Interferometer-based white light measurement of neutral rubidium - - PowerPoint PPT Presentation
Interferometer-based white light measurement of neutral rubidium density and gradient at AWAKE Fabian Batsch, Erdem z, Mikhail Martyanov CERN / TUM / Max-Planck-Institute for Physics 2 nd Wigner - AWAKE Workshop, Budapest, May 5, 2017 F.
May 05, 2017
CERN / TUM / Max-Planck-Institute for Physics
2nd Wigner - AWAKE Workshop, Budapest, May 5, 2017
May 05, 2017
Fig. Drawing of the Rb vapor source (by G.Plyushchev)
ɣ, p+, e- Diagnostic viewports
Measure instead vapor density at both ends
process
better 1 % accuracy
Goal: Measure optically Rb vapor density at both ends with ± 0.5% relative
accuracy and in a fully automated way
May 05, 2017
Measure instead vapor density at both ends
process
better 1 % accuracy
Goal: Measure optically Rb vapor density at both ends with ± 0.5% relative
accuracy and in a fully automated way
From: AWAKE Status Report 2016
May 05, 2017
Measure instead vapor density at both ends
process
better 1 % accuracy
Goal: Measure optically Rb vapor density at both ends with ± 0.5% relative
accuracy and in a fully automated way
n1= ?? n2= ?? gradient= ??
May 05, 2017
May 05, 2017
200°C, corresponding to a density range of 1014 - 1015 cm-3
pressure curve (5% abs. accuracy):
780.24 nm (D2 line) and 794.98 nm (D1)
in their vicinity
permittivity change
) ) log( exp( 1 ) (
3 1
DT T C BT A T k T n
B
A,B,C,D material- dependant constants
Index of refraction nir
D1
Vapor GS ir
N N n 1
D2
May 05, 2017
method adapted to vertical fringes
white light source, Mach-Zehnder- interferometer and spectrometer
travel through Rb
causes density-dependant change in periodicity of interference maxima.
Zehnder Interferometer
Wavelength [nm]
Wavelength [nm]
Index of refraction
D2
) cos( 2 ) ( ) ( ) (
2 1 2 1
I I I I Itot
May 05, 2017
) (
1
I ) (
1
I ) (
2
I
Use 1D spectrograph
to compensate inhomogeneous light distribution (caused by light source, light transport in fiber):
using FFT (by M. Martyanov, does not require reference spectra / noise filtered) possible
Itot(λ)
) cos( ) ( ) ( 2 ) ( ) ( ) (
2 1 2 1
I I I I Itot
Fig. Interferogram for n= 0 cm-3
May 05, 2017
Use 1D spectrograph
to compensate inhomogeneous light distribution (caused by light source, light transport in fiber):
Itot(λ)
) cos( ) ( ) ( 2 ) ( ) ( ) (
2 1 2 1
I I I I Itot
May 05, 2017
with A the amplitude, nl the density-length product λ1,2 transition wavelength r0 classical e- radius f 1,2 oscillation strength path length difference in interferometer
intensity spectrum near transition line , with A, n , fitting parameters
~ ) ( 4 ~ ) ( 4 ~ 2 cos ~ ) (
2 3 2 2 1 3 1 1
f r l n f r l n A S
S(λ)
Areas ignored by fit
Fitting parameters marked with ~
D2 D1
May 05, 2017
with A the amplitude, nl the density-length product λ1,2 transition wavelength r0 classical e- radius f 1,2 oscillation strength path length difference in interferometer
spectrum near transition line, with A, n , fitting parameters
completely equal arms ~ ) ( 4 ~ ) ( 4 ~ 2 cos ~ ) (
2 3 2 2 1 3 1 1
f r l n f r l n A S
Areas ignored by fit
Fitting parameters marked with ~
May 05, 2017
length difference constant for all measurements
spectrographs simultaneously, saved on local computer
calculation (done in a FESA class) -> density displayed in control room, density- time file saved in data base (implementation ongoing)
May 05, 2017
May 05, 2017
temperature stability of 0.1 K
l = 51 cm and valves to control Rb flow
Instruments & Methods in Physics Research A (2016), http://dx.doi.org/10.1016/j. nima.2016.02.005 (a) (b)
Valve Rb Valve Cold trap pipe
Photo: E.Öz
May 05, 2017
From vapor pressure curve: Measured values tracks vapor pressure curve at 0.6 – 3.8 % level
) ) log( exp( 1 ) (
3 1
DT T C BT A T k T n
B
A,B,C,D material-dependant constants Measured nRb vs. T:
± 5 % line May 05, 2017
accuracy < ± 0.5% <-> determine density gradient at (sub-) % level
simulate to equal vapor column length
May 05, 2017
Result: Both measurements differ by 0.1 % (up to 0.3 % , depending on temperature):
ARM 2 ARM 1
+ 0.5 %
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radiation, 120 m fiber length guide light between rack and cell ends
laser, filter and spectrograph
Plasma cell
Laser room
From: Jeremy, Blanc
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Interferometers
1 1
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Optic holder legs and diagnostic window:
Photo: CERN
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Optics Window
beam pipe
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7 % accuracy
every 40 sec)
7 % accuracy
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0.34 % accuracy
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0.33 % accuracy
reservoir and measured Rb vapor density
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Downstream Upstream
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accuracy.
spectrograph and cell separated -> light transport in 120 m fibers
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n1= 7.08 n2= 7.14 gradient= + 0.89 %
x 1014 cm-3
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nm for safety (peak in laser intensity at 1064 nm)
LASER
Fiber Collimator Beam dump
Collimator
Dielectr. mirror Beam dump
IR part 700-950 nm light in fiber Closed box
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Path length difference (pld), Amplitude A and Size of excluded data around transition line.
Determines spacing between intensity maxima Symmetric around transition line
A
May 05, 2017
Fit results plotted over excluded image regime:
May 05, 2017
Path length difference, Amplitude A and Size of excluded data around transition line.
An error of Δ A = 0.15 ≙ 19 % in amplitude leads to an error of 0.012 % in nRb Negligible
0.012 % of nRb
May 05, 2017
Path length difference, Amplitude and Size of excluded data around transition line.
with nRb ± 0.07 %
0.24 %
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May 05, 2017