Control of VEPP-4M Magnetic System
S.Karnaev, E.Kuper, A.Ledenev, B.Levichev, I.Protopopov, Io.Zaroudnev
Budker Institute of Nuclear Physics
Prospekt Lavrentjeva, 11, Novosibirsk, 630090, Russia
Abstract
The use of BINP-designed intelligent controllers for high-stability magnet power supplies allows us tosolve successfully the problem of beam acceleration in the VEPP-4M collider. There are two versionsof the controller. The first has sixteen-output sixteen-bit DAC; the second has a nineteen-bit DAC, anerror amplifier and a shunt. Both versions have digital linear interpolation and MIL/STD-1553interfaces. The aspects of the magnetic system control concerning hardware and software are described.
Results of a field behavior study in magnetic elements with the use of these distributed intelligent
controllers are presented.

1 INTRODUCTION
The VEPP-4M collider provides experiments with electron-positron beams, back-scattered compton γquanta and synchrotron radiation. The circumference of the accelerator is 366 m. The beams in the ringare guided by more than 100 dipole magnets, quadrupole lenses and steering magnets.
The collider is an accelerator with an energy range of from 1.8 GeV at injection to 6.0 GeV.
Therefore, a simultaneous rise of the magnetic fields by a factor of 2.5 - 3 is necessary for
performance of high-energy physics experiments within a range from 4.7 GeV to 5.4 GeV. This
beam energy rise is controlled by the Intelligent Device Controllers (IDCs) based on microprocessors.
Control of the IDCs is performed by the CAMAC-embedded ODRENOK computer [1], whichis integrated into the VEPP-4 control system [2]. ODRENOK loads predetermined settings into IDCsand controls them by commands via a MIL-1553-B multidrop data bus.

2 MAGNET STRUCTURE CHARACTERISTICS
The magnet system of VEPP-4M consists of arc cells (FODO), dipoles, quadrupoles, sextupoles andoctupoles. Auxiliary coils in dipoles, arc cells and steering magnets are used for beam orbit correction.
Moreover, quadrupoles have built-in coils for gradient correction. The VEPP-4M magnetic structure issymmetrical relative to the collision point. Table 1 gives an overview of the magnetic system.
Table 1- The VEPP-4 magnetic system
Magnet type

Number ofmagnets / pow. suppl.

Imax , (A)

Hmax , (kGs)

arc cell (DO, FO)dipole66 / 3
4/2
2/1
8 / arc cells
2/1
2/1
14 / 78/2
arc cells / 4
2/2
2/1
6/6
2/2
21 / 216/6

6400, 550, 170
1600
1100
6400
1500
1700
200 - 400
1350
500 - 700
2700
2000
25
88
25

5.3
6.6
14.5
9.5
6.7
7.5
2-4
8.1

quadrupole

sextupole
wigglersnake
sextupole
octupole
steering magnet
skew-quadrupole

Most of the elements are controlled by their own power supplies. All the magnets and power
supplies have been developed at BINP. The total number of power supplies is about 330.

All arc cells and some of the dipoles are powered by the same power supply. Quadrupole coils(F and D) in the arc cells are used for control of the tunes. The coils of the same type in all arc cells
are energized by the same power supply. The ring chromaticity is mainly regulated by the sextupole
coils in the arc cells. These coils are powered by four supplies.

3 CONTROL SYSTEM ARCHITECTURE
There are four data buses: two are connected to 16-output IDCs, the others are connected to singleoutput IDCs. Such configuration permits the use of common commands for IDCs ofeach type. For
example, “write in” command has
different format for IDCs of different
kind. Thirty remote modules may
be connected to each bus.
CAMAC embedded controllers
developed at BINP are used tocontrol the data buses. Thecontrollers are located in aperipheral
crate connected to ODRENOK
through a 1.6 Mbit/s serial link
driver and controller [3].
Derivation boxes
withoutFig.1 The configuration of the control system.
protective resistors are used tocouple the stubs to the data bus(Fig.2). The length of the stubs isless than 1 m.
The length of the buses is about 300 m. They pass over all the power supply control rooms aroundthe collider. The transmission rate is 1 Mbit/s. Each bus has a 50 ohms matching resistor at the end.

4 HARDWARE
To control the collider magnetic system, two types of IDCs are used. The first is based on the Intel
8086 microprocessor. It has 16 analog output signals produced by an integrated circuit DAC. Thesemodules are used in order to control low-current power supplies. Fig.2 shows a functional blockdiagram.
The second type is based on the Intel 8035
microcontroller. It has one control output,
which provides an amplified error signal.
This type is used for the control of middleand high-current power supplies. A
functional block diagram is shown inFig.3.

Fig.2 Block diagram of 16-output DAC.
Fig.3 Block diagram of single-output DAC.

Both versions have static internal RAM.
A sequence of up to 80 so-called "rows" can beloaded into the RAM. Each “row ” consists of 16
settings (or one setting for single-output IDC) and
one time setting. There exists the possibility ofdecreasing the time scale by a factor of 16. Thatenables control of the "fast" magnetic processes.
An example output signal is shown in Fig.4.
In addition, the microprocessor calculatesa duration in “tick-portions” for each “row” as a"calculate interpolation" command comes.
Sample-hold output circuits are controlled with afrequency of 256 Hz.
"Start", "stop" and"continue" commands are simultaneously sent toall IDCs connected to the same data bus for thesynchronization of magnet control.

Fig.4 Example of interpolated output signal.

Table 2Main features of IDCs
Version of IDC
16-output
single-output
Scale (binary)
14 + sign
19Output voltage (V)
± 6.5
8.192
Error.01%.001%
LSB
400 µV
15 µV
Interpolation time between two settings:“fast” scale
64 ms - 4 sfull scale
1 s - 63 s
Powerconsumption
V+5 V
+24

2A
0.2 A
0.05 A

2.5 A
0.2 A
0.05 A-24V

5 MAGNET CONTROLTo operate the magnetic system (handling, cycling, beam energy rise) different programs use theResident Executive Program (REP) [2]. REP loads tables or single settings into the IDCs and sends
control commands to IDCs via MIL-1553-B.
All of the magnets are made of non-laminated low-carbon iron. Magnetic field to coil current
ratio changes during the energy rise for two reasons. The first is a saturation of magnet yokes. Thissaturation may be corrected by a corresponding correction of the current. The second reason isassociatesd with a delay of magnetic flux penetration into the iron during the process of the energy
rise. The deviation of the ratio is determined from the equation (1).

d = (1 −H) × 100
H st,(1)whereHH st

is the dynamic magnetic field,
is the statis magnetic field at the same current.

The deviation larger than 0.2 may cause the “death” of the beam. This factor limits the speed of thebeam energy rise to 10 - 15 MeV/s, so that it takes 5 - 6 minutes to increase the beam energy from 1.8
GeV to 5.3 GeV.

The use of IDCs allows us to reduce the required time by a factor of 3 - 5. Supplementaryintermediate settings are loaded into the IDC’s memory to correct individual peculiarities of magnets.
These intermediate settings may be defined for each magnet from prior measurements. The magnetic
field of each magnet is measured with the use of the same hall-probe. An example of such correction for
one magnet is shown in Fig.5.

Fig.5 The magnetic field behavior with coil current rise in EM3 dipole.

6 REFERENCES[1] G.Piskunov, Autometria, N4 (1986), p. 32-38 (in Russian).
[2] A.Aleshaev et al., The VEPP-4 Control System, these Proceedings.
[3] V.Kozak, Preprint INP 88-24 (in Russian).