The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

A Measurement of Ripple Current in the by-pass resistor of Synchrotron Magnets: New Method for Mode Separation Measurement
TakashiAOKI, Masayuki KUMADA*, K.azuto NISHIKIGOURI, Tsutomu GUSHIKEN,Yoshinobu SANO, Tomohito TOGASHI, Eiichi TAK.ADA*, Koji NODA*, and Mitsutaka KANAZAWA*
Accelerator Engineering Corporation
2-13-1, Konakadai, lnage-ku, Chiba 263, JAPAN*National Institute of Radiological Sciences
4-9-l, Anagawa, Inage-ku, Chiba 263, JAPANAbstract

Detail of the measurement and further analysis arediscussed in the following sections.

We report a result of the ripple measurement in theby-pass resistor circuit of HIMAC ·synchrotron main
magnet coils. The result shows clear separation ofcommon- and normal- mode components in the frequency
domain. An interpretation of the data and relevantcharacteristic of the method such as ripple sensitivity arediscussed.

1 Introduction
In order to examine the validity of the designprinciple of the synchrotron power supply and magnet
load circuits, current ripple ratio was measured andevaluated for common and normal mode of the output
current.
In HIMAC, magnet coils were separately
connected for top and bottom pole piece, as shown in Fig.
1. [1] Since a ripple component of the magnetic field of
main magnets can cause an instability and loss of thebeam, it should be reduced as much as possible. We
need to understand the mechanism of ripple formation andtrans:rmss1on. One important clue was pointed out towatch the common mode current of the power-supply
magnet system. Although indirect compared with amagnetic field measurement, current measurement at bypass circuit of the so-called bridging resistor is expected
to be a clear demonstration of the mode separation andalso a handy sensor of high frequency component. An
initial result showed the components corresponding toboth the normal and common modes.
With anapplication of the technique of measuring sum anddifference in leads of top and bottom coil by-pass resistor,
two modes were indeed separated clearly.
Slray Capacitance 2nf

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Bridge Resistor 20 Q
Coillnducrance 4.6mH

2 Detection Method
Detection of a current ripple in the by-pass resistorlead was done by a single current probe. Figure 2 shows
an illustration of the mode-separation method.[2]Common mode current is nothing but the in-phase.
component of the current that runs through schematic lines
of upper and lower magnet coils in Fig. 1, while normal
one is the component that flows from positive output to theupper coils and to the lower coils and returns to thenegative port of the power supply. Thus, in the left
picture of Fig. 2,. oilly the normal mode component can bedetected because of cancellation of common mode· one.
The right side of Fig. 2 depicts the case where oilly thecommon mode is measurable.
The current probe was a Sony Tektronics P6034,
whose sensitivity and conversion ratio is 0.5 mA and 2V/A, respectively. Background noise was found tocorrespond to -130 dBVrms, or 0.63 1-!A in current. A
Hewlett Packard HP35670A four channel FFT analyzer
was fed with the p:robe signal, and analyzed for 10 Hz to52 kHz span. Calibration of the probe and analyzer wasdone with a small current source.

Fig. 2. Illustration of mode-separating detection by asingle current probe. Left: I+J sensing configuration.
Right: 1-J sensing.

3 Result of Measurements

Fig. 1. Equivalent circuit of coil ladder of magnets.

3.1 Mode Separation
A result of direct measurement is shown in Fig. 3,
without mode separation. 1200 Hz (normal) and 600 Hz(common) components are visible in both coils.
When the ripple signal was measured with the mode-50-

T~e 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

separation, the result demonstrated the clear separation ofnormal and common mode components, as shown in Fig.
4.
-50~=: f---------'---+ --+--+-+-+-+ --+-t+-----1
1-----+----1 --1--1--t-t- -+-t-t----i(a)3.2 Spatial Distribution ofthe common-mo~
In Ref. [2], it is calculated that the nomial mode

amplitude does not show the spatial dependence (at least
up to 1200Hz), while the common mode, by construction,varies amplitude along the position in the load. magnet·
ladder; largest at the nearest to the power supply, and nullat the farthest. The common m~ spectrum of Fig. 5
supports the calculation.
3.3 Comparison with result ofconventional method

