Proceedings of the Tomsk Polytechnic Institute named after S. M. Kirov, holder of the Order of the October Revolution and the Order of the Red Banner of Labor

Vol. 232 1975

Traveling-Wave Resonator with a High Gain Coefficient

E. T. Protasyevich, Yu. G. Shtein

(Presented by the seminar of the Microwave and Accelerator Engineering Sector of the Nuclear Physics Research Institute)

For effective acceleration of charged particles, large field strengths of the traveling electromagnetic wave are required. In this case it is necessary to use powerful microwave generators. The strength of the accelerating field can also be increased with the aid of traveling-wave resonators (TWR) [1]. It should be noted, however, that the TWRs used mainly for testing components of waveguide lines have a comparatively low gain coefficient, \(8 \div 12\ \mathrm{dB}\) [1]. This is explained by the fact that various tuning elements are installed in the resonator ring; these create additional inhomogeneities and reduce the gain coefficient to \(8 \div 12\ \mathrm{dB}\) [2, 3]. Therefore all elements that create additional reflections and losses of microwave power were removed by us from the TWR, which led to an increase in the gain coefficient by approximately a factor of two.

The resonant ring was made of rectangular waveguides with a cross section of \(44 \times 72\ \mathrm{mm}^2\). The length of the ring was \(3.5\ \mathrm{m}\). The intrinsic quality factor, measured by the resonance method at the resonant frequency of \(2834\ \mathrm{MHz}\), reached 18,000. After measurement of the intrinsic quality factor, using the formulas from [4], the value of the coupling coefficient of the resonator with the external line was determined. The calculated value of the coupling for the maximum gain coefficient was \(k_{\mathrm{c}} = 0.09\). Directional coupling of the ring with the external line was produced by two round apertures located on the narrow wall of the waveguide at a distance of \(\frac{3}{4}\lambda_{\mathrm{v}}\), where \(\lambda_{\mathrm{v}}\) is the resonant wavelength. As a result of measurements it was established that the directivity of the input coupler was \(24.9\ \mathrm{dB}\).

The gain coefficient of the ring was measured at a high power level by two methods. The first consisted in measuring the power with a directional coupler located directly in the ring; the second method consisted in determining the gain coefficient as the ratio of the power delivered to the load \(P\) to the power of the driving generator \(P_2\), according to the formula [5]

\[ M^2 = \frac{ \left( 1 - \sqrt{1 - k_{\mathrm{c}}^{2}}\sqrt{\frac{P_{\mathrm{n}}}{P_2}} \right)^2 }{ k_{\mathrm{c}}^{2} }. \]

It should be noted that the first method for determining the gain coefficient is simpler. However, when more complete information about the operation of the TWR is required, the second method is preferable.

The measurement arrangement for the resonator gain coefficient is shown in Fig. 1. The power of the generator, operating in continuous-wave mode, was 100 W; it was measured by directional coupler 4. The power delivered to the load was determined by the IM-4 power meter.

For the ratio \(\dfrac{P_{\mathrm{II}}}{P_2}\), equal to 0.18, the gain coefficient \(M^2\) reached 40.

The reflection coefficient in the ring, measured by directional coupler 3 (see Fig. 1), was 0.23.

Fig. 1. Functional diagram of the experimental setup: 1—microwave power generator, 2, 5—M4-2 power meter, 3, 4—directional couplers, 6—traveling-wave resonator, 7—matched load, 8—IM-4 power meter.

To eliminate the influence of the backward wave, a method was used in which a compensating device, consisting of a three-decibel waveguide bridge with two short-circuiting plungers, is placed in the load arm of the input coupler. With the aid of such a device it was possible to reduce the reflection coefficient in the ring to \(\Gamma = 0.11 \div 0.13\).

REFERENCES

1. V. E. Golyant, M. Ya. Mandel’shtam, Radiotekhnika i elektronika, 1959, 4, 4, 668.
2. S. I. Miller, Microwave I., 1960, 3, IX, 50.
3. A. N. Didenko, Yu. G. Shtein, Radiotekhnika i elektronika, 1973, XVIII, 3, 624.
4. J. Altman, Microwave Devices, Mir, 1968.
5. A. N. Didenko, Superconducting Waveguides and Resonators. Moscow, Sov. Radio, 1973.