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.

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

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

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

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

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.

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

Full Scale N2 System has reasonable flow rates and pumping requirements

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 99.998% attenuation of 8.26 keV x-rays Can be used in combination with gas attenuator Pneumatically actuated

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) 6906.442 3965.672 2398.152 1327.45 647.367 259.157 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 1.00000 0.3 0.95749 0.92714 0.88241 0.79772 0.62913 0.31424 0.01638 0.6 0.91679 0.85959 0.77865 0.63636 0.39581 0.09875 0.00027 0.9 0.87782 0.79696 0.68709 0.50764 0.24901 0.03103 0.00000 1.2 0.84051 0.73890 0.60630 0.40495 0.15666 0.00975 1.5 0.80478 0.68506 0.53500 0.32304 0.09856 0.00306 1.8 0.77057 0.63515 0.47209 0.25769 0.06201 0.00096 2.1 0.73781 0.58887 0.41658 0.20557 0.03901 0.00030 2.4 0.70645 0.54597 0.36760 0.16399 0.02454 0.00010 2.7 0.67642 0.50619 0.32437 0.13081 0.01544 0.00003 3.0 0.64767 0.46931 0.28623 0.10435 0.00971 0.00001

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

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

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

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-217980 (January 2006) D.D. Ryutov, R.M. Bionta, M.A. McKernan, S. Shen, J.W. Trent,

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.

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

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

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

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

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)

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.