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Glass Expansion Coils are produced from the highest quality raw materials and plated using prescribed methods to obtain maximum RF (Radio Frequency) transmission efficiency. There are numerous papers published on RF transmission with regard to conductor and coating resistance.
The generation of a plasma is the result of complex RF circuit design. Many of the environmental problems that affect the coil have little effect on the operation of the RF generator, except that corrosion produces an added load. In particular, corrosion of the coil has a significant effect on the performance and robustness of the plasma. Changing to a new coil reduces the load on the RF generator, since the current required to produce the same level of power is reduced as the resistance of the coil is reduced. Hence, having a coil with little or no corrosion has a positive effect on the longevity of the PA (Power Amplifier) Tube or RF FET's (Field Effect Transistor) due to the lower current required.
This can be explained by the conductivity of pure metals and oxides. Pure grade copper has an IACS (International Annealed Copper Standard) of 100, silver has an IACS of 105, and Gold has an IACS of 74. In referencing the ICAS system a higher IACS indicates higher conductivity. The following websites contain information on the IACS for various metals.
The formation of an oxide on the surface of the coil reduces the conductivity of the coil and hence it's performance. Depending on the instrument and the control feedback system, this can be seen by a reduction in the intensity for an element of the same concentration over the life of the coil.
From a conductivity point, the best material to make a coil from would be pure silver but, due to the high cost this is impractical. Copper is the next best material, but the copper must be of an acceptable grade and contain a minimum of contaminants. The copper used by Glass Expansion has a purity of 99.9% minimum. Once the base material is fixed, the environmental conditions under which the coil is placed need to be considered. The environmental conditions in the torch box include corrosive gases and high temperatures. These vary with instruments and plasma orientation (Radial or Axial). This leads to the use of plating of the copper coils to help prevent unwanted corrosion, with the two most common materials being Silver and Gold. The plating process is vital to the conductivity of the coil. There are many different types of plating procedures that result in varying degrees of conductivity. The procedure used by Glass Expansion does not use additive brighteners or corrosion inhibitors in the plating bath. This produces a dull coating, which is then manually polished to give a final bright appearance, resulting in a coil that retains the highest possible IACS rating.
Coils should be continually cleaned to remove the build up of surface oxidation. The rate of corrosion is higher for Copper, followed by Silver, then Gold. There are however, distinct advantages to using Silver. The plating process for Silver leaves a more even surface allowing fewer sites for chemical attack. The conductivity of Silver is much higher than that of Gold, but Gold does offer a greater protection in more aggressive environments.
Glass Expansion recommends coils in Silver and Gold due to better conductivity and corrosion resistance.