Development of an ECR Ion Source using liquid-helium-free superconducting solenoid coils

摘要

著者は，世界初の液体Heフリーの超伝導ソレノイドコイルを用いたECRイオン源の製作を行った。このシステムは，容易な操作性，少ない電力と冷却水の消費量，高いプラズマ閉じ込め磁場を同時に実現する画期的な物である。そして，この超伝導ソレノイドコイルによって実現できた高いプラズマ閉じ込め磁場と，長いプラズマチャンバー，プラズマチャンバー内壁上のアルミ酸化物の効果により，14.5 GHzを用いたECRイオン源としては，Xe35+で1μAという世界一の多価イオンビームを作り出せるイオン源となった。　加えて，バイアス電極法に関する実験により，バイアス電極法はイオン閉じ込め時間の最適化を行っていることを明らかにした。そして，バイアス電極法は，イオン源の最適化に非常に有効であることを明らかにした。　さらに，本イオン源の開発により，クラスターの液相から個相の相転移を伴う大クラスターの崩壌現象の観測が，世界で初めて可能になった。 Over the last several years, we have been constructing a facility for experiments to study the dynamic behavior of metal clusters in various processes, such as fission and multi-fragmentation. One of the main capabilities of this facility is the production of multiply ionized metal clusters through soft peripheral collisions with highly charged ions. The collisions occurs when the beam of metal clusters crosses a beam of highly charged ions. To produce the intense beam of highly charged ions for this experiment, we constructed an Electron Cyclotron Resonance Ion Source (ECRIS) using a liquid-He-free superconducting magnet. The new source uses Gifford-McMahon refrigerators for cooling the solenoid coils instead of liquid He. This feature allows us to avoid the higher costs in consuming liquid He, the complicated operation and bulkiness of ordinary superconducting magnets using liquid He. To implement this magnetic system efficiently, we determined the optimum configuration for the solenoid coils, the axial position of the yoke, in addition to the optimum operational parameters, such as the current in the coils. Also we determined the optimum shape of the hexapole magnet. A maximum axial magnetic field of 3 T and a radial magnetic field of 1 T on the plasma chamber wall are provided with this magnetic system. Microwaves of 14.5 GHz (maximum power of 2 KW) are used for heating the plasma. The new type ECRIS was named SHIVA (Superconducting without liquid He ECR Ion source for Various Atomic cluster experiments). In addition, experiments to determine the optimum conditions of the source have been preformed. SHIVA excelled in producing intense highly charged ion beams. We obtained a current of 5.7 μA of Xe32+, 3.8 μA of Xe33+ and 0.9 μA of Xe36+. These set world records for beam intensities of highly charged Xe ions extracted from an ECRIS using 14.5 GHz microwaves. In comparison with other high performance ECRISs using 14.5 GHz microwaves, we found that the long plasma chamber, the high magnetic field and the aluminium cylinder of SHIVA achieves longer ion confinement time and more intense beams of highly charged ions. The long plasma chamber, the high magnetic field and the aluminum cylinder on the wall of the plasma chamber are essentials in producing intense beams of highly charged ions.