In the past decade, the Thin Disk laser design was very successful as a high power laser design for cw lasers with good beam quality and high efficiency. Several numerical models show that the actually demonstrated output powers are far below the scaling limits due to amplified spontaneous emission (ASE). Also pulsed lasers based on Thin Disk amplifiers achieved comparable high average power, but only with medium pulse energies (about 100 mJ). Numerical models show that ASE effects limit the possible pulse energy to values of a few Joule, using the typical high power cw Thin Disk laser design. To evaluate designs for higher pulse energies a time resolved numerical model of pump absorption and ASE was developed and combined with a model of pulse amplification. The evaluation is focused on quasi-cw pumped pulse amplifiers. It includes the analysis of temperature, stress, deformation and thermal lensing, using finite element methods. This numerical model is used to develop thin disk designs for high output energies.