MEASUREMENT OF THE MAGNETIC FIELD OF A 300 MeV SYNCHROTRON

V. A. VIZIR, V. N. EPONESHNIKOV, B. N. KALININ, L. G. KOSITSIN,
P. P. KRASNONOSENKIKH, V. N. KUZMIN, Yu. K. PETROV

In the course of preparing the accelerator for startup, detailed studies were carried out of the most important characteristics of the magnetic field of the 300 MeV synchrotron. Special attention was devoted to measurement and compensation of the magnetic field at the moment of injection. Measurements of the field were also made at levels of 5000 and 10000 Oe. As a result of compensation of the magnetic field at the moment of injection, the radial distortions of the orbits, under the simultaneous action of all distorting factors, do not exceed $\pm 1.3$ cm, with a radial width of the region of operating values of the field fall-off index equal to 11 cm. Distortions of the mean magnetic median surface at the moment of injection do not exceed $\pm 0.2$ mm.

Below are described the most important stages of the magnetic measurements and the results of correction of the distortions.

Measurement and correction of azimuthal distortions of the vertical component of the magnetic field

Measurement of the azimuthal distortions of the vertical component of the magnetic field was carried out in the operating regime at levels of 18, 50, 80, 100, 5000, and 10000 Oe.

As a result of these measurements it was established that the field distributions in azimuth at $H_0 = 5000$ Oe and $H_0 = 10000$ Oe coincide with high accuracy. Therefore it may be asserted that the saturation effects of the steel of the magnetic circuit at $H_0 = 10000$ Oe have practically no influence. The orbit distortions at $H_0 = 5000 \div 10000$ Oe amount to $\pm 0.9$ cm.

Since at electron energies of 100–300 MeV the beam dimensions usually do not exceed 1.5 cm [1], the permissible orbit distortion in medium fields may be 2–3 cm. Consequently, the magnet assembly was performed with high quality, and the azimuthal distribution in medium fields is entirely satisfactory.

The total action of distortions of the natural magnetic field in the sectors and straight gaps at the moment of injection deflects the particle orbit from the mean radius by $\pm 5.7$ cm (Fig. 1).

The field distortions at the moment of injection are explained mainly by the influence of residual and eddy-current fields and were studied in detail in works [2, 3]. Compensation of these field distortions at the injection level was carried out by means of a constant field produced by the turns of the correction system [4, 5].

The magnitude of the radial distortion of the orbit was reduced to 1.3 cm (Fig. 1). As the field increases from 118 Oe to 100 Oe, the orbit distortion decreases, approaching 0.9 cm, and remains constant to the end of the acceleration cycle.

Fig. 1. Particle orbits

1 — 18 Oe, \(R = 95\) cm, without compensation;
2 — 18 Oe, \(R = 95\) cm, after compensation;
3 — 18 Oe, \(R = 100\) cm, after compensation;
4 — 1000 Oe, \(R = 95\) cm.

The apparatus and methods used in the magnetic measurements are described in detail in [6] and do not differ from the apparatus used in measurements on synchrotrons of a similar type [7].

Measurement and correction of the magnetic median surface

Measurements of the magnetic median surface were carried out in fields of 18 Oe, 40 Oe, 60 Oe, 80 Oe, and 100 Oe, at radii of 90 cm, 95 cm, and 100 cm. As a result of these measurements it was established that:

1) the magnetic median surface, averaged over the entire machine, lies 3.5 mm below the geometrical one;

2) the maximum deviations of the magnetic median surface at individual azimuths are \(\pm 2.5\) mm (Fig. 2);

3) no changes of the median surface with an increase of the field from 18 Oe to 100 Oe are observed;

4) the slope of the median surface along the radius does not exceed 0.3 mm;

5) the radial component of the field in the straight sections reaches 0.3 Oe;

6) the vertical oscillations of the orbit caused by distortion of the magnetic median surface in the sectors and by the presence of the radial component of the field in the straight sections amount to \(\pm 3\) mm (Fig. 2).

Compensation of the distortions of the magnetic median surface in the sectors and of the radial component in the straight intervals at the injection level was carried out by a constant field [4, 5]. As a result of this compensation it was possible to reduce the mean deviations of the median surfaces from the mean geometrical plane in the sectors to \(\pm 0.18\) mm, and the vertical oscillations of the orbit to \(\pm 0.9\) mm (Fig. 2).

Fig. 2. Vertical oscillations of the particle orbit and curves of the magnetic median surface at the injection level.
1 — before compensation, 2 — after compensation.

Fig. 3. Distribution of the field fall-off index with radius.

Measurement and correction of \(n\) in the operating regime

Measurements of the magnitude of the field falloff index \(n\) and of its distribution along the radius in the operating regime were carried out at fields of 18 Oe, 100 Oe, 5000 Oe, and 10000 Oe. The measurements were performed at 12 azimuths of each sector and were averaged over all azimuths.

As the measurements at fields of 5000 Oe and 10000 Oe showed (Fig. 3), the operating region in \(n\) decreases from 10 cm at a field of 5000 Oe to 8 cm at a field of 10000 Oe owing to saturation of the pole tips. The width of the operating region is practically the same at all azimuths.

Measurements of \(n\) at the moment of injection showed that the operating region is 10 cm at \(n_{\mathrm{av}} = 0.685\) at the mean radius. After correction of \(n\) at the moment of injection, the operating region was 11.5 cm at \(n_{\mathrm{av}} = 0.63\) (Fig. 3).

REFERENCES

1. F. A. Korolev, A. G. Ershov, O. F. Kulikov. Experimental study of electron oscillations in the 680 MeV synchrotron. Electron Accelerator at 608 MeV, Gosatomizdat, 1962.

2. V. N. Eponeshnikov. Influence of eddy currents on the change in the effective angle of a synchrotron with straight sections. Izv. uchebnykh zavedenii, “Elektromekhanika,” No. 6, 1959.

3. V. N. Eponeshnikov, V. P. Kirillov, V. N. Kuzmin. Dynamics of the effective sector angle in an accelerator with straight sections. Izv. vyssh. uchebn. zavedenii, “Fizika,” No. 1, 1960.

4. V. N. Eponeshnikov. Dissertation, NIIYaF EA TPI, 1959.

5. V. N. Eponeshnikov, B. N. Kalinin, V. N. Kuzmin. System for correction of the magnetic field of the 300 MeV synchrotron. Present collection.

6. V. A. Vizir, V. N. Eponeshnikov, B. N. Kalinin, V. N. Kuzmin, Yu. K. Petrov. Instruments for measuring the magnetic field in the 1.5 GeV electron synchrotron. Abstracts of reports at the VI Interuniversity Conference on Electron Accelerators, Tomsk, 27, 1966.

7. G. M. Bozin, L. V. Eremin, Yu. N. Metelnikov, V. E. Pisarev, K. N. Shorin. Magnet and characteristics of the magnetic field of the 680 MeV accelerator.