IR Vacuum Chamber and Assembly

Photo by Satoh
Designed by Kohriki

The 17th KEKB Accelerator Review Committee
KEK, 20-22 February 2012
K. Kanazawa
for
KEKB Vacuum GroupIR Technical Meeting Member
IR Installation Meeting Member
SVD/IR Mechanics Meeting Member

Contents
1. IP Chamber
Design Features – Fabrication Issues – Stress Analysis – Ridge –Connection to the QCS cryostat – Summary of new issues

2. Vacuum around IR
Vacuum components in the R side – Typical cross section of Ta pipe –Vacuum components in the L side – Rough estimation of pressure –Consideration on IR pressure – PID experiment at PF BL21 – Test Tabeam pipe – Summary of issues

3. Other issues
Mock-up for assembly simulation – Radiation shield – Schedule in 2012

4. Summary

1. IP Chamber
Design features•Minimize creation and trap of HOM.
•The central part and the branch for out going beam constitute a bent pipe of ID20mm.

BSC + ~2 mme-e+•The pipe for incoming beam start from ID20 mm. Then ID is gradually reduced
to about 9 mm to stop direct SR. Now the taper is longer than before.
•The inner surface of a pipe for incoming beam has ridges to prevent scattered
light from hitting the central Be part.

1. IP Chamber
Fabrication issues
HIP = Hot Isostatic Pressing

Manifold

WL: 210W (Ta)WL: 190 W (Ta)SRQ: ~ 50 W
SRB: 100 W

WL: 140 W (Ta)10~20 µm

WL: 70 W (thick Au)
SRQ: few W

WL: 270 W (Ta)SRQ: ? W
SRB: 30 W•A test for Be-Ti
brazing and Ti-Ta
HIP is now
undergoing.
•Ti is adopted forthe manifold insteadof stainless steel toreduce the stress(the followingslides).
Thin (1~2 mm) Au coatto reduce ohmic power
loss on Ti and Ta is anoption.

1. IP ChamberStress analysis
Outer shell of the central part
Stainless steel manifold
Equivalent stress: 159 MPa(Koike)
Outer shell of the central part
Ti manifold
Equivalent stress: 74 MPa•Under the temperature difference of 30°C between the inner and the outer Be pipecauses an equivalent stress of 159 MPa in a stainless steel manifold, which is about 80%of the yield strength of stainless steel (206 MPa).
•Therefore Ti is adopted instead of stainless steel for the manifold. The equivalent stressfor Ti under the same condition is 74 MPa while the yield strength of Ti is 170 MPa.

1. IP Chamber
Ridge shape (original idea)
Thin wall around here cannot has a ridge
structure. However, scattered photons fromthis part enter the central pipe with ashallow angle and give no serious effect.

A disk with adiameter of
24mm. The
position is thenearest end of aphoton production
region.

A disk with a diameter of
24mm in front of the Be
pipe.
This disk should be freefrom scattered photons.
The narrowest aperture must be aridge, of course.

Only single scattering is taken into account.
Test fabrication of the ridge

1. IP Chamber
Consideration on ridge shape
0.5 mm~ 6 mm?

Low risk for multiply scatteredphoton to escape forward?

Risk for multiply scattered photon toescape forward~ 16 mmsawtooth structure on L-side HER duct (only HER duct)

102 10

Against intuition, the loss factor ofthe shape with a vertical face islower. The shape of ridges will bechanged to similar one shown in theupper figure.

loss_z_sawtoothA (model07) [V/C]
loss_z_sawtoothB (model11) [V/C]
loss_z_w/o_sawtooth (model12) [V/C]10

loss_z [V/C]

1.5 10101 1095 1000 102468bunch length [mm]1012(K. Shibata)

The contribution of the tip scatteringon the top of a ridge is experimentallystudied by Z. Murakami and S.
Tanaka. Its effect in SR shielding is
not serious.

1. IP Chamber
Connection to the QCS cryostat•A model test of the original sealing mechanism wassuccessfully done.

BellowsCryostat
bodyBeam pipein thecryostat

BPM

Helicoflex-delta“Neckstopper”(Photo by Tanaka)•The use of radiation hard elastomer(EPDM) is investigated as a seal for theinsulation vacuum of the cryostat.
•This make it possible to keep theinsulation vacuum of the cryostat in
exchanging this connection pipe.
•This will be judged based on theestimation of radiation level at the seal.

seal

1. IP Chamber
Summary of new issues• The taper part for incoming beam is elongated.
• The material of the cooling manifold brazed to a Be pipe ischanged from stainless steel to Ti.
• The shape of ridges are reconsidered. More effective form
seems adoptable.
• In a connecting pipe to the QCS cryostat, the use of EPDM
seal is examined to increase a tolerance for double sealing andto make beam vacuum independent of cryostat vacuum.

