Stephen Kramer, VUV Ring Manager 1992-2002

1 BROOKHAVEN SCIENCE ASSOCIATES

Stephen Kramer, VUV Ring Manager 1992-2002

CSR Emission Studies in VUV/IR Ring NSLS


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


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


IR Developed in 1989 6 in 2004

MirrorsM1 and M2

Mirror M3Source point

eUHV

window

cone

bolometer(He)

lamellargratinginterferometer

chop


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


~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]


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


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


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


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


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


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


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


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


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.


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


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.


FIR Beam Port and Detectors Response


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


Documents

Stephen Kramer, VUV Ring Manager 1992-2002