ECH assisted start-up experiments at DIII-D supporting model validation for ITER plasma initiation*

Models for Electron Cyclotron (EC) pre-ionization to assist plasma formation in tokamaks are being developed. Experiments using 1-2 MW EC power for pre-ionization or burn-through, and similar experiments on C-Mod, AUG, JT60 and JET [1], are compared to provide comprehensive data for models and new insight into plasma formation physics. Pre-ionized plasmas have now been documented using Thomson Scattering, and typically achieve n_e=5-10x10¹⁸ m^-3 and T_e~100 eV. The pre-ionization increases the plasma current rate of rise for a given electric field on axis (E_axis) when loop voltage is applied. The plasma conditions in the pre-ionization phase show dependence on EC injection angle, EC power and EC energy and indicate that the necessary power is almost a factor of two higher compared with tangential injection compared to radial launch. Models for EC interaction include ray-tracing computation with reflections from the tokamak wall and are compared to experimental observations of fast camera data on the location of EC absorption and radial expansion of the plasma. Pre-ionization model simulations [1] of absorption and energy loss along the open field lines are compared to the measured ionization fraction of >50%, the plasma density, temperature, and their time evolution on 1-5 millisecond timescales with the application and turn-off of EC power. Contrast to observations, the simulations show a sensitivity of the pre-ionization fraction on the absorbed power, which is a few % only of the total EC power, providing insight in electron cyclotron absorption mechanisms. The results on EC pre-ionization and plasma formation agree with studies at other experiments [3] on the variation of EC power requirements with injection angle. ITER will use a toroidal inclination of the EC beam of >20◦ and based onDIII-D results estimating minimum power requirements of roughly 6.5MW for ITER.

[1] A.C.C. Sips et al., Nucl. Fusion 49 085015 (2009)

[2] H-T Kim et al., Nucl. Fusion 60 126049 (2020)

[3] J. Stober et al., Nucl. Fusion 51 083031 (2011)