Sloshing-Ion Distribution

Angled neutral beams create ions that peak away from the mirror midplane, shaping the plug potential and improving microstability.

Ions that pile up off-center

If ions were injected perpendicular to the field, they would concentrate at the mirror midplane and drive instabilities. Instead the plug beams are aimed at an angle, so injected ions have significant parallel velocity. These ions turn around near the throats, spending most of their time at the turning points and least at the center. Their density therefore peaks off-midplane — they slosh back and forth.

Why sloshing helps

• Density peaks near the ends of the plug, which is where the potential peak is wanted
• The midplane density minimum reduces drive for loss-cone microinstabilities
• The distribution supports the thermal barrier by keeping ions out of the barrier dip

Beam geometry sets the profile

The injection angle, energy, and the local mirror ratio together set where the ions turn and how sharply the density peaks. This is a tuning knob: the neutral-beam aiming is chosen so the sloshing peaks sit where the potential structure needs them. Getting this profile right is central to holding both the confining peak and the thermal barrier at once.

Sloshing-ion operation was demonstrated in tandem-mirror experiments in the 1980s, and it is a well-characterized technique. In the burner design it is modeled as the standard way to sustain the plug, and its beam requirements feed directly into the heating-power and vacuum budgets.