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

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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.