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M. R. Howells, Advanced Light Source
DIFFRACTION GRATING
MONOCHROMATORS
(PGMs)
Malcolm R. Howells
Advanced Light Source
M. R. Howells, Advanced Light Source
PLANE GRATING SYSTEMS: GENERAL
Focus condition for a spherical grating
cos2
r− cos
R
+
cos2
′ r − cos
R
= 0 let R ⇒ ∞ → ′ r = −r
cos2
cos2
Note that if r = ∞ then ′ r = ∞ irrespective of cff i.e . the grating then does no focusing
Using cff =cos
cos and recalling that M = −
′ r r
cos
cos → M = cff
Eliminating between
m
d0= sin + sin and cff =
cos
cos
sin =
m
d−
m
d
2 1
cff2 + 1−
1
cff2
2
1−1
cff2
r
r'
B
A
Grating
α β
Source point
Virtualimage
(of the grating alone)
M. R. Howells, Advanced Light Source
COLLIMATED LIGHT SX700 (CLSX700)
• This is the current and deservedly the most popular of many versions (used in ALS MES project)
• The original (Petersen) held fixed focus by maintainingand introduced the still-essential mechanism for positioning the plane mirror (next slide)
• CLSX700 has constant focus position irrespective of the value of cff which therefore becomes a user-controlled variable
• This can be used to track the efficiency maximum, suppress high orders or maximize resolution
• Sagittal focusing mirrors reduce sensitivity to manufacturing errors (Jark 1992) see next slide, othertypes were paraboloid (Kunz 1968), ellipsoid (Petersen 1982), elliptical cylinder (Nyholm 1985) orsphere (Padmore 1989)
cff = cos cos = 2.25
Kunz 1968Petersen 1982Follath 1997
M. R. Howells, Advanced Light Source
FORGIVENESS FACTOR
Requirement: ′ s T ≤1
2′ s T → 2 ′ r T ≤
1
2
′ r r
sT
T ≤1
4
sTr
′ s S ≤12
′ s S → 2 ′ r S ≤12
′ r
r sS
S ≤14
sSr
Reasons for the factor 2's:
1. Ray deviation is twice that of the mirror
2. Quadratic sum of and /2 is 1.1 implying a 10% error which we declare acceptable
M. R. Howells, Advanced Light Source
THE ZEISS MIRROR MECHANISM
• Reimer 1983• Pimpale 1991
• This mechanism, patented by BESSY and Zeiss, is a major reason for the success of this design
• The idea is to find a point about which to rotate the mirror such that the central ray always hits thegrating pole - this can be done with only a few microns error
• The easy and less accurate solution is continue the mirror surfaces at maximum and minimum angle tointersect the y axis and take the average of the two points - this will be near A (0, D/2) for small angles
• More accurate solutions (Pimpale 1991) are close to B (0, 3D/2) - best solutions are good within 10µmif D ≤ 8 cm and the grazing angle is less than 10° - B itself is a good solution to within 10 µm if D ≤ 3cm and the grazing angle is less than 10°
M. R. Howells, Advanced Light Source
SOME WORKING CURVES FOR PGM’S
M. R. Howells, Advanced Light Source
• 300/mm grating
• 79°<
M. R. Howells, Advanced Light Source
m
d= sin + sin so
= d cosm
d
dq=
d
d
d
dq=
d cos
m ′ r
Fractional bandwidth due to the entrance slit is
source
= qr
cos
m d≡ s
r A cff( )
Angularerror
Normalizedangulardispersion
In the small - angle approximation
A cff( ) ≅ dm
2
cff2 −1( )
NB - the grating changes the angular
spread by 1cff (cff is the magnification)
Follath 1997, 2001
source
=s
r A cff( )
exit
slit
= ′ s
′ r cff A cff( )
aberr
= ′ y ′ r
cff A cff( )
preopt
= po A cff( )
focopt
= fo cff A cff( )
grating
= gr 1 + cff( ) A cff( )
total
=
i
2
i∑ = 1
R
CONTRIBUTIONS TO THE SX700 RESOLUTION
M. R. Howells, Advanced Light Source
BESSY U125/1-PGM
Foc optic 0.05 sec
Slit 10 µm
Grating 0.2 sec
Preoptic 0.25 sec
Source 50 µm
Follath 2001
–ve order +ve order
BESSY U125/1-PGM
Preoptic 0.25 sec
Source 50 µm
Foc optic 0.05 sec
Grating 0.2 sec
Slit 10 µm
RESOLUTION CONTRIBUTIONS FOR A BESSY PGM
TOTAL
TOTAL
