As a pre-study for a 100 kW cluster injector planned for the application on nuclear fusion devices, an electrostatic high gradient accelerator for hydrogen cluster ions, limited in power to about 1 kW, has been set-up. It is operational since 1975. Thus far, acceleration voltages of up to 500 kV across a gap of 10 cm width have been achieved. Typical values of beam current and current density are 0.5 mA and 1 mA/cm2 resp