Deuterium-Tritium Levitated Dipole Fusion Power Plants

Abstract

Levitated dipole reactors offer an attractive path towards economic fusion power generation. The intrinsic decoupling of the confining magnetic field-generating REBCO magnets and the vacuum vessel offer unparalleled accessibility and maintainability, allowing for high plant duty factors and theoretically low electricity prices. In order to achieve rapid deployment of fusion power to the grid, the use of the Deuterium-Tritium (DT) fuel cycle is required due to its lower required plasma triple products. Historically, designs of levitated dipole fusion power plants have targeted advanced fuels as a DT device was seen to be infeasible due to the high fluxes of 14.1 MeV neutrons on the superconducting core magnet. This study presents high level designs for two feasible first-of-a-kind (FOAK) DT levitated dipole fusion power plants, the larger of which produces 667 MW of fusion power and is predicted to produce 208 MW of net electric power. Both designs consist of a heavily neutron-shielded, high-field REBCO core magnet capable of producing peak magnetic field strengths of 23 T while keeping peak mechanical strains below 0.4%. The neutron shielding is comprised of a layered structure of tungsten and boron carbide, which allows for 92% of the heat deposited in the neutron shield to be radiated out to the first wall while still providing sufficient neutron attenuation to give adequate REBCO conductor lifetimes. The core magnet REBCO coil is comprised of a small "sacrificial" section and a larger semi-permanent section. The sacrificial section, comprising ~20% of the coil, will have a neutron damage limited lifetime of ~1 year, after which the core magnet will be quickly removed from the vacuum vessel and replaced. This allows the damaged core magnet to be refurbished and reused, reducing cost and allowing for economic fusion power generation from a DT levitated dipole reactor.

Figures (23)

• Figure 1: Artistic render of a first-of-a-kind levitated dipole reactor. The core magnet pictured within the plasma in the center of the image is levitated in a large, simple two layer vacuum vessel. The levitation force is provided by a smaller magnet mounted at the top of the inner vacuum vessel. The inner vacuum vessel is surrounded by a tritium breeding blanket. The outer vacuum vessel is constructed from reinforced concrete and is designed to handle the atmospheric forces required for such a large vacuum vessel. Supporting systems and infrastructure are also depicted.
• Figure 2: A comparison of scale between the core magnet of a $\sim 500$ MW thermal power levitated dipole and the magnet systems of similar power output tokamaks. The vacuum vessel of a levitated dipole is approximately twice the diameter of ITER's outer vacuum vessel, however, it has been excluded from this diagram as the magnet systems are the highest cost component of any magnetic confinement concept.
• Figure 3: (a) Radial cross section showing the poloidal magnetic flux of a fusion power plant scale dipole at low $\beta$. The first closed flux surface is determined by the core magnet cryostat shell which acts as an inner limiter and the last closed flux surface is set by the vacuum vessel which acts as an outer limiter. The blue and green flux-tubes contain equal amounts of poloidal magnetic flux but the blue flux tube encloses a larger volume. When the two flux tubes interchange position, the volume of the blue flux-tube decreases and the plasma within is compressed and heated in accordance with Eq. \ref{['eq:pvgamma']}. Simultaneously, the volume of the green flux-tube increases and the plasma within expands and cools. (b) Pressure (green solid line, left axis) and magnetic field at the midplane (dashed orange line, right axis) from the equilibrium shown in (a) as functions of poloidal magnetic flux, $\Psi$. (c) Radial cross section showing the poloidal magnetic flux at high $\beta$ (solid lines) overlayed with low $\beta$ contours from (a) (dotted lines). (d) Total fusion power as a function of $\beta_0$ with the pressure peak location adjusted to ensure it remains two $\alpha$ gyro-orbits away from $\Psi_{\rm fcfs}$. The plasma expansion observed in (c) results in the maximum fusion power being achieved at $\beta_0\sim3$.
• Figure 4: Representative cross section of a DT fusion power plant levitated dipole core magnet. (a) Radiatively cooled tungsten tiles. (b) ${\rm B}_4{\rm C}$ and WC neutron shield structure. (c) Sacrificial section of the REBCO coil where the neutron flux is higher. (d) Permanent section of the REBCO coil. (e) Low-field region. (f) Hoop stress retaining structural overband. (g) Cryogenic slush reservoir. (h) Reservoir for cooling the neutron shield structure.
• Figure 5: Circuit diagram of a center-tapped transformer-rectifier type superconducting power supply. On the primary side (orange) are the primary windings and the power electronics needed to drive the power supply. A transition from room temperature to cryogenic conditions occurs accross the transformer. The secondary side (blue) consists of a pair of secondary windings, a pair of superconducting switches, and the core magnet itself.