Vision for the Future: BESSYVSR
A Variable Pulse Length Storage Ring*
A. Föhlisch, A. Jankowiak, J.Knobloch, P. Kuske
A. Neumann, M. Ries and G. Wüstefeld
HZB, Berlin (Germany)
* G. Wüstefeld et al., IPAC 2011
ICFA Workshop on Future Light Sources, March 5-9, 2012
Thomas Jefferson National Accelerator Facility, Newport News, VA (USA)
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
Outline
BESSYVSR
- Motivation
- Current limit of short bunches:
measurements & scaling laws
- Long. bunch focusing by sc-cavities
- Expected results
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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Motivation
why short e- bunches:
- time resolved, picoseconds X-ray experiments
- CSR for THz experiments
present situation at BESSY:
- dedicated low-a shifts, s=3ps
4 blocks of 3 days per year,
two operation modes:
40 mA (bursting) and 15 mA (stable)
future goal:
- simultaneously 15 ps & 1.5 ps bunch mode
up to 100x more current in short bunches
( 10 000x more THz power)
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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BESSY II synchrotron radiation source
BESSY II ring
1.7 GeV
240/p m
storage ring parameters
number of cells
beam current
nat. emittance
nat. moment. spread
2x8
<300 mA
6 nm rad
0.7x10-3
nat. bunch length
s0 = 13 ps (rms)
4 rf-cavities
0.5 GHz , 1.5 MV
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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Low-a optics
a tool to produce and study short bunches
definition of a
L  L0 1  a Δp p0 
relation s0 , a and V’
(=dV/dt=2pfrfV0):
σ 0  a V'
- BESSY THz-optics
short bunch, low current operation
15 ps  3 ps (rms)
700 fs are proven and analyzed
- MLS – ring of PTB
advanced, successful low-a optics
coherent THz radiation
short bunch studies
double-beam
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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Bunch length – current relation for the BESSY II ring
user
optics
10
THz
optics
1
s ~ Ib3/7
threshold from
coasting beam
0.1
1E-4 1E-3
1E-2
P. Kuske et al., PAC 2003
bunch length s / ps (rms)
fixed rf voltage amplitude of 1.35 MV
8 nC
0.1
1
10
measurements
theory
streak camera
Stupakov & Heifets
Fourier transform
spectrometer
PRSTAB 5, 054402 (2002)
THz bursting threshold
- beyond bursting threshold bunches
blow up in energy spread
- rule of thumb: (s, Ib)  (2s, 5Ib)
good agreement between
measurement and prediction !
single bunch current Ib/ mA
Are short bunches limited to low currents ??
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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Are short bunches limited to low currents ??
scaling law between a and I predicted by:
- Vlasov-Fokker-Planck simulation
- bunched beam theory (Sacherer)
- and coasting beam (Landau Damping)
recent papers:
K.L.F. Bane, Y. Cai, and G. Stupakov,
Phys. Rev. STAB 13, 104402 (2010)
and
Y. Cai, talk, IPAC 2011
‘Keil-Schnell’:
I | Z /n |  Fa(
||
0
Δp
) E /e
p
2
0

I~a
0
bunch length s
I~V’
for fixed s
 σ  a V'
increasing the rf-gradient V’ x 100
 a can be increased x 100
 I can be increased x 100
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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Scaling of CSR bursting threshold
threshold of instability for
CSR of parallel metal plates
CSR bursting measurements BESSY II
comparison with theory, good agreement
BCS: x=0.5+0.34c+dip
rms bunch length / ps
scaled bunch current
Y. Cai, talk, IPAC 2011
10
before 2003
8
6
4
Nov. 2011
coasting
beam
theory
2
Jan. 2012
BCS: x=0.5+0.34c+dip
1
1
10
100
1000
scaled single bunch current mA / MV
scaled bunch length
zero current bunch length caculated from
Voltage, a and long. tune
Voltage range: 0.6MV – 1.8 MV
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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BESSY II & sc-cavities
sc-cavities for bunch shortening
bunch length – current relation
sc-cavities (scheme)
100x enhanced rf-gradient
cavity V1,f1
cavity V2,f2
~ 5 m
straight
->J. Feikes et al, EPAC 2006
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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Simultaneously long & short bunches
Voltage / MV
present nc-cavity (power)
sc-cavity # 1 (focusing)
short
bunch
long
bunch
sc-cavity # 1 & 2 (focusing)
