Second Harmonic Electron Cyclotron Pre-Ionization in the DIII-D Tokamak

Second harmonic pre-ionization, i.e. the production of plasma before the application of a toroidal electric field, $E_\phi$, has been successfully demonstrated in the DIII-D tokamak using both the previously installed 60 GHz gyrotrons ($B_\phi \leq\,$ 1.05~T) and the present 110~GHz system ($B_\phi \approx\,$1.9~T). Pre-ionization and electron cyclotron (EC) assisted startup may be important in future devices such as ITER (at reduced toroidal magnetic field) and K-STAR where thicker vacuum liners and superconducting coils limit the maximum $E_\phi$ to values which are marginal for plasma initiation and burnthrough. In the DIII-D experiments the X-mode 2nd harmonic pre-ionization was reproducible, initially occurred at the 2nd harmonic EC resonance, and then filled the vessel volume. We will present the characteristics of these pre-ionized plasmas and parameter scans of EC power, neutral pressure, and toroidal field. The pre-ionization scenario will be discussed including modeling of single electron collisionless heating.