JPH058881B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention converts the transmission mode of electromagnetic waves by a circular waveguide between TE _po mode and TE _pn mode.
Regarding the TE _po -TE _pn mode converter, in particular, the periodic structure of the inner peripheral wall of the circular waveguide is improved in stages,
This makes the design and production easier. (Prior Art) Conventionally, as a type of heating method for fusion plasma, there is direct heating using the output of a high-power Gyrrotron. Therefore, the high frequency microwave output from the Gyrrotron is transmitted through the circular waveguide.
Although it is taken out in TE _po mode centered on TE ₀₂ ,
For heating, control, and measurement of fusion plasma
It is much more advantageous to use directly polarized microwaves such as HE ₁₁ mode. Therefore, conventionally, it has been common practice to convert the circular waveguide TE ₀₂ mode output of a gyrrotron into a linearly polarized HE ₁₁ mode via the TE ₀₁ mode. Furthermore, a circular waveguide mode converter between TE ₀₂ mode and TE ₀₁ mode has been commonly used as a simple oversized mode converter at the laboratory level. (Problem to be Solved by the Invention) However, as a circular waveguide TE _po -TE _pn mode converter for high power use in high frequency bands such as microwaves and millimeter waves, conventionally, M.Thumm A circular waveguide with a sinusoidal periodic structure as described in Int.Electron, Vol. 57, 1984, pp. 1225-1246.
TE _po mode to TE _pn mode converters were used exclusively. This mode converter converts the radius of the inner peripheral wall of the circular waveguide into the TE _po mode, as will be detailed later.
By changing the beat wavelength between the TE _pn mode in a sinusoidal manner and passing through the discontinuous part of the periodic structure of the circular waveguide,
This allows mode conversion between TE _po and TE _pn . However, there is a process that changes the radius of the inner circumferential wall of a circular waveguide into a sinusoidal waveform exactly as designed.
Manufacturing requires advanced technology and equipment, which is not easy, and in order to find the optimal dimensions and shape that will actually provide the highest conversion efficiency, based on the design values obtained as described below. The problem is that it requires an enormous amount of calculation to determine the conditions under which the required electromagnetic wave component is maximized by approximating a sine wave into an extremely large number of minute steps, and as a result only approximate solutions can be obtained. Ta. (Means for Solving the Problems) An object of the present invention is to solve the above-mentioned conventional problems, to be easy to design and manufacture, and to make calculations for optimizing design values relatively simple. The object of the present invention is to provide a circular waveguide TE _po -TE _pn converter consisting of discontinuous parts of a periodic structure in which an exact solution can be obtained. That is, the stepped periodic structure circular waveguide mode converter of the present invention is a circular waveguide with a discontinuous portion in which the pipe diameter repeatedly increases and decreases in at least one step at a predetermined period over a predetermined range. Consisting of a tube,
This is characterized in that the electromagnetic wave transmission mode is converted between the TE _po mode and the TE _pn mode. (Function) Therefore, according to the present invention, the structure is simpler than the conventional one, the design and manufacturing are much easier, and the cycle is such that design values can be reliably and easily optimized. A circular waveguide mode converter with a structure can be realized. (Example) The present invention will be described in detail below with reference to the drawings. First, a first example of the simplest configuration of a circular waveguide mode converter with a stepped periodic structure according to the present invention having a square wave periodic structure with only one step forming the periodic structure will be described.
