The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

ELECTRIC-DISCHARGE-BREAKDOWN PHENOMENA OF A GLASSWINDOW USED IN A FLUORESCENT SCREEN MONITOR
T. Suwada* and K. Kakihara, KEK, Tsukuba, Ibaraki, 305-0801 Japan
Abstract
A vacuum leakage occurred from a glass window(viewing window) used in a fluorescent screen monitor
cl?se to the summer shutdown of the KEKB operation
thts year, while the injector linac has been favorably
under operation. We have had no experience
ac_companied by such a vacuum breakdown of the glasswmdow due to an electric discharge during four years ofoperation ever since the commissioning of the KEKB
injector linac started in 1997. In this report we describe indetail the observational result and its cure on the vacuum
breakdown of a glass window used for the screen monitor.

1 INTRODUCTION
The KEK B-Factory (KEKB) project[l] started in 1994
to investigate CP violation in the decay of B mesons. The
KEKB injector linac[2] injects 8-GeV electrons and 3.5GeV positrons directly into the KEKB rings withasymmetric energies. One hundred and fourteen
fluorescent screen monitors in total are located along theinjector linac. They are utilized in daily operation in orderto easily check the transverse beam sizes and the beam
optics matching. Almost all of the screen monitors (oldtype) were replaced by new compact monitors with anaxial length of 85mm between the flanges. They werenewly developed for the KEKB injector linac[3]. Thirteenof the old-type screen monitors with the axial length of~60mm are being reused in the J-arc and energy-analyzer
hoes. The new screen monitors were firstly designed andmanufactured in 1994, and were finally installed in 1998
(started in 1994). The beam commissioning of the injector
linac started in October 1997 and also started in full scalein 1998.
We found a vacuum leakage from a viewing window ofa certain screen monitor, which is located at an electron
beam energy of about 6 GeV, during the KEKB operation,
close to this summer shutdown. After a detailed
investigation, it was found that the vacuum leakage wascaused by a breakdown of the viewing window due to an
intense electric discharge. Such a breakdown for theviewing window occurred for the first time four years
after the beam commissioning started. While we have noclear explanation for this phenomenon, it is important tosummarize our detailed observational results in order tobetter cure the vacuum breakdown.
*E-mail address: tsuyoshi.suwada@kek.jp (T. Suwada).

2 ELECTRIC-DISCHARGE BREAKDOWNPHENOMENA
2.1 Vacuum Leakage from a Viewing Window
Figure 1 shows the variation of a vacuum pressure
measured by an ion pump located at sector 3 during aboutfour days up to the start of the vacuum leakage from theviewing window. The degree of the vacuum started to
become worse in 18th in April and, after that, a rapid
growth of the degree of the vacuum continued until 19th,
and finally the linac operation became impossible due tothe interlock-system operation over the vacuum limit.
~is vacuum leakage was identified from the viewing
wmdow of the screen monitor located at sector 3 after an
immediate leakage test using a helium detector. Afterreplacing the viewing window by a new one, linac
operation was started again without feeding RF power
into the accelerator structures.

2.2 Observational Results
The viewing window used in the screen monitor
comprises a silica glass (Si02:68%, B20 3:18%, K20:9%,Al203:3%, other:2% [wt%]) with a thickness of 4.5 mm
and with a diameter of 38 mm based on the ICF70
standard. Figures 2 (a) and (b) show photo pictures of thisviewing window broken down due to the electricdischarge. Both pictures were taken from the window
surface in the vacuum side. Figure 2 (a) shows about
twenty main traces with a clear symmetry caused by theelectric discharge. Two kinds of the discharge traces arefound in the figure. One is a trace group generated fromthe glass centre; it grows in the direction of the inner sideof the metal flange with many fine branch traces. The
other trace group is also generated with many fine branch
traces from the inner corner of the metal flange; it grows
in the direction of the glass centre. Figure 2 (b) shows amicro~copic picture of the breakdown point penetrating
the wmdow glass in which a thick main trace is seen at
the centre of the picture. It was located about llmm apartfrom the glass centre. We found only one discharge trace
penetrating the window, which caused the vacuum
leakage. This penetrating trace is not a straight line, but aloosely curved line. The thickness of the penetrating trace
is barely observable by human eye and it is around a few
hundred J.Lm. We can also find several narrow discharge
traces developing transversely from the main discharge
point to the metal flange; they also have several narrowbranch traces. No transverse traces were found on theatmospheric surface side of the window glass.

