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

Development of Optical glass window for SR monitor by using a metal 0-ring sealing
Toshiyuki MITSUHASHI, Kenichi KANAZAWA and Sigenori HIRAMATUHigh Energy Accelerator Research Organization, 1-1 Oho,Tsukiba, Ibaraki 305 Japan

Abstract

A metal 0-ring sealed optical glass window for SR
monitor has been designed and constructed at the B factory
KEK. An optical material of fused-silica glass forrefractive-optical components is used as the optical grass
flat. To seal an optical glass flat, it is sandwiched by two
metal 0-rings from the both sides. We tightened up themetal 0-ring with 60% of full stroke for the sealing of thevacuum. The transmitted wavefront error is measured
with an interferometer (Fizeau type). The result of
remaining wavefront error is less than /J20. No sign ofwarp due to mechanical stress was detected in effective
diameter of 60mm.

2. Design of Window
The basic design of the glass window using a metal 0ring sealing is shown in Fig. 1. In our design of the glass
window, an optical glass flat is sandwiched by tow metal0-ring from both sides. The metal 0-ring locates on
vacuum-side flange serves as a function of vacuum sealing
and locates on the air-side flange serves to take a balanceof tighten up force.
(a)

1. Introduction
So called SR monitor in that mainly aided to measure
the beam size and the profile based on optical method is
one of most fundamental monitor in the storage ring [1].
Many apparatuses are adopted in the SR monitor such as
imaging system to make an image of the beam, the SR
interferometer to measure the beam size, and the streak
camera to measure a longitudinal beam profile, etc. In thismonitor, the visible SR beam is usually extracted from thestorage ring by a mirror. A vacuum tight glass window,
which has a good optical quality, is necessary for theextraction of visible SR beam from the vacuum duct ofaccelerator. A glass window made by the brazing iscommonly used for view port. The remaining wavefront
error of a transmitted light for such a brazed window isusually greater than /J8 due to thermal warp. The surface
flatness is also worse than /J8. For the purposes ofprecise optical measurement in the SR monitor such asinterferometry [2], error in the transmitted wavefront mustbe less than /JlO. By this reason, we cannot use thehermetic-sealed glass window for the extraction of visible
SR beam. To reduce the remaining wavefront error lessthan /JlO, we developed a metal 0-ring sealed optical
window at the B factory KEK. In this paper, a design,
preparation of glass flat and performances are described.

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(li0~:$U1!.at'0(ils""'~ulN_(b)Fig.l General design of the glass window. In this figure,(a): metal 0-ring, (b): vacuum-side conflat flange, (c):optical glass flat, (d): air-side flange.

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

By this structure, the optical glass flat is tightened up withequal forces from both side, and we can seal the vacuumwith minimum torque. Since we designed to attach theordinal conflat flange (ICF203) structure in the backside ofvacuum-side flange, we can easily attach this glasswindow to the SR extraction port in the accelerator. The
physical aperture is 80mm.

3. Optical performance test
The optical performance for transmitted wavefront of theglass window has tested by using an interferometer(Fizeaumeter) [3]. The set up of measurement is shownin Fig. 3. This test is performed in the effective apertureof 60mm in diameter.

3. Preparation of optical glass flat
We used a fused-silica glass material for refractiveoptical components such as lens. The diameter of theoptical flat is 100mm and thickness 19mm. The slope
error of surface of the optical flat is polished better thanThe surface quality is better than scratch and dig
A./10.
60/40. The parallel degree of both sides is better than 2".
As a result of performance test with an interferometer(Fizeaumeter), we obtained an error in the transmitted
wavefront less than A./20. The both sides of the glass flatis coated with a single layer of MgF2 as a anti-reflection
coating. The surface HV hardness of this coating isgreater than 65, and it is tight enough for metal 0-ringsealing.

2. Performance of vacuum sealing
We used the Helicoflex-delta ( le Carbone k.k ) as ametal 0-ring. We tightened up the flanges until to detect
the no sign of leakage. To test the vacuum sealing
performance of the glass window, we use a He leakdetector. As a result, we tightened up with 60% of fullstroke (0.9mm for each metal-ring) for the sealing of themetal-0 ring. A photograph of assembled glass window
is shown in Fig. 2.

Fig.2

A photograph of assembled glass window.

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Fig.3 Measurement set up for the transmitted wavefront
error by the glass window; (a) reflector, (b) testing window,(c) half mirror for reference, (d) beam splitter, (e) beamcollimator, (f) spatial filter, (g) observation screen, (d) ReNe laser. The components from (c) to (h) make the set upof Fiszaumeter.
A transmitted wavefront error of the glass window has
measured by using a flat mirror by following procedure.
1. Measure a reflected wavefront from the flat mirror withthe glass window,
2. Measure a reflected wavefront from the same flat mirror
without the glass window.
3. Subtract both wavefront.
The results of reflected wavefront from the flat mirror
with the glass window, reflected wavefront from the same
flat mirror without the glass window, and subtracted
wavefront error are shown in Fig.4 with a counter map anda 3-dimensional phase map. From figure 4, the peak tovalley of wavefront error of subtracted wavefront is 0.095A.,
and rms. wavefront error is 0.016A.. Since the probe lightis passing through the glass window two times, thetransmitted wavefront is given by a half of the subtracted
wavefront. Then the transmitted wavefront error is lessthan A./20. The quantity of remaining wavefront error
after the metal 0-ring sealing is almost same as theoriginal wavefront in the optical glass flat. We made fivesets of the optical glass windows for the SR monitors of B
factory, the remaining wavefront errors in 5sets were lessthan A./20. We can also detect the remaining warp in theglass due to the mechanical stress through a change ofcontrast of the interferograms. As a result, no sign ofwarp was detected inside of measured aria for all five sets

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001of the optical glass windows.

5. Conclusions
A metal 0-ring sealed optical glass window for SR
monitor has been designed and constructed for The SR
monitor of the B factory KEK. In our design of the glass
window, an optical glass flat is sandwiched by tow metal
0-ring from both sides. An optical material of fusedsilica glass for refractive-optical components is used as theoptical grass flat. The optical glass flat is typicallytightened up with 60% of full stroke for metal 0-ring, andno vacuum leakage is detected. The effective diameter ofglass window is 60mm. The transmitted wavefront error
is measured with an interferometer (Fizeau type). The
result of remaining wavefront error is better than A./. No
sign of warp due to mechanical stress was detected. Foursets of optical windows are used for the extraction ofvisible SR beam in the HER and the LER in last 3 years,
and no accident was happed.

(a)

REFERENCES[1] T. Mitsuhashi and M. Katoh, Proc. 5th European
Particle Accelerator Conference, 1669, (1996), Sitges(Barcelona).
[2] T. Mitsuhashi," Beam Profile and Size Measurement by
SR Interferometer " in Beam measurement, Ed. By S.
Kurokawa et.al., pp399-427, World Scientific(1999).
[3] M.V. Montravadi " Newton, Fizeau, amd Haidinger
interferometer" in Optical shop testing, Ed. By D.
Malacara ,pl-49,Wiley-Intemational (1992).

(b)

(c)

Fig.4 A result of transmitted wavefront error. (a):reflected wavefront from the flat mirror without the glass
window, (b): reflected wavefront from the same flat mirror
with the glass window, (c): subtraction of (a) and (b). Eachwavefront is shown with a counter map (left) and a 3
dimensional phase map (right).

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