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9.0 Introduction
Objectives
This section concerns those elements between the undulator end and the User
experiments. It is written based on the User and FEL physics requirements for the x-ray
optics and x-ray diagnostics, and the facility requirements (i.e. the facility protocols and
guidelines). Some of the User and FEL physics requirements are found in the document
“LCLS First Experiments (September 2000)”; more details have been obtained by
working with individual User group representatives.
From the known requirements, the appropriate hardware can be determined. It is
usually required to preserve the high transverse coherence, the time structure, and
sometimes the polarization, of the radiation. Both the x-ray optical components, and the
x-ray diagnostics, can be categorized as
a) those systems that are needed for FEL commissioning, or are required by
multiple User experiments
b) those that are specific to a limited number of the User experiments.
In the case of the diagnostics, those that are required for commissioning or are
required by multiple users are described first. Those specific to a particular experiment
are described in the section detailing the particular experimental layout and optical
requirements.
Experimental halls.
Two experimental halls are provided, one close to the undulator exit (Hall A, starting
~ 50 m from the undulator end) and one downstream of the undulator (Hall B, starting ~
400 m from the undulator end). The total experimental floor area is a requirement of the
first 6 experiments planned (including FEL physics); the locations are determined by
local access roads and topology. Optics in Hall B will experience a reduced power
density, by about a factor 15 (see below); this should allow many standard solutions and
materials to be applied to the optics. Hall A is necessary if maximum coherence is
important. Hall A is also necessary because a number of the optical elements and
instruments developed (at least conceptually) for the LCLS depend on close proximity to
the LCLS undulator for optimal operation or parameter values. These elements include
(and are discussed later):
a) planar take-off mirrors (if these were 300 m farther downstream they would
need to be substantially longer, up to 1 meter/facet);
b) refractive lenses (whether solid state or gaseous, such lenses are efficiencylimited by their aperture diameter - even at distances of ~50 m from the
undulator their operation will be marginal);
c) chirped-multilayer compressor optics (these are also efficiency-limited by the
beam diameter);
d) multilayer-based transmission gratings (with present techniques it is doubtful
that the required high-quality and efficient gratings with apertures greater than
~100 m could be built).
An additional reason for a close hall (Hall A) involves the transmission of the
spontaneous synchrotron radiation (SR) to experiments. Close to the undulator, a ~1 cm
aperture through the vacuum system should transmit a usable fraction of this spectrum.
This aperture is at the limit at which the differential pumping systems we have specified
can operate effectively. If the same SR cone needed to be transmitted to the far hall, the
vacuum aperture would more than double, necessitating a complete redesign of the
vacuum components.
These re-specified components would significantly more
expensive.
The large flight distance to Hall B, and associated beam wander, is not expected to be
a problem (the specification of beam "wander" in the LCLS design study report is ~10%
of the beam diameter, i.e. independent of path length). The angular acceptance for SASE
saturation through the undulator is of the same order as the beam divergence, so any
beam angle excursions larger than this value would probably quench any coherent output
(a similar constraint applies to the position of the beam axis). In addition to this, an
active monitor and feedback system coupled to positional and attitude controls on the
mirrors (and possibly the undulator) are planned to help stabilize beam movement.
Figure 9.0.1 illustrates one possible configuration of the halls, in which various
optical elements are indicated (all described later in detail). An important flexibility for
various experimental applications is the possibility of separating, or suppressing, the
spontaneous radiation with respect to the coherent line spectrum. This spectral-angular
filtering function can be performed by:
a) the absorption cell
b) take-off mirrors or crystals, and
c) by the horizontally and vertically tunable x-ray slits.
Front end
Undulator
exit
fast close
valve and
seat
differential
vacuum
pumping
slits /
collimators
differential
vacuum
pumping
collimators
differential
vacuum
pumping
monochromet
er spool
calorimeter
attenuator /
absorber
calorimeter
beam stop /
burn thru'
monitor
differential
vacuum
pumping
mirror
chamber 1, 3
degree
differential
vacuum
pumping
beam stop /
burn thru'
monitor
Hall A3
beam stop/
burn thru'
monitor
Optical
differential
vacuum
pumping
Hall A1
Atomic
physics
Hall A2
beam stop /
burn thru'
monitor
Hall A1
Plasma
physics
beam stop /
burn thru'
monitor
Hall A2
lead
FEL
physics
pinhole,
calorimeter
Hall A3
beam stop /
burn thru'
monitor
lead
mirror
chamber1
degree
differential
vacuum
pumping
beam stop /
burn thru'
monitor
Optical
differential
vacuum
pumping
Hall B1
Nanoscale
dynamics
collimators
differential
vacuum
pumping
Hall B2
beam stop /
burn thru'
monitor
Hall B1
Femto
chemistry
Hall B2
Hall B3
beam stop /
burn thru'
monitor
Biology
Hall B3
Diagnostic
beam stop /
burn thru'
monitor
Diagnostic
Diagnostic
lead
pinhole,
calorimeter
lead
lead
Figure 9.0.1 A schematic diagram of elements in the experimental halls.
Photon-induced damage.
