1. Primary Vacuum Window and Water Dump for the ILC-Photon Beam
P. Sievers/CERN. A. Ushakov/Univ. Hamburg. S. Riemann/DESY-Zeuthen.
LCWS 2016, Morioka, Japan
6 Dec. 2016

2. Content Introduction. A Stationary Window for the High Lumi Case.
A Tumbling Window for the High Lumi Case.
Intermediate Conclusion.
A possible Option for a Water Dump.
A Water Dump shifted by 1 km.

3. 1.Introduction
The FLUKA computations (A. Ushakov) for thin Ti-windows were made. They show, that the energy deposition EDD rises linearly with the thickness and the radial distribution is close to the profile of the incident beam.
The window is placed 48 m downstream of the target.

4. 2. A Stationary Window for the High Lumi Case
For very short pulses, the instantaneous temperature rises linearly with z.
However, already during the pulse of 1 ms, heat diffuses axially to become close to uniform across the thickness of mm after 1 ms.
Therefore during the gas convection cooling, the temperature in the thin window will decrease exponetially after each pulse and nearly uniformly over z.
This is no longer the case for thicker windows above 0.5 mm.

5. Stationary Ti-Window for High Lumi Case with 250 GeV and 2625Bunches.
Window thickness (mm)
0.1
0.2
0.5
1.0
PED D (J/g)
47.
94.
242.
484.
ΔT/Pulse (K)
47. (94.)*
94. (188.)*
470. -
Av. Power (W)
0.21
0.84
5.25
21.
Dpa/5000 h
70 !
88 !
Life time for 0.5 dpa (h)
36 !
29 !
No. of hits/5000 h
90 Mio.
* Excluding axial thermal diffusion during the pulse.

6. Stationary Ti-Window for High Lumi Case with 250 GeV and 2625 Bunches.
Window thickness (mm)
0.1
0.2
Steady state peak temp (oC) with He-gas cooling at 12 atm.
70.
144.
Steady state peak temp. (oC) with Ar-gas cooling at 5 atm.
100.
282.
Static stress at 12 atm. (Mpa)
67.5
33.6
Static stress at 5 atm. (Mpa)
28.
14.

7. 3. Improve the Situation with a Tumbling Window.

8. The time between hits of the same spot is 6. 4 s (0
The time between hits of the same spot is s (0.16 Hz), much longer than in the stationary window with 0.2 s. Therefore gas cooling is much simpler for the moving window. Optimize gas pressure and velocity.
To accommodate the tumbling diameter of 5. cm the diameter of the window has to be increased to 8 cm.
The radiation damage and the fatigue will be reduced by a factor 32.
The lifetime of the window will be increased by the same factor.

9. Results for the Tumbling Ti-Window and the High Lumi Case.
Window thickness (mm)
0.1
0.2
Steady state peak temperature (oC) at 1 atm He-cooling.
67.
114.
Steady state peak temperature (oC) at 5 atm Ar-cooling.
70.
144.
Static stress at 1 atm (Mpa).
20.
10.
Static stress at 5 atm (Mpa).
100.
50.
Thermal stress/pulse.
24 MPa
48 MPa
No. of cycles/spot over 5000 h operation
2.8 Mio.
Lifetime at 0.5 dpa (h).
1150 !

10. 4.Intermediate Conclusion
For stationary window.
The thicknesses above 0.2 mm lead to excessive temperatures.
Cooling by He-gas of a stationary, double walled window is possible for thicknesses of mm. Peak temperatures however close to 100 oC or above will occur for 0.2 mm thick windows.
Although the thermal stresses are low, fatigue, also by buckling , of the membrane can occur (see Song Jin-IHEP).
With 90 Moi. beam pulses/5000 h, severe radiation damage, fatigue and possibly chemical corrosion must be expected. The life time could be as low as days.

11. To prolongate the life time of the window, a «Tumbling Window» is suggested.
Peak temperatures are around 100 oC.
The mechanics for this tumbling movement is conservative.
An improvement of the life time by a factor of about 32 can be expected for all cumulative effects ( Rad. damage, fatigue, corrosion,…). Lifetime is about 1100 h.
One could imagine spreading the radiation damage over a larger circle or a spiral. A further factor of 2-3 may be gained. But this is mechanically more difficult.

12. The thin Ti-foils must be flawless
The thin Ti-foils must be flawless. Thin walled bellows work under cyclic loads, but they are not hit by a beam!
As an alternative, Beryllium of increased thickness may be considered as window material: lower density, longer radiation length, high heat capacity and thermal conductivity. Technology exists, but delicate and expensive.

13. 5. A possible Option for a Photon Water Dump.
For the photon beam only very thin windows can be used with limited pressure at the window.
The tumbling window solves part of the problem.
The “hot” cascade in the water, where most of the energy is deposited, extends over about 1 m and a radius of 5 cm. It contains 66% of the beam power of 86 kW.
Along the axis, the energy deposition is still very narrow, close to the beam size.

14. Energy deposition by the High Lumi beam in water (A
Energy deposition by the High Lumi beam in water (A. Ushakov), placed at 48 m downstream of the target.
Axial Position z (cm)
0.01
4.-5.
PEDD (J/g pulse)
~ 80.
760.
ΔT/pulse (K)
~ 10.
181.!

