1. magnetic confinement - ENEA - Fusione

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52 1. MAGNETIC CONFINEMENT 1.4 FT3 Conceptual Study configuration, in figure 1.51. The vacuum vessel is stainless steel to reduce activation. Indeed, FT3 is expected to produce more than 10 17 neutrons per shot at maximum performance. The proposed upgrade can make full use of all Target Plate the buildings, power supply and auxiliary heating systems as well as the diagnostics of FTU. FT3 will be installed in the FTU hall. A limited upgrade of the existing MFG3 from 200 to 330 MJ deliverable energy is necessary. The LH heating and the CD system (8 GHz, 6 MW) can be adapted to the FT3 port. The driven current is in the range 0.7-1 MA at n=10 20 m -3 . A limited upgrade of the ECH system (140 GHz 3.2 MW) is envisaged to allow a longer pulse length at a power capable of stabilising neoclassical tearing modes. All the FTU diagnostics can be adapted, with minor modifications, for use on FT3. New diagnostics have to be built for the X-point measurements and the plasma current profile measurement by the motional Stark effect. Support Structure Vacuum Vessel Fig. 1.51 - FT3 open divertor configuration. The most important upgrade in additional heating capability is the ICRH system (tunable in the range 70-90 MHz, 20 MW at the plasma), which will provide the fastparticle component in the H minority scheme at B=5 T and in the He 3 minority scheme at B=8 T. The FT3 construction and assembly is expected to last five years. The total cost is estimated to be about 100 MEURO. 1.5 PROTO-SPHERA 1.5.1 Introduction Chandrasekhar-Kendall-Furth (CKF) magnetic configurations are a novel approach to magnetic confinement fusion research. They are simply connected axisymmetric plasma equilibria containing a magnetic separatrix with ordinary X–points (B≠0). The magnetic separatrix divides a main spherical torus, two secondary tori on the top and bottom of the main torus and a spheromak discharge surrounding the three tori. Two degenerate magnetic X-points (B=0) are present on the symmetry axis at the edge of the configurations (fig. 1.52). Whereas CKF force-free fields cannot sustain any pressure gradient (∇ → ∧Β → → =0) and have a relaxation parameter µ=µ 0 j •Β → /B 2 constant all over the plasma, unrelaxed ((∇ → ∧Β → ≠0, (∇ → ∧Β → ≠0) CKF equilibria can be calculated with the boundary condition that µ=µ → 0 j •Β → /B 2 is constant only at the edge of the plasma. The surface-averaged value =µ 0 < → j •Β → /B 2 > will decrease from the edge of the surrounding spheromak to the axis of the main spherical torus. ST I ST CKF < β > ST = 102% S P I e Spherical Torus Secondary Torus Surrounding Discharge If the spheromak discharge is sustained by driving current along its closed flux surfaces, magnetic helicity, flowing down the gradient, will be injected into the main spherical torus through magnetic reconnections at the X- points. The gradient of the pressure profile will presumably be concentrated in the Fig. 1.52 - Unrelaxed CKF configuration.
1. MAGNETIC CONFINEMENT 53 1.5 PROTO-SPHERA b) 1.6 a) p (a.u.) µ (m-1) 1 0 20 0 3 c) d) IST/Ie 10 5 0 P UNSTABLE ST UNSTABLE STABLE 1 1.5 2 2.5 3 qst 0 Fig. 1.54 - Ideal MHD stability plot for β=1 unrelaxed CKF configuration, expressed in terms of the safety factor on the spherical torus magnetic axis q 0 ST and of the ratio of currents I ST /I e . 1.8 2.0 3.0 4.0 qst 95 q 0 0.3 R(m) 0 0 Fig. 1.53 - Unrelaxed CKF configuration, with I ST /I e =3. a) Flux coordinates and profiles on equatorial plane of b) pressure, c) and d) safety factor q. 0.2 R(m) same region where the gradient of has the largest variation (see fig. 1.53). Unrelaxed CKF equilibria with this kind of and pressure profiles are stable to all ideal MHD perturbations with low toroidal mode number (n=1, 2, 3), up to unity plasma beta values, β=2µ 0 Vol / Vol ≈1 (fig. 1.54). Unrelaxed CKF fusion reactors with the right helicity injection, β limit and energy confinement will allow an unimpeded outflow of a part of the high-energy charged fusion products. The charged fusion products will drift across the magnetic separatrix to the degenerate magnetic X-points (B=0) on the top/bottom of the configuration, easing direct energy conversion and the use of a burner as a space thruster. The high plasma β≈1 opens the possibility that plasma motions, i.e., radial electric fields, can sustain the magnetic field of CKF configurations. In the case of a CKF fusion reactor, the radial electric field can even be the natural result of losses of charged fusion products. To begin an experimental study of unrelaxed CKF configurations, a preliminary experiment is being proposed. The PROTO-SPHERA experiment will study the properties of a CKF configuration where a hydrogen forcefree screw pinch, fed by electrodes, replaces in part the surrounding spheromak discharge, while poloidal field coils replace the secondary tori. PROTO-SPHERA, with a longitudinal pinch current I e =60 kA, will produce a spherical torus of diameter 2×R sph =70 cm, aspect ratio A=1.2-1.3 and toroidal current I ST =120-240 kA. 1.5.2 Mechanical engineering PROTO-SHERA was designed in detail to define the load assembly (fig. 1.55). Table 1.IV gives the main engineering parameters of the machine. The plasma pulse duration of 1 s and the inter-pulse time of 5 min are key data. The machine is designed to operate at room temperature with a vacuum of ~1×10 -8 mbar. It can be baked up to ~90°C. Figure 1.55 shows the key components of the machine: electrodes (anode, cathode), coils, support structure and divertor plates, together with the protection plates that shield the coils from the hot electrodes. The vacuum vessel is 2 m in diameter and 2.5 m high, with a thickness of 18 mm. It
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ITALIAN AGENCY FOR NEW TECHNOLOGIES
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1. MAGNETIC CONFINEMENT 09 1.1 Toka
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