LCLS-TN-12-5-R1
Adjustable Aperture Slit Material Selection for LCLS-II
∗
J. Welch
September 28, 2012
Revisions
apertures are in use, a high-fluence FEL beam will
need to be very close to the slits, and it is therefore
prudent to make the slits of a design that is reasonably robust against high fluence damage.
1. Added SXR only configuration
Introduction
Material Recommendation
To understand the basis for the material selection of
the slits it is useful to consider the function intended
for adjustable apertures. In LCLS-II the adjustable Recommended material selections are given for two
aperture has basically the same functions as in LCLS- cases: (1) a design which will accommodate both the
SXR and HXR beams, and (2) a design that will
I, which are
only accommodate the SXR design. The SXR de• Define the angular range of spontaneous radia- sign is thinner but will not function well in the HXR
beamline, because the high energy photons will pention (SR) for K measurements.
etrate the thin B4 C layer and damage the underlying
• Allow a study of apertures by making a shadow contrast material.
in the SR that is projected downstream.
• Define a footprint on the offset mirrors for mirror
studies.
HXR and SXR Compatible
The simplest recommended package of slit materials for both the SXR and HXR beamlines is given
Table 1.The first layer the beam sees, B4C, is thick
In each of these applications there is not a neces- enough to keep the dose in the aluminum second layer
sity for the adjustable aperture slits to be able to at or below about 0.1 eV/atom . The aluminum is
withstand the highest fluence FEL beams. It is only just thick enough to provide better than 1% contrast
required that there is good contrast of SR on a down- for SR up to 25 keV. Additional thickness of either
stream YAG screen. The machine protection system layer would work as well.
can be used to insure that the adjustable aperture
slits are not accidentally exposed to high intensity SXR Only Compatible
FEL radiation. Nevertheless, when the adjustable
The simplest recommended package of slit materials
∗ Work supported in part by the DOE Contract DE-AC0276SF00515. This work was performed in support of the LCLS for an SXR-only slit design is given in Table 2. The
B4 C is only thick enough to protect the high-Z layer
project at SLAC.
• Clean up the beam profile by blocking large angle SR without blocking the main FEL beam.
1
By itself, it can provide good contrast up to x-ray
Table 1: Recommended materials and minimum
energies of about 8 keV.
thicknesses for the slits of the adjustable apertures
For FEL beams with a fundamental in the 8 keV
compatible with both SXR and HXR beams. .
to 13 keV range there is still significant pulse energy
transmitted through reasonable thicknesses of B4C.
Layer
Material Thickness
In this region aluminum provides good contrast and
[mm]
absorbs well, but not so well that it is easily damaged,
First
B4 C
15
so it makes a good second layer.
Second Al or Si
5
For SR with photons energy in the 13-25 keV range
and beyond (harmonics) any high-Z material can provide good contrast.
from FEL beams up to about 2500 eV. The high-Z
In Figure 1 are the results of a calculation of the
second layer provides adequate contrast for SR.
transmitted pulse energy and maximum absorbed
dose for the slit material package given in Table 1.
These calculations are described in Ref [1]. The iniTable 2: Recommended materials and minimum
tial FEL pulse energy was assumed to be 12 mJ and
thicknesses for the slits compatible with only SXR
the distance to the source was assumed to be 121 m.
beams. Any element with higher atomic number than
This analysis was performed to see how robust the
tungsten (e.g. Au, Pt) may be substitute for W, proslit package would be against direct accidental FEL
vided the mass/area is kept constant or larger.
beam strike, as well as how much contrast can be
expected.
Layer
Material Thickness
The plots in the first row in Figure 1 show the
[mm]
maximum dose and the transmitted pulse energy for
First
B4 C
1
the B4C first layer. FEL beams with a photon energy
Second W or Ta
0.1
below 2 keV can cause the dose in B4C to exceed the
safe working dose of 0.1 eV/atom. Damage may not
actually occur until substantially higher dose, but the
limit is not well known.
In the second row are the maximum dose and the
Discussion
transmitted pulse energy for the aluminum second
The SR spectrum for both the SXR and HXR beam- layer. Here the maximum dose is about equal to
lines is very broad and consists of a fundamental 0.1 eV per atom so the aluminum is at some risk of
ranging from 250 eV to 13 keV and significantly in- being damage by the highest fluence of hardest x-rays
tense higher harmonics up to more than one hundred and should be protected by the machine protection
keV. However harmonics with energy below about 25 system.
keV are going to interact most significantly with the
In Figure 2 the calculation has been extended to
YAG screen to form an image. For good contrast of 25 keV and demonstrates effective contrast for SR.
