  We present the design and implementation of a new, modular gas target
suitable for high-order harmonic generation using high average power lasers. To
ensure thermal stability in this high heat load environment, we implement an
appropriate liquid cooling system. The system can be used in multiple-cell
configurations allowing to control the cell length and aperture size. The cell
design was optimized with heat and flow simulations for thermal
characteristics, vacuum compatibility and generation medium properties.
Finally, the cell system was experimentally validated by conducting high-order
harmonic generation measurements using the 100 kHz high average power HR-1
laser system at the Extreme Light Infrastructure Attosecond Light Pulse Source
(ELI ALPS) facility. Such a robust, versatile and stackable gas cell
arrangement can easily be adapted to different experimental geometries in both
table-top laboratory systems and user-oriented facilities, such as ELI ALPS.
