New CCD driving technique to suppress anomalous charge intrusion from outside the imaging area for soft x-ray imager of Xtend onboard XRISM

Soft X-ray Imager (SXI) is an X-ray CCD camera of Xtend onboard the XRISM satellite which was successfully launched on September 7, 2023 (JST). At ground cooling tests of CCDs in 2021, we experienced an unexpected issue in which anomalous charges occurred outside the imaging area of the CCDs, and intruded into the imaging area seriously affecting the imaging and spectroscopic performance. Through experiments with non-FM components, we identified a charge intrusion path and developed a new CCD driving technique to prevent charge intrusion and keep spectroscopic performance that satisfies the requirements, even if this issue occurs in orbit. In this paper, we report an overview of the anomalous charge issue, the development of the new CCD driving technique, the performance by it obtained in the SXI and satellite thermal vacuum tests, and the results of cause investigation experiments.