Fueling modeling and control for ITER first plasma operation

The initial phases of operation on ITER aim to generate plasmas up to ~10 MA for several seconds. The ability to control the fueling gas via the Gas Injection System (GIS) and eventually the Pellet Injection System (PIS) is essential for the prefill & density control during the entire plasma discharge. The GIS control, as part of the plasma control system, is designed to handle the ITER specifications and plasma scenario requirements.

The GIS consists of 10 Gas Valve Boxes (GVB) each containing six Mass Flow Control (MFC) valves. Each GVB is connected to the Vacuum Vessel (VV) by a separate pipe (~17 m to ~32 m long). The pipes are the main source of delay of the fueling response, which affects the system controllability.

To evaluate the GIS design with respect to the fueling control requirements, a model of the system based on a 1-D description of the gas flow diffusion in each pipe has been developed. The minimum gas flow of the MFCs is too large to reach the prefill pressure target. A PWM solution for the control is proposed.

The model also computes the gas balance in the VV, and thus, computes the required prefill gas to reach the breakdown pressure and initial plasma density. The control effort to fuel the plasma at low density can be predicted by coupling the 1-D model with the particle balance equation that describes the plasma-neutrals interactions.

A GIS failure event handling strategy based on MFC equivalence is also proposed. The simulation results and future work to optimize the ITER prefill & fueling phases are reported here.