The 9th .Qymposium on Accelerator &ience and Technology, Tsukuba, Japan 1993

SYNCHROTRON RADIATION DAMAGE TEST OF INSULATING MATERIALSIN THE TRISTAN MAIN RING
Y .Ohsawa,K. Takayama,andT.Ozaki
National Laboratory for High Energy Physics in Japan(KEK) 1-l,Oho,Tsukuba-shi,Ibaraki-ken,305 Japan
H.Mitsui,R.Kumazawa,and T.Tanii
Toshiba Corporation 2-4 Suehiro-cho,Tsurumi-ku,Yokohama-shi,230 Japan
Abstract

manufactured in the atmosphere, by moldingBT(bismaleimide triazine) resin preimpregnatedglass-cloth, by means of heat shrinkable tape.
Type C is another resin-rich insulation system
which is manufactured under vacuum, bymolding polyimide preimpregnated glass-cloth,by asphalt compound.

Irradiation test of typical insulating materials foran accelerator magnet was carried out, using
actual radiation ill1 the TRISTAN MR(main ring)which was operated at the highest radiation levelin the accelerator society. Physical and chemicaldegradation of the insulating materials due toirradiation are quantitatively reported in thispaper. Some suggestions on future insulating
material and its manufacturing method are given.

For the purpose to knowing a change in theirelectrical properties such as tan8 ,an aluminum
bar with cross-section of 6(mm)x25(mm) andtotal length of 160(mm) was covered with each of
those materials in the same manner as actual coils.
To measure a change in their mechanicalproperties, on the other hand, samples wereformed in a laminate by molding the tapes in thesame manufacturing process as that of the barcoils. The plate sizes were 2 mm in thickness, 25
mm in width and 100 mm in length.

INTRODUCTION
TRISTAN is an e+e-colliding beam accelerator
with the collision energy in a range of ~"" 60
GeV. The MR was first operated in October 1986
at the beam energy of 25 GeV. Since then thebeam energy has been increased step by step from25 GeY to 30 GeV by employing RF accelerating
cavities [1]. Thereafter the radiation damage on
accelerator components, such as signal cable forbeam monitors, power cable for vacuum pumps,
various electric circuit components, and exciting
coil of magnets, has become increasingly serious

Table 1 Insulation constitution of test specimens.

[2].

Systematic studies of the radiation damage onelectrically insulating materials used for particle
accelerators are found in the literature [3];meanwhile, same kind of tests were carried in theTRISTAN MR [4] several years ago. However,
the irradiation dose was limited below 100 MGy
for both cases. Radiation damage beyond thislevel is current of big concern in the accelerator
society because irradiation beyond 100 MGy islikely to occur in localized regions of the existingmachines. Under such a situation, results of thefirst systematic studies performed in this doselevel are released.

TypeMaterial}\Manufacturing
process

Appliedmachine

Epoxy/mica/
E glass

VPI

TRISTJ\Nmain ring

BBT resin/5glass

Heat shrinkable PS ringtape-molding

cPolyimide/Tglasspressure-

Vacuum-asphalt

Nonemolding
E glass : Electric grade glass
S.T glass: Boron-free glass

TEST BENCH
All of test specimens were put in a radiation box
located in the steering magnet of the normal
section where the dose of 3.7-6.5xl0 Gy/h,consisting of direct synchrotron radiation and its
secondary, was seriously high in the MR. The
dose has been calibrated with the thermo-luminescence dosimeter method [2] by employing a test
beam with well monitored energy and current.
The samples taken out of the irradiation box atsome fixed time period were subjected to the tests
explained in the following section.

SPECIMENS
Three type of insulating materials for an exciting
coil as shown in Table 1 were chosen for thecurrent study. Type A is the so-called
VPI(vacuum-pressure-impre gnation) insulation
system which consists of glass-cloth reinforced
mica-paper tape impregnated with epoxy resin.
Type B is a resin-rich insulation system which is

TEST RESULTS AND DISCUSSIONSIn Fig. 1, the magnitude off BDV (breakdownvoltage) of the bar coils are shown as functions of-288-

dose. Fig.2 shows the relationship between
flexural strength of the laminate and dose .Fig.3 isthe relative dose dependence of residual BDV
and flexural strength .. The dose for halving BDV
and flexural strength, which is usually used as
criteria for insulation design, is listed in Table 2.
In the case of Type B, the degradation of theresidual BDV appears in the lower dose comparedto that of the mechanical property; this is contrary
to the popular view. At the highest irradiationlevel of 167 MGy, type A including mica reveals
the highest BDV but the lowest flexural strength
among three samples. In addition, it shows largeswelling; it is understandable from speculationthat the epoxy resin adjacently placed to the glasscloth is decomposed into gas and the amount ofgas increases with dose.

Table 2a-~

Irradiation dose for halving BDV and flexuralstrength obtained from Fig. 3.
TypeBDVFlexuralA

3.3 " 10 7

1.9B4.6X

107

1. 1 X 10 8c

1.1X

10 8

1.3120--...

~ 100c:~....,(/)

80----....

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

X

Xotre~th10 7

10 8..\:~
0 ·.

