Field and Coil Design for a Quadrupolar Mirror Hybrid Reactor

A 3D view of the theoretical device studied in this paper, with fission mantle, coolant outflow pipes and magnetic expanders at the ends

Journal of Fusion Energy – Field and Coil Design for a Quadrupolar Mirror Hybrid Reactor

The mirror machine with the entire coil set, where the circular coils reside outside the quadrupolar coils. Space is available for outflow/inflow of coolant from the fission mantle and power feed to radio frequency heating in the transition regions between the confinement region and the magnetic expanders

A vacuum magnetic field from a superconducting coil set for a single cell minimum B fusion-fission mirror machine reactor is computed. The magnetic field is first optimized for MHD flute stability, ellipticity and field smoothness in a long-thin approximation. Recirculation regions and magnetic expanders are added to the mirror machine without an optimizing procedure. The optimized field is thereafter reproduced by a set of circular and quadrupolar coils. The coils are modelled using filamentary line current distributions. Basic scaling assumptions are implemented for the coil design, with a maximum allowed current density of 1.5 kA/cm2. The coils are optimized using a local optimization method and the resulting field is checked for MHD flute stability and maximum ellipticity

The size of the machine is determined from several aspects. Strong magnetic field gradients are harder to produce with a thick fission mantle, which makes the coil system for a long-thin machine easier to build. The plasma radius needs to be wide enough to give sufficient plasma volume for power production and to confine alpha particles, and is set to 40 cm. A long-thin plasma column is achieved with a 25 meter long confinement region. A fusion power of 10 MW which corresponds to a neutron production of 3.6 × 10^18 neutrons per second could give almost 1.5 GW thermal power output with Q r ≈ 150. The 1.2 meter thick fission mantle provides enough space for fission materials, protection of first wall from fission neutrons, neutron reflectors, tritium reproduction and neutron shielding of magnetic coils. An available empty space could be used for control rods or adding of fission fuel if required. With ion cyclotron heating antennas located outside the confinement region, it is possible to avoid holes in the fission mantle, whereby almost all fusion neutrons enters the fission mantle and a very high Q r (Q r ≈ 150) is possible with reactor safety margins. Plasma is accessible only through the mirror ends.

3D-view of an array of three quadrupolar coils with varying currents

In this study, a vacuum magnetic field and a coil set producing that field has been derived for a quadrupolar mirror hybrid reactor with sloshing ions. The device with a 25 meter long confinement region is aimed for 10 MW power, which would correspond to a steady state 1.5 GW thermal power output. The magnetic field has been optimized for MHD flute stability, low flux tube ellipticity and low field gradients, and consists of a central part based on the Straight Field Line Mirror concatenated with another field to end the mirrors. A simple recirculation and magnetic expander region has been added to the confinement region. A set of circular and quadrupolar coils has been suggested to reproduce the optimized fields with satisfactory accuracy. The vacuum magnetic field produced by the coils has then been examined for flute stability and ellipticity. The obtained magnetic field satisfies the flute stability criteria with some margin, has a maximum ellipticity of about 20 and has smooth profiles (although with some ripple) for the axisymmetric and quadrupolar field components.

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