DESIGN OF THE ELECTROMAGNET OF A 300 MeV ELECTRON SYNCHROTRON

V. N. EPONESHNIKOV, L. G. KOSITSYN, V. N. KUZMIN, G. A. SIPAILOV

Introduction

The 300 MeV synchrotron was designed and built as a model of a 1500 MeV synchrotron; therefore, the design of the electromagnet of the 300 MeV synchrotron was chosen so as to model as fully as possible the necessary characteristics of the 1500 MeV synchrotron.

Since for the electromagnet of the 1500 MeV synchrotron a “racetrack” type design was chosen, with four straight sections and a C-shaped cross section of the plates in the meridional plane, with the gap facing outward, the same design was adopted for the electromagnet of the 300 MeV synchrotron. The principal characteristics of the electromagnet of the 300 MeV synchrotron are given in Table 1.

Table 1

Shape and Profile of the Poles

The shape and profile of the poles were selected experimentally on models of the electromagnet. As a result of the experiments carried out, a pole bevel angle was found that corresponded to the minimum stray-field coefficient. With a bevel angle of \(75^\circ\), the maximum induction on the orbit \(B_0 = 10.7\ \text{kGs}\) is achieved at an induction in the steel \(B = 15\ \text{kGs}\). The angular opening of the pole surfaces is \(2^\circ54'\).

On the radial edges of the pole surfaces, ledges are made which strengthen the field near the edges and thereby broaden the working region with an admissible value of the field fall-off index.

Fig. 1. Magnet-yoke plate

The magnet-yoke plate with the adopted shape and profile of the poles is shown in Fig. 1. The projections on the outer part of the poles were intended for attached betatron cores; however, during startup of the synchrotron the betatron cores were not used.

Magnetic Yoke

The electromagnet of the 300 MeV synchrotron consists of four identical sectors separated by straight sections. Each sector has a geometrical angle of \(83^\circ28'\). The straight sections are \(0.6\ \text{m}\) long. The magnetic yoke is made of electrical steel É42, \(0.35\ \text{mm}\) thick.

In order to simplify the assembly technology of the magnetic yoke, and also to obtain a more rigid structure, the plates were assembled into monolithic wedge-shaped blocks. Each sector of the magnetic yoke consists of six such blocks; in turn, each block consisted of 26 packets. To ensure the annular shape of the magnetic yoke, a packet was assembled from 10 whole, uncut plates and from 16 cut plates, one of each size (Fig. 2). During assembly of a block, insulating spacers made of cable paper were placed between the individual packets to avoid shorting of the plates. To obtain the geometrical angle of the magnetic-yoke sectors, \(83^\circ28'\), from the outer blocks

Fig. 2. Package and block making up the pole.

about 5.5 stacks were separated; as a result the middle 4 blocks occupy angles of \(15^\circ\), and the end blocks of each sector—\(11^\circ 44'\) each.

To obtain monolithic blocks, the following manufacturing technology was adopted. Burrs were removed from the stamped plates of all positions; then the plates were washed with gasoline and varnished. Varnish No. 3256 was used for varnishing. After threefold varnishing and drying, the plates went to assembly. The assembly of the blocks, their

Fig. 3. Base of the electromagnet

pressing and baking were carried out in a special press mold made of thick-sheet steel. During baking, which lasted 10 hours at a temperature of about \(140^\circ\text{C}\), the block was additionally pressed in the press mold by tightening bolts. After cooling, the block was removed from the press mold and tested for electrical and mechanical strength; after that magnetic measurements were made in order to determine its characteristics. During assembly of the magnetic core, the characteristics of the individual blocks were taken into account.

Each sector of the magnetic core is installed and completely fastened on a base, which is an arcuate plate 30 mm thick. The outer radius of the plate is 1.188 m, the inner radius is 0.315 m. From below, paired channels are welded to the plates along radii of 1.0 m and 0.4 m, by means of which the plates are reliably connected to the reinforced-concrete foundation. On top of the plate an inner fastening ring is welded, which along a radius of 0.36 m has 8 holes for studs, by which, during assembly, the clamping ring fitting into the groove of the magnetic core is fastened (Fig. 3).

Along a radius of 1.136 m, a support ring is welded to the surface of the plate, consisting of 8 sections; in each section there are 2 holes for clamping bolts. Between the support ring and the magnetic core an outer fastening ring with an inner radius of 1.073 m is placed. By means of the clamping bolts, and the outer and inner rings, the blocks of the magnetic core are fixed in the radial direction (Fig. 4).

To fasten the magnetic core to the base, clamping rings are inserted into the grooves of the magnetic core. The inner ring is pulled by studs to the inner fastening ring, and the outer ring, consisting of individual sectors, is pulled to the plate by bolts (Fig. 4).

Fig. 4. Fastening the magnetic core to the base

To compress the blocks in the azimuthal direction, two supporting No. 10 channels are welded in the lower part to the base plate. Holes with nuts welded to them are made in the channels. The pressure produced by the bolts is transmitted to the blocks through an end plate 12 mm thick, having the shape of the magnetic-core plates.

To secure the blocks in the upper part, tie arcs are used, which enter the grooves of the magnetic core. With their threaded ends, the tie arcs enter the lugs of the ribs of the end plate, forming a fastening frame by means of which the necessary pressure is created on the blocks of the magnetic core. The inner tie arcs (upper and lower) are structurally connected to one another.

Excitation winding of the electromagnet

The excitation winding is characterized by the following data: number of coils per pole \(N_p = 6\), number of turns in a coil \(W_k = 16\), number of coils for the entire electromagnet \(N_k = 48\), conductor cross section \(q = 4.7 \cdot 12.5 = 57.9\ \text{mm}^2\), average length of one coil turn \(l_w = 3.8\ \text{m}\), copper weight of the winding \(G_i = 1500\ \text{kg}\).

The coils of one pole are connected in series with one another; the coils of two poles of one sector are connected in parallel; between sectors a series connection is made; thus, the number of parallel branches is \(a = 2\). The connection diagram of the winding is shown in Fig. 5.

In each sector the voltage drop is about \(2.2\ \text{kV}\). The current input for both the upper and lower poles is made into the upper coil, and the output is from the lower one.

The coils were wound on a template. When winding the coils, the turn insulation was reinforced with one layer of micalenta. The micalenta was applied on the 12.5 mm side and fastened to the bus with taffeta tape wound with a gap.

Fig. 5. Electromagnet (top view)

With the number of turns in a coil \(W_k = 16\), the minimum number of turns per pole \(W_p\), and for the entire electromagnet with parallel connection of all coils, will be \(W = 16\). However, this gives rise to the inconvenience of bringing out the inner ends of the coils. In order to eliminate these difficulties, the coils were made doubled, with a rigid internal connection.

Fig. 5. Connection diagram of the excitation winding

The number of series-connected turns in the doubled coils is \(W = 32\). Each coil was insulated with five layers of LMCh-1 mica tape, 0.13 mm thick, half-lapped. After connection of the inner ends with silver solder, the doubled coils were given frame insulation consisting of five layers of mica tape applied half-lapped. During the insulation process the coils were subjected to triple compounding.

The excitation winding was fastened in the magnetic core by means of special jacks, which spread apart the coils of the upper and lower poles and were distributed uniformly over the entire perimeter of the coil. The output ends of the coils were mounted in distribution cabinets.

Conclusion

All units of the electromagnet of the 300 MeV synchrotron were manufactured in accordance with the design and technology developed at NIIYaFEA TPI, with the exception of compounding of the excitation-winding coils, which was carried out at NTGZ.