SR, we therefore seek materials that will block ener- The net fraction of transmitted beam through the
gies up to 25 keV. Good contrast is assumed to be aluminum is less than 1% for all photon energies in
less than about 1% transmitted to incident intensity this range, which indicate the package provides good
ratio.
contrast of SR.
Any high-Z material can easily provide contrast of
1% or better, but it will also easily be damaged. For
SXR compatible discussion
this reason a robust first layer of B4C is included.
B4C is a well-characterized material, can withstand In the SXR case the maximum FEL beam photon
very high fluences, and is relatively safe to work with. energy is only about 2 keV so there is relative little
2
eV/atom B4C
Fraction of mJ tranmitted through B4C
0.8
0.2
0.6
0.15
0.4
0.1
0.2
0.05
0
0
5000
10000
Xray energy [eV]
eV/atom Aluminum
0
15000
0.2
4
0.15
3
0.1
2
0.05
1
0
0
−6
x 10
5000
10000
Xray energy [eV]
eV/atom Tungsten
0
15000
0
5000
10000
15000
Xray energy [eV]
−8
Fraction
of mJ transmitted through Al
x 10
0
5000
10000
15000
Xray energy [eV]
−10
Fraction
of mJ transmitted through W
x 10
3 1: Calculation of fluences and dose for the materials6of the slits of the adjustable aperture over the
Figure
range FEL beam energies.
2
4
1
0
2
0
5000
10000
Xray energy [eV]
0
15000
3
0
5000
10000
Xray energy [eV]
15000
eV/atom B4C
Fraction of mJ tranmitted through B4C
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0
0
0.5
1
1.5
Xray energy [eV]
eV/atom Aluminum
2
0
2.5
4
x 10
1
8
0
0.5
1
1.5
2
2.5
4
Xray energy [eV]
x 10
−3
x 10Fraction of mJ transmitted through Al
0
0.5
6
0.5
4
2
0
0
0.5
1
1.5
Xray energy [eV]
eV/atom Tungsten
2
0
2.5
4
x 10
1
1.5
2
2.5
4
Xray energy [eV]
x 10
−3
Fraction
of
mJ
transmitted
through
W
x 10
1 2: Detail calculation fluences and dose for the various3 materials of the slits of the adjustable aperture
Figure
over the range of relevant SR energies.
2
0.5
1
0
0
0.5
1
1.5
Xray energy [eV]
2
0
2.5
4
x 10
4
0
0.5
1
1.5
Xray energy [eV]
2
2.5
4
x 10
penetrating high fluence energy to deal with. This
means the B4 C layer thickness can be reduced to the
point where the transmitted intensity at 2 keV is safe
for the underlying material. Good contrast for photon energies up to 25 keV can be provided by a thin
layer of high-Z material. See Figure 3 for calculation results for the recommended slit materials. The
right-hand plot of the bottom row shows that energy
absorbed in a 100 µm YAG screen is less than 0.1 %
over the range of harmonics of interest, so good SR
contrast is expected.
References
[1] J. Welch, Failsafe FEL Stopper and Collimator
Package, June 2012, LCLS-TN-12-2
5
eV/atom B4C
0.6
0.8
0.4
0.6
0.4
0.2
0
0.2
0
500
−9
1
x 10
1000 1500 2000
Xray energy [eV]
0
2500
eV/atom W
1
1.5
2
Xray energy [eV]
2.5
1
1.5
2
Xray energy [eV]
2.5
1
1.5
2
Xray energy [eV]
2.5
3
4
x 10
Fraction of mJ transmitted through W
0.01
0.4
0.005
0.2
0
500
−126
x 10
1000 1500 2000
Xray energy [eV]
0
2500
0
0.5
3
4
x 10
−3
x 10Fraction of mJ absorbed by YAG
eV/atom YAG
4
6
3
4
2
2
1
0
0.5
0.015
0.6
5
0
0.02
0.8
0
Fraction of mJ tranmitted through B4C
1
0
500
1000 1500 2000
Xray energy [eV]
0
2500
0
0.5
3
x 10
Figure 3: Calculation of fluences and doses for the slit materials for the SXR beam only.
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