Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
Microbunching Studies using IR Spectroscopy andTDS
Stephan Wesch, C. Behrens, R. Ischebeck, M. Röhrs,H. Schlarb, B. Schmidt, R. Tarkeshian
FLASH seminar – 25th November 2008
Overview
1 Motivation
2 Instrumentation
3 Measurements
4 Conclusion
IR CTR
- - - - - - ----------------- - - - ----------------
-
-
-
--
- --------------- - - - - - - --------------
-
-
--
- -
-
--
--
--
-
-
-
---
-
-
- -
-
-
- -
-
-
-
-
--
--
--
------
- - - - - - ----------------- - - - ----------------
-
-
-
-
-- -
-
-----
-------
-
- - - - -- --------------
-
-
--
-
-
-
--
--
-
-
-
-
-
-
-
-
--
-
-
-
-
--
-
-
-
-
-
-
--
--
--
-
-
-
-
3 5 10 20 30 40 50 60 700
1
2
3
4
5
Λ HÐmL
dU×d
Λ-1
HÐJ�
Ðm
L
E0 = 700 MeV, Q = 0.5 nC
Oncrest spectra:• coherent radiation in
spectral range ofλ = [3.3, 20] µm
• intensity level insame order as fromcompressedbunches
• >1% of the electronsinside the bunchradiate coherently
• strong fluctuations ofthe oncrest spectrum
→ nearly always present!
NIR CTRSingle shot (SASE conditions)
Average
1.0 1.2 1.4 1.60
200
400
600
800
Λ [Ð m]
inte
nsity
[a.u
.]
InGaAs spectrometer:• extension of IR spectrum
(λ = [0.9, 1.7] µm)• average intensity decreases down to
optical wavelengths• single shot spectrum consists of
extreme narrow and highly fluctuatingspikes
→ OTR measurements should confirmcoherent signals!
COTR (?)
Observation in June ’08 showed firstevidence of coherent signals in the opticalregime
−15 −10 −5 0 5 10 15 200
5
10
15
20
25
30
35Kly. Ampl. = 0.000;Pulse = 205; 2008−06−05T121514−meas−profile
time [ps]
long
. dis
trib
utio
n
datarms = 4.797pscal = −44.954fs/pixel
courtesy H. Schlarb
time [ps]
X [p
ixel
]
2008−06−05T121514−meas−profile
5 10 15 20 25
0
50
100
150
200
250
300
350
400
450
Questions:• reproduceable ?• special FLASH setting ?• location (longitudinal) in electron
bunch ?
Microbunching
Substructure inside the electron bunch, which is not related through the correletedenergy spread and can radiates coherently via CTR, CRD or CSR.
Simple LSC model:
?
noise
LSC LSCR56BC2 R56BC3
energy mod. energy mod.
density mod. density mod.
→ transformation from energy to density modulation is given by R56
→ substructure length should be varied by changing R56
→ spectrum shape and intensity should be influenced
Note:both R56 are mandatory for detectable IR radiationsee FLASH talk (08-04-08)
IR spectrometer
see H. Delsim-Hashemi FLASH talk (08-03-04)
Characteristics:• staging of 2 dispersive
reflective gratings• radiation detection by 60
pyroelectric senors (onlycoherent signals)
• single shot capability in 3wavelength ranges
i. 3.3 µm - 9 µmii. 9 µm - 22 µmiii. 25 µm - 65 µm
• CTR off-axis screen servedby TDS kicker
• port about 4 m upstream ofTDS screen
• beam transport with THzbeamline
TDS
TDS port + filter wheel + camera system
Port characteristic:• deflection of single bunch by
fast TDS kicker on screen• imaging radiation by Basler
monochrome camera• 2 color filters (Schott glas
BG39 / RG780) in filter wheelto resolve spectral content
TDS
Spectral response of camera × incoherent OTRintensity
