As the ions in the plasma are charged (the plasma is so hot all the negatively-charged electrons are stripped off the atoms, leaving them with a positive charge) they respond to magnetic fields. By setting up magnetic field lines toroidally around the interior of the tokamak, the ions and electrons in the plasma are forced to travel tightly around these field lines, preventing them from escaping the vessel. Extra fields help shape the plasma and hold it stable within the tokamak interior. The magnetic fields are generated by coils which surround the tokamak vessel – these are made of copper in the case of JET and a magnetic field is generated around these coils when large currents are passed through them. Careful design of the shape and orientation of the coils ensures the net magnetic field within the tokamak vessel is suitable to confine and control the plasma.