Modifying x-ray streak cameras for operation on igniting fusion experiments

The National Ignition Facility produced the first nuclear fusion experiment demonstrating net positive energy gain on December 5, 2022. The x-ray streak camera that measures the bang time and burn-width from this landmark experiment had an electronic failure and did not record data. The CCD sensor was replaced with a radiation hardened CMOS sensor that has since demonstrated successful operation on repeat ignition shots. Concurrently, an instrument artifact was identified that occurs when the signal consists primarily of energetic x rays >15 keV (common on burning plasma experiments). This artifact, which appears as a background pedestal, arises from the x-ray back-fluorescence generated by the solid metal accelerating mesh behind the photocathode in the streak tube. We have mitigated this background signal by limiting the sensitive area of the photocathode. Herein, the details of the modifications and the results are presented.