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Electrothermics by Javad Fouladgar

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3.7. Plasma initialization

At ambient temperature, the gas is not electrically conductive and it is therefore impossible to start the plasma by induction. Plasma start-up is then at low pressure by a discharge under the impact of an electrical field to create an ionized, conductive pocket of gas. The induction maintains and then extends this conductive pocket to the entire gas volume. In the case of cylindrical radiofrequency plasma, using an ignitor is enough to initialize the plasma. For the toric plasma, the initialization is much more problematic and requires more complex setups.

Figure 3.25. Plasma initialization with ignitor

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3.7.1. Initialization with a capacitive discharge

Figure 3.25 shows a principle schematic for the preionization of the plasma by an ignitor. The plasma must be at low pressure to allow a capacitive discharge with the ignitor’s voltage, which is of the order of a few kV. With this technique, the plasma’s preionization is done in the tube, but still occurs only on one side of the torus and does not go a full circle in the quartz. Indeed, as soon as one side discharges, the electrical resistance of gas becomes very small on that side, which leads to a fall in the ignitor’s voltage and does not allow the other side to ionize. However, for the induction generator to take over the ignitor, the current must go a full circle in the quartz.

Any trial with a capacitive ...

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