Control for Fusion Thermal Stability

An analysis of the thermal balance of a fusioning plasma from a control system perspective has been performed. The requirements for stability and the response characteristics of the thermal balance have been evaluated. The results show that open-loop equilibria are characterized by restrictively narrow stable operating temperature regimes and generally poor system performance. Closed-loop proportional feedback using the fuel feedrate and injection energy can be used to extend the stable operating temperature regime and significantly improve the system response. Thus, high open-loop temperature overshoots without neutral beam injection can be reduced to acceptable levels at temperatures as low as 20 keV, with a decrease in the settling time to under 30 sec. With 75 keV injection energy, acceptable overshoot can be obtained at plasma temperatures as low as 10 keV, with the time-to-peak below 20 sec and settling times less than 30 sec. It is still difficult to simultaneously satisfy overshoot and speed of response requirements at low temperatures with low feedback fractions. Additional improvement is available using proportional-integral-derivative (PID) control.