In translating the User and facility requirements to hardware, the attributes of the FEL
x-ray output must be considered. Detailed properties of both the coherent and
spontaneous radiation are available. In particular, the photon energy, the energy per
bunch, and the beam cross section, imply damage will be an important issue in
component design; these parameters are shown in Table 9.0.1 for locations
approximating the front of Hall A, and in Hall B. Only the coherent light poses a
problem; the spontaneous emission is divergent and will usually be reduced by an upstream aperture. It is also mostly at larger energies than the fundamental, and the energy
absorbed in optical components is low.
Normal incidence
The exact criteria for absorption and damage during pulses of such high intensity as
those to be produced by the LCLS are not known, and will be part of the first atomic
physics experiments, but photo absorption and possible subsequent melt are used here.
Table 9.0.2 shows results for different materials used in Hall A1 (atomic physics
experiments), for the worst case of 827 e.V. where absorption is largest [1]. Dose rates
given here are for normal incidence, relevant for transmissive optics, calculated from
photo-ionization cross sections (  phot oi onizati on), with the photon beam areal density  phot on
calculated for a propagated Gaussian beam:
dose  E phot on phot on phot oionization
Ephot on is the photon energy. Comparing the dose predicted in eV/atom, and the dose
required to melt, one finds Be, B4C, C and Si can be used without melt (although Si is
already at > 0.5 the melt limit). Probably BeB, Li and LiH can also be used. Two other
criteria must be considered for transmissive optics; first the material should be such that a
 phase change occurs in a reasonably large (> 5 m) distance, to allow construction.
Second, the distance required for this phase change should be shorter than the penetration
depth, so that a reasonable fraction of the light is transmitted.
Table 9.0.1. Parameters of the FEL x-ray photon output that are involved in estimates
of damage thresholds. Check divergence of SR
FEL photon
energy
0.828 keV (4.54 GeV electron
beam)
8.27 keV (14.35 GeV electron
beam)
Coherent
(fundamental)
Spontaneous
Coherent
(fundamental)
Spontaneous
Energy per pulse
(mJ)
3
1.4
2.5
22
Peak power (GW)
11
4.9
9
12
81
Photons/pulse
23x10
1.9x10
Divergence
(fwhm, rad)
9
Spot (fwhm) at 50
m (m) Hall A
610
130
Spot (fwhm) at
400 m (m) Hall
B
4400
570
Peak energy at 50
m (J. cm-2) Hall A
0.59
11.9
Peak energy at
400 m (J. cm-2)
Hall B
0.01
0.57
310
12
1
100
Table 9.0. 2. Parameters relating to material absorption, damage, and phase change
of 827 eV photons in Hall A1.
w orst ca se FEL photon energy = 8 27 e V, Hall A1 front
do se
(EV/atom )
me lt
(eV/ato m)
ph ase depth
(le ngth for a
1*  p hase
chang e) ( m)
5.2
pe netra tion
de pth (um)
Li
Li H
B
BeB
B4C
C
Si
0.001
0.001
0.005
0.005
0.14
0.22
0.38
Al
0.27
0.2
1.8
Cu
po lystyrene
0.72
0.11
0.35
0.5
2.7
0.36
1.4
0.6
0.86
0.74
0.9
1.1
47
35
5.1
4.3
1.6
1
1.6
pe netra tion
de pth / ph ase
de pth (large is
go od)
9.04
3.64
1.11
1.45
Grazing incidence
Calculations for grazing incidence mirrors include the effect of energy density
dilution through the angle (the footprint area increases), reflectivity, and deposition
throughout an e-folding depth for the photons. The results demonstrate an interplay
between atomic number, incidence angle, and photon energy. The absorbed energy
density is [2]:
A (eV / atom) 
Ppeak 2    i 1  R 
 2 
q
 Dw   p # 

Here Ppeak is the peak coherent power,  is the standard deviation of the temporal
pulse length, q is the electronic charge, Dw [cm] is the beam diameter at the optic, p
[cm] is the 1/e penetration depth of the light into the material in a direction normal to the
surface, # [cm-3] is the atomic density of the material, and R the reflection coefficient.
Following conventional analysis [1,3], we show A vs. i in Fig. 9.0.2 for three candidate
reflecting materials: Au (high-Z), Ni (medium-Z), and Be (low-Z), for three
representative values of the LCLS's coherent fundamental and ~3rd harmonic lines (900
eV, 8500 eV, and 30000 eV). Selecting, e.g., A  0.01, a criterion suggested by earlier
experimental work at SSRL [3], and safe with respect to melt, we may, for example,
select an Au-coated mirror with I ~ 0.0001 for energies < 3 keV, and a Be-coated
reflector with I ~ 0.0005 for all energies >3 keV.
100
A[eV/atom]
10
1
0.1
0.01
Au 1 keV
Au 8.6 keV
Au 30 keV
Be 1keV
Be 8.6 keV
Ni 8.6 keV
Ni 30 keV
Ni 1 keV
Be 30 keV
0.001
0.0001
10-5
10-6 -5
10
0.0001
0.001
0.01
0.1
Grazing Incidence Angle
Figure 9.0.2. Peak power energy loading of candidate LCLS mirror materials vs.
