Effects of Laser Pulse Shape on Operation of the Phase-Stable Proton Accelerator

• Main Authors: Hsiang-chien Liao, 廖祥鑑
• Other Authors: S. H. Chen
• Format: Others
• Language: zh-TW
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/60776764257004473272

碩士 === 國立中央大學 === 物理研究所 === 98 === 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.