Transcript S Kramer

CSR Emission Studies in VUV/IR Ring NSLS
Stephen Kramer, VUV Ring Manager 1992-2002
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Technical Design Parameters
NSLS VUV and Xray Light Sources Proposed in 1970’s
VUV ops 1982, Xray ops 1984 with >2300 Users/year
using >65 beam lines from Far-IR to Hard Xrays
VUV /IR Ring Parameters
Energy
0.8 GeV
Circumference
51m
Number of Periods
4 DBA
Length ID Straights
2- 2.25 m
Emittance (h,v)
160 nm, >0.4 nm
Momentum Compaction
.0235
Dipole Bend Radius
1.91 m
Energy Loss per Turn
19.8 KeV
User Top-off rate
4 - 5.5 hrs.
Ports VUV (UND) / IR
16 (2) / 6 IR
2
Energy Spread
RF Frequency
Harmonic Number
RF Bucket Height
RMS Bunch Length
Oper. /Max Current
Current per Bunch
Charge per Bunch
Touschek Lifetime
0.094%
52.88 MHz
9 (7 filled)
> 1.4%
150ps- 2ns
1/ 1.5 Amp
250-400ma
0.5nC
> 3 hrs
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VUV Period for Achromatic Optics
Peak x ~ 1.51 m, Doublet makes βx small in dispersion and large
in ID making Touschek lifetime small due to septum edge
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IR Developed in 1989  6 in 2004
e
Source point
Mirror M3
bolometer
(lHe)
UHV
cone
chop
window
Mirrors
M1 and M2
lamellar
grating
interferometer
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FIR Port at end of Dipole
U12IR extraction
U11/U12/U12IR
Two VUV ports ahead of FIR, at 7° and 22° into 45° dipole,
ρ = 1.91 m
dipole chamber shielding cut-off for CSR and ISR
fc ~ 24 GHz or 0.8 cm-1
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~1998 U12IR FIR Beam Saturated Detector
1000
20
D etector Signal [m V]
15
10
5
S
~
I
100
I
S~
10
1
0
0
20
40
60
80
Tim e [m s]
0
100
Frequency [GHz]
20
40
60
10
100
A verage Beam C urrent [m a]
80
15000
Scoherent / Sincoherent
D etector Signal [arb.]
2
Peak at
l = 7mm
10000
5000
0
0
1
2
-1
Frequency [cm ]
6
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Threshold Current Microwave Instability
Bunch length data showed a threshold of 100-120mA for
Microwave threshold at 745-800 MeV with Zbb/n ~ 1.8Ω
and Spear scaling
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Energy Dependence Microwave Instability
The microwave threshold based
on the Keil-Schnell Criterion
Ith 
 Eo  2  t
| Z|| / n |
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and  2  Eo 2 with
t
| Z /n|
 f(E,I)
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FIR Measurements of Spectra in 2-FIR
GHz
0
50
100
150
200
250
300
800
coherent SR
incoherent SR
400
0.5
Signal (coherent)
Signal (incoherent)
1.0
0
0.0
0
2
4
6
8
10
-1
Frequency [cm ]
Interference model for line structure in both ISR and CSR pattern
with Δf ~ 1cm-1 spacing in two similar beamlines with different
spectra ranges
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Reflection from Dipole Chamber Outer Wall
Could give a Sin2(2π f Δt) modulation to broadband ISR and CSR spectra
ΔL~1cm or frequency spacing ~ 30 GHz
Slope =2*sin(/2) ~ 0.4
Zero at ~30 GHz due to 180 degree phase shift at metal boundary
Path Difference [cm ]
1.10
1.05
1.00
Calculated
Measured
0.95
0.0
0.1
0.2
0.3
Horizontal Beam Position [cm]
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FIR Beam Coupler Has Cut Off ~30GHz
Optical coupling to FIR port cuts off below ~30 GHz, 4” beam window available 90°
Metal paddle to block FIR beam deflects microwaves down to window (ring VC limits)
Dipole chamber cut-off waveguide frequency TE1,0 ~ 2GHz and TM1,1 ~ 4 GHz
Microwave measurements confirm CSR peak ~42GHz ~1.4 cm-1 peak “C”,
but see two more “A and B”
Microwave horns for deflected light
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Time Dependence of Peak Signals
12
•
B and C are prompt signals
from the bunch limited by RF
diode ~1GHz BW
Revolution time To=170nsec
•
A signal 60 -100ns wide
delayed by ~30-50ns from
bunch
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Power in Peaks versus Bunch Current
•
•
•
•
•
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C &B signal linear
for Io < Ith
C&B quadratic for
Io > Ith
C&B Ey polarization
A signal always
quadratic or higher
beam impedance P~
I2 * R
A signal unpolarized
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A Peak - Wakefield Emission From Bellows
Vacuum bellows shield with
Cu convolutions 1cm gave
calculated beam
impedance 
RF microwave and charge
modulation shows 6.5GHz
but ~4GHz at current below
 Threshold current
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Micro-bunching measured with SC
Triggering the Streak Camera on the A peak signal yielded consistent micro-structure
with 150 psec modulation or 6.5 GHz, no other micro-bunch was significant.
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Warnock & Venturini CSR Signal + Interferr.
Adding a 0.8cm
delay for
reflection from
outer dipole
chamber wall
6
5
4
3
P(f)
2
Mod
1
0
0
20
40
60
80
100
f [GHz]
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Interference Pattern from Different Ports
3.5
3
2.5
2
1.5
U12IR
U10IR
1
0.5
0
15
25
35
45
f [GHz]
55
65
75
U10IR port has only 20° of bend ahead of port not > 23° for Interference pattern
from dipole. However pattern from upstream dipole less clear.
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FIR Beam Port and Detectors Response
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Summary and Status
•
•
•
•
•
VUV ring CSR emission above Microwave
Instability which scales with energy and α1
CSR and ISR show interference pattern from outer
wall broader than WG modes but related
Shielded ISR and CSR cut-off frequency appears
to be ~ 24 GHz but sharp cutoff, allows the 6 GHz
wakefield signal to be observed
Wakefield power growth proceeds CSR, shows
clear micro-bunching above CSR threshold
FIR users not interested in stable CSR emission
but maybe hard to achieve without RF upgrade
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