Microwave electric field

The dielectric properties of most foods, at least near 2450 MH2, parallel those of water, the principal lossy constituent of food (Fig. 1). The dielectric properties of free water are well known (30), and presumably serve as the basis for absorption in most foods as the dipole of the water molecule interacts with the microwave electric field. By comparison, ice and water of crystaUi2ation absorb very Httie microwave energy. Adsorbed water, however, can retain its Hquid character below 0°C and absorb microwaves (126). 

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If the molecular effects of the electric field are irrelevant to microwave heating of solutions, this assumption could be envisaged in the use of operating conditions very far from current conditions. On one hand, it will be necessary to use an electric field of higher amplitude, or to reduce the temperature according to the Langevin function. This last solution is obviously antinomic with conventional chemical kinetics, and the first solution is, currently, technologically impossible. It will, on the other hand, be necessary to avoid reaction media with dielectric loss. The molecular effects of the microwave electric field could, paradoxically, be observed for a medium which is not heated by the action of microwave irradiation. 

The values of e and e" of a food material play a critical role in determining the interaction of the microwave electric field with the material. A discussion of these interactions follows. A "map" of foods plotted against their dielectric parameters was introduced by Bengtsson and Risman (1971). Table 1 gives values for the dielectric constant, loss factor and penetration depth, and Figure 1 shows a "map" of these values for common foods. 

There are two major mechanisms by which the microwave electric field is converted to heat within a food. The first, the ionic interaction, comes from the linear acceleration of ions by the field. These ions are primarily from various salts within the product. The second interaction is molecular rotation of polar molecules, primarily water, as well as weaker interactions with carbohydrates and fats. 

Ions in a food oscillate transversely under the influence of the microwave electric field, colliding with their neighboring atoms or molecules. These collisions impart molecular motion which is defined as heat. Materials with mobile ions are conductive. The more available ions in a food, the higher the electrical conductivity. Microwave absorption in a food thus increases with its ionic content. The portion of microwave absorption due to ionic conduction can be described as a portion of the dielectric loss factor, ec. Geyer (1990) recently discussed this concept in his publication. 

The rate of temperature rise due to an microwave electric field is given by the equation... 

A simple extension of the Stark analysis given above enables one to derive an expression for the intensities of the electric dipole transitions. The oscillating microwave electric field is applied perpendicular to the static magnetic field, so that the Zeeman levels experience a time-dependent perturbation, represented by the operator... 

The cavity supports an infinite number of standing wave patterns, called modes, which are designated TEMnpq, where n, p and q are integers. The microwave electric field distribution can be calculated for any mode, from a formula given by Balle and Flygare [14]. However, the dominant modes are those of the type TEM0oq and for these the resonant frequencies are... 

It is, of course, also necessary to calculate the relative intensities of the hyperfine components of each rotational transition in order to assign the spectrum. As we have seen elsewhere, the perturbation due to the interaction of the microwave electric field E(t) with the molecular electric dipole moment may be represented by the effective Hamiltonian... 

Figure 10.73. Observed Zeeman pattern and theoretical reconstruction for a J = 3/2 —> 3/2 transition in HeAr+, with a rest frequency of 35 092.7 MHz [211]. The magnetic field was 4.85 G, using the TE10 mode with parallel ion beam and microwave propagation, but perpendicular microwave electric field and static magnetic field (AM/ = 1).

The perturbation due to the microwave electric field E(t) is given by the operator... 

In the microwave ion beam experiments described in this section, it is important to identify the microwave mode corresponding to the resonance line studied in a magnetic field. For a TM mode the microwave electric field along the central axis of the waveguide is parallel to the static magnetic field. We then put p = 0 in equation (10.161) so that the Zeeman components obey the selection rule AMj = 0. Alternatively in a TE mode the microwave electric field is perpendicular to the static magnetic field and the selection rule is A Mj = 1. This is the case for the Zeeman pattern shown in figure 10.73 each J = 3/2 level splits into four Mj components and the six allowed transitions should,... 

The setup for the first fine structure measurement is shown in Fig. 6(12). A slow e beam strikes a wall of a microwave cavity. The microwave electric field induces transitions from 2 S, to 2 Pj. Lyman a photons are detected in coincidence with IS annihilation y rays as the microwave frequency is swept through the resonance line. 

B. Magnetic field parallel to polarized microwave electric field... 

Dielectric Loss in the Presence of an External Magnetic Field. Figure 2 shows the temperature dependence of the dielectric loss at a microwave frequency of 24 kMc. (3). The upper curve represents the loss in a field of 2800 gauss perpendicular to the microwave electric field, hence a molecular... 

For measurements with rectangular wave guide an external magnetic field of 2500 gauss was applied parallel and perpendicular to the microwave electric field. For measurements with a coaxial line the dielectric loss was measured with a potential difference between inner and outer conductors in absence of any external magnetic field... 

Plasma of oxygen gas A. Radiofrequency electrical fields B. Microwave electrical fields 2.2.1 OH radicals from H202/Fe " at 100 -220°C (Fenton s reagent) A. Open beaker B. closed system ... 

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