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.