It is schematically shown in the figure. The illustrated configuration example consists of a circular waveguide with a radius a that acts as an oversized waveguide for the wavelength of the electromagnetic waves to be transmitted, and a circular waveguide with a length in the middle of which the radius is changed in a single step by ±Δa. 2
It is constructed by interposing discontinuous parts in which multiple stages of circular waveguides are connected in an alternating manner with their central axes coincident, and in actuality they have lengths with radii a + Δa and a - Δa, respectively. It is also possible to manufacture two types of circular waveguides by coaxially fitting them together, or by machining the inner peripheral surface of a thick cylindrical conductor with a radius not exceeding a-Δa using, for example, a boring lathe. It can also be manufactured by Deviation value Δa of waveguide radius in the above configuration example
As shown in Figure 2, the discontinuity of the sinusoidal periodic structure is determined by approximation from the conventional circular waveguide mode converter with a sinusoidal periodic structure in which the radius changes continuously in a sinusoidal manner. The radius deviation amount and the half-period length of the repetition deviation obtained as described below are used as initial values in the design of the radius deviation value Δa and the section length, respectively, and the optimum value is calculated using the mode matching method as described below. A computer is used to calculate the conversion rate, and the optimal design value that provides the highest conversion efficiency is determined. In other words, we will omit the details of the calculation process that was presented earlier at an academic conference, but we will explain the behavior of electromagnetic wave propagation at the discontinuous part of the circular waveguide shown in Figure 1. A scattering matrix is obtained by analyzing the transmitted power and reflected power using a mode matching method when the electromagnetic field is continuous even though the propagation mode is different before and after a step-like discontinuous surface where the radius suddenly changes. Find it as a formula,
The total transmitted power and reflected power in the case where such discontinuous surfaces continue in multiple stages are calculated as a continuous connection of the scattering determinant, and the above-mentioned radius deviation value Δa and section length are slightly changed from their initial values. , the above-mentioned total transmitted power and reflected power are repeatedly calculated, and the radius deviation value Δa and section length are determined for each optimal design when the highest conversion efficiency is obtained, where the transmitted power is maximum and the reflected power is minimum. value. Therefore, the above-mentioned initial values used as the starting point for optimizing the above-mentioned design values for the radius deviation value Δa and section length of the discontinuity in the circular waveguide mode converter of the present invention are different from the conventional It is determined as follows using the calculation method disclosed by M. Thumm for a waveguide mode converter. As shown in FIG. 3, with respect to the average radius _a0 in the circular waveguide discontinuity having a sinusoidal periodic structure, the deviation amount of the radius a(z) which is a function of the tube axis direction Z is Δa, When the periodic length of the shift is set equal to the beat wavelength λ of the circular TE _po mode and TE _pn mode that mutually mode convert, coupling between the transmission modes occurs at the discontinuous part of the periodic structure,
It comes to act as a mode converter. Therefore, the beat wavelength λ〓, which is equal to the period length of the radial shift, can be obtained by the following equation (1), assuming that the phase constants of the TE _po mode and the TE _pn mode are β _po and β _pn , respectively. λ〓=2π/｜β _po −β _pn | (1) On the other hand, the radial deviation amount Δa is expressed as
It is obtained from equation (2). a(z)=a ₀ {1+εsin(2πz/λ〓)}(2) In the above equation (2), ε is the number of repetitions N of the radial shift period forming the periodic structure of the discontinuity, which is usually 3 to 6. Appropriately select the total length of the discontinuous part within the range L=N・λ〓
Then, it is given by the following equation (3). ε=1/2・N・C _po,pn (3) Here, _po,pn is the following expression using the n-th and m-th zero points p _1o and p _1n excluding 0 in the linear Betzel function J ₁ . It is expressed by equation (4). By approximation from the sinusoidal periodic structure using each parameter obtained as described above, the initial values of the radius deviation value Δa and section length in the circular waveguide mode converter with the stepped periodic structure of the present invention can be calculated as follows (5) Set it like the expression. Δa=ε・a ₀ ,=λ〓/2 (5) Starting from the initial value set in this way,
The radius deviation value Δa and the section length are subjected to optimization calculations as described above, and the optimal setting value that provides the highest conversion efficiency is determined. In addition, in the above configuration example, the deviation value from the average radius a ₀ of the circular waveguide in the stepped periodic structure was vertically symmetrically increased or decreased to ±Δa, but the resulting highest conversion efficiency is practical. As long as it is 90% or more, it does not necessarily have to be symmetrical; for example, if there are restrictions such as the pipe diameter standards of two types of waveguides that are fitted alternately to form a stepped periodic structure. In such cases, two types of circular waveguides with standard tube diameters close to the optimal design value obtained as described above are used, and only the section length of the stepped periodic structure is set to the optimal design value. It is also possible to do so. When an electromagnetic wave in the circular TE _po mode is incident from one of the circular waveguides connected to the stepped periodic structure circular waveguide mode converter of the present invention according to the configuration example shown in the first figure configured as described above, the electromagnetic wave is The transmission mode TE _po changes into various modes at discontinuities in the stepped periodic structure. However, in a coaxial circular waveguide, only the electromagnetic waves in the circular TE _px transmission mode are generated, and in this stepped periodic structure, the input TE _po mode and the output TE _pn
