Development of non-linear density control compatible with ITER-like actuators

2023 Research Campaign, Prepare for ITER Task Force

Purpose of Experiment

In preparation for ITER, plasma control techniques should be evaluated on existing tokamaks in experiments that impose ITER-like conditions. In particular, control experiments should consider the characteristics and limitations of ITER’s actuator and diagnostic systems. The design of the gas fueling system in ITER requires that the injected gas flows through several meters of pipe before entering the vacuum vessel. This travel time will cause long delays and slow response times in the gas flow. The burden of this slowed actuation will degrade control performance and will make density control more challenging in ITER.
Because DIII-D’s gas fueling system does not experience delays and response times on the same timescale as ITER, ITER-like gas injection must be emulated in DIII-D through the Plasma Control System (PCS). The PCS, which is used to operate DIII-D, is responsible for data acquisition and actuator control. The Nerf Mode Program is an algorithm within the PCS that slows DIII-D’s gas injection by altering the PCS commands for the gas valves. The aim of this experiment is to verify the Nerf Mode Program’s emulation algorithm, which will enable future ITER-focused control experiments at DIII-D. In addition, existing and new density control approaches will be assessed using the Nerf Mode Program.

Experimental Approach

The experiment will begin by verifying the Nerf Mode Program in open loop. Using the Nerf Mode Program, different time delays and gas flow commands will be applied to DIII-D’s various gas valves, and the measured gas flow output will be compared to expectations for ITER. Then, PID control of the line averaged density will be tested with the Nerf Mode Program disabled. No time delay or lag will be imposed on the gas injection. The PID gains will be tweaked between shots until satisfactory tracking of the density target is achieved. Using the tuned PID gains, the PID controller will then be tested with ITER-like delays and response times imposed on the gas injection (the Nerf Mode Program will be enabled). The impact of the slowed gas injection will be analyzed, and the PID gains will be adjusted accordingly for the next few shots. If the density target cannot be tracked adequately regardless of the PID control settings, the emulated time delays and response times can be lowered in future shots. Finally, a Smith predictor controller will be tested with ITER-like gas injection.

See more details, including project leads, at U.S. Department of Energy, Office of Scientific and Technical Information (OSTI).