The 9th Symposium on Accelerator Science and Technology, Tsukuba, Japan 1993

MAGNETIC FIELD MEASUREMENTS AND DATA ACQUISITION
OF A MODEL MAGNET FOR THE B-FACTORY
Zhou Weruning* and Kuninori Endo
National Laboratory for High Energy Physics
1-1 Oho,Tsukuba-shi,Ibaraki-ken 305, Japan

ABSTRACTIn this paper we describe magnetic field measurements and the fielddata-acquisition system used to measure the model magnet for the Bfactory booster.
The results of the measurements indicate that the method adopted
here is good for acquiring field data . This type of measurement ishighly accurate and involves almost no. temperature coefficient. The
instrument is used not only for ac, but also de field measurements. Itis especially good for field measurements in the case of simultaneous
ac and de field excitation .

performed during the measurement. The data-acquisition systems
are shown in figurel.
The acquired data are plotted and displayed on a screen, stored in thehard disk of the computer and printed by a printer. If some acquired
data is incorrect, it can be found immediately.
MAGNETIC FIELD DERIVATIONThe synchrotron magnetic-field wave form is shown in figure 2. Its
wave form is expressed as
B(t) = Bdc + Bac sin rot,(1)

where B(t) is the magnetic field,
Bdc the de magnetic field,

DESCRIPTION OF THE DATA ACQUISITION SYSTEM

Bac the ac magnetic field (amplitude) andThe proposed B-factory booster will be a rapid cycle-type
accelerator. Its repetition rate is to be 50 Hz. A modified existing
dipole magnet for the TRISTAN -AR is being used in the simulation
research concerning the resonant magnet network and the power
supply. The present work is part of a study on the rapid-cyclesynchrotron magnet and power-supply system.
A measurement probe is attached to a table and has a small flip coilwith 2065 turns and is movable over a region of 700 mm x 1500 mm
on the measurement table. The positional error of the probe is about
l'10 !lni at each step and less than
65 11m which is the error
accumulated when traveling the entire table.
The computer controls dynaserv 1 through fineserv 1 in order tochange the probe position. I) An integrator controls dynaserv 2
through fineserv 2 in order to flip and flip back the probe coil. Afterthe induced voltage of the flip coil is converted to digital data in theintegrator , data is acquired by the computer.

±co the angular frequency of the ac field.
D·tc\;zD±

0

Figure 2 Magnetic field wave form
The inductive voltage of the flip coil in a magnetic field is calculatedusing the following· formula:e(t) = d<l>(t) = dNA(Bdc + Bac sin mt)COSCOrf,dtdte(t) = NA[-co1Bdcsinco1t +whereN(2 )(3)+Bac (cocoscotcoscort- co1 sin cot sin corf)].
Here N is the number of the turns of the probe coil,
A the average area of each tum of the probe coil,f the angular frequency of the flip andCO

co the angular frequency of the ac field.

T = 2rc
coand

(4)

when
Tr=kT

(k=1,2,3, ... ) .

(5)

Figure 1
The area of the magnet pole face is 200 mm x 1240 mm.
Measurements are automatically performed point by point at aninterval of one centimeter on the medium plane of the magnet. Dataacquisition and the movement of the flip coil are alternatively

Also* On leave of absence from the Institute of Modern Physics,(6)

Lanzhou, The People's Republic of China.

-276-Integrating last two terms from 0 to T r/2 are zero respectively inEquation (6),we obtainfT 1 !2E = Joe(t)dt = -2NABdc ·(7)When the probe coil is stationary (co f = 0) the induced voltage is

dNAB sinwt= NAB.cwcoswt
.u:
dt

(8)

J(9)

e,(t) =and

T/4

E, = -T/4 e,(t)dt = 2NAB.c ·When Bac= 0, the induced voltage of the flip coil is.
_ dNABdccosml_--NABdcm1 smm1t
e1 (t)-(10)

dt

(11)

andEvery E is converted to digital data corresponding to the magnetic
field, and is then stored.
CONTROL AND DATA-ACOUISIDON PROGRAM
All operations are controlled by programs stored in the computer.
The probe coil is returned to the coordinate origin of themeasurement table before carrying out any measurement. Dataacquisition and control programs are written in assembly and BASIClanguages, respectively.
The probe coil is flipped by 180 degrees, and then flipped back toorigin while waiting for the next flip. Analog-to-digital conversion iscarried out during the period of the coil flip. The data are acquired
on-line by the computer. The probe is moved by 6.x=10 mm withdynaserv 1 . This procedure is repeated until the measurement along
the x-axis has been completed. The probe coil is then returned to theorigin of the x-coordinate and moved by 6.y=l0 mm in the ydirection, and measurements along the x-axis are repeated.
The measurements are completed for a total of 1911 points over an
area of 120 mm x 1460 mm. Field mapping all over such an area
takes about 2.0 hours.
Acquired data are stored in the main memory during themeasurements. After all data have been acquired, they are
inunediately stored on a hard disk as a data file. The control and dataacquisition program as well as its block diagram are given in theappendix.

