A Thulium fiber laser pumped or InP diode laser stack pumped Cr:ZnSe thin disk cw multimode laser at 2.4 μm with an output power of 5 and 4 W, respectively, and with optical-tooptical efficiencies of 10% will be presented. An experimentally verified and numerically simulated thermal lensing induced and cyclic instability in the laser system will be shown. As a consequence, in order to prevent the lasing conditions in the resonator to be unstable, power scaling of a Cr:ZnSe thin disk laser is possible by enlarging the pump spot and reducing thereby the thermal lensing condition. Therefore, the instability is not initiated. As a conclusion, the investigated instability will show up in any laser active material which has a strong absorption of the pump beam, for instance in transition metal ion laser material systems in connection with any laser concept, like for instance in thin disk, bulk or slab designs.