1. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx LCLS X-Ray Absorber Richard Bionta, 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. LCLS FAC Meeting April 20, 2006

2. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Contents Introduction Requirements General Concept Gas Attenuator Pressure System Conceptual Design Passive Pumping Design Performance Analysis Gas Attenuator Prototype Solid Attenuation & Actuator Testing Instrumentation and Control Attenuator Subsystem Schedule Summary

3. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Updated Physics Requirements* * LCLS-PRD-1.5-003 “Physics Requirements for the XTOD Attenuator System”, 3/15/2006

4. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Attenuator envelope is 10 meters, with 6 m at high pressure Fast Valve X-ray Slit Ion Chamber Attenuator Fixed Mask Diagnostics Offset Mirrors

5. Baseline Plan: Use 6 meters of N 2 gas for E FEL 2 keV * For a transmission of 10 -4 Use Gas Use Solids Maximum N 2 pressure 20 Torr Maximum Be thickness 5.7 cm

6. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Nitrogen Pressure Requirements

7. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Attenuator Conceptual Configuration 6 meter long, high pressure N 2 section 3 Differential pumping sections separated by 3 mm apertures N 2 Gas inlet 3 mm diameter holes in Be disks allow 880  m (FWHM), 827 eV FEL to pass unobstructed Solid Be attenuators are inserted in the high-pressure section N 2 boil-off (surface) Flow restrictor Green line carries exhaust to surface

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. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Alignment concept using direct imager 20 Torr 0.7 Torr Gas Attenuator Gas Detector 1 Gas Detector 2 ~10 -6 Torr 6 m 10 cm 2 m 1.5 m WFOV Direct Imager

10. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Open valves to align sections 20 Torr 0.7 Torr Gas Attenuator Gas Detector 1 Gas Detector 2 ~10 -6 Torr 6 m 10 cm 2 m 1.5 m WFOV Direct Imager

11. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx 10 cm 1 window 1 window with Ta ring3 windows with Ta rings Low energy spontaneous through attenuator windows (40 mm diameter x 1 mm thick with 3 mm diameter hole through center) 10 cm 3 mm hole may be hard to see (limited by simulation statistics) but Ta rings allow easy identification of hole position

12. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx All valves open 3 valves closed 4.6 mR + 6 mm misalignment 3 valves closed 0.46 mR + 300  m 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. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Gas Attenuator Pressure System

14. 10-m 6 port 3-mm Aperture 720 L/s Q i = 0 sccm Pump Down Base Pressure Final outgassing rates of 10 -9 T-L/s-cm 2 is assumed Scroll Pump Speed vs Pressure is modeled Turbo Pump Speed vs Pressure is modeled System Pump down Performance

15. Chamber pressure log scale 18.8 Torr 4x10 -7 Torr Vacuum to full pressure with  = 50 seconds These calculations show 20 Torr achievable in central chamber with 3 mm holes… Inlet N 2 flow Q i = 5053 sccm Chamber pressure linear scale …while pressure in external beam pipe remains low.

16. Full Scale System has reasonable flow rates and pumping requirements

17. Attenuator Risks and Mitigations Pressure calculations are wrong, or system is not stable (pumping, temperature, flow) Testing underway in prototype, pressure ok, stability, temperature TBD Accuracy & Repeatability Unachievable 1% attenuation accuracy & repeatability is difficult to achieve at attenuation factors above 10 2 because of the very high precision in gas uniformity required. The requirement has been relaxed to 5%. Accuracy at this level requires calibration. Other sources of uncertainty (temperature, gas purity, absorption coefficient), Stresses on Be Diaphragm, Erosion of Be Diaphragm Calculations show these are not a problem Heating of the gas by FEL could be an issue Requires more study

18. Attenuator Risks and Mitigations (cont.) Contribution of the transition stages to uncertainties in the attenuation Under study, smaller transition better 3 mm aperture not large enough to accommodate FEL spatial envelope Larger aperture would limit achievable pressures Corrosion of the Be by the N 2 gas or ions This could be a serious problem but is hard to quantify

19. Ar could be substituted for N 2 to overcome some risks* Pressure of 600 cm of Ar and N 2 for 10 4 Attenuation Ar requires less pressure so could accommodate slightly larger apertures Ar will not corrode Be *LCLS-TN-06-1 "The Physics Analysis of a Gas Attenuator with Argon as a Working Gas." UCRL-TR- 217980 (January 2006) D.D. Ryutov, R.M. Bionta, M.A. McKernan, S. Shen, J.W. Trent,LCLS-TN-06-1

20. 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.

21. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Gas Attenuator Prototyping

22. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Prototype Gas Attenuator Has Been Completed Configuration Objectives Phase 1 Demonstrate stable control of gas chamber pressure (N 2 & Ar) Validate the vacuum design for intermediate flow Verify mechanical & thermal stability Phase 2 To measure the effect of aperture-nozzle geometries. Integration with Solid

23. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Prototype System is one-half of the full LCLS gas attenuator Gas Chamber Port 1,2,3 Scroll Turbo

24. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Prototype Assembly -1

25. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Completed Prototype Assembly with I&C

26. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Preliminary Test Results- N 2 Pump Arrangement V2 2 10 L/s Scroll V3 1 600 L/s Turbo V4 1 70 L/s Turbo

27. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Preliminary Results- N 2 -3 rd Stage Good Agreement with Calculation

28. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Prototype Preliminary Results Summary Computer Model Bench Marked and Validated Gas Flow (input flow, conductance) Pump Performance Pressure Distribution Met 20-Torr Design Goals Stable Short-Term Operation Up to 50 Torr Nitrogen

29. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Solid Attenuation and Actuator Testing

30. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Solid Attenuator Concept Overview 128 attenuation levels Seven beryllium slides 0.375, 0.75, 1.5, 3, 6, 12, and 24 mm thick Gives 0 to 47.625 mm in 0.375 mm increments Up to 20,000x attenuation of 8.26 keV x-rays Pneumatically actuated

31. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Solid Attenuator Features Repeatable within 0.1% of attenuation level Fast changes – couple seconds Safe - Only Be can intersect FEL (holder is open on end) Safe - Actuators fail in with internal spring Verified in place – micro-switches on actuator Takes up no space in Z (it’s inside gas cell) Be slides polished to optical finish

32. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Front View – Attenuating Pneumatic Actuator Attenuator Block Holder Custom Vacuum Vessel Adapter Flange Attenuator Block

33. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Front View – Open Spontaneous Radiation Clear Aperture

34. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx How Solid and Gas Work Together For 8.26 keV x-rays the range of the gas attenuation with Nitrogen at 0 to 20 Torr is 100% to 88.1% transmission (0 to 12% attenuation) The thinnest solid attenuator (0.375 mm) at 8.26 keV provides 92.5% transmission (8% attenuation) Overlap means gas fills in between solid levels – very fine resolution and large range

35. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Solid Attenuator will be in Gas Attenuator Solid Attenuator (Upstream end preferred to minimize effect of Compton scattering on downstream diagnostics)

36. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Solid Attenuator Actuator Testing Objective To determine the solid rotation angle, mainly about vertical axis Actuator Huntington UHV compatible pneumatic actuator; model L-2271-4-LL-2D-SM-EX, with spring to extend Setup Use custom test stand and representative test block. Cycle actuator and measure using CMM

37. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Solid Attenuator Actuator Prototype Measured angular repeatability of face of test block Used Coordinate Measuring Machine to determine precise location of face over 10 cycles Actuator Test Block CMM Probe

38. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Solid Attenuator Actuator Prototype Results Actuator repeats 11 times better than required Repeats within 0.2 degrees Requirement is 2.2 degrees for 5% repeatability at highest attenuation level Overall attenuation will repeat within 0.1% at the highest attenuation level due to angular repeatability Requirement is 5% repeatability

39. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Plan for Beryllium Procurement Make all attenuator slides from one batch Baseline Be has 200 ppm Fe One typical batch can yield two sets of slides Test attenuation length of Be Make Be slide of well measured thickness from batch Measure attenuation length of Be batch sample slide using x-rays

40. We plan to use polished O-30H Be for the solid attenuator blocks O-30H is the best Be for for polishing minimizing coherence effects

41. 2 sets of solid attenuator blocks can be fabricated for ~$30K (materials, cutting, and polishing)

42. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Instrumentation and Control

43. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Attenuator I&C Approach The control system for the attenuator will consist of a PLC that will be connected via a network to the global control system, EPICS

44. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Fault Tree with Control – Example 1

45. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Fault Tree with Control – Example 2

46. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx XTOD Attenuator Subsystem Schedule 1.System Concept Review(4/13/06) 2.Prototype Testing Results (5/06) 3.ESD (6/06) 4.Preliminary Design Review (12/06) 5.Final Design Review (2/07) 6.Procurement (6/07) 7.Assemble/Test (8/07) 8.FEE Beneficial Occupancy (8/07)

47. Stewart Shen shen2@llnl.gov April 20, 2006 UCRL-PRES-xxxx Summary Conceptual design meets requirements Design uses combination of gas and solid materials Computer gas flow model has been calibrated and is a powerful design tool Prototype testing is producing useful data Gas flow & pressure management is under control Project is on schedule to meet current baseline