# Optimization of DC Energy Storage in Tokamak Poloidal Coils
## 6 Complementarity in Power Demands of PF Coils  
The previous discussion was focused on PF systems but is mostly applicable to other coils. In the following, the specific characteristics of PF systems will be exploited. In tokamak systems, different PF coils may exhibit complementary energy demand profiles. This means that while some coils require high power input, others may have lower power demands or even release energy back into the system, as shown in Figure 2 for the DTT case. By analyzing these demand profiles, it is possible to design a shared DC storage system that optimally balances the energy requirements across multiple coils.  
Figure 7 shows the total power delivered to the load coils by all the PF PS systems. This power is integrated over time to show, in Figure 8, that most of the energy involved in the operations can be recovered because the effective losses in the process are very limited, especially if compared with the peak powers. For completeness, it is necessary to report that a part of the power goes to the plasma and to the passive elements, while a contribution is also provided by the CS coils. Probably, only the first DTT experiments could provide a quantification of the losses. However, even an incomplete recovery of the energy necessary for the tokamak operations is a relevant achievement.  
Figure 8: Total energy stored in the load coils and equivalent mutual couplings.  
Figure 7: Sum of the six instantaneous powers of the six PF coils shown in Figure 3.  
Figures 7 and 8 help to clarify that the total power is lower than the sum of the rated powers of each PS in Figure 3, suggesting that a common storage could reduce the overall power demand. This is due to a phenomenon of complementarity in the power demands of PF coils. One of the novelties of this paper consists of the exploitation of such a phenomenon.  
This characteristic is evident in DTT, leading to the significant findings reported in Section 9. Moreover, even though it is not easy to collect data for all the cases, some preliminary observations of the current scenarios of other tokamaks suggest that the approach proposed in this paper could be applicable to other situations. Figure 9 presents a relevant example: the first reference single-null scenario of JT-60 SA [55], the largest superconducting tokamak operating in the world. In Figure 9, the coils are named as in DTT, whereas in JT-60 SA, they are classified as \"equilibrium field coils\" with a different numbering [55]. These JT-60 SA coils are presently supplied by thyristor bridges without a DC link. A future paper will extend this analysis to other tokamaks and configurations.  
It is important to stress that the optimization moves from scenarios, such as those exemplified in Figures 2 and 9, that were conceived by the plasma physicists independently of any possible optimization of the electrical power or energy. Therefore, the results could be even more significant in the future, with an integrated approach considering these aspects from the beginning.  
The concept of complementary power and energy profiles is crucial to optimizing the design of DC storage systems. By leveraging the complementary profiles, it can reduce the footprint of the DC storage while maintaining the required performance.