷射與電漿作用後，電子被光壓推擠到電漿後端，並且在空間上產生電荷分離的電場。在電子層內的質子經由這個電場加速並形成具有近似單一能量的質子束，我們稱這個機制為Phase Stable Acceleration (PSA)。至今關於PSA機制的討論皆集中於使用方波雷射波形的探討，而也有學者歸納了PSA操作範圍的條件，為電漿密度及電漿厚度的乘積與雷射強度成線性關係。本研究群最近發現在使用方波雷射脈衝來產生單一能量的質子束，除了現有的PSA操作條件外，電漿密度還須大於雷射強度對應的門檻電漿密度。本論文使用PIC模擬研究不同雷射波形對此質子加速機制的影響。發現經由調整雷射波形可以使質子束品質更好，令其能量更集中。另一方面，調整雷射波形亦可增加原先PSA操作條件(使用方波雷射脈衝)的操作範圍。 Generation of high energy ion beam by laser irradiation on a solid thin foil target is an area of great interest in the research of laser plasma interaction. Several acceleration schemes have been considered in recent years. According to particle simulation studies, the most promising one is the so called phase-stable acceleration (PSA) scheme which proposes to use a circularly-polarized laser pulse interaction with an ultrathin target. The light pressure pushes the electrons forward, leaving the ions behind, until it is balanced out by the space charge electrostatic field generated in the process. For an optimal target thickness the electron sheath formed at the rear surface of the target can trap the ions and synchronously accelerated by the laser pulse leading to a monoenergetic ion beam production. Most parametric studies of the PSA scheme have been using the square laser pulse. From these studies, the operational regime of the PSA can be characterized by a linear relation between the normalized laser amplitude and the product of the plasma density and the target thickness. However, a recent study in our research group found that in addition to the existing PSA operation criterion, the target density must be larger than a threshold density for a given laser intensity in order to generate a monoenergetic ion beam. This thesis focuses on the effects of laser pulse profile on the operation of PSA scheme. The laser pulse of a trapezoidal profile (linear growth-plateau-linear decrease) in time, which somewhat simulates the laser pulse used in experiments is considered. In the parameter regime which satisfies the PSA criterion, our simulation results show an improvement in the ion beam quality with an increasing laser rising time. At the same time, the PSA operational regime can also be expanded using the shaped laser pulse to the cases that the plasma density is below the threshold density recently found.