400 500 600 700 800 900 10000.0
0.2
0.4
0.6
0.8
Λ HnmL
spec
tral
repo
nse
Ha.u
.L
~ BG39
~ RG780
-- no filter
Intensity variation in respect to camera gain
courtesy H. Schlarb
Plan
FLASH settings:• E0 = 780 MeV• φACC1 = φACC23 = φACC456 = 0◦
• Q = 0.75 nC (± 1.3%)
Procedure:
0. set BC currents
1. readjust phases
2. focus on TDS screen
3. take TDS images
4. take IR spectra
IR spectrometer I
IBC2 = 64 A
3 5 10 200.0
0.5
1.0
1.5
2.0
2.5
Λ HÐmL
dU×dΛ
-1
HÐJ�
Ðm
L
~ IBC3 = 30 A-- IBC3 = 50 A~ IBC3 = 60 A-- IBC3 = 70 A
IBC3 - variation:
→ increase of spectral intensity with R56
→ no shift of center wavelength
IBC2 - variation:
→ no significant intensity variation exceptfor IBC2 =64 A
→ no drift of center wavelength
IBC3 = 60 A
3 5 10 200.0
0.5
1.0
1.5
2.0
2.5
Λ HÐmL
dU×dΛ
-1
HÐJ�
Ðm
L
~ IBC2 = 57 A-- IBC2 = 60 A~ IBC2 = 64 A-- IBC2 = 70 A
IR spectrometer II
IBC3 - variation:
´
´
´
´ ´
í
í
í
í
30 40 50 60 700
5
10
15
20
25
30
IBC3 HAL
XÈF
long
È\H´
10-
3L
Λ Î @3.4, 13.9D Ðm
´
í
IBC2 = 62.5 A
IBC2 = 64 A
IBC2 - variation:
í
í
í
í
55 60 65 70 750
5
10
15
20
25
30
IBC2 HAL
XÈF
long
È\H´
10-
3L
Λ Î @3.4, 13.9D Ðm
í IBC3 = 60 A
Formfactor:
|F |2 = N−2 ·“
dUdλ
˛̨̨incoh.
”−1· dU
dλ
→ averaging F overλ = [3.4, 13.9] µm
Results:• increasing BC3 current
< F > increases up tosaturation (factor of 4!)
• increasing BC2 current< F > no significant effect
OTR images I
Videos
OTR images II
20 40 60 800
10
20
30
40
50
IBC3
inte
nsity
(a.
u.)
BG39 filter
20 40 60 800
2
4
6
8
10RG780 filter
IBC3
inte
nsity
(a.
u.)
20 40 60 800
20
40
60
80no filter
IBC3
inte
nsity
(a.
u.)
20 40 60 800
2
4
6
8
10RG780 filter, no streak
IBC3
inte
nsity
(a.
u.)
IBC2
=64A
IBC2
=57A
IBC2
=0AΣ ROI intensity:
→ intensity increaseswith decreasing BC3current for all filters!
→ low BC2 current �offers highestintensity in redregime
Note:problems withsaturation at oncrest
phases!
OTR spectra III
20 40 60 800
2
4
6
8
10
IBC3
rel.
fluct
uatio
n (%
)BG39 filter
IBC2
=64A
IBC2
=57A
IBC2
=0A
20 40 60 800
2
4
6
8
10
RG780 filter
IBC3
rel.
fluct
uatio
n (%
)
Σ ROI fluctuation:
→ rel. fluctuations 1% to 2% forBG39
→ about 3% up to 4% forRG780 filter
→ low current � setting showsmaximum fluctuations
Note:charge is stable around 1.3%
RG780 filter images at highcurrent show weak intensity
OTR spectra V
20 40 60 800
0.2
0.4
0.6
0.8
1
IBC3
rel.
ratio
BG39 − RG780
20 40 60 800
0.5
1
1.5RG780 − LOLA off
IBC3
rel.
ratio
IBC2
=64A
IBC2
=57A
IBC2
=0A
Intensity ratios:
→ for decreasing BC3 currentpronunciation in the redregime is significant
→ comparsion betweenTDS on - off reasonable
Conclusions
I. Coherent transition radiation signals reaches form the infrared into the opticalwavelength regime!
II. Spectra and TDS images show rapid fluctuating structure!
III. R56 dependencyIR: increasing intensity for increasing R56BC3
no obvious R56BC3 dependency
O: increasing intensity for decreasing R56BC3both R56 at minimum value create maximum intensitymore intensified signal in the red spectral part
IV. Problem of the determination of incoherent OTR!
V. Problem with camera saturation occurs!