Lab-scale system for microwave and plasma experiments

Summary form only given.The paper discusses the development of a lab-scale system designed for microwave and plasma experiments. It is suitable for any laboratory, which has an interest in doing research in microwave or plasma applications and in particular in biological microwave or plasma interaction experiments. We set out to investigate the field patterns using simulation software created by different types of feeds to a cylindrical chamber as the first priority in our quest for a lab-scale system that is easy to use and flexible for a wide range of microwave and plasma experiments. We aim for a single mode operation in a cylindrical cylinder. This restricts the maximum diameter of the chamber to 120mm at 2450 MHz before multi mode field patterns set in. This diameter can accommodate most Petri dishes. Microwave energy can be coupled to the chamber via the usual WR340 rectangular waveguide. Because of the orientation of the electric field into the waveguide, this method of feeding is found to limit the plane of maximum intensity to one quarter of a wavelength or 30mm above the metal base plate of the cylinder. Our preferred option is have the maximum intensity on the base plate where the sample would be placed. Therefore a waveguide coupling is unsuitable. The alternative option is to couple microwaves into the chamber via a coaxial cable. According to our simulation results, this method of feeding enables the electric field to be at right angles to the base plate where it can be present at its maximum or full strength. The coaxial cable feed is compatible with a magnetron construction so that the latter can be mounted directly on the top plate of the cylinder, which makes the construction easier and cheaper. For microwave experiments, it will be sufficient to place samples on the bottom plate and the precise microwave power or exposure is set through a switch mode power supply designed to control the magnetron. For plasma experiments, we divide the cylindrical - - chamber in two parts by a quartz plate. The top part remains at the atmospheric pressure and the bottom part can be evacuated to an appropriate pressure to generate plasma. This latter construction offers a more flexible way to treat samples at different pressures or under plasma. Stable plasma is easily initiated with our design. The microwave generator is equipped with variable power control for application of temperature feedbackcontrol and a PC interface to monitor the forward and reflected power. The complete system works well and the first has been used to perform biological experiments at Food Science Australia.