1. Proposed Offset Mirror System
J. Arthur
SiC 1.5mrad
Be 13mrad
SiC 1.5mrad
24mm offset
8.0m

2. The Problem
Unfiltered LCLS beam contains significant background from spontaneous harmonics (Ec ~ 150 keV) and from Bremsstrahlung from scattered electrons (GeV)
Much of this background lies directly on axis, and it is time-coincident with the FEL pulse
This is a nuisance for radiological shielding, and a huge problem for data taking

3. The Proposed Solution
Use total-reflection mirrors to deflect FEL radiation (and 3rd harmonic, up to 24 keV). Use collimating apertures to eliminate non-reflected background

4. Reflectivity vs energy for a flat SiC mirror at 1
Reflectivity vs energy for a flat SiC mirror at 1.5 mrad incidence angle
R > 95% for E < 24 keV
R < 10% for E > 28 keV
From Berkeley CXRO calculator: <

5. The mirror system will consist of two mirror pairs
The mirror system will consist of two mirror pairs. One pair will be inserted at a time. The SiC pair gives a cutoff at 24 keV; the Be pair gives a cutoff at 2 keV. Interlocks can be used, if necessary, to assure that one of the mirror pairs is always inserted.
SiC 1.5mrad
Be 13mrad
SiC 1.5mrad
24mm offset
8.0m

6. Technical Issues
Create new White Beam Optics Enclosure by moving shielding in FEE upstream
5mm apertures define FEL beam (plus core spontaneous)
Alignment is critical
Need high-quality mirrors
Risk of damage to mirrors from intense FEL pulse

7. Proposed layout of FEE 4 ft Fe 3 ft concrete 4 ft Fe SiC MIR Be MIR
GAS ATTEN
ADJ SLIT
ADJ SLIT
COLLIM
STOP
STOP
SOLID ATTEN
15m
11.6m
>2M SPACE FOR DIAGS
~0.6M SPACE FOR DIAGS

8. 4 ft Fe
4 ft Fe
3 ft
Concrete
3 ft
Concrete
24mm offset
0.65m 1m
8.0m 1m
9.0m
3.78m
2.13m
11.65m
2.13m
Pop-in camera location

9. Status of Proposal
Radiation Physics has examined shielding implications, presented preliminary report
Discussion between XTOD and XES regarding real estate in FEE, commissioning plan, etc.
Conventional Facilities implications are understood
Final decision expected soon