Runaway electron generation during tokamak start-up

Tokamak start-up is characterized by low electron densities and strong electric fields, in order to quickly raise the plasma current temperature, allowing fully ionize magnetic flux surfaces form. Such conditions are ideal for formation of superthermal electrons, which may reduce efficiency ohmic heating prevent a healthy thermal fusion plasma. This particular concern ITER where engineering limitations put restrictions on allowable fields limit prefill during start-up. In this study, we present new 0D burn-through simulation tool called STREAM (STart-up Runaway Electron Analysis Model), self-consistently evolves density, temperature field, while accounting generation loss relativistic runaway electrons. After verifying model, investigate under electrons can form tokamak as well their effects initiation. We find that Dreicer plays crucial role determining whether discharge becomes runaway-dominated or not, large number could plasma, thus preventing successful further ramp-up current. The be suppressed raising density via gas fuelling, but only if done sufficiently early. Otherwise seed have already been built up, avalanche even at relatively high densities.