The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Influence of External Magnetic Field on the Performance of Cold Cathode GaugesKazuo SUGII, Kumio IKEGAMI, Masao OTAKE, Shigeru YOKOUCHI, Akira GOTO, and Yasushige YANORIKEN (The Institute of Physical and Chemical Research)
2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan
Abstract

A cold cathode gauge is used to monitor the pressure ofthe vacuum chamber during the operation of the SRC sector
magnet. To examine the effect of environmental magnetic
field on the performance of cold cathode gauges, theexperiment using an electromagnet was carried out. The
result showed that the pressure measurement with a cold
cathode gauge was possible, although the change ofindication due to the increase of an external magnetic field
was large at a low pressure.
1 Introduction

We plan to use a nude B-A gauge and a cold cathodegauge (CCG) to measure the pressure of the vacuum
chamber of the RIKEN Superconducting Ring Cyclotron(SRC). A B-A gauge is used during the non-operation
period of the SRC; a CCG is used during the SRC operation,
since a B-A gauge is no use in a large leakage flux of theSRC sector magnet. The operation of a CCG makes use ofthe ionization by electrons trapped in an electric field and amagnetic field. Being constructed with a permanent magnet,
a CCG is expected to be usable even in a large external
magnetic field, by aligning it in parallel to the lines of theexternal field as precisely as possible. Measurement ofpressure using a CCG under an external magnetic field isactually reported [1]. In this paper we describe the changesof pressure indication for two kinds of CCG's available at amarket when an external magnetic field is loaded using anelectromagnet.

Fig. 1 Schematic diagram of the experimental setup.

Fig. 2 Appearance of the CCG's in the pole gap.
2 Experiment

2.1 Vacuum Gauge

Since the pressure required for the beam chamber of theSRC is estimated to be ofthe order of 10-6 Pa [2], a vacuum
gauge used in the SRC beam chamber is needed to have aminimum measuring range which is one order or more
lower than the order of the required pressure. Therefore, wechose two types of CCG's as follows: (1) Balzers: IKR060
and (2) Leybold: PR36.
2.2 Experimental setup

A schematic diagram of the experimental setup is shownin Fig. 1. A turbo-molecular pump (TMP. Osaka Vacuum,Ltd.: TH300) was set on a frame beside the electromagnet
for evacuating to ultra-high vacuum. A variable leak valve(VLV) and a mass flow meter (MFM) were connected withthe chamber to vary a chamber pressure. The two types of
CCG' s were mounted at the end of the vacuum pipe
extending from the side of the chamber and were placed in

the pole gap of the electromagnet. Figure 2 shows theconfiguration of the CCG's in the pole gap for testing in auniform region of the external magnetic field. Both CCG'swere laid so that the direction of magnetic fields generated
by the permanent magnets coincided with those of theexternal magnetic field. For testing in an external magnetic
field with a gradient, the CCG's were placed away from thepole gap at around the pole edge by shifting the wholeapparatus all together.
2.3 Experimental method

Pre-evacuation for degassing the chamber was carriedout before the measurement. In this operation, ultimate
pressures of 1.6 X 10-6 Pa and 1.9 X 10-6 Pa were indicatedwith IKR060 and PR36, respectively. The fluctuations ofPR36 indication were often observed which were supposed
to be due to instability of gas discharge. The magnetic field
gradient at the place where the CCG' s were set for thetesting in a non-uniform external magnetic field wasestimated from another field measurement to be about-284-

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

10·3 ,.--..--:-:---------~--

with a permanent magnet ( -0.1 4 T)--II-A------·---t.--_____. ~-----·----~-______..

..---------..

1!,__ with a permanent magnet ( -0.1 T)

IIII

0.2
0.4
0.6
0.8
External magnetic field (T)CCG : IKR060 (Balzers)

(a)

I(b)

External magnetic field
CCG: PR36 (Leybold)

Fig. 3 Change of pressure indication as a function of the field near the CCG' s without a permanent magnet when theexternal field is uniform: (a) IKR060 and (b) PR36.

_.._~~---·•~- -...-.

--/-+-----·~-(a)

~-..

