High-density, low temperature ignited operations in FFHR

New control method of the unstable operating point in the helical reactor FFHR makes the ignition study on the high density and low temperature operation possible. Proportional-integral-derivative (PID) control of the fueling with the error of the fusion power of e(Pf)=-(Pfo-Pf) can stabilize the unstable operating point. Here Pfo(t) is the fusion power set value and Pf(t) is the measured fusion power. Although the large parameter variation would lose its control due to the inherently unstable nature, it is possible to control the ignited operation by pellet injection with the pellet size between 12 mm and 16 mm. Unstable ignited operation is robust against disturbances such as impurity increments by fueling feedback alone. However, if the heating power feedback control is added, robustness to the disturbances is improved, and an operational regime with respect to the integration time and derivative time is expanded. (author)