Solenoid-free Start-up Modeling of 2^ndHarmonic Electron Cyclotron Heating and Current Drive

The QUEST ECH solenoid-free start-up experiment utilizing the 28 GHz gyrotron at 2^ndharmonic frequency has demonstrated remarkable efficiency and record start-up current values. An ST/tokamak start-up modeling is a highly-coupled non-linear problem as the magnetic field topology evolves dramatically from an open to a closed configuration, and the plasma temperature evolves from very cold collisional to very hot collision-less regime. For this task, we developed a grid-based modeling code where the plasma parameters, generated plasma currents, and resulting changing poloidal magnetic fields are evolved from the vacuum fields. Initially at low temperature, weak 2^ndharmonic ECH takes place requiring multi-pass absorption. The current generated in this phase is purely pressure driven. The grad-B drift driven current together with processional currents can then create a closed flux surface configuration where the bootstrap current can further enhance the plasma current. Once plasma temperature is sufficient, a single-pass absorption can rise sufficiently for ECCD to become dominant. This entire start-up process is a self-amplifying “explosive” non-linear problem, where a very rapid plasma current rise can be expected.