Multi-kW beams with high brightness offer advantages in material processing applications with large distance beam propagation such as remote welding. To achieve the combination of high power and low beam parameter product, the thin disk laser concept is widely used due to its power scalability. Nevertheless, the efficient generation of several kilowatts of output power per disk at beam parameter products below ~3 mmÂ·mrad is limited by an aspherical wavefront distortion in the disk and air turbulences in front of it. In the present paper the limiting factors are discussed and a novel method for compensation is presented. The compensating mirror consists of a silica substrate with a top-hat-shaped layer of 100 nm height to generate the desired phase-front correction and a conventional HR-coating on top. To prevent air convection in front of the thin disk crystal, the laser resonator was filled with helium. The experimental results yield a maximum output power of 3.4 kW and an optical efficiency of 49 % with a beam parameter product of ~2.6 mm•mrad (M2 ~ 8) at a cooling water temperature of 30 °C.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.