A conventional method is the voltage measurement
at the output point of the power supply, where positive andnegative output can be monitored simultaneously. The
mode separation was by an internal function of FFI'analyzer. The comparison of.the normal mode result isshown in Fig. 6. It is noteworthy that the normal mode
ripple spectrum in kHz range is similar at the by-passresistor current and the power supply output, whichindicates that the most of kHz region ripple is by-passed
to the resistor as intended.
3. 4 Ripple amount evaluation

Fig. 3. The ripple spectrum before the mode separationis shown for (a)upper coil, and (b)lower coil of a QF
magnet.
Further, the higher frequency region was· also
surveyed and is shown in Fig. 5. · Again, the frequency
relation between normal and cominon is seen clearly.

The result must be evaluated in terms of relativeripple of .the main .coil current. In the present work,
detected current was multiplied by the by~pass resistor
resistance, and the obtained voltage was divided by theinductance of the main coil as measured in a cold test, togive the amount of ripple current in the main circuit.
This current was then normalized to the full excitation·specification value of the current. The inductanceevaluation is problematic in this procedure, and is tested
with BM case for up to 1800 Hz that the coldmeasurement value can be applied as a constant value.
Quadrupole (Focusing) Magnets

The result was 0.5 ppm for normal mode (both 100and 1200 Hz), while for common' mode 0.1 ppm at 600
Hz. These values are consistent with the result froiD. theconventional method.
Dipole (Bending) Magnets

The normal mode was measured when active filter
was turned off. The resultant value was about 4 ppm..
As. for the common mode, the design features thesuppression of this. mode, and indeed the observed value
was 0.05 ppm at 50 Hz, which arises from the error andimperfection of the power supply construction.
Fig. 4. Th.e ripple spectra of (a)normal and (b)common
mode current. In (a), 1200Hz component is clearly seen,while (b) shows 600 Hz dominantly.

4 Discussion

The separation of normal and oommon modes in a-.51:-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997-55
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Fig. 5. Mode separated ripple spectrum of the QF magnet coils. Top pictures are for the coils nearest to the power
supply. Bottom pictures are for the farthest. In both c~ses, the left panel shows the normal mode, and the right, thecommon.
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design guide line of treating the common mode component
as applied to HIMAC power supply system.
To evaluate the frequency characteristic of themagnet coil, which strongly affects the result of present
current ripple measurement, we have been studying· abehavior of the (magnetic field) search coil output withand without by-pass resistor. Preliminary analysis of theratio in frequency domain seems to be helpful todetermine the frequency characteristic of the main coil.

Acknowledgment
The. authors would like to express their sincere
thanks to Profs. S. Matsumoto and K. Sato for invaluable
discussions. They are grateful for encouragements byDrs. K. Kawachi and S. Yamada. Their gratitude also
goes to AEC members, who helped us in various stages ofthe present work

References
Fig. 6.
The comparison of normal mode ripple
measurement. (a)voltage measurement at the output
terminal of the power supply, and (b )current measurementof the by-pass bridging resistors.
system of magnet coil and power supply was clearlyobserved. The normal to common attenuation is about
60 dB. The present result suggests the validity of the[I] K. Sato, Proc. 4th China-Japan Symp.(Oct.'90) p.151.
K. Sato et al., Proc. 8th SAST('91 )p.31.
M. Kumada et al., ibid., p.l99.
[2] T. Aoki et al., Proc. 2nd Symp. Power Supply Tech.
for Ace., INS-T-542, Mar. '96, p.23. (in Japanese)[3] M. Kumada, Ph.D. thesis, GUAS, 1996.

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