MAGNETIC FIELD CORRECTION SYSTEM FOR A 300 MeV SYNCHROTRON

V. N. EPONESHNIKOV, B. N. KALININ, V. N. KUZMIN

The need to correct the magnetic characteristics of the accelerator follows from the following considerations:

1. The assembly of the magnetic core is carried out with a certain accuracy; therefore there is always possible scatter in such important field characteristics as the average levels in the sectors and the median surface.

2. In designing and assembling the magnetic core it is impossible to take fully into account in advance the influence of such factors as eddy currents, residual magnetization, and the change in magnetic permeability with increasing field; therefore it is important to be able to correct their influence on the shape of the orbits throughout the entire acceleration cycle, and especially at times close to the moment of particle injection.

3. During the start-up of the accelerator it is necessary to correct a number of magnetic-field characteristics in order to weaken the influence of various devices introduced into the air gap of the electromagnet after magnetic measurements.

The system for correcting the magnetic characteristics of the 300 MeV synchrotron consists of the following circuits: correction of the radial component of the field in the straight sections, correction of the effective angles of the sectors, correction of the magnetic median surface, adjustment of the field fall-off index averaged over the entire machine, and expansion of the radial region of permissible values of the field fall-off index.

All magnetic-field characteristics are corrected by means of a constant field near the moment of injection. The correcting fields are produced by a system of turns located directly in the air gap of the accelerator, on the yoke of the magnetic core, and in the straight sections.

The pole windings are located in the air gap of the electromagnet directly on the pole surfaces. They include the turns of the median-surface correction windings, half-turns of the windings for expanding the radial working region and for adjusting the average field fall-off index.

The turns (PEl-0.75 mm wire) are laid in grooves of a frame made of electrical cardboard 1.75 mm thick. The grooves in the frame were made by means of a special circular device with a cutter. The grooves are cut along concentric circles relative to the equilibrium orbit every 3 mm with an accuracy of ±1 mm. In all, on the pole winding there are arranged—

63 turns were laid. The wire was coated with BF-4 glue and placed in the grooves. For mechanical strength, the top of the cardboard frame was covered with cotton fabric.

After preliminary degreasing of the magnet poles and drying, the frames were glued to the pole surfaces with BF-4 glue.

All connections of the turns in the winding were made with MGShV-0.5 mm² wire through terminal strips located on the end surfaces of the sectors. The windings for correction of the median surface are located on the upper and lower poles of the magnet and consist of 21 turns each. The turns are uniformly distributed over the pole at intervals of 9 mm. The direction of current in the half-turns of the lower pole is opposite to the direction of current in the half-turns of the upper pole. Adjustment of the median surface in each sector is carried out independently.

The windings for adjustment of the field decay index averaged over the entire machine are located on the upper and lower poles of the sectors and consist of 15 half-turns on each pole, placed uniformly along the pole winding at intervals of 9 mm. In order that the current in all turns have the same direction, 15 return half-turns are provided for each pole, located in the window of the magnetic core. To compensate the induced emf, 8 turns located on the yoke are connected in opposition into each winding. Adjustment of the mean field decay index is carried out simultaneously in all sectors by one and the same amount.

The windings for expanding the working region of the magnet consist of 11 turns located on each lower pole. The placement of the turns on the pole winding was carried out according to the results of magnetic measurements. The direction of the current passing through the outer half-turns of the windings is opposite to the direction of the current passing through the inner half-turns.

Expansion of the working region is carried out simultaneously in all sectors.

On the yoke of each sector, 20 turns wound with LPRGS-0.75 mm² wire are arranged symmetrically with respect to the midplane. These turns constitute windings for compensation of the 1st and 2nd harmonics of the magnetic-field asymmetry in the air gap and for compensation of the emf induced in the turns of the pole winding.

Windings of 17 turns (MGShV-0.5 mm² wire) are wound on the end blocks of each sector for correcting the magnetic angles of the sectors (yoke correction). Each pair of such windings in a sector is connected in series. The current in these windings is adjusted independently in each sector. In addition, to correct the vertical component of the magnetic field in the straight section, windings located on the ends of the sectors near the lower and upper poles of the end blocks are used. These windings are connected in series over the entire electromagnet and produce the main correcting field. Yoke correction makes it possible to eliminate the difference in the correcting field in individual straight sections.

To compensate the radial component of the magnetic field in the straight sections, special coils are used; these are slipped directly onto the vacuum chamber near the ends of each sector. In each straight section, a pair of coils is connected in series, and the current through it is adjusted independently.

All electrical circuits for magnetic-field correction are brought to a single control console, on which are concentrated measuring instruments, potentiometers for current regulation, chokes for suppressing induced currents, indicator lamps, and switches.

Power for the correction circuits is supplied by regulated semiconductor DC sources. The yoke field-correction circuits are powered by an electromechanical amplifier with a current of up to 5 A. The remaining correction circuits are powered by a DC motor-generator with a voltage of 40 V.

When the synchrotron is operated for optimum radiation, the following correction currents are set: median surface—40 mA, adjustment of the field index—30 mA, expansion of the working region—18 mA, first harmonic of the magnetic-field nonuniformity—100 mA, compensation of the radial component of the magnetic field in the straight sections—0–180 mA, correction of the effective sector angles: a) yoke—200–780 mA, b) end—2460 mA.