IGNITION OF A LOW FREQUENCY INDUCTIVELY COUPLED PLASMA AT ATMOSPHERIC PRESSURE
(Guy CHICHIGNOUD, Yves DELANNOY, Francois COCCO, Pascal RIVAT, Luigi NATALE)
Inductive plasmas used in process applications are classically coupled to high
frequency coils (several MHz), needing a high voltage power supply, and therefore difficult to
operate. In this work, plasma torches are operated at 400kHz with several solid state
generators, ranging from 50kW to 600kW. The plasma ignition at atmospheric pressure was
demonstrated, using a special circuit connected to the metallic confinement of the torch. The
paper explains the ignition process and its particularities for a low frequency (<1MHz), low
voltage (<5kV) power supply.
An arc is created between two copper water cooled walls in the torch, using a high
voltage 50Hz transformer operated for a short time. As shown by high speed videos, the arc
takes the shape of a rotating spiral, and this mode could be maintained for tens of seconds for
a low generator power. At high power, the induction coupling (H-mode) begins when as soon
as the arc approaches the form of a closed loop (figure 1), and increases the power transfer.
The rotating arc mode, yet demonstrated for high frequency, high voltage power
supply, was shown to use a current derived from the main power supply by capacitive effects
between the coil and the torch. At 400kHz, coupling capacitors had to be added to input a
small power into the arc, just enough to maintain it until it builds a closed loop to start the Hmode.
The paper explains why this arc evolves towards a rotating spiral and proposes a small
model for the current and electromagnetic forces applied to this arc.
You can read this article on Heat Processing_01/2017 (Vulkan Verlag).
Read more at: Heat Processing_ResearchDevelopment_01