# Optimization of DC Energy Storage in Tokamak Poloidal Coils
This paper will show that, for (superconductive) inductive loads, SCs are more advantageous than expected by a simple look at the classical Ragone plot. In fact, the optimization can exploit some specific characteristics and function principles of PF coils. The paper does not explicitly include the optimization of other types of coils that can induce PFs, but are not strictly PF coils, because they have different functions: the central solenoid (CS) [4], the vertical stabilization (VS), the fast radial control (RC), and the divertor (DIV) coils [15; 23]. Analogously, the PSs for the non-axisymmetric coils used for error field correction (EFC) and edge-localized modes (ELMs) [23] are out of the scope of this paper, also because they will be addressed in different papers. Nevertheless, it will be explained how the different coil functions imply a different optimization approach and results. For these reasons, even though the approach is generally applicable to all tokamaks and other nuclear fusion plants, the analysis must be carried out on real cases. Thispaper is focused on a last-generation tokamak named the Divertor Tokamak Test (DTT) facility [23; 24], under construction in Frascati, Italy, that is also one of the first public-private partnerships in this field [25]. DTT has six PF superconducting coils, which is a sort of standard in tokamaks, and all the other mentioned coils (CS, VS/RC, DIV) [23]. This paper explores innovative strategies for reducing the size of the DC storage stage in PF PS converters by employing SCs arranged in series and parallel configurations. For the first time, the proposed solution involves the shared utilization of the DC storage stage between pairs of PSs. This approach, validated through simulations, leverages the complementary energy demand profiles observed in certain pairs of coils, allowing for a more efficient and compact design. The findings of the study suggest that such an approach not only reduces the overall size of the DC storage stage but also enhances the performance, efficiency, and cost-effectiveness of the system. For example, an optimized power exchange with the grid can reduce other power quality problems affecting the tokamaks such as reactive power, harmonics, flicker, and oscillations. In particular, the huge power flow variations that could be a showstopper for the exploitation of tokamaks as an energy source [6; 7; 9; 12] could be internally managed. On the other hand, the paper will show some limitations and issues in the approach, such as the difficulty of optimizing the available power, energy, and voltage. The implications of this research can be extended to several applications in energy storage and conversion technologies, highlighting the importance of innovative design in meeting the complex demands of advanced energy systems. The rest of the paper is structured as follows. Section 2 presents the background of the operation of PF coils in a tokamak, providing the reference current and voltage scenario for such operations. Section 3 introduces the reasons for the need for energy storage based on SCs in PF coils. Section 4 describes the main parameters to be optimized for the design of the SC banks. Section 5 presents the possible optimization strategies and criteria for the design of the SC banks, with a specific focus on the PF PS characteristics. In particular, Section 6 proposes an effective optimization technique exploiting the complementarity of the power demands of PF coils. Different approaches are possible, but, after presenting them, the optimization is finalized by circuital simulations, taking into account also the effect of other tokamak elements. The main analytical developments to implement such circuital simulations are described in Section 7. Section 8 presents the parameter ranges and the choices for the simulation campaign. The obtained results are summarized in Section 9, clearly showing the advantages of the proposed approach. Finally, some practical considerations and the conclusions drawn from the study are presented in Section 10.