'-:::;;

60

OA
DB

40

6C

co 20liB

X
<l>

4-0>

"'30

>

20>

eA

0eE

-""'

c~

10-a(/)<l>=\c:o••.•••.•.•.•......... El.

a0

Fig. 3a.

Bo~~illL~~~Lliillill-L~=-~~

0105

109

106
Dose

Fig. 1"'0..

2: 600(Gy)

Relationship between BDV of bar coils anddose.

--"""---A..___~~""~ 4-00 . . . . . . . . . . . . . o .... 'G·. ·l!l ""B.
~"'c..

-;;;
ro~200X"'A

DoseFig. 2

Relationship between
laminates and dose.

(Gy)flexuralstrength·--.~~·.~

BDV

flcxurul strength.A.C

ro\0·. \\._···.\

roof-289-

00107
Dose (Gy)

10 8109

Dose dependences of residual BDV andflexural strength obtained from the results
shown in Fig. 1 and Fig. 2

Let compare type B and type C which are theglass-cloth reinforced resin-rich insulation
without mica, type C is apparently superior totype B with respect to the BDV and flexuralstrength, as seen in Fig.l This can be attributed tothe existence of voids in the insulating materials.
The oxidation tends more to develop in the void
contained insulating materials than in the voidfree one. type C is void-free because it wasprocessed in vacuum by the asphalt-compoundmolding technique. type B was processed in theatmosphere by the heat shrinkable tape molding
technique; therefore it contains voids .
It should be pointed out that in type B sample
voids generate more easily than in type C withirradiation ,as discussed below. Fig. 4 shows~tan8 characteristics as a function of dose.
Where, ~tan8 =(tan8@3kV-tan8@0.5kV). The
parameter ~ tan8 is known to reflect partial
discharges in void. It is reasonable to regard~tan8 as a measure of the amount of voids. As
seen in Fig.4, ~tan8 increases with dose in thelower dose region in type B than in type C. Thissuggests that due to gas evolution bydecomposition of the resin by irradiation,delaminations of the insulating materials occurredto generate voids. The down in iltano at 167
MGy for type B is supposed that the puncture of
the insulation occurred due to increase ofpressure in the closed void, and the void became
open to the atmosphere resulting in decrease involume.

insulating materials to decrease BDV and flexuralstrength; the voids took essential roles in theprocess.

CONCLUSIONS
From the irradiation tests of typical insulating
materials for magnet, using actual radiation in theTRISTAN MR (up to 167 MGy at the highest),
the following were made clear.
( 1)Epoxy/glass-cloth/mica-tape insulation
processed by the VPI method has higher BDV by
the aid of mica tape; however its flexural strength
remarkably degrades at the higher dose.
(2)The insulation processed by the asphaltpressure molding method under vacuum is far
superior both in electrical stability and mechanicalrigidity to the insulation processed by the heatshrinkable tape molding method.
(3)The insulation processed by the heat-shrinkabletape molding method shows that the decrease in
the BDV appears at a lower dose than for theflexural strength; this fact is contrary to thepopular view.
(4)Carbonyl absorbance is a useful index tomeasure the size of chemical decomposition
cause by irradiation.

1412~ 10'<:>8c:"' 6+'<l42

10 5Fig. 4

10 6107

Dose(Gy)

10 8Dose dependence of 1!. tan li of bar coils.

Fig. 5 shows the relationship between carbonyl
absorbance and dose for all samples. Their features, except forthe magnitude, are similar to each other. The ·carbonyl

As a concluding remark, studies of themanufacturing process as well as insulating
materials themselves are, indeed, important inorder to develop the higher radiation-resistantinsulation.

REFERENCES"'[1]Y.Kimura et al., Proceedings of the 6th
Symposium Accelerator Science and Technology, Tokyo,l987,pp15-17.
[2]T.Momose et al.,European Particle Accelerator
Conference,Rome,1988. The calibration was donein test beam operation for this purpose where asingle electron bunch of 0.33 rnA was stored withflat energy of 27 .5GeV, for a time period of180sec. A real time span of machine operation ismuch longer than the above time period. During
roughly three years operation of current interest,
the optics conditions such as tune or C.O.D. havebeen occasionally changed which led to changes
in beam orbit and beam profile, resulting innotable variation of the dose. Accordingly, themagnitude of total dose involves someuncertainty. Recent additional calibrations show it
is a factor of 2 or 3, assuming photon energy of150 KeV.
[3]P.Beynel et al., CERN 82-10(1982)[4]T.Ozaki et al., Proc.AC89,2015-2017(1989)u

JQ3L_~~illL-L~lill~~~~106

DoseFig. 5109

107(Gy)Relationship between carbonyl absorbance anddose.

Fig. 5 shows the relationship between carbonyl
absorbance and dose for all samples. Theirfeatures, except for the magnitude, are similar toeach other.
The. carbonyl absorbance isapparently proportional to dose. The increase of
the carbonyl group should originate from theoxidation reactions and scissions of molecules ofresin which are caused by the radiation. As seenin Figs. 4 and 5, the characteristics of samples forcarbonyl absorbance qualitatively coincide withthat for iltano.
Thus, carbonyl absorbance canbe regarded as a useful index giving the size ofchemical decomposition.
The authors arrived at a speculation that theseoxidation reaction and delamination degraded-290-