A New Method for Feedback Stabilization of a Microwave Power Supply

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Abstract
A new method has been developed for the feedback stabilization of a microwave power supply, with the intent to reduce emission noise in microwave-induced plasma atomic emission spectrometry (MIP-AES). The control method would be applicable to any microwave generator whose output power is affected by the input ac line voltage. In the feedback process, the effective line voltage is adjusted by means of a controllable inductance placed in series with a variable transformer at the input of the generator. In turn, this inductance is controlled by selecting the dc current in the primary winding of an auxiliary transformer, so the inductance in its secondary winding is altered. The long-term stability of the feedback-controlled system is verified and the range of stabilization is examined. Noise in the power output of the microwave generator is shown to be reduced at frequencies below 1.5 Hz. Although stable microwave power generation does not necessarily result in stable emission from a microwave-induced plasma source, an emission-based feedback signal is shown to improve the stability.