Ambipolar Diffusion
The self-regulating diffusion in which electrons and ions leave together at a common rate set by a self-consistent field.
Why species cannot leave separately
Electrons, being far lighter, would diffuse out of a plasma much faster than ions. But any charge separation builds an electric field that pulls electrons back and pushes ions out. In steady state the two species leave at the same rate: ambipolar diffusion. The self-consistent field enforces quasi-neutrality.
D_ambipolar = (mu_i D_e + mu_e D_i) / (mu_i + mu_e) ~ D_i (1 + T_e/T_i)Because ion mobility mu_i is much smaller than electron mobility, the ambipolar rate is close to the ion diffusion rate but enhanced by the electron temperature. The fast electrons effectively drag the slow ions out only as quickly as the ions permit.
The ambipolar field
The electric field that equalizes the fluxes is the ambipolar field. In a magnetized torus the condition is applied across flux surfaces, and the radial electric field it sets influences flow shear and turbulence suppression.
Unmagnetized versus magnetized
- In unmagnetized plasmas the constraint is simply equal parallel fluxes
- In a torus, neoclassical ambipolarity determines the radial electric field
- In a mirror machine, ambipolar potentials do useful confining work
How it is handled numerically
Transport codes impose flux ambipolarity as a constraint that determines the radial electric field, which then enters the flow-shear and turbulence models. In simple edge models the ambipolar diffusion coefficient replaces separate species coefficients.
Mirror relevance
In the Kronos burner, a D-3He tandem-mirror generator, ambipolar potentials are not a nuisance but a confinement tool: end plugs create potential barriers that confine ions electrostatically. The ambipolar balance between electron and ion end-loss sets the confining potential.