Why does a magnetohydrodynamic generator use an ionized gas or plasma?

A magnetohydrodynamic (MHD) generator uses an ionized gas or plasma because it must be electrically conducting to generate power through the interaction with a magnetic field.1

The underlying principle of an MHD generator is based on Faraday’s law of induction, which states that when a conducting fluid moves through a magnetic field, an electromotive force is established perpendicular to both the flow and the field. This force drives an electric current to an external circuit via electrodes.213

Because a neutral gas is not electrically conductive, it cannot interact with the magnetic field to produce this current. To achieve the necessary electrical conductivity—typically 10 to 50 siemens per metre—the gas must be heated to temperatures above 1,800 K to induce ionization. In many systems, this is facilitated by injecting a small amount of a readily ionizable "seed" material, such as cesium or potassium, which ensures the gas becomes a plasma capable of conducting electricity.24