Electrothermics

3.8. Conclusion

Designing a VLF annular inductive plasma transformer was tackled. This design requires the simultaneous solution of the electromagnetic, thermal, and flow equations. Assuming a very weak flow rate, the flow effect was ignored. The electromagnetic and thermal equations are linked to each other and include strongly linear terms.

A simplified 1D nonlinear model was used for a preliminary model sizing. This model allows us to calculate, with good preciseness, the minimum maintenance field and temperature within the torch as a function of the plasma radius.

To improve the model’s precision and take the system’s electromagnetic behavior into account, especially at start-up, a 3D model based on the (A–V) formulation of the electromagnetic equation was introduced. Using a voltage source by applying the surface impedance model allows us to calculate the non-uniform current distribution in the inductor cross-section. This non-uniformity led to a non-uniform distribution of the electric field when the magnetic circuit has only one arm. Using a magnetic circuit with several arms makes the electric field more uniform.

Many capacitive or inductive methods were used to initialize the plasma. Only an inductive method with a high voltage and a frequency of 450 kHz has given satisfactory results using a two-armed magnetic circuit. An inductive ignitor was proposed to generate the start-up voltage.