Numerical and experimental analysis of radiation from a microwave plasma source of the TIAGO type

Abstract

Unshielded microwave plasma sources radiate electromagnetic energy into space, which reduces the energy that can be used for plasma generation, contributes to discharge instability and is detrimental to laboratory personnel and equipment. We perform numerical analysis of radiation from a TIAGO torch, operating at 2.45 GHz, in which the plasma is generated at atmospheric pressure in the form of a flame at the tip of a metal nozzle. The analysis is carried out by solving the vector wave equation as for the antenna, with the assumption of axial symmetry and homogeneous electron density in the range of 1020–1022 m−3. We determine 2D electric field distributions inside a radiation sphere and radiation patterns for an unshielded torch and for a torch with shielding tubes with radii up to 100 mm and heights up to 200 mm. We also investigate the effect of the electron density, the tube height and radius on the reflected wave power, power absorbed in the plasma, radiated power and power entering the discharge. The results show that a tube of 25 mm radius (smaller than the cut-off radius) shields radiation very well, while the ratio of the radiated power to the entering power can achieve 85% for the unshielded torch and over 95% for a tube of 55 mm radius. In the experiment, we found that the powers required to ignite the discharge and to sustain it are about 80% greater and the plasma length is much shorter for a 55 mm radius tube than for a 25 mm radius tube, which we explain by the difference in the radiated power. The power density at a distance of 500 mm from the plasma with the entering power of 650 W depends on the direction and can exceed the permissible values several times. These results are consistent with calculations and indicate the need for appropriate shielding of the discharge.

A New Stripline-Based Atmospheric Pressure Microwave Plasma Sheet Source Designed for Surface Modification of Materials

“…The MPSS is an open structure, so EM radiation from it can be expected [ 34 ]. To take this fact into account in the calculations, it is assumed that the device is surrounded by an air-filled sphere representing the ambient space around the device (the radiation sphere), which is shown in Figure 4 .…”

“…The calculations are performed in a similar way as in our article [ 34 ] for TIAGO, another type of microwave plasma source. The electric component of the EM field in the calculation domain is determined from the vector wave equation, which follows directly from the Maxwell equations: ∇ × (∇ × E ) − k 0 2 ε r μ r E = 0, where E is a complex vector of electric field (phasor), ε r is the relative complex permittivity of a medium, μ r is the relative permeability of a medium (assumed 1 for all the media), k 0 = ω/ c is the wave number, ω = 2π f is the angular frequency, and c is the light velocity in vacuum.…”

“…The feeding coaxial line is powered by the fundamental, transverse electromagnetic (TEM 00 ) mode with an assumed power at the device input plane P inc . Equation (1) is linear with regard to E , therefore, with the assumed plasma parameters, the choice of P inc values has no effect on the normalized electric field distributions and power ratios [ 34 , 36 ].…”

“…Such plasma parameters can be expected from the measurements presented in the paper [ 19 ]. However, the results should be almost identical for other values, as long as the n e / s ratio is the same [ 34 ]. The height of the plasma sheet is—according to the experiment −40 mm and it protrudes 3 mm down from the dielectric box.…”

“…In such a structure, EM radiation is to be expected, as it is for other open microwave plasma sources [ 33 ]. Because our device is powered at several hundred watts, unwanted radiation may reach values that are harmful to human health [ 33 , 34 ]. To find ways to reduce this unfavorable effect and shield the radiation, we modified the standard stripline by placing additional metal plates on its open sides.…”