Transcript Velocity bunching experiment at @SPARC

Velocity bunching
experiment @ SPARC
Daniele Filippetto
on behalf of SPARC team
Outline
•
The velocity bunching concept
• SPARC hardware overview
• VB experiment @ SPARC
• Emittance degradation by solenoid
misalignment
• Conclusions
D. Filippetto HBEB-MAUI_09
• The velocity bunching concept:
• the beam is injected in a long accelerating structure at the 0 crossing field phase
•Injection at low energies where The beam is slower than the phase velocity of the
RF wave (typically the first LINAC after the gun)
• it will slip back to phases where the field is accelerating, but at the same time
it will be chirped and compressed.
•Compression and acceleration take place at the same time within the same linac
section
At SPARC the beam is accelerated
from 4-5 MeV up to 20-25 MeV
(instead of 60-65)
D. Filippetto HBEB-MAUI_09
Peak current vs RF compression phase
HIGH-C
1300
1200
Average current (A)
1100
MEDIUM
COMPRESSION
1000
900
LOW COMPRESSION
800
700
600
500
400
300
200
100
-95
-90
-85
OVER-C
-80
RF compressor phase (deg)
SPARC nominal case
Initial parameters:
1 nC beam
10 ps long
D. Filippetto HBEB-MAUI_09
-75
-70
-65
-60
... What happens to the transverse plane?
If the transverse emittance can be preserved during the
longitudinal focusing, the beam brightness can be increased
SPARC
L. Serafini, M. Ferrario, “Velocity Bunching in PhotoInjectors” ,
AIP CP 581, 2001, pag.87
may avoid the phase space degrading effects observed in magnetic
compression experiments on photoinjector-derived beams
D. Filippetto HBEB-MAUI_09
SPARC overview:
Diagnostic and
Matching
Undulators
150 MeV
S-band
linac
u = 2.8 cm
Velocity
Bunching
10m
Kmax = 2.2
r = 500 nm
15m
Long
Solenoids
Seeding
D. Filippetto HBEB-MAUI_09
THz
Source
S-band
Gun
• Iron joke (blue) for field lines guiding
• 1 single and 4 triplet coils surrounding two LINAC section,
indipendently powered
D. Filippetto HBEB-MAUI_09
Diagnostic hardware:
Time measurement resolution:
 50 fs / pixel @ 150 MeV
 33 fs / pixel @ 100 MeV
Spectrometer system:
Θ=14
deg; parameters:
SPARC
typical
s yB @Lm=26.7cm;
mm
70
Ld=2.13m;
Pixel
@ 1 . 5size=30um;
V
MV
DEFL
Energy
E @ 150
MeVresolution:
about 15keV @ 150MeV;
@
f RF 2 . 856 GHz
Overall resolution (RMS):
L @ 4m
screens
10-4≤ DE/E ≤ 10-2
stB _ RES =
Dipole magnet
D. Filippetto HBEB-MAUI_09
syB
VDEFL
wRF L
E / e RF
@
90 fs @ 150 MeV
60 fs @ 100 MeV
Quadrupole
triplet
deflector
VB run @ SPARC:
Laser parameters:
Yrms =350um
TFWHM=7.3ps
Energy @ cathode = 170uJ
Gun parameters:
Gun Input Power=7.5MW
Gun Peak Field=105MV/m
e-Energy out of the gun=4.7MeV
Working inj.phase=30 deg.
e-beam charge @30deg=280pC
D. Filippetto HBEB-MAUI_09
x 10
-5
7.3 ps FWHM
2
intensity (Arb. Units)
Xrms =358um
2.5
1.5
1
0.5
0
0
5
10
time (ps)
15
20
C-Factor Vs RF compressor phase:
First linac section used as compressor
C=3 chosen for characterization measurements:


