1 brookhaven science associates stephen kramer, vuv ring manager 1992-2002 csr emission studies in...
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Stephen Kramer, VUV Ring Manager 1992-2002
CSR Emission Studies in VUV/IR Ring NSLS
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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 GeVCircumference 51mNumber of Periods 4 DBALength ID Straights 2- 2.25 mEmittance (h,v) 160 nm, >0.4 nmMomentum Compaction .0235Dipole Bend Radius 1.91 mEnergy Loss per Turn 19.8 KeVUser Top-off rate 4 - 5.5 hrs.Ports VUV (UND) / IR 16 (2) / 6 IR
Energy Spread 0.094%RF Frequency 52.88 MHzHarmonic Number 9 (7 filled)RF Bucket Height > 1.4%RMS Bunch Length 150ps- 2nsOper. /Max Current 1/ 1.5 AmpCurrent per Bunch 250-400maCharge per Bunch 0.5nCTouschek Lifetime > 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
MirrorsM1 and M2
Mirror M3Source point
eUHV
window
cone
bolometer(He)
lamellargratinginterferometer
chop
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FIR Port at end of Dipole U11/U12/U12IRU12IR extraction
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
0 20 40 60 80 1000
5
10
15
20
De
tec
tor
Sig
na
l [a
rb.]
Time [ms]
10 1001
10
100
1000
De
tec
tor
Sig
na
l [m
V]
Average Beam Current [ma]
0 1 2 30
5000
10000
15000
Peak at = 7mm
Sco
here
nt /
Sin
cohe
rent
Frequency [cm-1]
0 20 40 60 80
Frequency [GHz]
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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
22 2
||
f(E,I) | / | | / |o t t
th o
EI and E with
Z n Z n
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FIR Measurements of Spectra in 2-FIR
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
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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
• 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
• 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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0
1
2
3
4
5
6
0 20 40 60 80 100f [GHz]
P(f)
Mod
Warnock & Venturini CSR Signal + Interferr.
Adding a 0.8cm delay for reflection from outer dipole chamber wall
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Interference Pattern from Different Ports
0
0.5
1
1.5
2
2.5
3
3.5
15 25 35 45 55 65 75f [GHz]
U12IR
U10IR
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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