Electrothermics

2.6. Conclusion

A simulation of the plasma applicator taking the inductor and, possibly, the cold crucible into account can calculate the electrical impedance equivalent to the whole charge precisely.

The coupled electromagnetic and thermal equations are solved on a PC using MATLAB®. The adaptive mesh and numerical methods used (MM, FDM and FVM) make this program fairly fast: the convergence time is less than a few minutes. This voltage formulation is locally validated by the measurements taken on an inductive plasma installation of 32 kW operating at 5 MHz.

This model of the load (composed of the plasma, the inductor, and the cold crucible) can be coupled then to a simulation of the supply generator.

A new simulation of the HF generator, feeding an inductive plasma torch, is developed using an optimization program. This optimization program converges toward the steady operating state and requires no simplifying hypothesis. Setting up this model requires that the triode installed in the generator be perfectly simulated (i.e. its nonlinearities must be considered). Its great advantage is that it converges in a few seconds no matter what the state of the plasma, loaded or at rest: in other words, it is independent of the load impedance.

We can now state, seeing the results, that:

– The generator’s resonance frequency is not the resonance frequency of just the oscillating circuit, but also that of the feedback and impedance matching circuit.

– In practice, a very weak phase , added ...