Useful in a hybrid scheme with magnetic compressor (SPARX case)
C=15
Less sensitive to relative phase jitter
240 fs rms
C=3
1000 fs rms
Maximum energy
D. Filippetto HBEB-MAUI_09
E-beam parameters @ LINAC exit, C=1:
147.5MeV
0.16MeV
0.11%
280pC
3.01ps
30Amps
159.6 keV*mm
x 10
13
LONGITUDINAL TRACE SPACE
-2.5
-2
Time [ps]
Max energy on crest
Total DErms
DE/Erms
Charge
Bunch Length RMS
Slice Peak Current
Longitudinal emittance
-3
-1.5
-1
-0.5
146
147
148
149
Energy [MeV]
150
Beam slice current profile
35
4
x 10
-4
SLICE ENERGY SPREAD
25
3
20
DE/E rms
Slice current (Amps)
30
15
10
5
0
-2000
2
1
0
2000
Z (um)
4000
D. Filippetto HBEB-MAUI_09
6000
0
-5
0
Time (ps)
5
Effect of solenoid:
TW solenoids OFF Vs ON (660Gauss)
Hor. (red) and Ver. (blu) emittance
C=1
TW sol on
6
(mm mrad)
TW-SOL on:
εx=1.85um
εy=1.65um
Best emittance after solenoid
scan with TW-SOL off:
εx=1.4um
εy=1.5um
4
2
0
156
158
160
162
Gun solenoid current (A)
sx (red) sy (blue) ( mm)
Beam envelope @ 161 A with TWsol ON
1000
800
Isol=161 A
600
400
0
10
20
Distance from the cathode (m)
D. Filippetto HBEB-MAUI_09
TW solenoids Off VS ON, slice emittances:
1.6
TW Solenoids Off
1.4
TW Solenoids ON
2.5
70
TW Sol OFF
60
TW Sol ON
2.0
1.2
1.0
0.8
30
0.6
20
0.4
1.5
Mismatch
40
Current (Amps)
Horizontal Emittance (um)
50
1.0
0.5
0.2
10
0.0
0
0.0
0
1000
2000
3000
Z position along the bunch (um)
4000
0
1000
2000
3000
4000
Z position along the bunch (um)
•The solenoid misalignment leads to an increase of the projected
emittance, which is not found looking at the slice emittances;
•the mismatch parameter is similar in the two cases;
•The difference is due to slice centroid misalignement (will be treated more
in detail further on in the presentation);
•A beam based alignment is mandatory to reach lower projected emittances;
D. Filippetto HBEB-MAUI_09
Beam after compression @C3
Bunch charge
No compression
Compression @C3
280 pC
280pC
Injection phase (S1) 0 deg (on crest)
-87deg
Beam Energy
101 MeV
147.5 MeV
Total energy spread 0.11%
1.1%
Bunch length
3.01ps RMS
0.97ps RMS
TW Solenoid field
0
450 Gauss
140
120
Slice Current compressed
Slice current uncompressed
Slice Current (Amps)
100
80
60
40
20
0
0
D. Filippetto HBEB-MAUI_09
800
1600
2400
Z (um)
3200
4000
4800
Beam after compression @C3
3
Emittance (mm mrad)
H emittance
V emittance
2.5
2
For a compression factor C=3: Bsl=1.1x1014 A/m2
1.5
Gain
of a factor 3.7 on the maximum slice current (30 Vs 110)
1
156
158
160
Gun solenoid current (A)
Loss of a factor 1.15 on the minimum slice emittance (1.2 Vs 1.4)
14)
Gain
of
a
factor
2.7
on
the
slice
Max
Brigthness
(0.41
Vs
1.1x10
Emittance without TW solenoids
(Gun solenoid current=157Amps):
ΔB/C=0.9
Ex=6.2 mm mrad
Ey=4.0 mm mrad
D. Filippetto HBEB-MAUI_09
• Low charge/max Compression Case:
Bunch length rms=
Longitudinal emittance=
Laser spot size rms=
60pC
1.95 ps
54.2 keV*mm
250um
Compression factor
Bunch Charge=
20
15
10
Extreme
compression WP
5
0
-95
D. Filippetto HBEB-MAUI_09
-90
-85
Phase (deg)
-80
Beam @ C-17 (TW sol 45Amps):
Energy=97.6 MeV
DE/Erms =1%
Ipeak=217.5 Amps
Ex=1.52 mm mrad
Ey=1.62 mm mrad
B ≈ 2x1014 Amps/m2
TW solenoids OFF
Gun sol Current(151Amps):
Ex=4.1 mm mrad
Ey=3.4 mm mrad
D. Filippetto HBEB-MAUI_09
Proj.
emittance
• Critical point:
Proj. emittance degradation due to solenoids misalignment
•The solenoid force is energy dependent:
KL=qB0/2m0cβγ
• strong energy-time correlation in VB conditions
• different focusing forces for different time slices
• if the beam is propagating off axis respect to the magnetic field,
the slice centroids will experience time dependent kicks
Lower Energies
higher Energies
Induced longitudinal-transverse correlation, proj. emittance increase
D. Filippetto HBEB-MAUI_09
Example: 1mm solenoid misalignment (H)
Effect on transverse beam shape along the Linac:
Out linac1
On crest
measurements
VB conditions
Out linac2
PARMELA runs simulating
the two TW solenoids
1mm off axis respect to
the rf cavity, on crest
and in the VB conditions
D. Filippetto HBEB-MAUI_09
VB conditions
Effect on emittance measurement:
QS for slice emittance
RFD on
same quad
currents
X
time
Y
QS for projected emittance
RFD off
X
Simulated X e Y vs phi at linac output
Beam dimensions
higher emittance value
4
x 10
2.5
X-phi
Y-phi
2
1.5
1
0.5
0
D. Filippetto HBEB-MAUI_09
-60
Quad strength
-40
-20
0
20
40
60
Slice centroid spread exclusion:
Projected emittance from slice
αn, βn, γn, εn twiss
parameters of slice n
Emittance (mm mrad)
3
2.5
H. proj. emitt.
V. proj. emitt.
H. emitt from slice
2
1.5
1
156
158
160
Gun solenoid current (A)
D. Filippetto HBEB-MAUI_09
Slice centroid contribution to the emittance:
M.Ferrario, V.Fusco, M.Migliorati, L. Palumbo,Int. Journal of Modern Physics A ,Vol 22, No. 3 (2007) 4214-4234

