SELECTION OF THE TYPE OF ACCELERATING SYSTEM FOR AN IRON-FREE WAVEGUIDE SYNCHROTRON OF SEVERAL GeV

P. I. GOSKOV

Before proceeding to the design and construction of waveguide synchrotrons for high energies, it is necessary to obtain experimental data on the operation of such accelerators in the intermediate-energy region; that is, it is first necessary to develop, build, and investigate the operating characteristics of a waveguide synchrotron of several GeV. The experience accumulated as a result will make it possible to create considerably more powerful waveguide synchrotrons.

In connection with the development of iron-free accelerator technology, especially at the Institute of Nuclear Physics, Siberian Branch of the Academy of Sciences of the USSR, the creation of a waveguide synchrotron for high energy with an iron-free magnetic system is at present the most realistic. In this case, it is first necessary to solve the following problems: for such an accelerator, select the most suitable waveguide system; study its electrodynamic properties; after which, proceeding from a definite design of the waveguide accelerating system and its electrodynamic properties, determine the required high-frequency excitation power.

Solving these problems will make it possible to proceed in earnest to the creation of waveguide synchrotrons for high and superhigh energies.

In the present work, on the basis of investigations carried out at the Tomsk Polytechnic Institute of the electrodynamic properties of various waveguide slowing structures for an iron-free waveguide synchrotron, the most efficient accelerating system is selected.

As applied to such a synchrotron, the waveguide accelerating system must satisfy the following basic requirements.

1. The waveguide accelerating system must have a simple structure and manufacturing technology.

2. The accelerating system used must provide the possibility of comparatively simple structural placement of the waveguide structure along the orbit; that is, it must be well matched structurally with the magnetic system.

3. The use of a periodic structure must not lead to large distortions of the guiding magnetic field.

1. The presence of a waveguide structure must not lead to the appearance of electrical contact, for direct current, between the current-carrying buses.

2. The waveguide accelerating system, in its operating mode of oscillation, must not have superposed modes of oscillation.

3. The operating wavelength corresponding to the chosen operating regime of the waveguide accelerating system must lie in the range of optimal wavelengths \((\lambda = 10 \div 100\ \text{cm})\), i.e., those corresponding to the greatest shunt impedance of accelerating systems and to the powerful microwave sources available.

4. The use of a waveguide accelerating system must not lead to an increase in the magnetic gap, since, owing to the need to create a strong magnetic field, its value must not exceed \(2 \div 3\ \text{cm}\).

5. In the region of small slowdowns

\[ \left(\frac{c}{v_{\phi}} = 1\right) \]

the waveguide system must have a sufficiently high shunt impedance, since this will lead to a reduction in the high-frequency power consumed.

1. Unlike microwave devices and linear accelerators, the waveguide accelerating system of an electron synchrotron must have a high shunt impedance for large interaction spaces.

The points listed above require no special explanation. It is only necessary to note that some of the enumerated requirements are contradictory. For example, it is contradictory for the system to have a large interaction space and a high shunt impedance; for the system to operate in the indicated range of wavelengths, to have a high shunt impedance, and to permit a magnetic gap of the order of \(20\ \text{mm}\); for a system satisfying a large number of the indicated requirements to be simple from the technological standpoint, etc.

From the standpoint of the absence of distortion of the controlling magnetic field, the greatest interest is presented by such waveguide accelerating systems in which the current-carrying buses are located inside the interaction space of the waveguide systems.

However, in this case, in the decimeter wavelength range it will be very difficult to obtain, in such systems, accelerating electric fields with a large longitudinal component between the buses. This is explained by the fact that, as the interaction space increases, the shunt impedance of waveguide systems drops very sharply. Since the geometrical dimensions of the buses will be comparable with or considerably larger than the dimensions of the waveguide slowing structures, and the distance between the buses is small, the electric field of waves having a component along the buses will “sag” only weakly into the space between the buses. Therefore such waveguide accelerating systems can be used only at decimeter wavelengths, when the dimensions of the waveguide slowing structure are considerably larger than the transverse dimensions of the current-carrying buses.