15. Improving the Situation
To avoid pile up of the temperature between the pulses, a vertical flow in the water of some cm/200 ms is sufficient. Say 0.1 m/s is still a laminar flow.
To prevent boiling, a preload of 12 atm was suggested.
At the contact of the stationary beam window with the cooling water, turbulent flow (several m/s) is required. The beam window with a thickness of mm of close to 100 oC is in contact with rather cold water.
Its life time due to radiation damage is very short.
This is not a comfortable situation. Can we do better?

16. Consider an Option: Water Dump at 1 atm.
Whenever a EDD of about 340 J/g is reached, (see A.Ushakov), the water will start to boil.
The vapor created along the axis may push aside the surrounding water.
A hole will be created already DURING the beam pulse.
The remaining beam pulse will continue to drill.
Where does this process end?

17. A simple Model: Assume an EDD decreasing linearly with z from 760 J/g to zero at z=100 cm.

18. This should be calculated more precisely.
However, one may expect that the depth of the hole due to boiling at the end of the pulse will be about 1. m.
Additional water length is required to reduce the energy density along the axis, till a safe, metallic end stop can be used.

19. Pressure Waves in the Water
The beam causes linearly rising thermal expansion of the water volume, up to 1 ms. This is partially prevented by the surrounding, unheated water and by the mass inertia of the heated volume itself.
This causes pressure amplitudes and waves with positive and negative (depletion) values already during the pulse.
At “negative” pressures, the water may “brake up”, start to boil and create a hole.
Such negative pressures of -1.5 atm have been reported for the TESLA-water dump (M. Schmitz).
To prevent this, some pressure preload in the system may be required.
But the limit for the window is about 5 atm.

20. Sketch of the Water Curtain Dump

21. Conditions to be verified.
The dump must have sufficient length to tolerate “drilling” holes by evaporation along the axis.
The water splash is expected to occur rather into lateral than into axial direction.
For this, sufficient “free” buffer volume must be provided around the curtain in the tank to avoid pressure bumps reaching the wall of the tank.

22. From the EDD vrs. z ( Ref. A. Ushakov) one can estimate the water volume which exceeds the boiling point at 1 atm and during one pulse.
Considering High Lumi, this is the case in a central region with a diameter of 2 mm and a length of below 1. m.
The average power deposited there is about 2. kW.
This leads to a production of a vapor volume of about 1.5 l/s at 100 oC and 1 atm. This is ok, my water boiler in my kitchen has a power of 1.6 kW.
The required mass flow in the closed water circuit should be at least 10 kg/s. 30 kg/s would be better.

23. 6. Discussion and Conclusion.
A tumbling window, in combination with a water curtain dump at 1 atm could be an option.
By placing a tumbling window about 10 m upstream of the water dump, its replacement and the maintenance of the tumbling mechanics can be envisaged.
Direct contact of the window with water is avoided.
The contact of the window with air and some humidity can be controled.
The water and vapor is completely confined in a closed circuit.

24. The production of water vapor in the tank of the dump must be tolerated.
The response of the water to the beam pulse must be studied in more detail, using professional FEM-codes, comprising dynamic effects, depletion waves and phase change in water.
At what time during the pulse does this occur?
What preload is required to prevent this?
Will there be an explosion or just a slow pressure puff?

25. Radiation Damage and the End Stop.
Radiation damage in the primary window is reduced by spreading the beam energy over the tumbling window. Life time was about 1150 h. Could be improved.
However, the possibility of easy exchange must be foreseen.
“Radiation Damage” in the water can be tolerated. The water is “reconditioned” continuously online, by the remote water treatment plant. This is one of the advantages , as compared to stationary solid dumps, suffering from radiation damage.

26. Over a length of 5.6 m of water, the photon beam is attenuated by a factor of about 10^4.
This would allow to use safely a downstream thick Ti-window and a Cu end stop with sufficient radiation resistance for 5000 h lifetime (Ref. A. Ushakov).
Also the temperature rise and stresses in a thick 1 mm Ti-end window are tolerable.
This would then allow to use a water cooled Cu-end stop outside of the water tank.
The total length of the water tank and the slender end pipe of about 10 cm width could be about 7.6 m, including the initial hot evaporation zone of about 2 m.
Adequate shielding around and behind the water tank must be foreseen.
Safety aspects (water leaks), production of Tritium, Be 7,….have to be considered.

27. 7. Shifting the Tumbling Window and the Water Dump by 1 km.
Window thickness (mm)
0.1
0.2
0.5
1.0
ΔT/Pulse (K)
6.2
12.5
62.7
129.
Steady state temperature ( C)
26.2/ 1 atm He cooling
32.5/ 1 atm He cooling
82.7/ 1 atm
He cooling
188./ 1 atm He cooling / 5 atm He
Thermal stress/pulse (MPa) ok
32.
66.
Static stress (MPa)
100./ 5 atm
200./10 atm
50./ 5 atm
100./10 atm
20./ 5 atm
40./ 10 atm
10./ 5 atm
20./10 atm
Life time ( h)
8000.

28. Due to the enlarged beam size at 1 km, a factor of 7
Due to the enlarged beam size at 1 km, a factor of 7.5 is gained in the performance of the tumbling window.
Similarly, the temperature rise/pulse in the water is reduced from 181 K to 24 K. Thus, no boiling!
The dynamic response of the water to the beam pulse will be less violent. But must still be studied.
Shifting the water dump by 1 km will be very helpful!

29. Thank you for your Attention.