Since the radius a(z) of the circular waveguide exhibits a periodic change in period length that approximates the beat wavelength λ〓,
Only the electromagnetic waves of the desired circular TE _pn mode are extracted from the other circular waveguide, and mode conversion between the TE _po mode and the TE _pn mode is achieved. In the configuration example shown in the first diagram, according to the present invention, the step-like periodic structure that periodically changes the radius of the circular waveguide is increased or decreased by only a single step, and is made into a square wave-like periodic structure, but the sine-wave periodic structure It is also possible to create a multi-stage step-like periodic structure by approximating from
Furthermore, the plurality of stages can be made into a step-like shape with each step being equal, or each step can be made into an unequal step-like shape. Of course, the optimum design values can be obtained using the same mode matching method as in the above. Therefore, in any case, although it is based on approximation from a sinusoidal periodic structure, it is an optimization of the design value for a stepped periodic structure with a finite number of stages, so it is an optimization of the design value of the sinusoidal periodic structure itself. In contrast, an enormous amount of calculation is required due to countless minute step-like approximations, and only an approximate solution can be obtained.However, it is possible to obtain the optimal design value as an exact solution by calculating a much smaller amount. Next, specific numerical examples of the stepped periodic structure circular waveguide mode converter of the present invention will be explained. (1) Circular TE ₀₁ → TE ₀₂ mode converter with frequency 35.5GHz. Figure 4 shows a concrete numerical example of the structure of a circular TE ₀₁ → TE ₀₂ mode converter in an oversized waveguide system with a frequency of 35.5 GHz and a waveguide radius of 16 mm. For the specific example shown, if the beat wavelength λ = 60.8 mm between the TE ₀₁ mode and the TE ₀₂ mode, and the number of repetitions of the radial shift period N = 4, the deviation amount Δa = 1.28 for the average radius a ₀ = 16 mm. mm. Therefore, considering the ease of obtaining material waveguides and the ease of processing, we adopted material waveguides with radii of 1.7 mm and 14.5 mm with radius deviation values Δa of +1.0 mm and -1.5 mm. In this case, optimization was performed starting from the above-mentioned initial value λ = 60.8 mm of the period length 2 of the stepped periodic structure, and the period length 2 was set to 59.7 mm. A circular waveguide with a radius of 16 mm is connected before and after the discontinuous part of such a structure, and connecting flanges are attached to both ends of the waveguide. Tables 1 and 2 below show the power ratios of transmitted power and reflected power for each transmission mode in the above specific example for initial values and optimal design values, respectively, and the modes obtained by the initial values are shown in Tables 1 and 2 below. While the conversion efficiency is approximately 85%, the mode conversion efficiency obtained with the optimal design value is approximately 93%.
The actual measurement results were almost the same. In addition, in a conventional mode converter with a sinusoidal periodic structure based on the same initial value, the mode conversion efficiency is about 90%, and if optimization is performed using the approximation described above, it is expected to reach about 97%. As a mode converter, a conversion efficiency of 90% is sufficient for practical use, so conventionally, mode converters with a sinusoidal periodic structure were initially developed without performing an extremely large amount of calculations and optimization. Design and manufacturing were performed based on these values, but in addition to requiring considerable skill in precision machining, the problem was that if the actually manufactured product deviated from the design values, the desired mode conversion efficiency could not be obtained. It was hot.

【table】

[Table] (2) Circular TE ₀₂ → TE ₀₁ mode converter with a frequency of 56 GHz. Figure 5 shows a concrete numerical example of the structure of a circular TE ₀₂ → TE ₀₁ mode converter in an oversized waveguide system with a frequency of 56 GHz and a waveguide radius of 12.8 mm. In the illustrated example, the optimum deviation amount Δa for the average radius a ₀ = 12.2 mm is ±0.6 mm, material waveguides with radii of 13.4 mm and 12.2 mm are used, and the period length of the stepped periodic structure is 64.6 mm. Repeat this for 5 cycles and attach connecting flanges to the circular waveguides at both ends. The power ratio of transmitted power and reflected power for each transmission mode in the above-mentioned specific example is shown in Table 3 below with respect to the optimum design value, and the mode conversion efficiency obtained with the initial value is approximately 95%. On the other hand, the mode conversion efficiency obtained with the optimal design values is improved to about 98%.

