High-power laser facilities can be mostly divided into two classes of systems[1], with (i) energetic/low-repetition-rate systems on one hand for facilities, such as the National Ignition Facility (NIF)[2] or Laser Megajoule (LMJ)[3], and (ii) low-energy/high-power/high-repetition-rate systems on the other hand[4–6]. However, the past decade has been marked by numerous efforts to populate the intermediate 100 J to kJ energy/high-power/high-repetition-rate class. L3-HAPLS[7], DIPOLE[8] and L4-ATON[7] are the first laser facilities in this latter category. These laser systems were developed to provide new directions for high-energy laser–matter interaction experiments due to the significant increase in experimental data generated; however, a renewed interest has recently been driven by NIF fusion shots[9–11] that underscore the need for high-energy/high-repetition-rate laser facilities that could open the route towards inertial fusion energy (IFE). Heat management is at the core of these energetic recurrent systems, particularly during the amplification of the laser beams. Different thermal management technologies have been investigated, including cryogenic cooling[8], high-speed gas-flow[12] and liquid cooling[13,14]. Liquid cooling offers a relatively simple and cost-effective solution for heat extraction[15], but several difficulties have to be addressed. The coolant must be transparent at both pump and emission wavelengths, have a low absorption to reduce losses, a weak nonlinear index of refraction and ideally have a broad compatibility with materials, including the amplifier medium, as well as presenting a low hazard to facilitate implementation[16]. From an optical point of view, liquid cooling channels need to induce small optical aberrations from large period (power, astigmatism, …) down to mid and small spatial millimeter-scale periods[17,18]. Liquid-cooled amplifiers are currently used in facilities as pump laser or main beam amplifiers[7,19,20], which has motivated developments for improving their performances. In particular, (i) a thermo-hydraulic-mechanical-optical model was developed to provide a complete and multi-physics model of these amplifiers[21]. During the first step, the gain and heat generated by optical pumping are calculated using a combination of a phenomenological lamp model[22], heat transport and calculation of the population of the different atomic levels. In the second step, the spatial distribution of the heat is used in a COMSOL software model that includes the computer-aided design of the amplifier cell and the description of the coolant flow to compute by ray tracing the laser wavefront deformation induced by thermo-mechanical-hydraulic effects[21]. (ii) A liquid-cooled amplifier test-bed was designed and built to characterize mid- to large-scale spatial frequency distortions in amplified wavefronts and compare these with model predictions[23]. In addition, knife-edge Foucault measurements were also performed in a single-pass configuration to investigate (0.1–10 mm) small- to mid-scale spatial frequency distortions, in particular those induced by liquid flow[24]. Here we report on the amplified optical wavefront performances in the mid- to large-spatial-scale range (1–100 mm) of a neodymium phosphate liquid-cooled amplifier cell pumped by flash-lamps built as a test-bed for liquid-cooled amplification. The amplifier cell was qualified at different repetition rates from 1 shot per few minutes to 1 shot per minute. Emphasis is herein placed on assessment of the mid-spatial-scale (1–10 mm) distortions in amplified wavefronts commonly observed in fluid-cooled amplifiers[25,26]. Such wavefront defects in large-aperture multi-slab laser systems are likely to degrade the focal point quality and, in the worst case, damage optical materials due to Kerr effects and/or amplitude modulation during laser beam propagation[17,27].