Electrothermics

3.2. Different types of very low-frequency applicators

Maintaining the plasma requires a minimal effective voltage U_ei, 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);

H_e is the magnetic field of an inductor with n coils, length l, and with current I (H_el = 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), H_e 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 U_ei 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).

Figure 3.2. Plasma transformer and torch

If we keep a cylindrical ...