重离子储存环CSRe上O5+离子的激光冷却和精密激光谱学

Precision spectroscopy of highly charged ions (HCI) based on heavy ion accelerator is one of the most important research directions in the field of atomic physics. The precision spectroscopy of HCI can be used not only to test the standard model of physics, e.g. strong field quantum electrodynamics (QED), but also to provide key data for astrophysics and fusion plasma physics. The combination of lasers and heavy ion storage rings to carry out precision laser spectroscopy of HCI will provide a novel research platform. Laser cooling of highly charged heavy ion beams could reach a much lower momentum spread and much faster cooling speed as compared to the traditional cooling methods, and in addition, precision laser spectroscopy of relevant transitions in highly charged ions can be performed simultaneously during the laser cooling experiments. Hence, it will significantly improve the accuracy of the experimental spectroscopy and lead to the experimental breakthrough. We propose to perform two kinds of experiments at the heavy ion storage ring CSRe based on this project: (a) a cw laser with a wavelength of 220 nm will be used to cool the relativistic Li-like oxygen ion beam at a beam energy of 280 MeV/u at the CSRe, the longitudinal momentum spread of △p/p <1×10^-6 will be achieved, and the dynamics of this ultra-cold ion beams as well as the mechanism of phase transition will be investigated; (b) precision measurements of the 2s1/2-2p1/2 and 2s1/2-2p3/2 transitions of O^5+ ions based on the laser cooling experiments will be performed, the experimental accuracy will reach 2×10^-5. The combination of advanced lasers and heavy ion storage rings to perform laser cooling and carry out precision laser spectroscopy of HCI on this basis is a new method, and is directly relevant to laser cooling and precision laser spectroscopy of highly charged and relativistic heavy ions at the future large facility HIAF, in China.

基于重离子加速器的高电荷态离子精密谱学实验研究是当前原子物理的重要研究方向之一。高电荷态离子精密谱不仅可以用来检验基本物理模型，例如强场量子电动力学QED，而且能够为天体物理和聚变等离子体物理提供关键参数。激光结合重离子储存环为开展高电荷态离子精密测量实验提供了新机遇。相比重离子储存环上传统的离子束冷却方法，激光冷却可以获得极低温度（mK）离子束，从而减小离子束多普勒展宽造成的谱学实验误差，在激光冷却的同时开展精密激光谱学实验有望在实验精度和实验方法上获得突破。本项目拟在兰州重离子加速器冷却储存环CSRe上开展两方面研究：(a)使用波长为220nm激光对能量为280MeV/u的类锂O5+离子束实现激光冷却，获得△p/p<1×10^-6的超冷离子束，深入研究其动力学性质和相变物理机制；(b)在此基础上开展O5+离子的2s-2p能级跃迁精密测量，实验精度达到2×10^-5。

在本项目的资助下，我们利用一束220nm连续激光在兰州重离子加速器冷却储存环CSRe上实现了能量为275MeV/u类锂16O5+离子束的激光冷却实验，是目前激光冷却实验中离子电荷态最高，能量最高，跃迁波长最短的实验。实验得到离子束纵向动量展宽约为1×10-6的超冷离子束，通过扫描激光频率实现了大范围动量展宽离子束的激光冷却，利用连续激光结合电子冷却作用力实现了对连续束的激光冷却，同时观测到电子冷却和激光冷却效果，实验结果表明在电子冷却基础上开展激光冷却不仅能够获得动量展宽小一个量级的离子束，并且能够开展高精度激光精密谱学实验。该实验结果得到国际同行的强烈关注，收到ICPEAC2019会议和COOL19会议的大会邀请报告。为了全面理解激光冷却实验结果，我们发展了储存环单粒子追踪模拟方法，实现了重离子储存环上激光与连续束相互作用肖特基频谱模拟工作，发现激光与离子束相互作用范围极大地受到离子束发射度影响，完成了激光冷却压缩束肖特基频谱的模拟工作，首次发现压缩束的相干成分会对肖特基谱产生重要影响，能够解释激光冷却实验的观测结果，为下一步超冷离子束动力学和激光精密谱学实验奠定基础。此外，为了更好的实时探测激光冷却离子束动力学过程并开展精密激光谱学实验，我们设计、测试、安装了一套专门用于重离子储存环的新型前冲式极紫外荧光探测系统，该探测器系统具有单光子响应效率，其探测光子波长范围为50~100nm，并成功开展了在线测试实验，将用于CSRe上的激光冷却和精密激光谱学实验。本研究工作发展的实验方法将用于未来强流重离子加速器HIAF上的更高电荷态相对论能量重离子的激光冷却和精密激光谱学实验。