Microinstabilities and Drift Modes

Even when MHD-stable, mirror plasmas can develop small-scale instabilities driven by the loss cone; the burner's design suppresses them.

Instability at small scale

Large-scale MHD stability, achieved with a magnetic well, is necessary but not sufficient. Mirror plasmas can also suffer microinstabilities — short-wavelength modes driven by the non-thermal features of the distribution, especially the empty loss cone. These modes can enhance transport and scatter ions into the loss cone faster than collisions would, degrading confinement.

Loss-cone-driven modes

• The loss cone is an empty region of velocity space — a free-energy source
• Drift-cyclotron and Alfven ion-cyclotron modes are the classic concerns
• They can pump ions into the loss cone, raising axial losses
• Suppressing them is essential to reaching design confinement

How the design suppresses them

The main tool is the sloshing-ion distribution: by aiming beams so ion density peaks off the midplane and the midplane is relatively empty, the distribution is shaped to reduce the free energy driving these modes. Warm plasma streaming through the plugs and careful control of the distribution further stabilize them. This is why beam geometry is treated as a stability tool, not just a heating choice.

Microstability is one of the more physics-intensive aspects of mirror operation and a legitimate open area the burner program treats carefully. It is characterized in the design study using distributions from the modeled operating point, and it is high on the list of phenomena the test unit is meant to measure directly.