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

Nondestructive DC Beam Current Monitor with NANO-Ampere ResolutionYuuichirou SASAKI, Kichiji HATANAKA, Kenji SATO, Tetsumi TANABE*,Shinpei ONO*, Kouji NODA**, Kei SHINADA***, Yasuharu YAMADA*** and Kouichi NAKANO***
Research Center for Nuclear Physics, Osaka Univ.,
Osaka 567 JAPAN*Center for Nuclear Study, School of Science, Univ. of Tokyo,
Tokyo 188 JAPAN**National Institute of Radiological Sciences,
Chiba 263 JAPAN***Technology Research Laboratory, Shimadzu Corporation,Kyoto 619-02 JAPANAbstract

A beam intensity monitor using a SQUID is being
designed in order to measure the ion beam intensity ina non-destructive manner. Prior to the design procedure, some fundamental tests were performed with models. Measurements were performed to investigate sensitivities, shielding effects, and noise levels of the system.
Results are described, and the design of a cryostat is alsopresented.
1 Introduction

There are various detection systems developed tomeasure beam intensities from accelerators. Scintillators
cover the region up to 106 particles per second (pps),
corresponding to electrical currents up to 1 pA. Current transformers of the fluxgate type can cover the region over some Jl.A in a non-destructive manner. Theregion in between can be covered by ionization chambers (IC) and secondary emission monitors (SEM). But
these systems more or less distort the ion beam due tothe energy loss and the beam emittance growth by scatterings. Another type of beam transformer, using theprinciple of a Cryogenic Current Comparator 1 (CCC),
is being designed at the Research Center for Nuclear
Physics (RCNP), Osaka University in collaboration with
the National Institute of Radiological Sciences (NIRS).
In the design, a DC-SQUID is adopted to improve theresolution. The nA resolution is expected.
2 A SQUID System

If a de bias current is fed to SQUID, the voltage V,changes periodically depending on <I>x with <I>o period.
Where <I>o is the flux quantum; <I> 0 = 2.07 X 10- 15 Wb.
<I>p is proportional to the beam current h. The current
I1 is fixed under the condition h = 0, and the flux
<I>x is locked. When h is fed, the feedback current If
flows to keep the flux <I>x constant. This change in If isproportional to the current h.
In the present design, a double washer DC SQUID
developed as a magnetic-flux sensor for a multichannel
biomagnetometer at Shimadzu Corporation 2 was usedas the magnetic flux sensor. The measuring principle isthe same as described above.

beam current

lbFig. 1 Circuit diagram of a SQUID and flux lockedloop.

Figure 1 shows the circuit diagram of a SQUID anda flux locked loop. SQUID, the pick up, the input andthe feedback coils are coupled.

M;,Ip + Mf,If = <I>x

where Lp is the pick up coil inductance, L; the input
coil inductance (200 nH), MJi is the mutual inductance
between the input coil and the feedback coil (6 nH),M;, between the input coil and SQUID (5 nH), MJs
between the feedback coil and SQUID (2.2 pH), Ip is thecurrent through the input coil, If through the feedback
coil and <I>p is the flux which penetrates the pick up coil,<I>x penetrates the SQUID washer.

3 Test with Small Size Models

Models were fabricated to examine the performanceof the system. Figure 2 shows one of them. The system consists of a magnetic shielding, a pick up coil, anda SQUID circuit. The beam current is simulated witha one turn loop around the magnetic shielding. The
loop current induces an azimuthal magnetic field. Because of the shielding structure, the azimuthal component of the magnetic field can p'enetrate into the shielding with small attenuation, while other components arestrongly attenuated. In order to realize high sensitivity, it is necessary to reduce noises around the pick up-255-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City,coil and the SQUID. Measurements were performed toinvestigate sensitivities, noise structure s, and shielding
effects of the system using a FINE MET (Hitachi Metals,FT-3L, FT-3M) or an amorpho us (VITROVAC 6025-F)
toroidal core surround ed by 4 turns of NbTi ·wire.

1997

3.2 Noise
The origins of noises are supposed to be electromagnetic noises, mechanical movements of the system,
and from the characteristics of toroidal cores. The frequency of main electromagnetic noise is 60Hz. Mechanical movements of the system give 1-30 Hz noises. Table
2 shows the measured noise levels using four kinds of atoroidal core. The noise level with aFT-3M core is 2 orders of magnitud e larger than those with a VITROVAC
6025-F or a FT-3L core. This may come from the factthat the lemanen t magnetic flux density of FT-3M ishigher than those of VITROVAC 6025-F and FT-3L.
Table 2The measured noise levels using four kinds of a toroidal
core. The low pass filter was not installed.

...

Kind ofa toroidal core

NoiselevelFeedback
resistance

VITROVAC 6025-F

40 mV

4.7 kn

FT-3L

15 mV

1 kn

FT-3M

1500 mV4.7 kn

au

18 mV

1oo knPick upFig. 2 Schematic drawings of the model. The magnetic shielding is a folder type with five ringtype shield elements. A FINEME T or an amor-.
phous toroidal core surround ed by 4 turns of
NbTi wire is used as the pick up coil.

