ICP - Generator

Two basic types of electronic circuits are commonly used to produce the RF energy required to operate an ICP:
1. The fixed-frequency crystalcontrolled oscillator and

2. The free-running variable frequency oscillator.

Either type of circuit is fully suitable for operating a plasma configured for ion generation. Radio-frequency generators that are used to operate ICPs are basically simple circuits with a limited number of components, which produce an alternating current at a specific frequency. These generators must be capable of operating with up to 2 kW of output power to adequately sustain an atmospheric pressure argon plasma.

The basic frequency of this generator is controlled by a piezoelectric crystal in the feedback circuit of the oscillator.

The crystal-controlled oscillators for ICPs typically operate at a frequency of 13.56 MHz; the power supply circuitry includes a frequency doubler to provide a typical plasma operating frequency of 27.12 MHz.

Although other frequencies have also been used, this is the most commonly utilized crystal-controlled frequency in plasma systems.

At the higher frequency of 27.12 MHz, the coupling between the generator and the plasma is more efficient, which makes the plasma more robust with regard to sample introduction stability.

A thermionic amplifier tube is used to provide the high-power output needed to operate an atmospheric pressure plasma.

Crystal-controlled systems usually require a servomotor-operated variable capacitor in an impedance-matching network to maintain tuning of the system and minimize reflected RF power, which extends the functional life of the power tube.