Ashok Menon

New inductively coupled plasma for atomic spectrometry: the microwave-sustained, inductively coupled, atmospheric-pressure plasma (MICAP)

A novel inductively coupled plasma (ICP), termed the microwave-sustained, inductively coupled, atmospheric-pressure plasma (MICAP), has been developed that operates at microwave frequency (2.45 GHz). To sustain the new plasma, a dielectric resonator ring (fabricated from an advanced technical ceramic) is coupled with a 2.45 GHz microwave field generated from a microwave-oven magnetron. The microwave field induces polarization currents (small shifts in the equilibrium positions of bound electrons) in the resonator that generate an orthogonal magnetic field, analogous to that produced by electrical current within a traditional ICP load coil. This magnetic field is capable of sustaining an annular plasma in either air or nitrogen that can readily accept solution samples in the form of a wet aerosol produced from a conventional nebulizer and a spray chamber. An initial analytical evaluation of the MICAP with radially viewed optical emission spectrometry (OES) revealed that limits of detection ranged from 0.03–70 ppb with relative standard deviations from 0.7–2.0%. In addition, the new plasma exhibited good tolerance to solvent loading, and was found to be capable of accepting a wide variety of organic solvents directly and salt solutions up to 3% w/w concentration. Combined, the results suggest that the MICAP could be a competitive, simpler alternative to traditional, radiofrequency argon ICP-OES.

Dielectric resonator antenna for high power RF plasma applications

Summary form only given. Some of the numerous applications of the inductively coupled plasma, at both atmospheric and low pressure, include plasma manufacturing, optical and mass spectroscopy, gasification and plasma reforming, semiconductor fabrication, in-space propulsion, gas lasers, ion sources, and fusion. A coil of copper or silver-plated tubing is typically used to couple the radio-frequency power into the plasma. Although this technology has been used extensively for many decades, an RF coil suffers from limitations which negatively affect the quality of the generated plasma and increase the complexity of the plasma source. These limitations include high ohmic losses in the antenna conductor which necessitate fluid cooling, high levels of capacitive coupling which necessitate RF shielding, high inter-turn voltage which limits the maximum power due to electric breakdown, external tuning capacitors which add to the size and cost, material properties of copper which demand a vacuum or thermal barrier between the coil and the plasma, etc.We will present a remarkable alternative to an RF coil which addresses all of the limitations mentioned above. It increases the maximum power limit by an order of magnitude while operating at a very high efficiency and producing high quality uniform plasma, without the need for RF shielding, and is fully compatible with high vacuum, high purity, and high temperature environment. This is possible due to the outstanding electrical, thermal, and mechanical properties of the advanced technical ceramics. We have used such ceramic materials to construct ring shaped dielectric resonators whose dielectric polarization currents replace the conduction currents of a copper coil. Polarization currents offer many advantages over conduction currents in plasma applications. One of the most outstanding properties of a dielectric resonator is that it completely eliminates capacitive coupling as the resonator maintains exactly zero RF potential even under full power. We will describe the construction and optical plasma diagnostics of prototype plasma sources for both atmospheric pressure thermal plasma at 2.45GHz in air and Nitrogen1 and low pressure cold plasma at 430MHz in Argon at power levels up to 1kW. In addition, we will show conceptual solutions for the implementation of dielectric resonator antennas in many of the inductively coupled plasma applications.