3.1 Sensitivity
Responses were measured with 600 nA, de current.
Sensitivities are independ ent of the number of shielding elements, but they depend on the kind of a toroidal
core. Table 1 shows obtained sensitivities. These values
are consistent with those expected from the calculation.
When a FINEME T or an amorphous toroidal core wasused, sensitivities were about 1 m V/nA-kn. With an aircore, sensitivity was lower (1/500 of magnitud e) thanthose with a VITROVAC 6025-F or a FT-3L toroidalcore.

Noise spectra were measured. It was found that eachcore has noises of character istic frequencies. Noises at
15, 30, 32, 33, 40, 72, and 80Hz appeared in case of aFT3L toroidal core (see Figure 3). In case of a VITROVAC
6025-F toroidal core, a noise at 32 Hz appeared , and inaFT-3M toroidal core at 33 and 90Hz.

-20.0
dAVIIIz

VITROVAC 6025-F

Sensitivity
3 mV/nA

Feedback
resistancei_\

Table 1
Sensitivities obtained with models.

Kind ofa toroidal core

! :' I 1:las -Gd. 7969 d~V\Ii~ !I!I -66.7969 BVIH~ I I

1\0.92 mV/nA

1 kn

FT-3M

8.3 mV/nA

4.7 knau

0.14 mV/nA

100 kn:<.

i·-......I::!

; ! !! I
I ! :!
I-120.0d8VIIIz

IItcs.om:!

! : j!
; :·~·!i":~<Gaa>
dBMg:''\4.7 kn

FT-3L!

'i::'.

'i ::.';

iI'!i i'--:.,~~ j ' ! l r:''.

Ij~:; '1/';~v~·:;' ,,Ill!

·~,..~U.

IIil~~l1'~'~'''·~

I!: lI;I I ! I·Jt

[ j.

.

I' I

FllE!liiLNCY liz

I! I·i100.0

Fig. 3 The noise spectrum measured with a FT-3L
toroidal core. The low pass filter was not installed.

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

A low pass filter whose cut-off frequency was 10 Hz
was installed in the system. When the low pass filter
was used, the noise levels were become 1/3 of. magnitude smaller than those values in Table 2. Main effectis that 60 Hz electromagnetic noise was reduced to 1/3of magnitude. The measured sensitivities were the sameas those in Table 1. Figure 4 shows a series of 10 nA
test pulses with the low pass filter. The signal indicatesa movement of the base line in the order of 0.5 nA/s.

-400.0rnVTable 3
The shielding efficiency.

!

~- ___!_~-~-____ _L_'I'.

Shieldingelements

Blli

Bj_f

1 ring20 nA

730 nA

5 ring

9 nA

30 nA

2 coaxial

less thanless than

8 X 10- 3 nA

2 X w- 2 nA

Xa

-500.0mV

u.0

TIME sec

8.0

ceramics are inserted into both the beam tube and theliquid He container tube to avoid effects from wall currents. Side walls of the cryostat can be dismounted toexchange a shield and a SQUID system. Three layers ofheat radiation shields, two super insulations and a copper shield, are equiped to minimize the heat conduction.
Stock volume of liquid He is 15 I. The cryostat is now
under construction.

Fig. 4 10 nA pulses, 0.5 s pulse width with a low
pass filter. A FT-3L toroidal core is used.
(horizontal axis: 0.8 sec/div., vertical axis: 10mV /div.)=

F=r;:::: ~3.3 Shielding Efficiency
The shielding efficiency was studied for three cavities with a VITROVAC 6025-F toroidal core. They are
the cavities which have one ring type shielding element,
five ring type elements and two coaxial type elements,
respectively. Each cavity has been exposed to uniform
external magnetic fields with Helmholtz coils wound outside of the cryostat. An external field of 10- 5 T gave
signals corresponding to 10-700 nA (see Table 3). Whenthe cavity which has five ring type shielding elements
was exposed to a transverse external field of 10- 5 T, thesignal corresponding to 30 nA was yielded. This value
is lower by 1/5 times than expected from the calculation, but it is still higher by an order of magnitude thanthat required for the environment in which this system
will be installed. When the cavity which has two coaxial
type elements was exposed to a transverse external fieldof 10- 5 T, the change of signal could not be observed.
The theoretical prediction is 5 x 10- 4 nA. This smallchange cannot be resolved in our experiment because of
the limits of measurement.

4 The cryostat~I(_ .J.
... ~~~Ir- · - - ·..

I'I(_ · ~.jFig. 5 Cryostat under construction.

References[I] A. Peters et al., "A Cryogenic Current Comparator
for Nondestructive Beam Intensity Measurements",
Proc. 1994 the Fourth European Part. Accel. Conf.,
London, (1994) 290.
[2] K. Shinada eta!., Shimadzu Review 51 (1994) 217.

A schematic drawing of the cryostat is shown in Figure 6. A warm hole of 110 mm in diameter is requiredin order to pass the beam. Isolation gaps using A/2 0 3-257-