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2.3 Fluorescent Screen Monitor and Its
Radiation Environment
A mechanical drawing of the screen monitor is shownin Fig. 3 and a typical device arrangement around thescreen monitor is shown in Fig. 4. The driving system of
the screen monitors used in the injector linac is almost thesame as that used in other linac facilities. The driving
operation of a fluorescent screen is performed by apneumatic cylinder controlled by compressed air with apressure of about 5 kgf/cm2 using an electromagneticvalve. The initial position of the screen is set to beparallel and 13 mm above to the beam line. The screen isdriven at an angle 45 degrees to the beam line by anoperator's request. Fluorescent light is sent down througha viewing window mounted 65 mm below from the beamline, and is guided to a CCD camera installed at the floor
level by using two aluminium-coated optical mirrors.
During the first investigation we had some doubt
concerning a bad mechanical drive of the monitor, andthus inferred the generation of a high-radiation
environment from the screen due to a beam loss. This
inference was immediately rejected after testing themechanical system both in local control and under remotecontrol.
Next, we checked the dark currents generated from theaccelerator structures, which also could cause a highradiation environment at the monitor position. The
radiation environment on the linac beam line is expected
to be higher than that before the KEKB project started
because the pulsed RF power from a high-power klystronwas boosted to be nearly double and the acceleration
gradient was also boosted to be 20 MV/m from -7 MV/mby using a pulse-compression device. Figure 4 shows thearrangement of the accelerator structures around thescreen monitor on the beam line. As can be seen from thefigure, it is expected to easily generate a high radiation
environment due to the dark currents at the monitor
position located only 270 mm from an output coupler ofthe upstream accelerator structure. Figure 5 shows typicalfluorescent images (a) for this monitor and (b) for anothertypical monitor under the typical operational condition
without beams. The fluorescent intensity for the former
screen is clearly stronger than that for the latter screen.
At present, we understand that such high-intensity darkcurrents might cause a high radiation environment at thisscreen
monitorand,especially,
low-energy
electromagnetic radiations might hit the window surface
synchronously with a RF pulse having a repetition rate of
50 Hz. It is thus expected that a large amount of soft
(relativelylow-energy)
electromagnetic
radiation
accumulated on the window surface, which caused
electric-discharge breakdown.

comprises the same silica glass coated with a thin film of
stannic oxide (Sn02). The thin film was spread out on thewindow surface by a spray and was fixed by a hightemperature process. The electric resistance between theglass and the metal flange was measured to be about 950
n. The new viewing windows will be replaced at threelocations under a high-radiation environment during thesummer shutdown.

4 CONCLUSION
A vacuum leakage from a viewing window used in afluorescent screen monitor occurred close to the summer
shutdown this year for the KEKB operation, while theoperation of the injector linac was progressing well. It
was found that this vacuum leakage was caused by anelectric-discharge breakdown of the viewing window due
to high charged-up on the window surface under a highradiation environment. It was inferred that the highradiation environment was generated by high-intensity
dark currents from the accelerator structures. In this report
some detailed observational examples and theirmeasurements were summarized. However, at present wehave no clear idea concerning the basic physical process
for the vacuum breakdown and have also no idea whysuch a breakdown was caused four years after theinstallation.
Cures are in progress for a viewing window coated by athin film with a well conductivity.

5 REFERENCES[1] S. Kurokawa, et al., "KEKB Design Report" KEK
Report 90-24 (1991).
[2] I. Sato, et al., "Design Report on PF Injector Linac
Upgrade for KEKB" KEK Report 95-18 (1996).
[3] T.Suwada, A.Enomoto, T.Urano and H.Kobayashi,"Mechanical Design of the New Screen Monitor",

Proc. the 20th linear Accelerator Meeting in Japan,
Osaka, 1995.
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3CURESDate

For the reason described in the previous section, we
found a ctire by using a new viewing window withelectrical conductivity on its surface in order not toaccumulate electrical charge. The new viewing window

Figure 1: Variation in a degree of the vacuum in sector 3
measured by an ion pump during about four days up to
the start of the vacuum leakage from the viewing window.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Figure 4: Arrangement for the screen monitor
peripheral devices. The screen monitor is installed
between two 2m-accelerator structures.

Figure 2: (a) Digital (top) and (b) microscopic photo(bottom) pictures of the generated electric-discharge
traces on the glass surface of the viewing window.

Mi«o Solenoid Valve

CylinderFigure 3: Mechanical drawing of the screen monitor. The
beam goes through the monitor from the reverse side to
the front in the left drawing, and from the right side to thein the right drawing.

Figure 5: Typical photo pictures of the fluorescent screen
images generated by dark currents from the accelerator
structures, (a) the screen image (top) of the monitor
accompanying by the vacuum breakdown and (b) thescreen image (bottom) of another typical monitor.

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