  Stellarator magnetic configurations need to be optimized in order to meet all
the required properties of a fusion reactor. In this work, it is shown that a
flat-mirror quasi-isodynamic configuration (i.e. a quasi-isodynamic
configuration with sufficiently small radial variation of the mirror term) can
achieve small radial transport of energy and good confinement of bulk and fast
ions even if it is not very close to perfect omnigeneity, and for a wide range
of plasma scenarios, including low $\beta$ and small radial electric field.
This opens the door to constructing better stellarator reactors. On the one
hand, they would be easier to design, as they would be robust against error
fields. On the other hand, they would be easier to operate since, both during
startup and steady-state operation, they would require less auxiliary power,
and the damage to plasma-facing components caused by fast ion losses would be
reduced to acceptable levels.
