The D.I.S electron beam ion source EBIS-C1 can operate at a pressure of up to 1×10^-7 mbar with the partial pressure of the desired particles being in the range of 5×10^-9 mbar to 5×10^-8 mbar for most optimized operating conditions. To allow for that partial pressure while remaining below the maximum pressure and reducing the disruptive effect of residual gas, a base pressure below 5×10^-9 mbar is advised.