Consequently, at centimeter wavelengths in iron-free waveguide synchrotrons it is more expedient to use accelerating systems of another type, i.e., those in which the waveguide slowing structure is located inside the magnetic gap, which, as already indicated above, must not exceed \(2 \div 3\ \text{cm}\).

Therefore, in the latter case, accelerating systems based on smooth and iris-loaded waveguides, used in operating

waveguide synchrotrons at the Research Institute of Nuclear Physics of Tomsk Polytechnic Institute [1, 2] prove unsuitable, since with their use the height of the magnetic gap will be no less than 6 cm. In addition, their use is also undesirable because they will substantially shield the rapidly varying controlling magnetic field.

The indicated difficulties can be avoided only by using pin-type slow-wave structures, for which it is characteristic that, with a small waveguide height, large wavelengths can be obtained. These structures are also of interest because they can be placed directly on one or both current-carrying buses, and in this case there will be no dc electrical contact between the current-carrying buses. The fact that such pin-type slow-wave structures can be made as a single unit with the current-carrying bus ensures a good structural combination of the magnetic and accelerating systems.

Of the known pin-type slow-wave structures, the most suitable for this purpose are waveguides with pin combs fastened in various ways on the broad walls of the waveguide, namely: staggered-pin structures, opposing rows of pins, and simple comb structures [3]. Of these, two-row pin structures are of greatest interest, since their shunt impedance is considerably higher than that of the corresponding homogeneous ones (in the interaction space).

When the indicated structures are used, the smallest magnetic gap can be obtained in the case of ordinary two-row pin combs. In particular, in the decimeter range, when they are used, the magnetic gap may be 2 cm [4], whereas when staggered-pin structures and structures of the type of opposing rows of pins are used, the magnetic gap will be \(\geq 2.5 \div 3\) cm [3, 5].

In addition, another important factor speaking in favor of two-row pin combs is the following.

Since in all two-row pin structures excitation of in-phase and antiphase waves is possible, in order to achieve sufficiently good separation of the operating mode of oscillations from neighboring (parasitic) oscillations—required so that the ultrahigh-frequency power goes mainly into exciting only the required type of oscillation—it is absolutely necessary to avoid superposition of the dispersion curves of both types of waves. In this case, waveguide accelerating systems based on pin combs, in which the dispersion curves of the in-phase and antiphase waves are separated in frequency range, have undoubted advantages over waveguide systems based on staggered-pin structures and opposing rows of pins, for which the superposition of oscillations having a longitudinal component at the center of the interaction space with oscillations for which the longitudinal component is absent is characteristic. Another advantage of waveguide accelerating systems based on two-row pin combs is that they will be fastened only to one current-carrying bus, whereas in the case of two-row staggered-pin structures and opposing rows of pins the pins must be fastened to both current-carrying buses.

Thus, it follows from the foregoing that, as the accelerating system for a waveguide iron-free synchrotron of several GeV, one should choose a waveguide accelerating system based on a two-row pin-comb slow-wave structure, since in them it is possible to excite wavelengths of the order of 10 cm with the smallest magnetic gap. At the same time, the technology for manufacturing the accel-

the accelerating system will be the simplest, and, in the operating type of oscillation, superposed (parasitic) oscillations will be absent.

Such a system will most fully satisfy the requirements listed.

References

1. A. A. Vorob'ev, A. N. Didenko et al. Atomic Energy, 18, 6, 1965.
2. A. A. Vorob'ev, N. P. Didenko et al. PTE (in press).
3. P. I. Gos'kov. Atomic Energy, 18, 3, 1965.
4. P. I. Gos'kov. In: VUZ Proceedings, “Physics,” 3, 1965.
5. P. I. Gos'kov, L. N. Bezmaternykh. Radio Engineering and Electronics, 5, 1965.