Spectroscopic Investigation of a Microwave‐Generated Atmospheric Pressure Plasma Torch

Abstract

The investigated new microwave plasma torch is based on an axially symmetric resonator. Microwaves of a frequency of 2.45 GHz are resonantly fed into this cavity resulting in a sufficiently high electric field to ignite plasma without any additional igniters as well as to maintain stable plasma operation. Optical emission spectroscopy was carried out to characterize a humid air plasma. OH-bands were used to determine the gas rotational temperature Trot while the electron temperature was estimated by a Boltzmann plot of oxygen lines. Maximum temperatures of Trot of about 3600 K and electron temperatures of 5800 K could be measured. The electron density ne was estimated to ne ≈ 3 · 10 20 m −3 by using Saha's equation. Parametric studies in dependence of the gas flow and the supplied microwave power revealed that the maximum temperatures are independent of these parameters. However, the volume of the plasma increases with increasing microwave power and with a decrease of the gas flow. Considerations using collision frequencies, energy transfer times and power coupling provide an explanation of the observed phenomena: The optimal microwave heating is reached for electron-neutral collision frequencies νen being near to the angular frequency of the wave ω.

Microwave Plasmas at Atmospheric Pressure

“…Some radially resolved profiles of plasma parameters obtained by applying Abel inversion are presented in [22]. First the excitation and gas temperatures of the 2.45 GHz air plasma followed by the gas temperature measurements at the 915 MHz air plasma will be discussed.…”

“…Afterwards the excitation and gas temperature as well as the electron density of the H 2 /Ar discharge in the 915 MHz plasma torch will be investigated. In the center the excitation temperature reaches values of up to 5800 K. This maximum excitation temperature is almost independent of the supplied microwave power and of the gas flow [22,23]. Some radially resolved profiles of plasma parameters obtained by applying Abel inversion are presented in [22].…”

“…Some radially resolved profiles of plasma parameters obtained by applying Abel inversion are presented in [22]. Again the maximum gas temperature is almost independent of the supplied microwave power and of the gas flow at around 3600 K [22,23]. In axial direction between z = 48 mm and z = 70 mm the plasma is not accessible due to constructive reasons of the cylindrical resonator.…”

Design and optical study of a microwave plasma torch in nitrogen used for the evaporation of aluminium wires

“…This type of plasma has been an object of continuous attention and development. In general, microwave discharges (MWDs) are characterized by high flexibility of design and adaptability to particular applications. The design of MWDs has considerable influence on its operation range, for example, available pressure ranges, types of useable feed gases, power dissipation processes, excited discharge volumes, and also vacuum compatibility, and might strongly influence the discharge parameters, including the plasma composition, gas temperature, and electron energy distribution function.…”

Generation of Microwave Capillary Argon Plasmas at Atmospheric Pressure

“…Atmospheric microwave plasmas can operate over a wide range of plasma density regimes from low-density glow-discharge-like plasmas [21], [22] to higher-density arc-discharge-like torch plasmas [23], [24]. Although the atmospheric pressure microwave plasmas have been studied over the past two decades, they have not been fully developed for the synthesis of materials near ambient pressures.…”