Processes for the simultaneous and selective growth of single walled and multiwalled carbon nanotubes in a single set of experiments are disclosed. The processes may include preparing a graphite electrode rod containing catalyst selected from Fe, Co, Ni, and a mixture thereof, acting as an anode. The process may include preparing another graphite electrode rod, each electrode having a distal and a proximal end. The process may include placing the above said two electrodes parallel to each other and their axis being substantially aligned in a chamber. The process may further include creating a DC-arc discharge inside the chamber by applying a DC-current voltage. The process may further include an cooling assembly having a cooling coil that surrounds the two electrodes. The cooling assembly may be used to maintain a temperature gradient that permits the depositing of single walled and multiwalled carbon nanotubes simultaneously in one experiment.