3.2. Different types of very low-frequency applicators
Maintaining the plasma requires a minimal effective voltage Uei, which is induced around the plasma’s circumference.
This voltage is given by the rate of variation of the magnetic flux enabling the discharge:
[3.1]
where:
R is the radius of tube (m);
is the magnetic permeability in a vacuum;
is the magnetic field’s angular frequency (Rd.s–1);
He is the magnetic field of an inductor with n coils, length l, and with current I (Hel = nI).
We know that there exists an upper limit to the numbers of ampère-turns at the primary level (linked to the inductor’s geometrical and mechanical disposition), He is then bounded by this limit of nI.
By maintaining a constant cross-section (for cost reasons), the only way we can bring the radiofrequencies (RF) toward the VLF with the same voltage Uei is by increasing the magnetic permeability in all or a part of the magnetic field lines’ trajectory. This augmentation can be done by introducing a magnetic core in the device (Figure 3.2).
If we keep a cylindrical ...
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