M. R. Howells, Advanced Light Source
Petersen 1986
SGM –veorder
SGM +veorder
WORKING CURVES FOR cff=2.25 ETC
M. R. Howells, Advanced Light Source
BESSY CLSX700'S: SUMMARY
M. R. Howells, Advanced Light Source
BESSY U125/1-PGM BEAM LINE
Follath 2002
Effective total length = 27 m
M. R. Howells, Advanced Light Source
BESSY PGM RESOLUTION DEMONSTRATION
• Doppler width 0.4 meV
• Monochromatorcontribution 0.65 meV
• Resolving power=1.0x105
• Rotation increments- grating: 17 nrad- mirror: 9 nrad
• Measured with 1200/mmgrating at cff =10 to 12
Picture from R. Follath, BESSY
M. R. Howells, Advanced Light Source
1200mm
300/mm
Where we are(925/mm)
Where they are
BESSY PGM'S: RESOLUTION SUMMARY
Where we'd like to be
M. R. Howells, Advanced Light Source
Mono angles
75eV 2000eV
Cff = 1
Cff = 10
thermaldistortionn
good higher ordersuppression
goodefficiencywith deepgrooves
goodefficiencywithshallowgrooves
resolutioninsensitive to slopeerrors
mechanical limits
largeincludedangle
smallincludedangle
mechanical limits
Parameters for this study:150 lines/mm 25nm groove depth (operating from 75 eV to 800eV)(low dispersion)
1200 lines/mm 6nm groove depth (operating from 300 eV to 2000eV)(high dispersion)
Slide from TonyWarwick
M. R. Howells, Advanced Light Source
OPTICAL PATH FUNCTION: VARIED-LINE-SPACING GRATINGS
d(w) = d0 1 + v1w + v2w2 + ...( ) n w( ) = ni00wi
i=1
∞
∑
F = Cijkijk∑ ,r( )wil j zk + Cijk
ijk∑ , ′ r ( )wil j ′ z k + m
d0nijk
n100 =1n200 = −v1 2
n300 = v12 − v2( ) 3
n400 = −v13 + 2v1v2 − v3( ) 4
nijk = 0 if i or k ≠ 0
Now the groove spacing d(w) and number n(w) are functions of w and are also expressed as power series
So the optical path function becomes
Evidently the use of VLS can benefit aberrations of the form i00:
Defocus, coma, spherical aberration but not ones with j, k ≠0
M. R. Howells, Advanced Light Source
WHY DO VLS GRATINGS WORK?
The spherical - grating focus condition is now:
F( )200 =1
2w2
cos2
r−
cos
R
+
cos2
′ r −
cos
R
−
v1m
d0
= 0
So setting R = ∞, using the grating equation and switching to grazing angles a, b
1
′ r =
cos a − cos bsin2 b
− 1
r
sin2 a
sin2 b=
1
rv1r
−2sina + b
2
sin
a − b2
sin2 b −
sin2 a
sin2 b
Now approximating sin a ≅ a etc
1
′ r =
1
r−v1r
a2 − b2
2b2 −
a2
b2
Evidently if v1 =−2r
then ′ r = −r
IDENTICALLY (for all values of a, b
and thus . Moreover if, in addition,
v2 =1
r2 then coma and sperical aberration
are corrected as well under the same small -
angle assumption
B
Source point r
r'
A
Grating
α β
Virtualimage
This works equally well in converging or diverginglight (principle of reversibility) although the literaturediscusses only converging.
M. R. Howells, Advanced Light Source
-4.0E-06
-2.0E-06
0.0E+00
2.0E-06
4.0E-06
6.0E-06
8.0E-06
1.0E-05
0 20 40 60 80 100 120 140
Wavelength (Å)
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
0 500 1000 1500
Wavelength (Å)
DOES v1 = –r/2, r' = –r GIVE OPTIMUM FOCUSING?
• No - the usual procedure is to write the focus equation twice for two wavelengths of exact focus and solve for r' and v1 -(this requires a fixed included angle or at least a fixed focusing principle) - the nominal focal distance then changes by asmall amount (2% in the above case) - this gives locally better focus although not much improvement over a large range
• Note that all of the design studies in the literature except one (Koike 1995) consider only fixed-included-angle cases andoffer, sometimes very good, although still approximate solutions but they have SGM-like disadvantages.
• Now that the variable-included angle schemes are available we have exact focus solutions such as CLSX700 or classicalSX700 although the latter does not give control of the included angle to the user.
