TF coil uses NbTi superconductor with subcooled superfluid liquid helium at 1.8 K and atmospheric pressure. There are four cryogenic vertical field coils inside the TF coils and four superconducting coils at the outside of the TF coils to maintain the elongated plasma. The ohmic heating coils are located inside the central core of the reactor system. Since the magnets require a pulsed power supply, each reactor is supplied with a 2-MWh superconductive energy storage unit. Since ohmic heating is effective only at relatively low plasma temperatures, auxiliary heating is necessary to raise plasma temperatures to ignition conditions (when fusion reactions will sustain themselves without further heat input). The NUWMAK design employs radio-frequency (RF) supplementary heating in the ion cyclotron range of frequencies in order to ignite the plasma. The design entails launching a fast magnetosonic wave into a 50-50 DT plasma and heating the ions at the second harmonic cyclotron frequency of deuterium. The NUWMAK reactor operates with a cycle length of 245 seconds: 225 seconds of burn followed by 20 seconds of down time. The heat stored in the blanket material provides energy to the coolant during the down time, reducing the cyclic variation from 70% to 30% of the maximum energy to the turbines. Since a 30% variation is still unacceptable, a steam drum is used and the feedwater temperature is adjusted. Figure 3.13 is a schematic of this load-leveling system, with which constant electrical output can be achieved. Simultaneous operation of the two reactors is not necessary for constant electrical output. Two reactors produce 4566 MWt with a net electrical output of 1320 MW. Since the burn time is 92% of the cycle time, the net thermal efficiency is 31.5%. Fig. 3.13 Schematic of NUWMAK Load-Leveling System
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