COMMISSIONING OF A 300 MeV SYNCHROTRON

V. P. Anokhin, V. N. Eponeshnikov, L. G. Kositsyn, V. A. Kocheturov,
B. A. Solntsev, V. I. Tolmachev

The commissioning of the accelerator comprised four main stages. At the first stage, correction of the magnetic field with respect to the electron beam was carried out [1]. Observation of the first turns of the beam was performed by means of movable fluorescent screens and shadow flags [2]. The principal difficulties were caused by distortions of the median plane of the magnetic field in the straight sections. Beam deviations due to these distortions reached \(\pm 15\) mm, which constituted a considerable part of the working-aperture cross section, \(40\ \text{mm} \times 100\ \text{mm}\). Radial distortions of the orbit amounted to \(20\) mm. Compensation of the distortions of the median plane made it possible to reduce the vertical oscillations of the beam to \(\pm 3\) mm. By selecting the correction of the first harmonic of the magnetic-field distortion, the radial orbit distortions were reduced to \(5\) mm.

The second stage consisted in obtaining the calculated duration of the quasi-betatron regime [2]. To study the process of electron capture into the quasi-betatron regime, NaI(Tl) crystals with a photomultiplier and an internal betatron target were used. By selecting the initial injection conditions, the calculated duration of the quasi-betatron regime was obtained.

At the third stage, the main attention was devoted to selecting the initial frequency of the accelerating voltage and to adjusting the law of frequency variation. Experience in commissioning the accelerator at a fixed frequency showed that the initial frequency had to be maintained with an accuracy of \(0.5\text{–}0.7\%\) within the range \(28.2\text{–}28.4\) MHz. Owing to the influence of eddy currents in the electromagnet of the ferrite bias, the dynamic characteristic of the ferrite in the initial part lies below the static characteristic, and this difference is related to the magnitude of the modulating current and to the rate of its rise (Fig. 1).

This circumstance made it considerably more difficult to select the waveform of the modulating current and the initial frequency. The solution to the situation that arose was the use of two independent excitation circuits for the electromagnet of the ferrite bias. The first circuit forms a current whose maximum value does not change when the rate of rise is varied. By selecting the elements of this circuit it was possible to obtain a duration of the synchrophasotron accelerator of about \(200\ \mu\text{s}\). Approximately after \(150\text{–}180\ \mu\text{s}\) the second circuit is switched on. By adjusting the parameters of this circuit and choosing the switching moment, the acceleration process was brought up to \(330\ \mu\text{s}\). Owing to the deep frequency modulation, the requi-

…requirements on the accuracy of the modulation law proved to be stricter than in existing synchrotrons. The permissible deviations do not exceed 1–2%. To satisfy this condition, very careful adjustment of the parameters of the shaping circuits was required. The build-up time of the accelerating voltage, owing to the very low \(Q\) of the broadband resonator, is 2–3 \(\mu\)sec; therefore the requirements on the instant at which the voltage is switched on are easily met—it is sufficient for the broadband generator to be switched on in the middle of the interval between the injection pulse and the end of the quasi-betatron regime. This setting of the switching-on instant was carried out with an oscilloscope. During tuning it was found that the largest number of electrons is captured for acceleration when the duration of the quasi-betatron regime is less than the calculated value and amounts to 3 \(\mu\)sec.

Fig. 1. Dynamic and static modulation characteristics of the self-oscillator

The duration of the synchro-phaseotron regime, 330 \(\mu\)sec, corresponds to an energy of the accelerated electrons of 4.8 MeV. After the accelerating voltage was switched off at this stage, circulation of the beam in the chamber was observed for 12 \(\mu\)sec, after which the beam went to the internal betatron target.

At the fourth stage, the electrons were recaptured into the synchrotron regime. Of substantial importance here was the matching of the instants of switching off the first channel of the high-frequency path and switching on the second channel, as well as the magnitude of the rise time of the envelope of the accelerating-voltage amplitude on the narrow-band resonator. The required accuracy of matching, for a rise time of 10 \(\mu\)sec, is 1–2 \(\mu\)sec. At present the dependence of the intensity of the accelerated beam on various parameters of the synchrotron is being studied.

REFERENCES

1. V. P. Anokhin, L. G. Kositsyn. Beam and radiation indication system in the 300 MeV synchrotron. In the present collection.

2. V. P. Anokhin, V. N. Eroshnikov, L. G. Kositsyn. Correction of the magnetic field by the beam and the quasi-betatron regime in the 300 MeV synchrotron. In the present collection.

3. B. A. Solntsev, V. I. Tolmachev. Accelerating system for the synchro-phaseotron regime in the 300 MeV synchrotron. In the present collection.