Plasma Conductivity & Seeding in MHD

An MHD generator only works if the flowing plasma conducts well enough; conductivity, not flow, is often the binding constraint.

Conductivity is the currency

The current an MHD stage can draw scales with the electrical conductivity of the flowing plasma. In a fusion-exhaust MHD channel the working fluid is the plasma itself — already ionized and hot, so it conducts far better than the combustion gases used in classical fossil-fired MHD. That is a genuine advantage: the burner does not need to inject an alkali seed to create ionization the way a combustion MHD generator does.

How conductivity varies

Plasma conductivity rises steeply with temperature (roughly as temperature to the three-halves power in the classical Spitzer picture) and falls with impurity content, because high-charge impurity ions scatter electrons. Along the conversion channel the plasma cools as it gives up energy, so conductivity drops downstream. The channel geometry and electrode placement must account for this falling conductivity profile.

Why seeding is largely avoided

• The fusion exhaust is already fully ionized — no alkali seed is needed to make it conduct.
• Avoiding seed removes a contamination source that would otherwise recycle into the plasma.
• Impurity control still matters: sputtered electrode material lowers conductivity and must be minimized.

Consequences for design

Because conductivity falls as the plasma cools, most of the MHD conversion is done in the hot, high-conductivity inlet region, and the cooler downstream flow is better handed to thermionic or thermal recovery. This is another reason the burner uses a train rather than one converter: each mechanism is placed where the plasma state suits it best.