2. Vacuum around IR
Layout of vacuum components in the R side
The beam pipe in the cryostat is made of Ta (redpart in the figure). The maximum thickness of thispipe is 4 mm.
Pumps are located outside the cryostat.
The major gas load for the incoming
positron line is photon-desorbed gas.
That for the outgoing electron line isthermal outgassing.

electronTa beam pipes are Au
coated to reduce photon
desorption and ohmic
power loss.

positronElectron cloud in the positron beam line is expected tobe suppressed by solenoid fields (~50 G is necessary).

2. Vacuum around IR
Typical cross section of Ta beam pipe

Vertically elongated aperture
in QC1P(Larger aperture is preferable
but it is not available in thepresent design.)

BPM

2. Vacuum around IR
Layout of vacuum components in the L side

positronelectron

The layout issimilar to the Rside.

2. Vacuum around IR
Rough estimation of pressure
Thermal only
P0 = 2 ×10 −7 Pa
PIP = 8 ×10 −7 Pa
Photon - desorption dominant

PIPP0 = 2 ×η PapositronP0Example: Incoming positron line
Assumptions:•The equation for a long tube gives good estimation
for a short tube
•At IP pressure is maximum•P0 is determined by the total outgassing of thecryostat beam pipe and the pumping speed there (50l/s).
•The thermal outgassing rate is 1 10-11 Torrl/s/cm2(1.33 10-8 Pa m3/s/m2).

PIP = 11 ×η Pa

η : photo - desorption coefficient

The difference between P0 and
PIP is determined by the local
outgassing and the conductanceof the beam pipe. It doesn’t
depend on the pumping speed
outside the cryostat.

2. Vacuum around IRConsideration on IR pressure•At least, the pressure around the IP (±5m) must not affect the overall average designpressure of the ring (1×10-7 Pa). Therefore the target pressure in this region must be
less than 1 × 10-7 × 3000/10 =3 × 10-5 Pa. A consideration on the beam-gas Rutherford
scattering requires much lower pressure in the QC magnets where β is large.
• The main gas source in this region is photon-desorbed gas due to the direct SR fromthe last bend. If a photo-desorption coefficient η is assumed to be 1 × 10-6 molecules/
photon, The average pressure of this region will be a few × 10-6 Pa.
•To realize this low photo-desorption coefficient, it is investigated to use Au coat for a
Ta beam pipe in the cryostat, which is expected to show a lower photo-desorption
coefficient compared to other metals with a surface oxide layer.
•Au will also work to reduce ohmic power loss on a wall.
•To check this idea, the photo-desorption coefficient of Au coating and Ta will bemeasured at PF (Photon Factory) of KEK.

2. Vacuum around IR
PID measurement at PF BL21Cuεcr = 4 keV(Y. Tanimoto)Measurement of the photo-desorption
coefficient of Cu, Ta, Au plated Ta is now
underway at PF BL21 with a cooperationof Dr. Y. Tanimoto.

News
20 Feb 2012

Y. Tanimoto

2. Vacuum around IR
Test Ta beam pipe

A test Ta beam pipe with cooling channels, a taper, and conflat flanges are now infabrication.
From the impression on the preparation of Ta flanges for PID experiment, the technology
to produce XHV Ta beam pipe seems to be not established yet.

2. Vacuum around IR
Summary of issues••

Adopt a 1 ~ 2 µm thick Au plating for the Ta pipe in the cryostat to obtain alow photo-desorption coefficient (η ~ 10-6 molecules/photon) and to reduce
the ohmic power loss.
A test model of Ta beam pipe is now under fabrication. At present thetechnology to produce an XHV beam pipe seems unsatisfactory.

3. Other issues
Mock-up for assembly simulation
A mock-up is now being prepared tosimulate assembling and installing of SVD
and IP chamber.

(Kohriki)3. Other issues
Radiation shield
Allover radiation shield for detector components, electronics, and human must be considered indetail.
The discussion starts soon.

Example of one of additional shield (blue shaded part)
Existing concreteshield

Detector area

3. Other issues
Schedule in 2012
FY2012(H24)
1 2 3 4 5 6 7 8 9 10 11 12 1 2 3Reshape the front end of the accelerator bridge
Belle rotation

Draw beam lines in Tsukuba Hall
Belle rotation

Fabrication of QCS magnetsQCS

Test Ta pipe
PID experiment

Fabrication of Beast II IP chamber
IP chamber
Mock-up preparation(Not a complete list)

4. Summary• The design of the IP chamber is now in the almost final stage.
• Within ±3.8 m around IP, the major outgassing is due to photodesorption process by the direct SR from the last bend, an idea
to reduce the photo-desorption coefficient using Au coating is
now investigated.
• Mock-up to simulate assembling IR components is being
prepared.
• Radiation shield is now an important issue to require thediscussion on overall structure around IR.