E-H mode transition of an inductively coupled plasma torch at atmospheric pressure

Inductively coupled plasma torches need high ignition voltages for the E–H mode transition and are therefore difficult to operate. In order to reduce the ignition voltage of an RF plasma torch with a metallic confinement tube the E–H mode transition was studied. A spark generator was used to initiate an arc and the E–H mode transition of the plasma was then filmed using a high-speed camera. The electrical potential of the metallic confinement tube was measured using a high-voltage probe. It was found that an arc between the grounded injector and the metallic confinement tube is maintained by the electric field (E-mode). The transition to H-mode occurred at high magnetic fields when the arc formed a loop. An equivalent circuit for the set-up is proposed in order to assess the relative importance of the forces involved in the curvature of the arc (Lorentz force and drag force). As a result, the ignition voltage could be significantly reduced by connecting the metallic confinement tube with a capacitor to the RF generator. Consequently, conditions to ignite an inductive plasma at atmospheric pressure operating a solid state power source can be achieved.