螺线管磁场约束下激光等离子体产生与传输的研究

Laser ion source (LIS) can produce pulsed beam with the highest intensity within microseconds compared to the other ones, especially refractory heavy metal elements. Ion beams produced by LIS may satisfy the demand of next generation accelerator facility and single turn injection of heavy ion cancer therapy facility. But the beams produced by LIS have bad quality and short pulse width, which will cause mismatch with later stage accelerator and low ion capture efficiency. Aiming at the contradiction of laser produced ions: the pulse width increases with plasma drift distance while current intensity decreases, DC and pulsed solenoids are planned to be employed in this program to confine the expansion of laser produced plasma in vacuum, and some key scientific problems need to be solved: the physical mechanism of the transport of laser produced plasma under solenoidal field confinement; the optimization of beam’s temporal structure and injection efficiency with direct plasma injection scheme; the improvement of beam quality and stability of laser produced ions in magnetic field. The accelerated C6+ ions with pulse width more than 2 μs and particle number 1E9-1E10 ppp may be achieved after pre-injector by this study, which will satisfy the single turn injection of heavy ion cancer therapy facility. This study's development will promote the miniaturization of heavy ion cancer therapy facility and build important research foundation for the High Intensity heavy ion Accelerator Facility (HIAF).

激光离子源是在微秒量级产生脉冲束流最强的离子源，尤其是难熔重金属元素离子，激光源产生的束流可能会满足新一代加速器装置以及重离子治癌装置单次单圈注入的需求。但是激光源产生的束流品质较差，脉宽较短，导致其与后级的加速器装置匹配较差，离子被俘获的效率较低。针对激光离子束脉宽随等离子体漂移距离增长而束流随漂移距离急剧衰减这一矛盾，本项目计划采用直流或脉冲型螺线管对激光等离子体在真空中的膨胀过程进行约束，重点解决几个科学问题：螺线管磁场约束下激光等离子体传输的物理机制；直接等离子体注入模式下束流时间结构和注入效率的优化；外加磁约束后激光离子束束流品质及稳定性的提升。希望通过本研究，加速1E9-1E10 ppp，脉宽大于2 μs的C6+离子，以满足重离子治癌装置单次单圈注入模式对预注入器的要求。本项目的开展将促进重离子治癌装置向小型化发展，同时也为未来先进重离子加速器装置HIAF奠定重要的研究基础。

基于直接等离子体注入方案的激光离子源可以应用于紧凑型的碳离子治疗装置，使用该方案，可以实现全剥离的碳离子束单次单圈注入同步加速器，显著减小整个装置的规模和成本。本项目主要研究内容围绕激光等离子体电流密度与脉宽随着漂移距离的变化关系，主要研究进展与结果总结如下：随着螺线管磁场强度的增加，C离子束的半高全宽增长了约9倍后趋于饱和；电荷量和峰值流强随着磁场增加而后趋于饱和；实验结果显示，螺线管磁场改变了激光等离子体的横向分布；离子束流强随漂移距离的衰减速度随螺线管磁场的提高而减缓；离子束脉宽与漂移距离的线性关系仍然成立，但增长系数增大；针对C靶做的稳定性测试显示，引入磁约束后对激光离子源的稳定性并没有影响；螺线管约束的引入，提高了高电荷态离子的产额；同时针对特定的漂移距离，可以通过优化磁场强度来优化特定电荷态的产额；在直接等离子体注入方案的漂移段引入螺线管约束，实现对注入离子束脉宽等注入条件的调节与优化。