1. XTOD Attenuator Status
DOE Review of the LCLS Project
(SC4)
October 25, 2006
Richard Bionta, Stefan Hau-Riege, Keith Kishiyama,
Donn McMahon, 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. Content Physics Requirements Aperture Arrangement Gas Attenuator
Solid Attenuator
Integrated Attenuator Design
Prototype Testing
Project Management Issues

3. Design based on Physics Requirements*
* LCLS-PRD “Physics Requirements for the XTOD Attenuator System”, 3/15/2006

4. Attenuator is part of Front End Enclosure (FEE)
Slit
Solid
Attenuators
Spectrometer
Package
Collimator 1
Total Energy
Fixed
Mask
Beam Direction
Gas Attenuator
Gas Detector
Gas
Detector
Direct Imager
(Scintillator)
FEL Offset Mirror System

5. Gas Detector / Attenuator Conceptual Configuration
N2 boil-off (surface)
Gas detector
Flow restrictor
4.5 meter long, high pressure N2 section
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
Solid attenuators
N2 Gas inlet
Green line carries exhaust to surface
Status Attenuator:
PRD done
SCR done
Prototype done
ESD draft
PDR in preparation
Gas detector

6. Prototype Gas Attenuator runs well at 0-20 Torr

7. Gas detectors share differential pumping with the Gas Attenuator
Optical band pass filter
Photo diode
Gas Attenuator high pressure section
Gas detector
~1 Torr
~10-3 Torr
~0.1 to 2 Torr N2
~10-3 Torr
20 Torr
~10-6 Torr
~20 cm inner diameter
non-specular reflective surface (e.g. treated Al)
Single shot, non destructive, measurement of u
2 m
5Torr no problem according to Stew
Optical ND filters
Photo tube
LCLS X rays cause N2 molecules to fluoresce in the near UV

8. 3-mm Apertures in Transition Stages with Bellows to allow transverse positioning of opening in window
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 and for ease of alignment.

9. Alignment concept using direct imager
Gas Detector 1
Gas Attenuator
Gas Detector 2
~10-6 Torr
2 Torr
2 Torr
~10-6 Torr
20 Torr
10 cm
4.5 m
10 cm
2 m
2 m
WFOV Direct Imager
1.5 m
1.5 m

10. Open valves to align sections
Gas Detector 1
Gas Attenuator
Gas Detector 2
0.7 Torr
~10-6 Torr
20 Torr
0.7 Torr
10 cm
4.5 m
2 m
10 cm
2 m
WFOV Direct Imager
1.5 m
1.5 m

11. (40 mm diameter x 1 mm thick with 3 mm diameter hole through center)
10 cm
1 window
1 window with Ta ring
3 windows with Ta rings
Low energy spontaneous through attenuator windows
(40 mm diameter x 1 mm thick with 3 mm diameter hole through center)
3 mm hole may be hard to see (limited by simulation statistics) but Ta rings allow easy identification of hole position

12. Direct imager image of saturated low energy FEL + Spontaneous
All valves open
3 valves closed
4.6 mR + 6 mm misalignment
0.46 mR mm misalignment
Direct imager image of saturated low energy FEL + Spontaneous
With this method it is easy to locate the FEL and systematically align the apertures with the FEL wherever it is located

13. Attenuator Risks and Mitigations
Pressure calculations, and system stability (pumping, temperature, flow)
Retired. Proved by prototype testing results.
Accuracy & Repeatability Unachievable
Repeatability proven.
Accuracy within 5% likely achieved, assuming local temp fluctuation is negligible.
Possible Optical Distortion caused by gas plume disturbance
Calculation shows it is not a problem.
Contribution of the transition stages to uncertainties in the attenuation
In PDR, shorter transition stages will be pursued.
Corrosion of the Be by the N2 gas or ions
This could be a problem, Ar gas is the alternative,
or replace the Be with B4C.

14. Prototype Integrated Attenuator System
Gas Attenuator
Gas Detector Cell

15. Gas Detector Cell
Last Stage (Stage 6)

16. Completed Prototype System @ LLNL Vacuum Lab
Gas Flow Control-1
Gas Flow Control-2
Gas Attenuator
Gas Detector Cell
Last Stage

17. Integrated System Vacuum Modeling Pressure Distribution
Normal Operation
Gas Attenuator 20 Torr
Gas Detector 2.5 Torr
Pressure Distribution
Attenuator Pressure 20 Torr
V Torr
V x10-3 Torr
V-4 (GD) 2.0 Torr
V x10-3 Torr
V x10-6 Torr
Qi sccm
Qg 171 sccm
RESULTS
In response to step gas inputs of Qi and Qg,,
1.Attenuator pressure reached design level (20 Torr)
2.Gas detector pressure reached design level (2.0 Torr)
3. Last stage pressure met boundary conditions (~3x10-6 Torr)

