We propose a high voltage dc-dc converter for a CW (continuous wave) $CO_2$ laser system using a current resonant half-bridge inverter and a Cockcroft-Walton circuit. This high voltage power supply includes a 2-stage voltage multiplier driven by a regulated half-bridge series resonant inverter. The inverter drives a step-up transformer and the secondary transformer is applied to the voltage multiplier. It is highly efficient because of the reduced amount of switching losses by virtue of the current resonant half-bridge inverter, and also due to the small size, low parasitic capacitance in the transformer stage owing to the low number of winding turns of the step up secondary transformer combined with the Cockroft-Walton circuit. We obtained a maximum laser output power of 44 W and a maximum system efficiency of over 16%.