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

1999财政年度进行了KrF准分子激光器的研制，用于生产低发射度光束，并进行了激光离子源的运行试验。结果，我们获得的光束电流远高于Nd: YAG激光器获得的光束电流。MCP探测器与CCD相机耦合的应用，使时间分辨发射度测量成为可能。通过这一测量，我们发现在一个光束脉冲期间，从源形成的光束的发射度和轮廓会发生巨大的变化。然而，即使使用短波长的准分子激光，我们也无法获得低温等离子体所期望的较低发射度。从这一结果我们发现，光束发射度与等离子体的流体动力运动和激光穿透深度等多种复杂现象有关。研究非线性空间电荷力引起发射度增长的光束输运实验始于2000年。然而，由于光束的发散性很强，几乎不可能将光束有效地注入静电四极通道。为了改善这种情况，我们测试了一种具有球形二极管结构的光束提取系统。然而，由于过度聚焦效应，光束发散仍然很强。与这些实验研究并行，我们开发了一个粒子计算机代码，描述了每个粒子在非线性空间电荷力下的运动。利用该程序研究了均匀密度分布和非均匀密度分布的梁的性能。对于非均匀光束，不仅发射度有较大的增长，而且强度分布也有明显的均匀化。从这些结果中，我们通过数值验证了观测到的发射度增长可以归因于非均匀光束中包含的非线性空间电荷场能量的释放。

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.