(TE) grazing incidence angle and LCLS energy.
Diffraction
Assuming a crystal to be cut to thicknesses equal to the extinction depth, and operated
in a symmetric diffraction geometry, then a conservative estimate of power and energy
loading can be obtained. Assume: 1) the angles of incidence of the radiation (with
respect to the surface plane) are large enough to enforce negligible reflection; 2) on the
average, the diffracted beams travels through approximately the same distance in the
material as the 0th order beam; and 3) on average, the absorption depth, ta, for the
diffracted beams is the same as for the incoming beam. Then the energy loading is:
(eV / atom) 
Ppeak 2 
qD2w #t a
Selecting Si, C*, and Be as the candidate materials, we tabulate A for selected
crystal plane orientations in Table 9.0.3. The lower absorptivities of the lower-Z
materials result in substantially lower energy loading, in fact down to levels comparable
to those that were presumed marginal for the mirror material. On the other hand, even at
1.5 Å, it is evident that carbon (i.e., diamond), ostensibly the material of choice, is
beginning to experience substantial loading. At wavelengths > 2-3 Å, the corresponding
numbers for all three materials would begin to warrant serious concern.
Table 9.0.3. Selected crystal and energy loading parameters under LCLS beam
conditions at 1.5 Å. Dw = 100 m. Assumed crystal thickness is ~te.
Material
Lattice Spacing dH [Å]
1st Order
Resolution [Dl/l]
A
Diffraction
(x10-6)
[eV/atom]
Angle [°]
Be (002)
1.7916
24.1.75
22.8
0.014
Be (110)
1.1428
41.02
7.1
0.009
C* (111)
2.0593
21.36
59.7
0.069
C*(220)
1.2611
36.49
19.3
0.042
Si (111)
3.1354
13.84
135
2.011
Si(220)
1.9200
22.99
57.7
1.232
Table 9.0.4. Summary of suitable materials for optical components, without any FEL
radiation attenuation
transmi ssion
grazing in cidence
crystaldiff raction
multi layers
Front ha lls
0.8 keV
8 keV
Margin ally B e, C, Li,B e, B,C, Si,
Si
andlo w-z
compoundso f the
above
Au
Be
(ext reme
incidence)
none
Be, C*
Alllo w to
mod erate z
Alllo w to
mod erate z
0.8 keV
W M, Au OK
Rear hall s
8 keV
W,M, Au OK
W,M, Au OK
W,M, Au OK
Alllo w to
mod erate z
W,M, Au OK
Alllo w to
mod erate z
W,M, Au OK
Table 9.0.4 summarizes the possible materials that can be used, if there is no FEL
radiation attenuation. In Hall B the increased spot size reduced the energy density by a
factor ~15, and more standard materials are possible at all photon energies. For example,
W, M and Au have doses < 0.3 eV/atom for both 8 and 0.8 keV photon energies.
Continued R&D into the topic of x-ray photon-material interaction and damage is
imperative to reduce the risk of component damage associated with many of the User
experiments. First, not all known physics has yet been included in the modeling
described above; see section 9.6.0. Second, and more importantly, the remarkable photon
areal densities available are expected to instigate new processes, which are the specific
topic of the atomic physics experiments (see "LCLS First Experiments (September
2000)". The effects of the intensity spikes within a single FEL ~ 250 fs pulse, the spikes
having characteristic times < 1 fs and intensities ~ 5 times the nominal value, are
unknown.
Absorbers and attenuators
Gas, liquid or metal attenuators (see section 9.5.2.3, and 9.6.2.5) will be constructed
to reduce the FEL beam intensity, both as an experimental control, and to allow the use of
standard optical component and diagnostic designs. It is emphasized that, while a design
for a gas attenuator has been provided, a basic question is how the actual LCLS pulses,
whose intensity and degeneracy parameters lie well outside the regime of weak-field
interactions, will interact with candidate absorbing media. We note the gas absorption
cell [4] can be used for initial studies of scattering of the LCLS pulses by absorbing
media. The chamber design, predicated on the initial use of xenon, can includes ports for
line of sight fluorescence detection, as well for the introduction of external magnetic and
electric fields. Due to its location inside the FFTB tunnel, provisions for a detector
shielding enclosure have been included.
References
[1] R. M. Bionta, “Controlling Dose to Low Z Solids at LCLS”, LCLS note LCLS-TN-00-3
[2] R. Tatchyn, "LCLS Optics: Technological Issues and Scientific Opportunities," in Proceedings of the
Workshop on Scientific Applications of Short Wavelength Coherent Light Sources, SLAC Report 414;
SLAC-PUB 6064, March 1993
[3] R. Tatchyn, P. Csonka, H. Kilic, H. Watanabe, A. Fuller, M. Beck, A. Toor, J. Underwood, and R.
Catura, "Focusing of undulator light at SPEAR with a lacquer-coated mirror to power densities of 109
watts/cm2," SPIE Proceedings No. 733, 368-376(1986)
[4] D. Ryotov and A. Toor, "x-ray attenuation cell", LCLS TN-00-10 (2000)