Development of trapping energetic ion beams by using rf ion guide

RIKEN加速器设施中的弹丸碎片分离器(RIPS)提供种类繁多的RI束流，对其化学性质或寿命限制较少。然而，束的能量和质量不足以用于低能束实验，特别是对于俘获实验。我们一直致力于通过使用大型气体捕集器和RF离子导引系统来减缓RIPS提供的高能RI-光束，从而实现捕获RI-离子的精确原子光谱分析。高能束(~GeV)首先通过降解器，然后在大型氦气室(~100Torr)中停止和加热。He的高电离势保证了大多数离子最终以单一电离态结束。随后，我们利用离子电荷作为把手，施加直流电场将RI-离子拉到位于出射孔处的阴极上，而阴极上的射频电场使它们远离电极表面。我们测试了两种类型的阴极电极，并根据它们的结构将它们命名为RF-漏斗和RF-地毯。在RIPS装置上进行了射频离子导系统的在线测试，该装置提供了典型强度为-106个原子/S的70 MeV/u Li-8离子束，当束流强度较弱时，目前为止获得了1%的总效率。实验结果表明，随着束流强度的增加，效率降低。通过在电池中使用冷阱来减少杂质，这一效应被部分补偿。

The projectile fragment separator (RIPS) at the RIKEN accelerator facility provides a wide variety of RI-beams with less restrictions on their chemical property or lifetime limit. The beam energy and quality is, however, not adequate for low-energy beam experiments, in particular for trapping experiments. We have been working on technical developments toward precision atomic spectroscopy of trapped RI-ions by slowing down the energetic RI-beams provided by the RIPS using a large gas-catcher cell and an rf ion guide system. The high energy beam (〜GeV) is first passed through a degrader and then stopped and thermalized in a large He gas cell (〜100 Torr). The high ionization potential of He guaranties that most ions end up in the singly ionized state. Subsequently, using the ion charge as a handle, we apply dc electric fields to pull the RI-ions to a cathode electrode located at an exit hole, while rf-fields at the cathode electrode keep them away from the surface of the electrode. We tested two types of cathode electrodes and named them rf-funnel and rf-carpet depend on their structure. On-line tests of the rf ion-guide system have been performed at the RIPS facility which provides a 70 MeV/u Li-8 ion beam with a typical intensity of -10^6 atoms/s. An overall efficiency of 1% was obtained so far, when the beam intensity was weak. A decrease of the efficiency was observed as increasing the beam intensity. This effect was partially compensated by decreasing the impurities by employing a cold trap in the cell.