18. Integrated System Vacuum Modeling Pressure Distribution
Stand-by Operation
Gas Attenuator 0 Torr
Gas Detector 2.5 Torr
Pressure Distribution
Attenuator Pressure 1.95x10-4 Torr
V x10-5 Torr
V x10-4 Torr
V-4 (GD) 2.0 Torr
V x10-3 Torr
V x10-6 Torr
Qi 0 sccm
Qg 171 sccm
RESULTS
In response to sudden drop of gas input of Qi,,
1.Attenuator pressure was lowered to ZERO level (10-4 Torr)
2.Gas detector pressure maintained at design level (2.0 Torr)
3.Last stage pressure still met boundary conditions (~3x10-6 Torr)

19. Prototype Tests (10/10/06)
Pressures of Gas Attenuator and Gas Detector can be operated independently.

20. Comparison of Measurement & Calculation
Value
GA-P
(Torr)
GD-P
GA-Flow
(sccm)
GD-Flow
Stage-2 P
Stage-3P
Stage-5 P
Stage-6 P
Measured 20 2
1600
220
.84
4.77x10-3
2.42x10-3
4.33x10-6
Calculated*
1506
171
1.02
1.86x10-3
4.74x10-3
3.55x10-6
* Pump Speed S(p) was extrapolated from catalogue values

21. Purpose of Solid Attenuator
Attenuates high energy x-rays from to levels of current x-ray sources
Diagnostics may not be damaged
Experiments may be performed without damage
Attenuates x-rays whose energy is too high for gas attenuator to attenuate to level desired
Works in series with gas attenuator – effects may be superposed
Overlaps in attenuation levels with gas attenuator to allow large dynamic range with fine levels

22. Solid Attenuator Concept Overview
64 attenuation levels
Six highly polished Be slides
Each twice as thick as the last (0.5 to 16 mm)
Up to 2,300x attenuation of 8.26 keV x-rays
Millions of attenuation levels when used with gas attenuator
Pneumatically actuated

23. Front View – Attenuating
Pneumatic Actuator
Custom Vacuum Vessel
Adapter Flange
Attenuator Block
Attenuator Block Holder

24. How Solid and Gas Work Together
For 8.26 keV x-rays the range of the gas attenuation with Nitrogen at 0 to 30 Torr is 86.7% to 100% transmission
The thinnest solid attenuator (0.5 mm) at 8.26 keV provides 87.6% transmission
Overlap means gas fills in between solid levels – very fine resolution and large range

25. X-ray Energy (eV)
8267
7000
6000
5000
4000
3000
2000
Be Attenuation Length (mm)
4060
3051
1875
1057
539
224.9
64.8
Gas Attenuator Trans Torr N2 (4.5 m)
86.7%
78.6%
67.7%
50.4%
25.8%
4.0%
0.0%
Thickness (mm)
Transmission
0.0
100.0%
0.5
87.6%
83.7%
74.9%
60.0%
36.7%
9.1%
1.0
77.5%
71.1%
57.4%
37.4%
14.5%
1.0%
1.5
68.5%
60.3%
44.0%
23.3%
5.7%
0.1%
2.0
60.6%
51.2%
33.7%
2.3%
4.0
37.0%
26.6%
11.6%
2.2%
8.0
13.8%
7.2%
1.4%
16.0
1.9%
0.5%
31.5
0.04%
0.00%

26. What if maximum Gas Pressure is 20 Torr?
If maximum pressure is 20 Torr there will be small gaps in achievable transmission levels
At 2 keV – No gaps from 100 to 0.1%
At 3 keV – 11.7 to 9.1%, 1.06 to 1.0%, etc.
At 4 keV – 40.6 to 36.7%, 14.9 to 14.5%, etc.
All requirements are met – at least 3 steps per decade (8 desired)
30 Torr operation will be explored

27. Solid Attenuator Actuator Prototype
Measured angular repeatability of face of test block using CMM
Repeats within 0.2 deg.
2.2 degrees required
Will repeat within 0.1% of attenuation level
5% required
Actuator
Test Block
CMM Probe

28. Attenuator DP Stand – Preliminary Design

29. Integrated Attenuator System ~ 8.5 m

30. XTOD Attenuator Subsystem Schedule
System Concept Review (4/06)
Prototype Testing Results (10/06)
ESD (10/06)
Preliminary Design Review (12/06)
Final Design Review (3/07)
Procurement (6/07)
Assemble/Test (9/07)
FEE Beneficial Occupancy (11/07)
Operational (2/08)

31. Summary
Integrated Attenuator System design is workable and offers numerous advantages.
Risk areas are identified.
Obtained excellent prototype testing results.
Vacuum computer modeling yields satisfactory results.
No technical problems are foreseen.
Project schedule meets current program plan.