The BESSY Low Alpha Optics and the Generation of Coherent Synchrotron Radiation

J. Feikes, K. Holldack, H.-W. Hübers*, P. Kuske, G. WüstefeldBESSY and * DLR, BERLIN

see contribution in ICFA Beam Dynamics Newsletter No. 35, December 2004

‘Frontiers of Short Bunches in Storage Rings’ Frascati, INFN, 7-8 November 2005

wuestefeld@bessy.de

Content

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

1. Low alpha optics 2. Coherent radiation3. Bunch-length current relation4. Limits of short bunches 5. Upgrade option: short bunches at BESSY II

content

the machine opticsoptics parameter reg.user

optics

low alpha

optics

circumference / m 240 240

number of cells 2x8 16

nom. Energy / GeV 1.7 1.7

tunes Qx / Qy 17.8 / 6.7 14.7 / 6.2

nat. chrom x / y -53 / -27 -35 / -27

nat. emitt / nmrad 6

synchr. freq. / kHz 7.5 7.5 … 0.35

mom. com. factor 7.2E-4 7E-4 …1E-6

plus & minus

nat. bunch length rms / ps 12 12 … 0.7

main parameters

short bunches by low manipulation ²fs = control value

very stable machine operation, good life time 20 mA and 20 hours

application: coherent THz radiation, ICFA No. 35, article by U. Schade et al. short x-ray pulses at BESSY, accepted PRL, A. Krasyuk et al., 2005

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

The BESSY Low Alpha Optics

single & multi bunch1.25 MHz to 500 MHz10 A < I< 0.1 mA(2mA~10 electrons)

10

4 sextuple familiesfor beam dynamicscorrections

α=-3·10-6

fs=350Hz

synchrotron frequency and alpha

synchrotron frequency fs as a function of rf frequency

- fs increases strongly with deviating rf frequency - optics tuned by sextupoles (long. chromaticity)

extracted momentum compaction factor

- fit to measured data

2210

03.0,0,103 216

0 rms

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

Longitudinal tune

=0.1%

temporal resolved THz pulse of pulse length of few 100 ps:multiple reflections in THz beam line

H.-W. Hübers et al.Applied Phys. Lett.87,184103 (2005)

InSb-bolometer

resolution of single turns, =1s

1 turn

1 turn

bursting CSR

stable CSR

CS

R in

tens

ityC

SR

inte

nsity

time

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

THz Detectors at BESSY

Typical values Si-Bol. InSb HEB

NEP (W/Hz1/2) ~10-13 ~10-12 ~10-10

Rise time (ns) ~106 ~1000 ~0.03

Frequency (THz) 0.1 - 15 0.1 -1.5 0.3 - 6

Detector parameters

H.-W. Hübers et al.,ICFA Newsletter No. 35

hot-electron bolometer HEB

resolution of single bunches, =30ps

Spectral range of CSR

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

power spectrum analysis by Martin-Puplett Fourier transform spectrometer

Brilliance of the BESSY THz spectrum

10 gain710 gain

7

1 10 100 1000 1000010-14

10-9

10-4

101

low alpha stable CSR, 18 mA

user optics burstig CSR, SB 15 mA

0.1THz 1THz

black body, 1200 K, 10 mm^2

incoherent radiation250 mA, user optics

BR

ILLI

AN

CE

[W

/mm2 /s

r/(0

.1%

bdw

)]

WAVENUMBER [cm-1]

raw data power spectruminterferogram source comparison

user optics, single bunch 15 mA

Power spectrum and bunch shape of stable bunches

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

CSR bunch length measurement

- 700 fs, 300 nA- 870 fs, 140 nA- 1.2 ps, 140 nA

- fit

Fourier transform spectroscopy,coherent and incoherent THz radiation interferogram

Fourier transform of interferogram power spectrum, raw data includes f()-factor

experimental dataTHz pulses

Theorymachine parameters

Haïssinski equation and CSR wake bunch shape n(z)

Fourier transform of bunch shape form factor g()

Kramers-Kronig analysis shape of THz pulse 122 /1 z

Gaussian relation between bunch length and spectral band width

normalized power spectrum

power spectrum coherent • f()power spectrum incohere.• f() = P / Pcoh. incoh.

incohere. power P = p Ncoherent power P = p N (N -1)g() norm. power spectrum P / P = N g()coh

incoh

coh

incohee

e

e

no significant difference was found in thespectral distribution for <0 and >0

CSR spectra at fs=1.75 kHz, =3 psdifferent 400-bunch currents

raw data, not normalized

wavenumber 1/cm

0

5

109.03mA8.10mA6.70mA4.50mA3.20mA2.20mA

0 10 20 30 400

10

9.96mA8.12 mA7.00mA5.00mA1.00mA

inte

nsity

/ a.

u.in

tens

ity /

a.u.

