BROADBAND MEASURING SETUP FOR STUDYING THE ELECTRODYNAMIC CHARACTERISTICS OF WAVEGUIDE SYSTEMS

P. I. GOSKOV

The successful performance of all the experimental investigations of the electrodynamic characteristics necessary in the development of waveguide synchrotrons—dispersion, coupling resistance, quality factor, shunt resistance—of various waveguide systems is possible only if an appropriate universal measuring setup is available. Such a setup must satisfy the following basic requirements:

1. The setup must make it possible to measure all electrodynamic characteristics of waveguide systems.

2. The setup must allow measurements to be carried out over a wide frequency range and over a broad interval of electrodynamic properties of waveguide systems.

3. The setup must be simple to operate and must provide the possibility of continuous monitoring of the correctness of its operation.

4. The methods used must ensure the required measurement accuracy.

5. The measurement methods must be of the same type; i.e., as far as possible, all the necessary measurements should be reduced to the measurement of one or two quantities.

6. The methods employed must ensure simultaneity and independence in the measurement of all electrodynamic parameters. This, in turn, will ensure a high measurement rate.

7. The methods used must permit full or partial automation of the measurement process, since only automation can ensure the maximum speed and simplicity of experimental investigations of the electrodynamic characteristics of waveguide systems.

Among the known measuring setups in the microwave range, not a single one, as applied to the task of a comprehensive investigation of the electrodynamic characteristics of various waveguide systems, satisfies the requirements listed above. First of all, it should be noted that these setups do not allow all the electrodynamic parameters of waveguide systems to be measured, still less simultaneously and by a method of the same type. Most known setups are cumbersome and complicated to operate, and use labor-intensive and not very accurate measurement methods. As a rule, these setups are narrow-band. Consequently, they generally cannot be used for investigating waveguide systems at

based on pin structures, since the latter are very broadband systems. In particular, in the microwave range some of them may have a passband on the order of 20–30 cm [1, 2].

A broadband setup for the experimental investigation of the electrodynamic characteristics of waveguide systems, satisfying the above requirements as completely as possible, can at present be constructed only if either the pulsed-excitation method or the combined-probe method proposed by the author is used. However, when carrying out a very large volume of experimental investigations of various types of waveguide systems, it is advisable that the setup permit measurements by both methods. This is explained by the fact that, in the case of waveguide systems with a weak relative intensity of the accelerating electric field, visual methods do not provide the required accuracy. In this case, only the use of the combined-probe method and a channel for heterodyne frequency measurement can ensure, up to a certain limit \(I_{z\min}\), sufficient accuracy. The quantity \(I_{z\min}=\frac{E_z^2}{W}\) is then, obviously, determined by the minimally distinguishable shift of the resonance frequency, i.e., \(\Delta f_{p\min} \simeq 1\ \text{kc}\), which can be observed only in the case of very weak electric fields \((I_z \to 0)\). Waveguide systems with such properties are, of course, of no interest for acceleration purposes. Consequently, the combination of two methods for measuring electrodynamic parameters will make it possible to carry out a broad range of experimental investigations of the properties of waveguide systems.

The minimum operating range of such a measuring setup should be \(\lambda = 10\text{–}30\ \text{cm}\) (1000–3000 Mc). Covering it with a single microwave generator is very difficult, especially when standard generators are used. In designing the measuring setup we proceeded from the condition of making it as rapidly as possible. This can be achieved only by making maximum use of standard equipment. Therefore, to obtain the required frequency range as the source of microwave power, two standard-signal generators were taken: GSS-15 with the range \(\lambda = 14.8\text{–}31\ \text{cm}\) and GSS-27 with the range \(\lambda = 7.8\text{–}15.6\ \text{cm}\) (Fig. 1). Thus, owing to the use of these generators, the operating range of the setup was from 7.8 cm to 31 cm. In view of the use of two measurement methods, the generators must operate in two modes: in the continuous-generation mode and in the external pulse-modulation mode.

As an external modulator it is convenient to use a two-polarity rectangular-pulse generator 26-I, operating in the internal-modulation mode. In principle, the modulators present in both generators can be used, i.e., they can be used in the internal-modulation mode. However, this is not expedient, since then, in order to extinguish part of the transient characteristic when using the pulsed-excitation method, some alteration of the radio circuits of the GSS-15 and GSS-27 would be required. In addition, the 26-I generator has a broader tuning range for the duration and repetition frequency of the pulses.

To delay the quenching pulse relative to the pulse modulating the microwave source, a small-sized pulse generator, MGI-1, is used. It is triggered by a negative pulse from the 26-I. From its output, the negative pulse is fed to the modulating elec-

trode of the ÉO-7 oscilloscope, as a result of which part of the transient response is blanked. By adjusting the “delay” on the MGI-1, we can set an arbitrary blanking level. When operating in the shock-excitation mode, the signal from the resonant mock-up of the waveguide system is fed through the detector section and amplifier to the input of the oscilloscope. The amplifier is used in the case of weak signals.

Visible labels in the block diagram:

• broadband beat-signal amplifier
• frequency meter ICh-7
• resonance indicator M95
• heterodyne wavemeter ShGV-S
• mechanism for moving the probe
• resonant mock-up of the waveguide system
• detector section
• microwave generator GSS-27
• microwave generator GSS-15
• amplifier 28-IM
• modulator 26-I
• sawtooth-pulse generator MGI-1
• ÉO-7

Fig. 1. Broadband measuring setup for studying the electrodynamic characteristics of waveguide systems

When operating in the continuous-generation mode, a sensitive pointer microammeter M95 or M198 is used as the resonance indicator. With a sufficiently strong signal, the 28-IM measuring amplifier may be used. For precise measurement of the drift of the resonant frequency, a heterodyne frequency-measurement channel is used, consisting of the ShGV-S heterodyne wavemeter, a broadband beat-signal amplifier, and the ICh-7 frequency meter. Instead of the standard broadband amplifier (for example, USh-10), in this-

the setup uses a more compact amplifier (assembled according to a known circuit) with a passband of 4 MHz.

The use of standard equipment naturally increased the dimensions of the setup, but it made it possible, in a very short time, to assemble and tune it and to carry out all the necessary studies of the electrodynamic characteristics of a large number of different pin systems. This is an unquestionable advantage of setups of this type.

The results obtained were used at the Research Institute of Nuclear Physics and Electrophysics of the Tomsk Polytechnic Institute in developing the preliminary design of an iron-free waveguide synchrotron for 2 GeV.

REFERENCES

1. P. I. Gos'kov. Izv. TPI, vol. 139, 1965.
2. P. I. Gos'kov. Izv. TPI, present collection.
3. a) P. I. Gos'kov, P. I. Matyazh. Byull. izobr., No. 10, 1965.
   b) P. I. Gos'kov. Izv. vuz. “Fizika”, 3, 1963.
4. P. I. Gos'kov. Author’s certificate, class 21d, 36, application No. 901145/26-25.