Study on the Divergence of High-Current Ion Beams due to Nonlinear Space Charge Force

在1999财政年度，进行了用于产生低发射度光束的KrF准分子激光器的研发和激光离子源的运行测试。其结果是，我们得到的束流比用Nd：YAG激光器得到的束流高得多。MCP检测器与CCD相机耦合的应用使时间分辨发射度测量成为可能。通过测量，我们发现在一个脉冲过程中，源的束流发射度和轮廓会发生很大的变化。然而，即使用短波长的准分子激光，我们也不能获得低温等离子体所期望的较低发射率。从这个结果中，我们发现，光束发射度取决于许多种复杂的现象，如流体动力学运动的等离子体以及激光的穿透深度。光束传输实验的研究由于非线性空间电荷力的发射度增长开始在2000财政年度。然而，光束的发散是如此之强，以至于几乎不可能有效地将光束注入静电四极通道。为了改善这种情况，我们已经测试了一个光束提取系统与球形二极管结构。然而，由于过聚焦效应，光束发散仍然很强，在这些实验研究的同时，我们开发了一个粒子计算机程序，它描述了非线性空间电荷力下每个粒子的运动。利用这个程序，我们研究了具有均匀密度分布和非均匀密度分布的光束的行为。对于非均匀束流，不仅可以观察到大的发射度增长，而且可以观察到强度分布的显著均匀化。从这些结果中，我们数值验证了所观察到的发射度增长可以归因于释放的非线性空间电荷场能量的非均匀束。

In fiscal year 1999, R＆D on the KrF excimer laser for the production of low-emittance beams and the operation test of the laser ion source were performend. As a result we obtained beam currents which were much higher than those obtained with a Nd : YAG laser. Application of an MCP detector coupled with a CCD camera enabled time-resolved emittance measurement. By this measurement we found that the emittance and profile of the beam form the source can change dramatically during a beam pulse. However, even by the excimer laser with a short wave length, we could not obtain the lower emittance expected for low-temperature plasmas. From this result, we have found that the beam emittance depends on many kinds of complicated phenomena such as hydrodynamic motion of the plasma as well as the penetration depth of the laser.Beam transport experiments for the investigation of emittance growth due to non-linear space-charge force started in fiscal year 2000. However, the divergence of the beam was so strong that it was almost impossible to inject the beam efficiently to the electrostatic quadrupole channtel. To improve this situation, we have tested a beam extraction system with a spherical diode structure. Nevertheless the beam divergence was still strong, owing to theoverfocusing effect.In parallel with these experimental investigations, we have developed a particle computer code which describes the motion of each particles under the nonlinear space-charge force. Using this computer program we investigated the behaviors of beams with a homogeneous density distribution and an inhomogeneous one. Not only large emittance growth, but also remarkable homogenization of the intensity distribution was successfully observed for the inhomogeneous beam. From these results we numerically verified that the observed emittance growth could be attributed to the release of the nonlinear space-charge field energy contained in the inhomogeneous beam.