tot
n, x

=  s x s x  s xs x
2
'2

' 2
[] =
1
Np
Np
 []p =
n =1
Ns
N p =  N ps ;

env
n
=
s
2
s
s
'2
s
 s ss
s =1
' 2
s

Total
n
=
  []
s =1 p s =1
ps
;
x = xs  xs ;
uses the slice centroid
2
'2
'2
 ncent =  s cent
, s  s cent, s  s cent, s  s cent, s
 ncross = 
1
Np
N s N ps
different from 0 only if slice centroids
do not lie on the same axis
'2
2
'
'
'
s s2  s cent

s

s

2
s
s

s
s
,s
cent, s
s
s s
cent, s cent, s
2
       
env 2
n
cent 2
n
εxenv=0
D. Filippetto HBEB-MAUI_09
cross 2
n
εxcent=0
correlation between slice centroid spread
and single slice dimension in ph.sp.
εxcross≠0
Measured H. Projected emittance @157A (red dot)= 2.3um
•From
εxenv
=1.5 the
um slice emittance with the quad scan, the values of alpha
andumemittance for each slice are calculated at one precise
εxcentbeta
=0.52
position
εxcross=1.72 um
Transverse phase space distorsion
due to also
beamthe
misalignment
2 +(ε env)2=2.34um
the
measurements
system for slice centroids
εxtot•From
=√(εxenv
)2+ QS
(εxenv
)
x
0.14
(both in X and X’)
can be written and solved (first order system)
0.12
x 10
0.10
3
-4
Emittance (mm mrad)
0
-6
0
2.5
12
Mean + RMS divergence (rad)
2
-2
x 10
-5
0.08
•All the
can be calculated
Slice centroids
Vs 3Z emittance terms
10
Slice mean divergence Vs Z
0.06
4
X centroid + sigma (m)
-4
divergence (mrad)
6
0.04
H.
0.02
proj. emitt.
V.
0.00
proj. emitt.
H. emitt from slice
-0.02
8
6
4
2
0
-0.04
-2
-0.06
2
200
1.5
1
-4
400
600
-0.08
-0.8
800
1000
Z position (um)
-0.6
1200
156
158
160
Gun solenoid current (A)
D. Filippetto HBEB-MAUI_09
-0.4
1400
-0.2
1600
-6
0.0
0
X (mm)
0.2
200
0.4
400
0.6
600
800
1000
Z position (um)
1200
1400
1600
Conclusions:
• Demonstrated transverse emittance preservation in the VB regime
for medium compression factors;
• Preliminary studies on high CF show an emittance decrease, but
still work to do to fully compensate.
• Higher total energy spreads make the beam emittance sensitive to
magnetic components misalignment (quads, sol., etc...)
• The slice centroid spread contribution to the projected emittance
can be isolated and measured
Next steps
• BBA on TW solenoids
• emittance study as function of TW solenoid fields (field shaping)
• Longitudinal phase space detailed studies (slice DE)
• THZ production, ICS experiments, FEL single spike, laser comb
D. Filippetto HBEB-MAUI_09