Using Particle-in-Cell Simulations to Understand Space Charge Broadening in Pulse Dilation Drift Tubes

Gated X-ray framing cameras at the National Ignition Facility (NIF) are used to image the temporal evolution of X-ray emission from high energy dense plasmas in inertial confinement fusion (ICF) experiments. To resolve the < 100 ps duration of X-ray emission during the peak fusion burn, pulse dilation is applied to temporally magnify this time history [1]. When pinhole focused X-ray’s strike the framing camera photocathode, a ramped voltage in time is applied that creates a velocity gradient in the electron packet along the emission axis. The electrons then expand as they travel down a zero electric field drift region and are then intercepted by the back end of the framing camera detector. The expansion of the electron packet is primarily due to the applied pulse dilation; however, some contributions originate from interactions between the electrons themselves [2] and possibly interactions between the electrons and the drift tube boundary. Here we present 3D particle-in-cell simulation results of the single line of sight (SLOS) diagnostic [3] drift tube to determine the predicted contributions of electron packet expansion due to space charge broadening, edge currents, and electromagnetic wave modes.



[1] S. R. Nagel et al. Rev. Sci. Instrum. 87, 11E311 (2016)

[2] C. Trosseille et al. Rev. Sci. Instrum. 93, 023505 (2022)

[3] K. Engelhorn et al. Rev. Sci. Instrum. 89, 10G123 (2018)