Microwave absorption coefficients
Knowledge of absorption coefficients of metals at microwave frequencies is required as input for the modelling work. It is also essential to assess the thermal loads as a function of microwave power density.
Intrinsic conductivities of metals are well known. To compute the theoretical absorbed fraction of power on an ideal smooth surface one requires the intrinsic conductivity, the angle of incidence and the operating frequency. In practice effects like surface roughness and coating have a large impact though and these may double or triple the absorbed fraction. As a consequence, in this work absorption coefficients of actual ITER materials are assessed both by modelling these additional aspects, and by measurement in a resonator facility. Figure 3 shows photo of a resonator presently in use in our microwave lab.
Fig. 3. Resonator facility under development to measure stray radiation absorption coefficients. To the left a microwave source injecting microwave power in the copper sphere. The intensity of the microwave power in the sphere is measured by a microwave detector diode shown on the top of the sphere. The aperture in the sphere allows to i) insert samples in to the resonator which will, due to the losses, drop the intensity in the resonator, and ii) allows to vary the size of the aperture (considered to be fully absorbing surface). thus providing a mechanism to calibrate a decrease in intensity of the resonator with absorption. Work in collaboration with Max-Planck-Instituut W7-X (Greifswald):, Heinrich Laqua and Dmirty Moseev and our students Gijs Akkermans and Rico Geraerts.
The single pass absorbed fraction of power in a ceramic can be found with knowledge of the “loss tangent δ”, the permittivity, the operating frequency and the material thickness. Knowledge of the loss tangent δ (conduction current density over displacement current density) is the number that is hardest to obtain and it is frequency depend as well. But even with good knowledge of tan δ multiple reflections inside the sample will increase the single pass absorbed fraction. In addition, the absorption is modified when the sample is emerged in a stray radiation field enclosed by metal walls such as the vacuum vessel. Assessment of absorption of dielectrics is therefore a topic of study too, both by modelling and as by measurement.