ACCELERATING DEVICE OF A 300 MeV SYNCHROTRON

B. A. SOLNTSEV, V. I. TOLMACHEV

To provide a uniform frequency characteristic over the frequency range 28.2–37.5 MHz, the accelerating device must have a quality factor no higher than 3. Since the principal frequency changes occur in the initial part of the acceleration cycle over several hundred microseconds, while the entire acceleration cycle lasts 30 msec, it proved expedient to divide the entire high-frequency path, including the accelerating device, into two channels. The accelerating device of the first channel must provide acceleration during the process of frequency variation—it must have the required passband and therefore requires a comparatively large pulse power. In the remaining part of the cycle, any resonant system may be used and, since the electromagnet has straight-line sections, a cavity resonator of the usual toroidal type may be used.

The greatest difficulties are presented by the choice of a broadband accelerating system. The practice of constructing proton synchrotrons suggests the possibility of using two types of accelerating devices—cavity resonators loaded with ferrite, and drift tubes. A cavity transformer with ferrite would undoubtedly be the best solution and would make it possible to reduce the required excitation power. However, even the best ferrites in the frequency range of interest to us have an intrinsic quality factor not exceeding 8–9.

Thus, it is impossible to obtain a large power gain with a ferrite resonator. In addition, the construction of the resonators would require the development of ferrites of a special shape, which is economically impractical in the present case. Drift tubes, widely used at present, being an aperiodic system, are structurally convenient. The equivalent shunt resistance of a drift tube with a standing wave [1] is

\[ R_{\text{sh}} = 2 Z_0 \left( \frac{\varphi_{\text{e}}}{2} \right)^2, \tag{1} \]

where \(Z_0\) is the characteristic impedance,
\(\varphi_{\text{e}}\) is the equivalent angle of the tube.

The directly conducting coating of the vacuum chamber may be used as the drift tube. In this case, at the final frequency of the range \(\varphi_{\text{e}} = 2\pi\). In the remaining region the tube length proves to be less than \(2\pi\), thus varying from

$0.76\pi$ to $2\pi$. Since the modulus of the voltage transfer coefficient of the drift tube [1]

\[ |n| = 2\sin \varphi_e/2, \tag{2} \]

the frequency characteristic is highly nonuniform. Figure 1 shows experimentally obtained curves characterizing the change in the input resistance of the cavity over the range with one and two accelerating gaps. As is evident from the figure, the resistance of the cavity has a

Fig. 1. Frequency characteristic of the accelerator cavity

complex character and has a maximum value near the final frequency. A serious problem is ensuring electrical contact between the conducting coating of the cavity and the feeding feeder at high power levels. Here a system of ring electrodes [2] can be used, but the intrinsically low efficiency of the drift tube [low $z_0$ in (1)] is further reduced by the fact that the reactance of the input resistance of the system cannot be compensated in the frequency range by a section of two-wire line.

Taking these circumstances into account, in the 300 MeV synchrotron a toroidal cavity resonator has been used as the broadband accelerating system; its passband is broadened by shunting it with an active resistance. To accommodate the accelerating device in the synchrotron, one straight section has been allocated; therefore the broadband resonator is paired with a narrowband one. Figure 2 shows the design and geometrical dimensions of the paired resonator. In both resonators, in order to reduce the overall dimensions in the capaci-

the input part concentrated capacitances formed by capacitors of the KVKB type are introduced.

Shunting of the resonator, as experience has shown, is more effective the higher the characteristic impedance of the resonator. As a result, the wide-band resonator has a more developed

Fig. 2. Resonator design

Fig. 3. Resonance characteristics of a wide-band resonator.

inductive part and occupies a larger portion of the straight section. With such geometrical dimensions, it is more correct to regard the narrow-band resonator as a radial line. This circumstance affects its tuning, since the value of the capacitance introduced into the gap depends on its location. The quality factor of the narrow-band re-

of the resonator and its shunt resistance are respectively \(Q = 160\), \(R_{\text{sh}} = 250\ \Omega\). To provide an accelerating-voltage amplitude of \(1000\ \text{V}\), a power of at least \(2\ \text{kW}\) is required.

Figure 3 shows the resonance curve of the wide-band resonator before and after shunting. The required passband is obtained with a shunting resistance of \(35\ \Omega\). For shunting, UNU resistors of \(75\ \Omega\), located inside the resonator, were used. The power required to obtain an accelerating voltage of \(600\ \text{V}\) is \(6\ \text{kW}\). The resonators are excited by means of coupling loops. The resonators are located in close proximity to one another, and therefore coupling between them is unavoidable due to the electric fringing fields in the gaps. Although the resonator frequencies are widely separated (the natural frequency of the wide-band resonator is \(30.5\ \text{MHz}\), and that of the narrow-band resonator is \(37.5\ \text{MHz}\)), it proved necessary to carry out the recapture process before the revolution frequency of the electrons in the orbit falls within the passband of the narrow-band resonator. Although in this case a fraction of the accelerated electrons is lost, the narrow-band resonator is not excited by the electron beam, and the total number of captured electrons does not decrease.

REFERENCES

1. Yu. M. Lebedev-Krasin. “Radiotekhnika i elektronika,” 1, 7, 940, 1956.
2. A. A. Vorob’ev, B. A. Solntsev, V. N. Titov. Izv. vuzov, “Radiotekhnika,” No. 2, 246, 1959.