The biased end plates could also assist to more precisely determine the positioning of drift surfaces in the confinement region. Biased end plates, placed at the end tank outside the confinement region, is a tool to control the radial electric field. In such a magnetic field, in combination with a weak radial electric field, each guiding center is forced to move close to its mean magnetic surface, thereby suppressing neoclassical transport effects. The ellipticity depends sensitively on the gyro center magnetic drift in the confinement region, and the optimal result approaches a state where the magnetic drift is made as small as possible, which corresponds to the SFLM (Straight Field Line Mirror) field. Magnetic shaping is made to find the minimal ellipticity.
In a minimum B field, the projection of a flux surface on planes perpendicular to the magnetic axis evolves from a circular shape at the mid-plane to a highly elliptical shape as the mirror ratio is increased. A previous calculation of an idealized quadrupolar mirror field in Ref Ågren and Moiseenko (2017) is extended to a magnetic field which smoothly evolves to expander regions beyond the mirror throats.
A mirror machine with a minimum B field for MHD stability is an option for a steady-state compact fusion neutron source design.
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