[Table] In order to verify the results of the above-mentioned optimization of the design values for the mode converter of the present invention, for example, for the circular TE ₀₁ → TE ₀₂ mode converter of the above-mentioned specific numerical example (1), the step-like periodic structure When the number of repetitions N of the radial shift period is increased sequentially, electromagnetic waves are radiated into the air from the discontinuous end face opening, and the respective far-field radiated electric field strengths are measured, and from the measurement results, the following equation (6) is used. The absolute value |A _o | of the amplitude of the electromagnetic wave in the TE _pn (m=1, 2, 3) mode was determined to calculate the power in each propagation mode. |A _o | ^z = |E|θ=θ _n | ² R ² /f _n (θ _n ) ² (m=
1, 2, 3) (6) Here, R, θ are the polar coordinates of the measurement point with the radiation point as the origin, and E is the component of the far radiation electric field regarding the coordinate angle of the radiation aperture end face. Figure 6 shows the schematic arrangement of the far-field radiation electric field measurement system described above, and shows the measurement results of the power ratio of each propagation mode of TE ₀₁ , TE ₀₂ , and TE ₀₃ when the number of radial shift cycle repetitions N is increased sequentially. Figure 7, Figure 8,
Each is shown in FIG. In the measurement system shown in FIG. 6, the 35.5 GHz electromagnetic wave from the Gunn oscillator 1 passes through the reflected wave absorption circulator 2 and the frequency meter 3 in sequence, and is supplied to the variable attenuator 4 to adjust the power. After measuring the supplied power and reflected power through the directional couplers 5 and 6, respectively, they are supplied to the mode converter 7, and the rectangular TE ₁₀ mode for the measurement system is converted into a circular
Convert to TE ₀₁ mode and add mode filter 8
Only the circular TE ₀₁ mode electromagnetic waves taken out through the pipe are supplied to the test mode converter 10 having the configuration shown in FIG. 4 through the tapered pipe 9 for pipe diameter conversion. The detector 11 detects the far-field radiated electric field from the end face opening of the test mode converter 10, analyzes the radiation pattern, and calculates the power of each propagation mode TE ₀₁ , TE ₀₂ , TE ₀₃ . On the other hand, in FIGS. 7 to 9 showing the above-mentioned measurement results, the solid line indicates the theoretical value, and the dotted line indicates the measured value. Further, the first point in each figure indicates each power ratio in the input, and the last point indicates each power ratio finally obtained as shown in Table 2. (Effects of the Invention) As is clear from the above description, according to the present invention, the circular waveguide TE _po -TE _pn mode converter, which is often used for direct heating of fusion plasma, etc.
By using an extremely simple step-like periodic structure instead of the conventional sinusoidal periodic structure, which requires considerable technology to manufacture through precision processing and a huge amount of calculations to optimize, we have achieved the following remarkable results. It can be said that it is effective. (1) The periodic structure is extremely simple compared to the conventional sinusoidal periodic structure, and manufacturing is much easier. (2) By using an oversized circular waveguide as a mode converter, an exact solution can be obtained with a much smaller amount of calculations than with the conventional sinusoidal periodic structure, which requires a huge amount of calculations to obtain only approximate solutions. Optimum design can be easily carried out, and the degree of freedom is large. (3) When a sinusoidal perturbation of the pipe diameter in a TE mode converter using a circular waveguide is realized by an easy-to-manufacture step-like perturbation, in the conventional technology, the fundamental wave component of the step-like perturbation is a sinusoidal perturbation. However, as explained above based on calculations and experiments, the approximation for such a sinusoidal perturbation has low mode conversion efficiency, and the present invention improves the period and period of the step-like perturbation. On the contrary, higher conversion efficiency can be obtained by shifting the amplitude from the value in the sinusoidal perturbation. (4) Since the discontinuous deviation due to the periodic structure from the average radius of the circular waveguide is too large, it can withstand a certain amount of high power.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view schematically showing a configuration example of a circular waveguide mode converter with a stepped periodic structure according to the present invention, and FIG.
The figure is a vertical cross-sectional view schematically showing the configuration of a conventional circular waveguide mode converter with a periodic sinusoidal structure. FIG. 5 is a vertical cross-sectional view showing a specific example of the design values of the invention mode converter, FIG. 5 is a longitudinal cross-sectional view showing another specific example of the design values, and FIG. 7 to 9 are characteristic curve diagrams showing the measurement results of the power conversion efficiency for each propagation mode. 1... Gunn oscillator, 2... Circulator for absorbing reflected power, 3... Frequency meter, 4... Variable attenuator, 5, 6... Directional coupler for power measurement, 7...
Rectangular-circular mode converter, 8...mode filter, 9...tapered tube, 10...test mode converter, 11...detector.

Claims (1)

[Claims] 1. Consisting of a circular waveguide with a discontinuous portion in which the pipe diameter repeatedly increases and decreases in at least one stage at a predetermined period over a predetermined range, and in TE _po mode.
A circular waveguide mode converter with a stepped periodic structure, characterized in that it converts electromagnetic wave transmission modes between TE _pn modes. 2. The circular waveguide mode converter according to claim 1, wherein the at least one stepwise repeated increase/decrease is a rectangular wave-like increase/decrease. 3 The predetermined period is set to the TE _po mode and the
The predetermined range of pipe diameter increase/decrease is set based on the beat wavelength between the TE _pn mode and the number of repetition cycles of the pipe diameter increase/decrease, each phase constant of the TE _po mode and the TE _pn mode, and the electromagnetic wave. The stepped periodic structure circular waveguide according to claim 1 or 2, wherein the waveguide is set based on the n-th and m-th zero points excluding 0 of a first-order Betzel function representing transmission. mode converter. 4. Claims 1 or 2, characterized in that the predetermined period and the predetermined range set as described in claim 3 are optimized using a mode matching technique. Stepped periodic structure circular waveguide mode converter as described in .