High-energy nanosecond regenerative amplifiers are essential front-end drivers for large-scale laser facilities. However, in conventional stable resonators, the inherently limited mode volume of Gaussian beams restricts energy scaling beyond the hundred-millijoule level. Small beam waists lead to premature optical damage and severe nonlinear phase accumulation (B-integral). While expanding the fundamental mode area is a desirable solution, in Gaussian-mode resonators the presence of thermal lensing fundamentally constrains the achievable mode radius, and strong competition from higher-order transverse modes degrades the beam quality.

In this work, we propose and numerically investigate a large-mode-area regenerative amplifier design that incorporates an intracavity diffractive phase mirror to enforce a customized flat-top super-Gaussian eigenmode. By exploiting the principle of phase-conjugated self-reproduction, a 7 mm × 7 mm square super-Gaussian (8th-order) mode is established as the fundamental mode of the resonator. This approach not only effectively compensates for the influence of thermal lensing but also provides strong mode-discrimination capability, thereby preserving excellent wavefront quality while operating with a significantly expanded mode volume.

Comprehensive numerical simulations based on the Fox–Li iterative algorithm are performed to evaluate the robustness of this scheme against practical engineering imperfections. A systematic tolerance analysis indicates that the resonator maintains acceptable beam quality—characterized by an intensity non-uniformity below 5% and phase errors well within a small fraction of a wavelength—under the combined influence of representative fabrication errors, alignment inaccuracies, and thermal perturbations. Key findings include that the mode profile remains robust against typical thermal focal length fluctuations, and that the dominant sensitivity resides in angular misalignment of the diffractive optic, while moderate surface figure errors and lateral displacements introduce only gradual degradation. Across all investigated scenarios, the customized super-Gaussian mode consistently exhibits a significantly reduced peak fluence compared to an equivalent Gaussian mode carrying the same energy, thereby greatly extending the damage-threshold margin.

The simulation framework and tolerance guidelines established in this work provide a solid foundation for the ongoing experimental verification of DOE-based large-mode-volume regenerative amplification, paving the way toward robust, compact hundred-millijoule front-end systems with high spatial beam quality.