This is new section 6, undulator performance. Redo of old section 5.
6.1 OK
6.2.1 line 13 ‘K=0.17’
Line 14 ‘0.34 photons/beam electron.’
Omit 3’rd line from end. Replace with ‘Due to losses from collimator and undulator
misalignment E166 measures only a lower limit of 0.2 photons/electron for the undulator
photon flux.
6.2.2
Photons from the undulator are measured by Silicon Tungsten detectors S1 and PRT
whose absolute calibration is determined by GEANT simulation as described in section
3.4.2. The four PRT detectors are located immediately upstream at the face of the
collimator C2 with edges tangent to the 3 mm diameter collimator aperture as shown in
Figure X.(use photograph from old version) These detectors sample that portion of the
flux that does not enter the collimator aperture. They also provide a measurement of the
centering of the undulator beam. Photons exiting the 15 cm long collimator strike the
conversion target producing positrons and are then measured by detector S1 prior to
incidence on the Fe analyzing magnet where the photon transmission asymmetry is
determined. With suitable correction the sum of these fluxes yields the number of
photons per beam electron produced by the undulator.
The angular and energy distribution of the undulator photons is described in Section 2.
Combining this distribution with the measured vertical and horizontal emittance of the
electron beam it is possible to simulate the photon distribution at the collimator face.
Figure XX shows the simulated acceptance as a function of aperture diameter assuming
that the photon beam is centered on the collimator so that 93% of the photons should
enter the collimator.
5000
4000
3500
3000
2500
2000
1500
1000
500
0
0.97685
0.96042
0.94766
0.92975
0.89863
0.85147
0.77116
0.62685
0.36335
0
Collimator Acceptance
Fraction of Photons into C2
Aperture
1.2
1
0.8
0.6
Series1
0.4
0.2
0
0
1000
2000
3000
4000
5000
6000
Collimator Aperture (microns)
The conversion target removes an additional 7.3% of photons before they are counted by
S1. The total detected photon flux is therefore given by sum of PRT and corrected S1
signal.
However, throughout the data taking period it was necessary to steer the electron beam so
that it passed cleanly through the undulator to minimize background noise in the CsI
detector caused by beam tails striking the undulator protection collimator C1. The
steering resulted in a small displacement of the photon beam from the aperture center
resulting in reduced transmission through C2. Displacement is manifested by inequality
in the PRT counter signals. As an example Figure YY shows the left right and up down
variation for 420 runs at spectrometer setting 100 amperes
Series 1 up/down Series 2 left/right
PRT Assymetry
4
3.5
3
2.5
Series1
Series2
2
1.5
1
0.5
0
0
50
100
150
200
250
Run Sequence Number
300
350
400
450
The beam displacement is primarily in the x direction and from simulations corresponds
to displacements of from 500 to 1100 microns at the collimator. The average y
displacement is less than 100 microns. The correspondence with S1 and total PRT signal
is shown in Figure ZZ .
Series 1 Collimator Aperture
Series 2 Stopped by Collimator
Series 3 Total
Undulator Photons per Beam Electron for 420 Runs at Spectrometer
Setting 100 Amperes
0.25
0.2
0.15
Series1
Series2
Series3
0.1
0.05
0
0
50
100
150
200
250
300
350
400
450
Run Sequence Number
As expected, increasing left right ratio leads to reduction in S1 signal and a
corresponding increase in PRT signal with, however, the sum remaining fairly constant at
about 0.2 photons per electron about 60% of the expected undulator performance.
Averaged over the entire experiment we find the undulator flux to be 0.199 ± 0.008
photons per beam electron. We attribute this discrepancy to imperfect collimator
alignment. While the collimator aperture diameter was chosen sufficiently large to
accommodate collimator tilt for a centered photon beam, displacement of the beam
toward the aperture edge causes serious loss of transmission due to tilt. Simulation shows
that a misalignment of 3.3 milliradians results in a 3% loss if the beam is centered but is
47% for beam displaced by 1000 microns. Therefore, we know that the E166 undulator
production is greater than 0.2 photons per beam electron from the E166 undulator but we
are unable to directly confirm the design value of 0.34.
Last paragraph line 3 ‘of the photon flux reaching the conversion target by nearly 30%
from 0.061 to 0.079 photons/electron. (see figure nn)
Photons/beam electron
S1 photon flux.Spectrometer setting 180 amperes. Ferrofluid introduced
at run 132
0.1
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
Series1
0.01
0
0
50
100
150
200
250
300
350
Sequence number
Photns/beam Electron S1,PRT,
Total
6.2.3 Intensity …….
Use as is. Add at the end. ‘The K factor is indeed approximately linear but the absolute
magnitude of K reflects the fact that we measure only a lower limit for the undulator
flux.’
0.25
0.2
0.15
Series1
Series2
Series3
0.1
0.05
0
0
500
1000
1500
-0.05
Undulator Current
2000
2500
K value. Upper curve total flux,
Lower curve photons through
collimator
Dependence of Undulator K value on Current
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0
500
1000
1500
2000
2500
Undulator Current
6.2.4 Undulator Steering
Wording OK. Omit last line
Beam deflection X,Y BPM 6130
DEFLECTION (MICRONS)
5
0
-5 0
500
1000
1500
2000
2500
-10
Series1
Series2
-15
-20
-25
-30
-35
Undulator current