Preferential Shielding of Magnetic Field Components on SPARC Disruption Mitigation System

Due to the nature of magnetic field interactions with electically conductive materials, one field direction is generally of most interest in the design of an electro-mechanical device that is to operate in a strong magnetic field.  Magnetic shields are often designed to reduce the absolute strength of the magnetic flux density within the shield to zero. However, due to the magnetic field strength near fusion relevant tokamaks such as SPARC, any reasonable amount of magnetically permeable material would become saturated.  It is then worthwhile to minimize the magnetic flux in the direction of interest, to the degree possible.

The SPARC Disruption Mitigation System (DMS) consists of 6 massive gas injection valves mounted near to the tokamak, distributed with 120 degree toroidal spacing above and below the midplane, and with an up-down relative clocking of 60 degrees toroidally. This system is primarily responsible for mitigating thermal and electromagnetic loads, complementing the runaway electron mitigation coil. 

The valves are electromagnetically actuated; eddy currents are induced in the circular valve flyerplate which repel the driving field.  External fields from the equilibrium coils that are perpendicular to the surface normal of the flyerplate will subject the plate to torquing forces during operation. These forces will cause the moving parts to wear, reducing the lifetime of the valve, and raising the potential for dust or chips being injected into the vessel volume.

 

By deviating from the standard ‘solid enclosure’ geometry of most magnetic shields, we preferentially shield out the perpendicular component of the magnetic flux density within the shield, increasing the effective shielding of our saturated material.  We observe a concomitant reduction in shielding of the axial field in the region of interest. In essence, we are adjusting the angle of the field within the shield, rather than minimizing the total strength.