short & long bunches
sum voltage
rel. long. phase position / ns
0.5 GHz, 1.5 MV
V’=Vxfrf= 0.75 MVGHz
1.5 GHz, 25 MV
V’=Vxfrf= 37.5 MVGHz
1.75 GHz, 21.4 MV
V’ = Vxfrf= 75 MVGHz
- flexible fill pattern, I<300 mA
- 15 ps & 1.5 ps pulses simultaneous at all beam ports
- all IDs available
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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MAD tracking
single particle tracking, BESSY II user optics & two sc-cavities
0
-20
-40
-300
-200
-100
0
100
200
300
rel. long. position / ps
2
1
0.7ps
(rms)
0
-1
-2
-20
-10
0
10
20
rel. long. position / ps
40
4%
20
Dp/p acceptance
short & long bunch
momentum
acceptance > +/- 4%
0
-20
-40
rel. momentum / 0.001
4%
20
rel. momentum / 0.001
long
40
rel. momentum / 0.001
rel. momentum / 0.001
short
-300
-200
-100
0
100
200
300
rel. long. position / ps
quantum limit
2
1
0
-1
-2
-20
-10
0
10
20
short & long bunch
quantum excitation
& damping
10 damping times
rel. long. position / ps
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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More advanced scheme: double beam
chromatic orbit length:
L  L0 (1  a Δp p0 )
orbits of equal length L=L0:
I) Δp p0  0
II)
a0
2 solutions if a=0, a1=0
a  a0  a2 (Δp p0 )2
Δp/p 
0
double beam scheme
long
puls
   a /a
a0>0
beam port
short
puls
F
0
2
a0<0
2 sc-rf cavities & low a optics
- double beam scheme combined
with two sc-rf cavities
long and short bunches
longitudinally and transversely
separated
alternatively:
short bunches are kicked on separate orbits, like the quasi-SB operation
G. Portmann, et al., Proc. 2008 BIW, Tahoe City, CA (2008).
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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More advanced scheme: double beam
measurements at MLS
vertical displacement
e- beam, beam profile image
photon beam image (beam port exit)
beam current: 170 mA
lifetime: 10 h
6cm
horizontal displacement
transverse separation of photon beams
double beam can be easily produced at the MLS (low-a optics)
 good life time,  high currents
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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HZB expertise
BERLinPro and BESSYVSR:
- cavity design for BESSYVSR is similar to BERLinPro,
1.3 GHz structures to be scaled to 1.5 GHz and 1.75 GHz
- high current beam interaction with sc cavities to be
studied at BERLinPro
BERLinPro
BESSYVSR
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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Summary
long & short bunches simultaneously :
- 175 long bunches, 15 ps < 300 mA
- 175 short bunches, 1.5 ps (rms)
- at all beam ports available
- all IDs available
- present transverse user optics applied
expected results:
beam
parameter
present
350 bunch
filling
THz optics
BESSYVSR
175 bunch
filling
user optics
bunch length (rms) / ps
3
1.5
0.7
0.3
0.04 (0.03 nC)
0.8
0.14
0.02
14
(140)
24
3.5
current (sb) / mA
current (mb) / mA <300
BESSYVSR
175 bunch
filling
THz optics
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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MLS measurements
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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Measurements from SPEAR3
J. Corbett et al.
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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Measurements from DIAMOND
priv. comm. Ian Martin, DIAMOND
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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Pcoh / Pincoh
BESSY: sub-ps bunch
diagnostics at low currents
10
6
- 870 fs, 140 nA
- 700 fs, 300 nA
- 1.2 ps, 140 nA
10
5
- Gaussian fit
10
4
10
3
5
10
20
wave number / cm -1
applied currents below bunch
lengthening /deforming effects
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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11
CSR in frequency domain
N e = 10
power spectrum analysis
Ne  form factor
7
10 gain
5 10 15 20 25
wave number cm-1
Pcoherent / Pincoherent
power spectra by
Fourier transform
spectroscopy
detector signal / a.u.
power spectra
7
10 gain
5 10 15 20 25
wave number cm
-1
BESSY II user optics, single bunch 15 mA
G. Wüstefeld et al., BESSYVSR, ICFA Workshop FLS, March 5-9, 2012, Jefferson Lab
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