强流激光源高电荷态离子的产生优化以及与射频四极加速器的匹配研究

This project will aim at optimizing the production of high charge state heavy ion beams from high intensity laser ion sources and the matching of laser ion sources to radio-frequency quadrupole (RFQ) accelerators, so as to improve the efficiency of the laser ion source and provide high quality intense short pulsed heavy ion beams for the downstream accelerator facility. The whole sesearch consists of five parts: (1) based on the systematic experimental study on the influence of the laser ion source paramenters, like the laser energy, pulse duration, incidence angle, focusing spot pattern and target conditions, on the laser plasma properties, like the charge state distribution, beam current intensity, ion plulse duration, expanding angle and energy spread of ions, and the comparison with the hydrodynamic model of expanding plasma, to optimize the production of the desired ion species; (2) using magnetic confinement (solenoid or permenant magnet ring) to decrease the expanding angle of the laser plasma and to prolong the ion pulse duration; (3) to design the extraction system of multi-electrode suitable for ion beam extraction from nonstationary plasma flux; (4) to optimize the laser beam transport line and the incidence conditions so as to improve the long-term stability and repeatability of laser ion sources; (5) based on the measurement results of the ion energy spread, to design the corresponding modulated extraction voltage to decease the ion energy spread, in which way the requirement of RFQ for the energy speread of the injected ion beam could be meet.

本项目旨在通过研究强流激光离子源高电荷态离子束的优化产生以及与射频四极加速器的匹配问题，达到提高激光离子源运行效率并为后级加速器提供高品质的强流短脉冲高电荷态离子束的目的。具体的研究内容及思路如下：（1）系统测量激光离子源的各项参数（激光能量，脉宽，入射角，焦斑，靶材性能等）对所产生的等离子状态（电荷态分布，束流强度，脉宽，发散角，能散）的影响，利用流体动力学模型模拟等离子体膨胀过程，对照实验测量结果，进而优化所需电荷态离子束；（2）利用磁约束（螺线管或永磁环）手段，压缩等离子体膨胀的发散角并适当延长离子束脉宽；（3）设计适合强流脉冲束的多电极引出系统；（4）通过优化激光光路、入射条件等，提高激光离子源运行的长期稳定性和重复性；（5）结合（1）中得到的关于激光源产生离子束的能散测量结果，设计相应的调制引出电压，降低引出束流的能散，以匹配射频四极加速器对注入束流能散的要求。

本项目的研究内容为利用激光离子源产生强流高电荷态重离子束以及激光离子源与射频四极（Radio-Frequency Quadrupole- RFQ）加速器的匹配问题。具体研究内容及其结果如下：系统研究了激光离子源的各项参数对激光等离子体加热以及高电荷态离子产额的影响并对其进行全面优化，产生了一批强流高电荷态重离子束：其中较轻的高电荷态离子（质量数小于50）流强达到10^10 ppp，较重的高电荷态离子（质量数介于50到120之间）达到10^9 ppp；利用螺线管约束激光等离子体，有效压缩了激光等离子体的发散角，在不影响高电荷态离子相对产额的前提下，显著提高离子束流强和脉宽；研究了影响激光离子源重复性的主要因素并对其进行优化，得到脉冲重复性分别好于5%和2%的碳束和镍束；开展利用纳秒量级双激光脉冲打靶产生高电荷态离子束研究，在保证高电荷态离子产额不变的前提下，降低了离子束总束流峰值流强并延长其脉宽；基于直接等离子体注入模式（Direct Plasma Injection Scheme- DPIS），利用RFQ成功加速激光离子源产生的C6+离子束至7.12 MeV，峰值流强达到14 emA；针对这种注入模式发展了一种时间分段的模拟方法，在此基础上对DPIS过程进行优化，最终使被加速的C6+离子束流强増至4×10^10 ppp（particle per pulse-粒子每脉冲），或者使加速后C6+离子束脉宽延长一倍。这一系列成果对于研发满足未来大型重离子加速器和紧凑型重离子治癌装置的预注入器具有重要意义。