Stabilization of Unstable Low-Temperature and High-Density Operation in the Force-Free Helical Reactor

A new control method for the unstable operating point in the force-free helical reactor (FFHR) is proposed for low-temperature and high-density ignited operation. While in the stable ignition regime, the error of the fusion power of e'_DT(P_f) = +(P_f0 - P_f) is used to obtain the desired fusion power with proportional-integral-derivative control of the fueling, we have discovered that in the unstable ignition regime, the error of the fusion power with an opposite sign of e'_DT(P_f) = -(P_f0 - P_f) can stabilize the unstable operating point. Here, P_f0 is the fusion power set value, and P_f is the measured fusion power. Around the unstable operating point, excess fusion power (P_f0 < P_f) supplies fueling, increases the density, and then decreases the temperature. Less fusion power (P_f0 > P_f) in the subignited regime reduces the fueling, decreases the density, and then increases the temperature. While the operating point rotates to the clockwise direction in the stable ignition boundary, it rotates to the counterclockwise direction in the unstable ignition regime. Using this control algorithm, it is demonstrated that the operating point can reach the steady-state condition from an initial very low-temperature and low-density regime. The fusion power can also be shut down from the steady-state condition without any problems. Furthermore, characteristics of the stable and unstable ignition regimes are compared for the same fusion power, and control robustness to changes with various parameters has been studied.