DC field measurements are carried out after waiting for a warm-up
time of 40 minutes. Both the accuracy and the reproducibility of themeasurements are better than± lx10 4 .
When the probe coil is stationary, the induced voltage appears acrossthe coil due to the time-dependent magnetic field. If the voltage isintegrated from the bottom to the top of the ac field, Bac is obtained.
If the probe coil is flipped by 180 degrees, only Bdc is obtained underboth ac and de excitations of the magnet.
Both Bac and Bdc are obtained using integrator "b", in which theinduced voltages are converted to frequency by two VFC's (voltage tofrequency converter).
The frequency of the induced voltage is about 50 Hz in a B ac
measurement. The timing circuit, which is based on the same
frequency, generates two timing signals. Their periods correspond to24.5 and 49.5 times the B ac cycle, respectively. The .frrst signal
determines the integration time, and the second determines themeasurement cycle. The initial time of every measurement cycle isdeferred by ten milliseconds. The measurement cycle is just one
second.
A total of 24.5 cycle's pulses converted from the induced voltage ofthe probe coil are counted by a counter . If the induced voltages are
symmetric in both half cycles, the pulses of the positive and negative
half .cycles of the induced voltages cancel each other every cycle.
The counter gives no output during the first 24 cycles. Only the last
half cycle's pulses of the induced voltage are counted by the counter
during the integration time.
After waiting for the integration time and an additional 101
microseconds, the output register transfers data. After another 101
microseconds delay, data are acquired. The probe coil is moved andthe next measurement cycle starts.
When the probe coil is flipped during a magnet field measurement,from equation (3) the induced voltage has two parts caused by Bac
and Bdc. The pulses caused by Bac are canceled in the counter. No
contribution is made to Bdc· The measurement procedure is similar tothatofBac·The relations between the magnetic field, inductive voltage, flip andintegration time in the integrator "b" are shown in figure 3.
B(t)Oe(t):"\~

os"1vvt"[e(t)"\~~-Acqilo

I .-====:!

o \;j~V~

flip
50Hz_
Ref.

MEASUREMENT
Two kinds of integrators ("a" and "b") are used for themeasurements. When no ac field exists, we use integrator "a" for defield measurements. Data acquisition follows the procedure described
below:A trigger pulse from the computer is transmitted to integrator "a".
Both gates of the ADC (analog to digital converter) and the accumulator are opened. The integration timing circuit begins to work,
and within 20 milliseconds the flip coil starts to flip. As soon as theintegral time, which is selected to be within the 0.5 to 0.9 second
range, reaches pre-setted one, the integration timing circuit and ADC
are closed. After 10 microseconds the output register transfers data.
Another 100 microseconds later the accumulator is cleared. Then,
within 20 milliseconds the flip coil starts to flip back to the original
position. The probe coil is moved and the next new measurement
cycle starts.

AA

Tvg

y · -LJ_ LJ_j·I
Bac __jf--------'

gateD.B

1~gat~;ft'e_jpuis~

strobeIIIA

A~cLV v;vBa:
DA

rv _v

/"'\v

/

1hvfAoA L
J! 1saoI:~
I

I~~----.·'tl0.45 s'i-c;\24.5 cycle)
11

4, 1~ lQln<,....

1:~====~-B-~~,e~~==~===~~~1 rub~~Figure 3, Time chart of the flld,voltage,llip and integration time

Bdc was measured with integrators "a" and "b". The difference
between the two kinds of azimuthal magnetic-field distributions at-277-y=O is plotted in Figure 4. The maximum difference is 2.5 X w-2 at
the fringing region. The magnet center is located at 66 em on theabscissa.

ACKNOWLEDGMENTS
It is our pleasure to thank associate professor T.Kubo andMr.Y.Ohsawa for their valuable help in this work.

0.005

*<.>

REFERENCE

0······-········-·······

0)

5...

-0.005~-0.01

Q-0.015~-0.02l!

illl!lll!

il!

!

::::::!!

lAPPENDIX

l·······i·······-·t········!--·······i--··--···i·········i--·······;········ ·······r·······-0.025 03366

99

Longitudinal distance(em)

:

132

165

Figure 4 Difference in the Bdc field whether Bac is applied or not*) difference(normalized) = Bdc <Hac not exist)-Bdc<Bac exist).

The effective magnetic field length is calculated by the following
formulae:and(1) The words "dynaserv" and "fmeserv" are the brand names of theservo motor and its controller, respectively. Products of Yokogawa
Precision Co .

Bl= [B(x)dx(13)== (B_, + B_.-.1 +... +B0 +... +B,_ 1 + B,)tll

L= Bl .

1, Control and data-acquisition program block diagram.
2, Program list:
Program BDC. BAS is for control and data acquisition.
Program READBDC. BAS is for reading data from the disk file.
Program PLOT. BAS is for plotting both the radial and azimuthal
field distributions.
Program LENGTH . BAS is for calculating of the effective
magnetic length between y=6cm and y=-6cm .
Program PLODIF. BAS is for plotting the difference in Bdc when
B ac is applied or not .
All of the above program are stored on the hard disk of thecomputer (PC-9801 ns/t ).

(14)

B.
where Ba is the average field at the central region.
-x to x is the range of the measurement ( x is far from themagnet fringe ), and~x is the measurement step .
The effective magnetic field lengths under lac and Ide excitation areabout one centimeter longer than those under only Ide excitation in
the useful radial region. They are listed in the table below:
Table Effective magnetic field length in emyldc=1000A,Iac=566 A

Idc=1000A, Iac=OA-6

126.81126.01-5126.93

126.27-4127.15

126.44-3127.29

126.54-2

127.37126.62-1

127.67126.660127.72

126.671127.61

126.66
126.632127.57

3127.53

126.564127.47

126.465127.57

126.336127.29

126.10Figure 5 Program BDC.EAS block diagram-278-