# A Combined Electromagnetic and Mechanical Approach for EU-DEMO Toroidal Field Coils
## 2 DEMO: The Design and the Rationale Behind  
The functioning of DEMO, or more in general of a tokamak (\"toroidal plasma kamera b matrintikh katyinkax\") machine, is based on the interaction of huge magnetic fields. The attainment of the necessary conditions to initiate the fusion reactions is constrained by three parameters: the density of the plasma particles \(n\), the plasma temperature \(T\) and the energy confinement time \(\tau_{E}\). These parameters are intertwined and different relations, as e.g., [PERSON]'s law, describe the theoretical processes to obtain the fusion reactions, stabilize them and draw energy. The challenge of the magnetic confinement machines consists in increasing \(\tau_{E}\) by trapping a suspended plasma at a very high temperature (\(10^{8}\) K) inside a closed configuration. The elemental closed magnetic configuration is given by a toric solenoid, with poloidally wound coils that generate a toroidal field. The force lines exerted are coaxial and centered on the torus axis, with the toroidal magnetic field decreasing linearly along the radial direction. Each particle linked to the magnetic flux lines is subjected to an orthogonal force, which produces a drift centrifugal acceleration. Unfortunately, this leads to a charge separation within the plasma, which creates an electric field and generates an electromotive force that contributes to the drift of the particles. Therefore, the pure toroidal configuration is not sufficient for the plasma confinement. In order counteract the drift acceleration, a poloidal field is superimposed to the toroidal one. This additional field has a lower intensity than the toroidal one, but a greater radial gradient so that the resulting magnetic field has too-helicoidal force lines where the pitch of each helix is greater than the torus radius, leading the flux lines to close on themselves after some turns around the torus axis. In a tokamak, the poloidal field is generated by an ideal wire defined by the plasma itself. The plasma is the secondary of a transformer, while the primary circuit is a solenoid placed on the machine central axis. Considering [PERSON]'s law, the electromagnetic induction is obtained by varying the current on the central circuit. It is important to mention that, since the reverse of the plasma current cannot occur, it is necessary that the magnetic flux and therefore the current vary with a monotonous increasing function, but an infinite current is obviously impossible to achieve. It is therefore necessary to operate the machine in a pulsed regime. Once this configuration is obtained, the plasma particles, however, tend to expand radially under the action of hydrodynamic forces generated by the current that flows through the plasma itself. Indeed, the particles tend to concatenate the minimum magnetic flux available. This trend is contrasted by introducing a vertical magnetic field interacting with the plasma current by means of external annular coils that surround the toroidal magnets. By properly directing the current in these coils, it is ensured that the resulting action of the hydrodynamic force is directed towards the torus axis. In DEMO, shown in Figure 1, the toroidal field configuration is obtained with sixteen D-shaped magnets (TF) [5], the vertical field by six poloidal annular coils (PF) [6; 7] and the primary transformer circuit by one central solenoid (CS), formed by a stack of five independent modules [8]. Each magnet system in DEMO includes superconductive coils made of Cable-In-Circuit Conductors (CICC) [9]. The final function of the poloidal system (CS and PF) consists in driving the plasma current to the operative limits and keeping it stable inside the vessel. One \"plasma scenario\" has, usually, an early stage first, when the currents of the CS modules vary rapidly to induce the plasma current. Then, in the following time points, a quadrupolar field component counteracts the radial and the vertical expansion of the plasma and produces an ovality of its shape.