Carbon nanotubes can be made by using an electric arc discharge between two graphite electrodes, suspended in an inert gas, like helium.
An inert gas is used, and oxygen is excluded, to prevent oxidation of the carbon atoms.
The carbon in the negative electrode (cathode) sublimes and carbon nanotubes are formed on the positive electrode (anode).
If the anode contains a metal catalyst, and a pure graphite cathode is used, single walled carbon nanotubes are formed.
Alternatively an electric arc can be formed between metal electrodes suspended in a hydrocarbon solvent. The solvent is decomposed and the carbon atoms form carbon nanotubes form on the anode.