The BESSY (and MLS) Low Alpha Optics andthe Generation of Coherent Synchrotron Radiation
J. Feikes, K. Holldack, H.-W. Hübers*, P. Kuske, G. Wüstefeld
BESSY and * DLR, BERLIN

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

G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007

abstract & content

Abstract
The BESSY II optics is tuned to a low alpha mode for bunch length shortening.
About 1mm short bunches emit coherent synchrotron radiation in the THz range.
Details of the machine optics and measured THz signals are discussed.
Plans for the presently commissioned MLS ring * for short bunch generation arepresented.

Content
1. Low alpha optics
2. Coherent radiation
3. Bunch-length current relation
4. Limits of short bunches
5. Upgrading idea: short bunches at BESSY II* thanks to the PTB and BESSY commissioning team

G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007

BESSY II and MLS storage rings

Berlin-Adlershof (south east of Berlin)

BESSY II foot print

Europe's most modern Technology Park
The BESSY II ring:energy 1.7 GeV
circumference 240 m
number of cells 16 / DBA
rf frequency 500 MHzMLS76 m

new : Metrology light source MLS
the MLS ring:energy 0.2 – 0.6 GeV
circumference 48 m

Scientific employes: 6500 (+ 6500 students)
New media: 1500 employes
others: 4300 empoyes

number of cells 4/ DBA

16 m

rf frequency 500 MHz

G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007

the BESSY Low Alpha Optics

Low alpha optics for bunch length manipulation
the machine optics
tune parameters

QDoptics parameter

reg.useroptics

low alphaoptics

tunes Qx / Qy

17.8 / 6.7

14.7 / 6.2

nat. chrom ξx / ξy-53 / -27-35 / -27

S- 4 sextuple families for beam dynamics corrections- single & multi bunch 1.25 MHz to 500 MHz rep. ratecurrent per bunch 10 μA < I< 0.1 mA- very stable machine operation,
good life time 20 mA and 20 hours
QD

S

G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007

longitudinal chromaticity

condition for stable beam operation: α ≠ 0synchrotron frequency and alphasynchrotron frequency fs as a function of rf frequency- fs increases strongly with deviating rf frequency- optics tuned by sextupoles (long. chromaticity)
fs=350Hz

extracted momentum compaction factor α- fit to measured data α = α 0 + α 1δ + α 2 δ
α=-3·10-6δrms =0.1%2

α 0 = −3 ⋅10 −6 , α 1 = 0, α 2 = −0.03

See also: Control of the bunch length on an electron storage ring
H. Hama, S. Takano and G. Isoyama, NIM A329 (1993)
G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007

low alpha tuning for the MLS

low alpha tuning (simulation) for the presently commissioned MLS ring
Metrology Light Source (MLS) of the Physikalisch-Technische Bundesanstalt (PTB),
next to the BESSY II site, expected values:

α-value / 0.01

User optics α=0.02 , bunch length σ= 4.5 mm at 600 MeV
THz optics α=0.001, bunch length σ= 1.0 mm at 600 MeV
applied rf: 500 kV, 500 MHz

dipolequadrupole
sextupole
octupole

2 chromatic sextupoles
3 sextupole, one octupole(curvature of α)

3 chromatic sextupoles(slope of α)

circumference / m 48energy / MeV 200 - 600
100 MeVmicrotron

momentum deviation / %scheme of MLS ringnon. lin. low alpha tuning

see also R. Müller et al., Infrared Phys. Technol. 49 (2006) 161
G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007

THz detectors at BESSY
CSR signals & fast THz detectors

CSR intensity

InSb-bolometer
hot-electron bolometer HEB

bursting CSR

CSR intensity

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

bunch revolution
1 turnH.-W. Hübers et al.,ICFA Newsletter No. 35800 nsstable CSR

1 turntemporal resolved THz pulse ofpulse length of few 100 ps:
multiple reflections in THz beam line

resolution of single bunches,
detector τ=30ps

timeresolution of single turns,
detector τ=1μs

first strong CSR signals
T. Nakazato et al.,Phys. Rev. Lett. 63, 1245 (1989)

Detector parameters
Typical values

Si-Bol.

