Transcript BLM Dosimetry Simulations in the LCLS Undulator Magnets using MARS SLAC

BLM Dosimetry Simulations in the
LCLS Undulator Magnets using
MARS
Jeff Dooling
Argonne National Laboratory
SLAC
Redwood Room A/B, SLAC
Thursday, June 19, 2008
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
1
Jeff Dooling
[email protected]
What damages undulator magnet material
(NdFeB)?
Neutrons
Hadrons
Heat
—electrons and photons by themselves do not
appear to lead to damage except in very
high dose
—can electrons be used as a proxy to
estimate damage from the other sources?
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
2
Jeff Dooling
[email protected]
BLM Dosimetry
Calibrate—compare radiator electron fluence
(Cerenkov signal) with neutron fluence
(magnet damage) in the LCLS undulators
using MARS
Examine the calibration with differing loss
scenarios; e.g., US foil strike, halo scraping
in the FEL, beam misalignment or offsets.
Does the calibration ratio change?
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
3
Jeff Dooling
[email protected]
First, get the geometry right!
Below is an earlier model showing a slice through the fused silica radiator
should
be opened
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
4
Jeff Dooling
[email protected]
Correcting the geometry
Al. radiator housing ap. specified with negative length
y-z view
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
length ignored
x-y sect.
5
Jeff Dooling
[email protected]
Corrected LCLS undulator geometry in MARS
aperture specified correctly
y-z view
x-y sect.
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
6
Jeff Dooling
[email protected]
First Undulator results—considerations
PCMuon just upstream
1-mm equivalent Al foil target at OTR33,
85 m upstream
13.64 GeV electrons, g=26,690
Opening angle of shower q~1/g=37.46 mrad
Bremsstrahlung height < 85 m(37.46x10-6)
~3.2 mm
Simulations conducted with 108 macroparticles
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
7
Jeff Dooling
[email protected]
First Undulator results—considerations, con’t
Twiss parameters,
en=10-6 m-rad = bge =ge (g=26,690)
for the moment,
exey=e=en/g=3.8x10-11m-rad
From MAD model (LCLS04Nov07),
bx,min=27 m/rad, by,min=23 m/rad
x’max=(ex/bx)1/2=1.2 mrad, y’max=(ey/by)1/2=1.3 mrad
x’max,y’max<<1/g, Will quads focus shower?
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
8
Jeff Dooling
[email protected]
First Undulator results—geometry
Quad ap. has been corrected and is
reflected in present data
PCMuon (Fe) just upstream
Location of XYZ Histogram regions in the pole/magnet material
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
9
Jeff Dooling
[email protected]
Neutron fluence and electron spectral fluence
magnet neutron fluence distribution
(top + bottom, so should divide by 2)
radiator electron spectral fluence
Magnet volume where “peak” neutron flux is evaluated
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
10
Jeff Dooling
[email protected]
Radiator signal from Cerenkov
Using Frank-Tamm formula
Assuming ultra-relativistic form which varies by
less than 4 percent at 2 MeV (lower limit)
Fitting ln (SPE) vs. ln (E) simulation data with
polynomial (typically 3rd order)
Using average optical coupling and quantum
efficiencies over the wavelength range of interest
(200-600 nm)
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
11
Jeff Dooling
[email protected]
Radiator electron spectral fluence
—first radiator 0.2, 0.5, and 1.0 nC on foil
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
12
Jeff Dooling
[email protected]
Summary of 1st Radiator/Undulator data—Radiator signal; peak
undulator neutron fluence is also shown
Bunch
charge
(pC)
Electron
Fluence
(104cm-2)
200
(106)
Qout
(nC)
Iout
(A)
Peak
Neutron
Fluence
(104cm-2)
0.46
0.23
18.5
7.39
0.45
500
1.19
0.60
48.3
19.3
1.15
1000
2.40
1.21
97.0
38.8
2.30
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
Npe
13
Jeff Dooling
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More first undulator results
—electron spectra with and without W enhancer
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
14
Jeff Dooling
[email protected]
More first undulator results
—electron spectra varying rad. height w/o W enhancer
BLM signal ~ IspeVrad
Ispe,rrVr,rad=0.75 IspeVrad
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
15
Jeff Dooling
[email protected]
Multi-undulator model
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
16
Jeff Dooling
[email protected]
Radiator/Undulator locations
1
7
13
19
e-beam &
shower
25
31
location numbers
refer to und. magnets
and upstream BLM
radiators
magnet/pole mixt.
magnet
Al support struct.
Ti strongback
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
17
Jeff Dooling
[email protected]
Electron fluence spectra every 6th undulator—
without the W enhancer
Radiators 7, 13,
19, 25, & 31
First radiator
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
18
Jeff Dooling
[email protected]
Electron fluence spectra every 6th undulator—
with the W enhancer
Radiators 7, 13,
19, 25, & 31
First radiator
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
19
Jeff Dooling
[email protected]
Comparison in the DS radiators—with and
without the W enhancer
Radiators 7, 13, 19, 25, & 31 w/ enh.
Radiators 7, 13,
19, 25, & 31
w/o enhancer
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
20
Jeff Dooling
[email protected]
Comparison of radiator electron fluence with magnet peak neutron
fluence without enhancer
Rad. Pos.
z (cm)
“1”
13.24
“7”
2420.84
“13”
4828.44
“19”
7236.04
“25”
9643.64
“31”
12051.24
FE
(105cm-2)
0.191
0.887
1.315
1.700
1.782
2.186
FN
(104cm-2)
2.322
0.422
0.865
0.805
1.127
1.081
FN/FE
1.216
0.0476
0.0658
0.0474
0.0632
0.0495
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
21
Jeff Dooling
[email protected]
Comparison of electron radiator fluence and peak neutron magnet
fluence with W enhancer
Rad. Pos.
Z (cm)
“1”
13.24
“7”
2420.84
“13”
4828.44
“19”
7236.04
“25”
9643.64
“31”
12051.24
FE
(105cm-2)
0.236
2.398
2.835
4.111
4.480
5.369
FN
(104cm-2)
2.226
0.263
0.868
0.681
0.819
0.837
FN/FE
0.943
0.0110
0.0306
0.0166
0.0183
0.0156
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
22
Jeff Dooling
[email protected]
Work to do
Add quadrupoles—presently coded into MARS;
needs to be tested and debugged
Add proper input distribution to electron beam (e.g.,
KV, Gaussian, other)
Add halo distribution (second distribution)
Look at beam offsets with distributions and halos
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
23
Jeff Dooling
[email protected]
Summary
Calibration of FN/FE changes substantially after
the first radiator/undulator (highest neutron fluence
here with foil shower—PCMuon?)
Calibration ratio more stable for DS r/u
E-spectrum altered with enhancer
Enhancer modifies the environment around the
radiator and to a lesser extent the US, center
magnet section (reduces neutrons)
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
24
Jeff Dooling
[email protected]
Acknowlegments
Thanks to Bingxin Yang of Argonne and Heinz-Dieter
Nuhn and Alberto Fasso of SLAC for many helpful
discussions.
LCLS Undulator Magnet Irradiation
Sensitivity Workshop
Thursday June 19, 2008
25
Jeff Dooling
[email protected]