Tuning and matching the TM1 cavity

The cylindrical resonant cavity operating in the TM010 mode is increasingly being used to generate a microwave-induced plasma (MIP) for elemental analysis by atomic emission spectrometry. In this study, the concepts of tuning and impedance matching for the Beenakker type TM010 resonant cavity are discussed. Proper placement and penetration depth of an antenna coupling probe appear to be a preferable approach to matching the TM010 microwave resonant cavity to a microwave generator. This approach precludes the use of external matching devices that add complexity to the system. In our system, a long probe penetrating 92% into the cavity well and positioned 14 mm from the center of the cavity allowed operation of a critically coupled argon plasma at 13–15 W as recorded from the generator’s calibrated power meter. Additionally, a detector is described that permits measurement of the electric field inside the cavity. The electric field inside the cavity decreases substantially upon ignition of a burning plasma. Aided with a tesla coil, a minimum electric field of 150 V cm−1 allows ignition of an argon plasma. After ignition, the electric field typically reduces to 30 V cm−1 and is not strongly affected by increases in input power. The effect of impedance matching and the electric field caused by increasing the input power to the cavity is also examined.