InSb

HEB

NEP (W/Hz1/2)~10-13~10-12~10-10Rise time τ (ns)~106~1000~0.03

Frequency (THz)

0.1 - 15

0.1 -1.5

0.3 - 6

G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007

spectral range of BESSY CSR

from Fourier spectra to power spectrainterferogram

raw data

power spectrum

source comparison
0.1THz710 gain

1THz

user optics burstig
CSR, SB 15 mA

incoherent radiation
250 mA, user optics

low alpha stable
CSR, 18 mA

black body, 1200 K, 10 mm^2-4

102

710 gain

BRILLIANCE [W/mm/sr/(0.1% bdw)]

110-910-14

10110100

1000

10000-1

WAVENUMBER [cm ]user optics, single bunch 15 mA
power spectrum analysis by Fourier transform spectrometer

brilliance of the BESSY THz spectrum
in cooperation with Dr. U. Schade, BESSY

BESSY offers 4 low alpha shifts of 3 days / year
application: coherent THz radiation, ICFA No. 35, article by U. Schade et al.
short x-ray pulses at BESSY, PRL 95, A. Krasyuk et al., 2005

G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007

single bunch CSR-intensity I

transition from stable to bursting CSR, user optics

CSR from deformed, but

temporal emission spectrumof CSR bursts (user optics)

rel. THz power signal

start of burstingmeasured CSRsignal
parabolic fit

1.6

1.7

back groundlevel 1.8

single bunch current in mA

single bunch current / mA

stable bunch, fs=7.2 kHz

bursting frequency / kHz
at spectrumsanalyzer

G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007

single bunch CSR-intensity II

bunch current / mAbunch current / mA

spectrums analyzer records, centered around 1.25 MHz rev. frequency

fs=7.2 kHz

fs=5 kHzCurrent and temporal
1.25 MHz

1.20 MHz1.30 MHz

1.20 MHz

1.25 MHz1.30 MHzemission dependenciesof CSR radiation at

frequency / MHz

frequency / MHzbunch current / mA

low alpha parameter fs

bunch current / mAdifferent settings of the

fs=3 kHz1.20 MHz

fs=2.1 kHz
1.25 MHzfrequency / MHz

1.30 MHz

1.20 MHz

1.25 MHz1.30 MHzfrequency / MHz
frequency / MHz

G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007

bunch length and current relation

bursting data
user optics
13 ps
THz optics
3 ps
sub-ps optics
700 fsbursting instability
Stupakov & Heifets

streak camera dataTHz data
bursting databunch length / psbunch length - current relation

σ~I 0.384
σ~I 3/8

σ~I 3/7
single bunch current / mA

results at bursting threshold:- eff. / naturale bunch length σ/σ0= 1.5

empirical scaling relation between bunch
length σ, synchrotron frequency f and current I:(σ / σ 0 ) 4 = ( f / f 0 ) 4 + ( I / I 0 ) 3 / 2- eff. bunch length · unstable mode σki =2πσ/λ i =5- bunch length ~ current relation σ~I ªa=3/8 from experiments, a=3/7 from theory

G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007

limits of short bunches I
longitudinal-horizontal couplings effect in the MLS ring
longitudinale bunch length is chromatic H dependent.
tz-z’

x-x’

z-z’

x-x’

MAD tracking simulation,
5
8·10 turns = 10 damping
times & quantum excitation

rf-voltage:
σ= 1.0 mm
σ= 0.5 mm

0.45 MV
45.0 MV

σ= 1.0 mm
σ= 0.1 mm

H=1.2 rad·mbunch length / mm

H=0.035 rad·mbunch length / mm

ultra short bunches are only
Emax=600 MeVcircumference = 48 m

possible at small H locations
see also : Y. Shoji inscheme of MLS ring

Phys. Rev. ST Accel. Beams 8 094001 (2005)
Phys. Rev. ST Accel. Beams 7 090703 (2004)
G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007

limits of short bunches II

BESSY II, user optics: MAD-simulation of electron diffusion due to radiation damping
initial value: no spread inphase space, only natural
spread in momentum distribution

180° in phase long. spacelong. bunch length
spread of σ=0.4 mm80 turns around machine

conclusion:
radiation damping limits the multiple usage of- ‘laser sliced’ electrons for short x-rays- ‘laser sliced’ dip as a THz-source
bunch rotation 180°
quantum excitation

num ber of elec trons / a.u.

9

σ-rms=0.4 mm=1.2 ps

8
7654
32

longitudinal delay / ps↓

1-5-3.3 -1.60
1.6 3.3
longitudinal delay / ps5.0↓↓

relative m om entum dev iation ‰↓bunch rotation
90 degree

G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007upgrading of the rf-gradientidea of enhanced THz radiation and short X-ray pulses at BESSY II

rf-upgrading: 1.5 GHz & 50 MV
sub-ps bunches!

I ∝σz

8/3dVrf / dz

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

rms-bunch length / psapplying the scaling law for bursting threshold:user optics

present rf
0.

THz optics

5

user optics

upgraded rf

1G.5&Hzz
GH1&.5M50V

13 ps

V3 psM

1.3 ps0.3 psTHz optics
bursting thresholdbursting thresholdsingle bunch current / mA

G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan,2007

conclusion

Conclusion:

the low alpha optics is a scheme to extends the photon
spectrum of storage rings to intense THz and short X-ray pulses
coherent THz radiation as a diagnostics tool delivers sensitive
and new information on beam dynamics

G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007