1. Keith Kishiyama, Marty Roeben Dimitri Ryutov, John Trent and
LCLS-XTOD Attenuator
LCLS Lehman Review
February 8-9, 2006
Keith Kishiyama, Marty Roeben
Dimitri Ryutov, John Trent and
Stewart Shen
This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.

2. Contents Requirements Gas Attenuation Passive Pumping Design
Performance Analysis
Solid Attenuation
Argon option
Prototype Plan
Project Schedule
Conclusions

3. Attenuator Physics Requirements
Be able to reduce the FEL beam intensity by up to 4 orders of magnitude at any energy within the design range (0.8 to 8 keV)
Provide stable, reproducible (to within 1%) attenuation for repeated FEL shots.
Can be configured to allow unobstructed passage of all radiation through the stay-out-zone
Implied Requirements
Provide matched boundary vacuum conditions

4. Baseline Plan: Use 6 meters of N2 gas for EFEL < 2 keV Use solid Be blocks for EFEL > 2 keV
Maximum N2 pressure 20 Torr
Use Gas
Use Solids
Maximum Be thickness 5.7 cm
* For a transmission of 10-4

5. Attenuator Conceptual Configuration
6 meter long, high pressure N2 section
3 Differential pumping sections separated by 3 mm apertures
3 mm diameter holes in Be disks allow 880 mm (FWHM), 827 eV FEL to pass unobstructed
N2 Gas inlet
Solid Be attenuators are inserted in the high-pressure section

6. 3 mm Apertures on Bellows to allow transverse positioning of opening in window
Y stage
Gate valve
Bellows
Be disk on gate valve
X stage
Y motion
Be disk on gate valve survives FEL hits. Disks are transparent to high energy spontaneous, allowing alignment of hole using WFOVDI camera in FEE. Gate valve removes window when gas attenuator not in use.

7. Our calculations show 20 Torr achievable in central chamber with 3 mm dia. holes
While pressure in external beam pipe remains low.
Central chamber reaches 18.8 Torr
Pressure in external beam pipe 4x10-7 Torr
6 port 3-mm Aperture
Total pumping speed 720 L/s
Input gas flow = 5053 sccm N2

8. Full Scale N2 System has reasonable flow rates and pumping requirements

9. Solid Attenuator Concept
Occupies central cell of gas attenuator
256 attenuation levels
Eight beryllium slides
Each twice as thick as the last (0.3 to 38.4 mm)
Up to % attenuation of 8.26 keV x-rays
Can be used in combination with gas attenuator
Pneumatically actuated

10. Combination of gas and solid attenuators allow fine tuning of attenuation at all FEL energies
Beryllium
X-ray Energy (eV)
8267
7000
6000
5000
4000
3000
2000
Attenuation Length (microns)
72.958
Attenuation Length (mm)
6.91
3.97
2.40
1.33
0.65
0.26
0.07
Transmission thru Gas Attenuator at max pressure
0.8812
0.8066
0.7067
0.5438
0.3003
0.0575
0.0001
Thickness (mm)
Transmission
0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3.0

11. Attenuator requires 10 meters in the FEE
Fast Valve
X-ray Slit
Ion Chamber
Attenuator
Fixed Mask
Diagnostics
Offset Mirrors

12. Attenuator Risks and Mitigations
Pressure calculations are wrong, or system is not stable (pumping, temperature, flow)
To be tested in a prototype gas attenuator, under construction
Accuracy & Repeatability Unachievable
1% attenuation accuracy & repeatability is difficult to achieve at attenuation factors above 102 because of the very high precision in gas uniformity required. We suggest reducing the requirement to 5%
Heating of the gas by FEL beam, Other sources of uncertainty (temperature, gas purity, absorption coefficient), Stresses on Be Diaphragm, Erosion of Be Diaphragm
Calculations show these are not a problem

13. Attenuator Risks and Mitigations (cont.)
Contribution of the transition stages to uncertainties in the attenuation
Under study
Aperture not large enough to accommodate FEL spatial envelope
Would limit achievable pressures
Corrosion of the Be by the N2 gas or ions
This could be a serious problem and is hard to quantify

14. Ar could be substituted for N2 to overcome some risks*
Ar requires less pressure so could accommodate larger apertures
Ar will not corrode Be
Pressure of 600 cm of Ar and N2 for 104 Attenuation
*LCLS-TN-06-1 "The Physics Analysis of a Gas Attenuator with Argon as a Working Gas." UCRL-TR (January 2006) D.D. Ryutov, R.M. Bionta, M.A. McKernan, S. Shen, J.W. Trent,

15. Ar could work up to 8 keV at 60 Torr
Ar at 60 Torr would eliminate the need for the solid attenuator, but the absorption edge at 3 keV and added safety concerns about heavy gasses, makes Ar less than ideal. We keep Ar as a backup option and will test its use in the prototype.

16. Prototype Gas Attenuator under construction
Objectives
Phase 1
Demonstrate stable control of gas chamber pressure (N2 & Ar)
Validate the vacuum design for intermediate flow
Verify mechanical & thermal stability
Configuration
Phase 2
To measure the effect of aperture-nozzle geometries.
Integration with Solid

17. Prototype System will be one-half of the full LCLS gas attenuator
Turbo
Gas Chamber
Scroll
Port 1,2,3

18. Some parts have been assembled to provide preliminary data
Chamber- 1
Chamber- 2
Turbo
Chamber- 3
Pump Calibration Test Setup
Scroll

19. Initial Testing shows some deviations from model at high pressures
Measurement and Calculations

20. XTOD Attenuator Project Schedule
Prototype Testing Results (5/06)
ESD (6/06)
Preliminary Design Review (12/06)
Final Design Review (2/07)
Procurement (6/07)
Assemble/Test (8/07)
FEE Beneficial Occupancy (8/07)

21. Summary Attenuator conceptual design uses combination of gas
and solid schemes.
Argon can substitute for N2 to mitigate some risks.
Computer gas flow model has been calibrated.
Prototype Testing will start soon.
Project is on schedule.