• Therefore retrofitting VLS to a standard SX700 makes some sense to give user control of the included angle howevernote that the focal length or the focal distance of the mirror would have to change.
• Conclusion: VLS role in general-purpose monochromators is becoming minor but it is still strong in spectrographs.
M. R. Howells, Advanced Light Source
0.00001
0.0001
0.001
0.01
0.1
1
1 10 100 1000 10000Wavelength (Å)
Defocus
1 µr slope error
Mirror coma
Source size
Total
• VLS solution with cffchosen for maximumefficiency (between 1.5and 2.5)
• Wavelengths of exactfocus: 20 Å and 80 Å
• Source: size 50 µm anddistance 15 m
• Beam height: 2mm
• Inside (+ve) order
• 1200/mm
VLS RETROFIT TO AN SX700
M. R. Howells, Advanced Light Source
MES PROJECT CALCULATIONS
First, second and third orderefficiency calculations for alaminar grating 150/mm
M. R. Howells, Advanced Light Source
MES PROJECT CALCULATIONS
Source-size-limited resolving power fora source of size FWHM = 40 µm atdistance 12 m with a 150/mm grating
M. R. Howells, Advanced Light Source
MES PROJECT CALCULATIONS
Cylindrical mirror slope errorneeded to get a resolving power of7500 with the 150/mm grating
M. R. Howells, Advanced Light Source
MES PROJECT CALCULATIONS
Grating slope error needed toget 7500 resolving power with150/mm grating
M. R. Howells, Advanced Light Source
Mirror cooling for high heatload at low energy (75eV)
slope error for glidcop mirror 75eV
-2.00E-05
-1.50E-05
-1.00E-05
-5.00E-06
0.00E+00
0 100 200 300
mm
rad
Cff=1.25 dT=4.29
Cff=2.5 dT=1.99Cff=5 dT=1.42
slope error for glidcop mirror 300eV
-6.00E-06-5.00E-06-4.00E-06-3.00E-06-2.00E-06-1.00E-060.00E+00
0 100 200 300
mm
rad
Cff=1.25 dT=1.44
Cff=2.5 dT=0.40
Cff=5 dT=0.26
slope error for silicon mirror 300eV
-1.20E-06-1.00E-06-8.00E-07-6.00E-07-4.00E-07-2.00E-070.00E+00
0 100 200 300
mm
rad
Cff=1.25 dT=2.50
Cff=2.5 dT=0.68Cff=5 dT=0.43
slope error for silicon mirror 75eV
-5.00E-06
-4.00E-06
-3.00E-06-2.00E-06
-1.00E-06
0.00E+00
0 100 200 300
mm
rad
Cff=1.25 dT=7.80
Cff=2.5 dT=3.45Cff=5 dT=2.42
absorbed power density300eV
0
0.02
0.04
0.06
0.08
0.1
-200 -100 0 100 200
mm
W/m
m2
Cff=1.25, 2theta=173.9
Cff=2.5, 2theta=176.9
Cff=5.0, 2theta=177.5
absorbed power density75eV
0
0.05
0.1
0.15
0.2
0.25
0.3
-200 -100 0 100 200
mm
W/m
m2
Cff=1.25, 2theta=167.9
Cff=2.5, 2theta=173.8
Cff=5.0, 2theta=175.0
The rms slope error (µrad) of the pre-mirror corresponding to a resolvingpower R=7500 (FWHM) from the150l/mm grating.
11.5
5
1
500 1000eV
1
2
3
4
Cff=1.25Cff=2.5
Cff=5.0
M. R. Howells, Advanced Light Source
JENOPTIC SX700 MONOCHROMATOR
Three gratings
UHV rotary encoder
Grating drive arm
Mirror drive arm
Plane mirror
M. R. Howells, Advanced Light Source
New implementation of SX700monochromator
Legs filled with epoxy granite
Heavy pump below bellows onseparate support
6-strut alignment
Lightweight rigid honeycombtable
Aluminum structural vessel
External sine-bar drives andlinear encoders
Seal joint below beam heightfor alignment access
External grating changer
M. R. Howells, Advanced Light Source
Monochromator chamber,vacuum test Oct 2001
M. R. Howells, Advanced Light Source
monochromator scanningmechanism with integral cooling
Pressure equalized water feed
Pressure-equalized water feed
Silicon plane mirror
Double grating
Water lines on sine bar
Water manifold with no flexingparts
Grating carriage with roll andyaw adjusters for double
grating
Invar structural frame