Manfred Thumm* and Lambert Feher

Forschungszentrum Karlsruhe, IHM, 76021 Karlsruhe, Germany
*and Universität Karlsruhe, IHE, 76128 Karlsruhe, Germany

E-mail: manfred.thumm@ihm.fzk.de, Phone: ++49 7247 822440, Fax: ++49 7247 824874

From the point of view of standard microwave technology at the frequencies 0.915 GHz and 2.45 GHz, the need for using higher ISM frequencies like 5.85 and 24.15 GHz for materials processing applications has to be carefully verified with respect to special physical/engineering advantages or to limits the standard technology meets for the specific problem. Costs evaluation of industrial millimeter (mm)-wave systems have to be competive, not only to conventional heating, but also to standard microwave solutions. The most important issue here is the availability of mm-wave generators appropriate in power, size and efficiency. The present paper reports on the state-of-the-art and future of modern 24.15 GHz sources like magnetron, extended interaction oscillator (EIO), klystron and gyrotron for industrial processing of materials.

In general, the physical and technological advantages of mm-waves compared to standard microwaves (decimeter waves) are very well known:

• Better coupling of nonmetallic, dielectric materials to the electromagnetic field due to the higher frequency and the increased loss tangent (stronger absorption).

• Highly increased spatial field homogeneity in the applicator due to the shorter wavelength (applicators with stochastic field distribution).

• Millimeter waves can be focused and targeted much better (spatial selective processing).

• Higher densities and dissociation rates in plasma-chemical processes are achievable.

However, a crucial point for new industrial mm-wave systems at very high ISM frequencies is the availability of affordable sources with appropriate power, size and efficiency. Recent pub-lications show a strong increase on available power sources at the ISM frequency of 24.15 GHz.

The most common microwave sources for industrial applications are the magnetron (e.g. kitchen microwave oven) and the klystron (e.g. radar applications). As higher frequencies, the EIO and the gyrotron attract due to their unique properties. This paper tries for an elementary comparison

Fig. 1: left: Power vs. size [W/cm³], right: Power vs. weight [W/kg]

of the significant differences of these continuous wave (CW) vacuum electron tubes (single component) at 24.15 GHz with typical power levels < 10 kW (industrial power level) in terms of the tube's size, weight and efficiency for a single component.

As a result to their physical interaction principles, the EIO and the magnetron are most efficient for compact applications, where the component size is strictly limited. If one considers the component’s weight related to its microwave power, the EIO concept as well turns out to be the most lightweight source at 24.15 GHz (see Fig. 1).

In terms of produced mm-wave power, the gyrotron is the most efficient source at 24.15 GHz. With a single stage depressed collector efficiencies of 50%-60% can be achieved. The EIO shows here the lowest performance. Another clear disadvantage of the EIO is obvious, if one considers the necessary high accelerating voltage per power (see Fig. 2). The gain in compact dimensions and weight is reduced by the need of bigger power supplies.

Fig. 2: left: Efficiencies of 24.15 GHz tubes, right: Accelerating voltage vs. power [kV/kW]

The present state-of-the-art of industrial gyrotrons for technological applications in the frequency range from 23 to 30 GHz is summarized in Table 1:

The use of gyrotrons appears to be of interest if one can realize a relatively simple, low cost device which is easy to use (such as a magnetron). Gyrotrons with low magnetic field (operating at the second harmonic of the electron cyclotron frequency) which can be provided by a permanent magnet (PM) system, low anode voltage, high efficiency and long lifetime are under development.