Design of a low-energy-spread 12 MeV S-band thermionic electron linac based on longitudinal phase-space shaping

Compact high-energy electron linacs for industrial non-destructive testing and computed tomography require sub-millimetre focal spots to achieve high spatial resolution. In the present 12 MeV system, the chromatic sensitivity of the downstream quadrupole triplet motivates a stringent exit-energy-spread target. We present an acceptance-guided longitudinal phase-space-shaping strategy for a thermionic S-band standing-wave linac. Output-conditioned back-mapping reconstructs a V-shaped longitudinal response at the entrance of the nineteen-cell β = 1 section, which is used as a physics-based matching reference for the bunching-section design. The selected axisymmetric design produces a 12.4 MeV beam with a capture efficiency of 39.1% and establishes the non-uniform target axial-field profile for the three-dimensional RF structure. PARMELA tracking using the CST electromagnetic field representation and independent CST particle-in-cell (PIC) calculations of the same complete three-dimensional structure predict dominant-peak relative FWHM energy spreads of approximately 0.48%, within the adopted 0.5% beam-quality target. Independent commissioning measurements give horizontal and vertical optical transition radiation (OTR) focal-spot FWHM values of 0.26 mm and 0.36 mm, respectively.