:

~c-1·~"--with a permanent magnet ( -0.14 Twith a permanent magnet ( -0. 1 T)

0.2
0.4
0.6
0.8
External magnetic field (T)CCG: IKR060(b)

0.2
0.4
0.6
0.8
External magnetic field (T)CCG: PR36

Fig. 4 Change of pressure indication as a function of the magnetic field near the CCG' s without a permanent magnet
when the external field is not uniform: (a) IKR060 and (b) PR36.
-0.74 Tim at the pole-gap field of 1 T. The leakage flux ataround the TMP was estimated to be as small as 6 X 1o-3 T
at the pole-gap field of 1 T. We judged that this small fieldwould have no effect on the performance of the TMP.
The measurement was performed under the following
four conditions: (1) without a permanent magnet and in a
uniform external magnetic field, (2) without a permanent
magnet and in a non-uniform external field, (3) with apermanent magnet and in a uniform external field, and (4)with a permanent magnet and in a non-uniform externalfield. For the measurement without a permanent magnet,
the pressure at a zero field was measured using apermanent magnet. The magnetic field at the pole-gap

center was increased from 0.1 T. The pressure was changedby adjusting the flow rate of nitrogen gas into the chamberwith the VLV and MFM.

3 Experimental results
Figure 3 shows the change of pressure indication as afunction of the field near the CCG' s without a permanent
magnet when the external field is uniform. Only when theexternal field was zero, the pressure was measured using
CCG's with a permanent magnet. In the case of IKR060,
the change of indication is small although the indicatingvalue increases as an external field increases at a low-285-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

r-. _,._..., -·-·------+-+---<~--.-... ...........

-+--+-·-~

~

N.
t'\

....
__..

~
~

............
~

~

....

~v-- ~~ ~- p.---.

1.5

0.5

'I'

1.5

0.5

22

Magnetic field (T)CCG: IKR060

Magnetic field (T)CCG: IKR060

Fig. 5 . Change of indication as a function of themagneticfield near IKR060 with a permanent magnet whenthe external field is uniform.
pressure. In the case of PR36, the indicating value
decreases rapidly as an external field increases, and finallythe measurement becomes error. During the measurement
the fluctuation of indication and/or the representation oflower limit on the meterpanel were found when thepressure at a zero field was below 2 X 1o-5 Pa. Thisphenomenon is considered to come from the instability ofgas discharge.
Figure 4 shows the change of pressure indication as afunction of the magnetic field near CCG's without apermanent magnet when the external field is not uniform.
In the case of IKR060, the tendency of the change issimilar to the result obtained in a uniform magnetic field.
In the case of PR36, the change of indication is more stable
than that obtained in a uniform field, but is still larger thanthat for IKR060. The cause of this instability is considered
to come from that the axis of PR36 was not aligned
precisely in parallel to the lines of the external field, since
the position of PR36 was more or less away from themedian plane of the magnet (see Fig. 2).
Figures 5 and 6 show the change of pressure indication
aas function of the magnetic field near IKR060 with apermanent magnet. The results in the case of a uniform
field are shown in Fig. 5. It can be seen from this figurethat when the pressure at a zero field was above 10-5 Pa thepressure indication decreases suddenly in the range of 00.25 T eventually the measurement becomes error. Above
0.3 T, the change of indication is small. The results in thecase of a gradient field are shown in Fig. 6. It is seen thatthe change of indication shows a similar tendency as inFig.5.

Fig_ 6 Change of indication as a function of the magneticfield near IKR060 with a permanent magnet whenthe external field is not uniform.

Conclusion

We judge that we can use CCG's in the leakage flux of
the SRC sector magnet unless we will pursue a precise
measurement. It would be better to use a CCG without apermanent magnet because the environmental magnetic
field during the SRC operation is expected to beapproximately below 0.3 T, in which range themeasurement with a permanent magnet becomes error forunknown reasons. However, we still need to figure out aproblem that the measurement without a permanent magnetbecomes error when the environmental field is very small(0-0.1 T).
Acknowledgement

The authors would like to thank LEYBOLD Co. Ltdand Hakuto Co. Ltd for providing us with the technicaldata.
References[1] N Yasumitsu, Proc_ 8th Meet. on Ultra High Vacuum
Techniques for Accelerators and Storage Rings, KEK,
(1992) 75.
[2] K. Sugii et al., "Vacuum System for RIKENSuperconducting Ring Cyclotron", in this proceedings.

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