< 0

> 0

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

Example of THz power spectra taken at IRIS (BESSY)

higher currents expand the spectrum to higher frequencies:

-stable bunch deformation-unstable bunch bursting

normalized

by I²

Pcoh/Pincoh

CSR spectra at fs=1.2 kHz and different currents

Fourier transform measurements at the BESSY IRIS infrared beam line,cooperation with Dr. Ulrich Schade (BESSY), Dr. Jongseok Lee (Seoul National University)

THz optics3 ps

user optics13 ps

sub-ps optics700 fs

bursting instabilityStupakov & Heifets

streak camera data

THz databursting data

2/30

40

40 )/()/()/( IIff

scaling relation between bunch length synchrotron frequency f and current I:

sin

gle

bu

nch

cu

rren

t /

mA

bursting frequency / kHz

temporal emission spectrum of CSR bursts (user optics)

bunch length - current relation

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

Bunch length and current relation

bursting data

~I 0.384

~I 3/7

~I 3/8

single bunch current / mA

bunc

h le

ngth

/ p

s

results from bursting threshold:

- eff. / naturale bunch length = 1.5

- eff. bunch length · unstable mode k =5

- bunch length ~ current relation ~I ª

a=3/8 from experiments, a=3/7 from theory

0

ii

linear coupling matrix, long.-horiz. ,the chromatic H-function couples the planes:

)'(' 1561551521512 zmzmxmxmz

126121622126111621152151 )()( xmmmmxmmmmxmxmz

1000

1

0

0

565251

262221

161211

mmm

mmm

mmm

Mx-z transfer matrix, no energy change

use transformation properties of dispersive vector (D,D’) to replace m16 and m26

12 xMx

x’ D’

x1/2

DH1/2

Hxx

222111 sinsin HxHx HHz

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

Limits of short bunches I

similar expression for m55 & m56 - terms, if z’ and are included!

0.45 MV45.0 MV

rf-voltage:

scheme of presently constructed MLS ring of the PTB, next to BESSY site

Emax=600 MeVcircumference = 48 m

longitudinale bunch length is chromatic H dependent

x-x’ x-x’z-z’z-z’

port IIH=1.2 rad·m

port IH=0.035 rad·m

bunch length / mmbunch length / mm

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

Limits of short bunches II

quadrupoles ~ sextupoles ~ d/doctupolesd /d

22

low alpha tuning:

=1.0 mm =0.1 mm

=1.0 mm =0.5 mm

t

MAD tracking simulation,8·10 turns = 10damping times & quantum excitation

5

BESSY II, user optics: MAD-simulation of electron diffusion due to radiation damping

¯

¯longitudinal delay / ps

rela

tive

mo

me

ntu

m d

ev

iati

on

‰

bunch rotation90 degree

¯longitudinal delay / ps

nu

mb

er

of

ele

ctr

on

s

/ a

.u.

bunch rotation 180° quantum excitation

¯

2

3

4

5

6

7

8

9

1

0 1.6 3.3 5.0-5 -3.3 -1.6

-rms=0.4 mm

conclusion: radiation damping limits the multiple usage of - ‘laser sliced’ electrons for short x-rays - ‘laser sliced’ dip as a THz-source

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

Limits of short bunches III

180° in phase long. space

80 turns around machine

initial value: no spread in phase space, only natural spread in momentum distribution

long. bunch length spread of =0.4 mm

Upgrading of the rf-gradient

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

option for enhanced THz radiation and short X-ray pulses at BESSY II:

upgrading the rf-gradient by a 1.5 GHz, cw superconducting rf-structure placed into one straight ID-section

applied scaling law for bursting threshold: dzdVI rfz /3/8

figure copied from F. Sannibale et al, ICFA Newsletter No. 35

BESSY II 710 Auser optics & upgrade

upgrading by a factor of 201000 x more THz power!

upgrading by a factor of 100sub-ps bunches!

single bunch current / mA

rms-

bunc

h le

ngth

/ ps

user optics

user optics

THz optics

THz optics

0.5 GHz &

1.5 MV

1.5 GHz &

50 MV

bursting threshold

bursting threshold

present rf

upgraded rf

13 ps

3 ps

1.3 ps

0.3 fs

Conclusion

G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

Conclusion:

the low alpha optics (as one possible scheme) extends the usage of storage rings to intense THz and short X-ray pulses

coherent THz radiation as a diagnostics tool delivers sensitiveand new information on beam dynamics