µPQ – ultra-compact microwave plasma source for atmospheric pressure
Fig. 1: The compact microwave plasma source with opened housing.
Fig. 2: Measured frequency and output power of the voltage controlled plasma source power oscillator with 50 Ω load.
Fig. 3: Measured plasma temperature as a function of the distance to the surface to be treated.
Industrial interest in atmospheric-pressure plasmas is growing steadily. Eliminating the vacuum technology increases cost efficiency and simplifies handling and process sequences. For many applications, however, the high temperature of atmospheric-pressure plasmas is problematic, which is conventionally reduced by pulsed operation. Another way to maintain a low temperature is to generate the plasma using microwave frequencies. In this case, due to the inertia of the heavy gas ions, almost exclusively the light electrons are accelerated. This results in a relatively cold and, in contrast to pulsed operation, continuous plasma of high intensity, which can be used efficiently in a variety of applications, particularly for surface treatment such as activation or ultra-fine cleaning.
This is why FBH has developed an ultra-compact microwave plasma source for atmospheric pressure. It delivers a potential-free microwave plasma with a plasma power in the 15 to 20 W range, which is sufficient for many applications. The source is compact and very handy. Its housing with a size of only 114 x 33 x 25 mm³ contains a resonator for plasma excitation, a power oscillator operating in the license-free ISM band around 2.45 GHz, and the control and monitoring electronics (Fig. 1).
The circuit design of the microwave power oscillator is optimized with respect to both reliable plasma ignition and efficient plasma feeding, with an FBH gallium-nitride transistor responsible for generating the microwave power. The operating frequency of the power oscillator can be varied by a control voltage (Fig. 2). On the one hand, this frequency controllability allows the compensation of manufacturing tolerances of the high-Q resonator. In addition, frequency can be adjusted to the two different operation modes for plasma ignition and burning, which further increases the reliability and efficiency of the plasma source. The developed power oscillator achieves high output power over the entire control range (see Fig. 2).
Due to the low temperature of the generated plasma even temperature-sensitive materials such as plastics can be treated without any problems. At a distance of 10 mm from the plasma source, temperature falls below 100° C (Fig. 3). The activation and adhesion improvement of various polymers such as PP, PE and acrylic glass have already been proven in industrial test runs, using air as process gas. Due to the special circuit design of the microwave power oscillator, the plasma source also works with nitrogen, oxygen, argon and forming gas.
Despite its outstanding properties and manifold possibilities, the microwave plasma source is extremely easy to put into operation. All what is needed is a DC voltage of 48 V, a gas supply and water cooling, which can also be replaced by air cooling if required. The plasma source can be used both as a stand-alone solution and as part of industrial equipment. The source and its potential will be presented in detail in a lecture at the 36th ak-adp workshop [1].
[1] Anwenderkreis Atmosphärendruckplasma – ak-adp, 36th workshop „Atmosphärische Plasmen – Effektives Werkzeug von A(ktivierung) bis Z(ukunftstechnologie)“, postponed from March 25/26 to July 1/2, 2020, 06886 Lutherstadt Wittenberg; flyer available at https://www.ak-adp.de/wp-content/uploads/2019/11/Flyer-akadp-36Workshop_web-1.pdf