plasma breakdown is the moment at startup when neutral gas ionizes and becomes a conducting 📝plasma.
Every discharge begins the same way: the 📝vacuum vessel of a 📝Tokamak is filled with a thin neutral gas — typically 📝deuterium — and the 📝central solenoid ramps its flux to induce a toroidal electric field. Stray free electrons accelerate in that field, strike neutral atoms hard enough to knock loose more electrons, and the population multiplies in an avalanche — the same Townsend mechanism that strikes the arc in a fluorescent lamp. Within milliseconds the gas transforms from insulator to conductor, current begins to flow, and 📝ohmic heating takes over the climb in temperature.
The word marks a boundary, not a phase: breakdown is the threshold event that 📝ramp-up follows, as current builds toward 📝flattop. It is also a real design constraint. Breakdown demands a specific window of gas pressure and electric field, and superconducting machines — whose thick conducting structures slow field penetration — often lean on assistance, using a pulse of 📝electron cyclotron resonance heating (ECRH) to pre-ionize the gas and make breakdown reliable at modest solenoid swing. A discharge that fails to break down cleanly never becomes a plasma at all, which makes this brief moment the gatekeeper of every experiment.