Velocity bunching experiment @ SPARC
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Transcript of Velocity bunching experiment @ SPARC
Velocity bunching experiment @ SPARC
Daniele Filippettoon behalf of SPARC team
D. Filippetto HBEB-MAUI_09
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 acceleratedfrom 4-5 MeV up to 20-25 MeV(instead of 60-65)
D. Filippetto HBEB-MAUI_09
100200300400500600700800900
1000110012001300
-95 -90 -85 -80 -75 -70 -65 -60
RF compressor phase (deg)
Ave
rage
cur
rent
(A)
LOW COMPRESSION
OVER-C
MEDIUM COMPRESSION
HIGH-C
Peak current vs RF compression phase
SPARC nominal caseInitial parameters: 1 nC beam 10 ps long
D. Filippetto HBEB-MAUI_09
If the transverse emittance can be preserved during the longitudinal focusing, the beam brightness can be increased
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
SPARC
... What happens to the transverse plane?
D. Filippetto HBEB-MAUI_09
S-band Gun
Velocity Bunching
Long Solenoids
Diagnostic and Matching
Seeding
THz Source
150 MeV S-band linac
10m
Undulatorsu = 2.8 cmKmax = 2.2
r = 500 nm15m
SPARC overview:
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• 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:
Dipole magnet RFdeflector
Quadrupoletriplet
screens
Spectrometer system:Θ=14 deg;Lm=26.7cm;Ld=2.13m;Pixel size=30um;
Energy resolution:about 15keV @ 150MeV;
Overall resolution (RMS): 10-4≤ DE/E ≤ 10-2
Time measurement resolution:
SPARC typical parameters:
mLGHzf
MeVEMVV
m
MeVpixelfsMeVpixelfs
RF
DEFL
yB
4856.2
1505.1
70
100@/33150@/50
MeVfsMeVfs
LeE
VRF
DEFL
yRESt
B
B 100@60150@90
/
_
D. Filippetto HBEB-MAUI_09
VB run @ SPARC:
0 5 10 15 200
0.5
1
1.5
2
2.5x 10-5
time (ps)
inte
nsity
(Arb
. Uni
ts)
7.3 ps FWHMLaser parameters:
Xrms =358um
Yrms =350um
TFWHM=7.3ps
Energy @ cathode = 170uJGun 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
C-Factor Vs RF compressor phase:
Maximum energy
First linac section used as compressor
C=15
240 fs rms
C=3
1000 fs rms
C=3 chosen for characterization measurements:
Useful in a hybrid scheme with magnetic compressor (SPARX case) Less sensitive to relative phase jitter
D. Filippetto HBEB-MAUI_09
-5 0 50
1
2
3
4x 10-4 SLICE ENERGY SPREAD
Time (ps)
DE/E
rms
E-beam parameters @ LINAC exit, C=1:
Max energy on crest 147.5MeVTotal DErms 0.16MeV DE/Erms 0.11%Charge 280pCBunch Length RMS 3.01psSlice Peak Current30AmpsLongitudinal emittance 159.6 keV*mm
-2000 0 2000 4000 60000
5
10
15
20
25
30
35
Z (um)
Slic
e cu
rrent
(Am
ps)
Beam slice current profile
LONGITUDINAL TRACE SPACE
Energy [MeV]
Tim
e [p
s]
146 147 148 149 150
-3
-2.5
-2
-1.5
-1
-0.5
x 1013
D. Filippetto HBEB-MAUI_09
Effect of solenoid:TW solenoids OFF Vs ON (660Gauss) C=1
156 158 160 1620
2
4
6
Gun solenoid current (A)
(mm
mra
d)
Hor. (red) and Ver. (blu) emittance
0 10 20
400
600
800
1000
Distance from the cathode (m)
x (red
) y (b
lue)
(m
)
Beam envelope @ 161 A with TWsol ON
Isol=161 A
Best emittance after solenoid scan with TW-SOL off:εx=1.4umεy=1.5um
TW-SOL on:εx=1.85umεy=1.65um
TW sol on
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TW solenoids Off VS ON, slice emittances:
•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;
0 1000 2000 3000 40000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0
10
20
30
40
50
60
70
TW Solenoids ON
Horiz
onta
l Em
ittan
ce (u
m)
Z position along the bunch (um)
TW Solenoids Off
Cur
rent
(Am
ps)
0 1000 2000 3000 40000.0
0.5
1.0
1.5
2.0
2.5
Mism
atch
Z position along the bunch (um)
TW Sol OFF TW Sol ON
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Beam after compression @C3No compression Compression @C3
Bunch charge 280 pC 280pCInjection phase (S1)
0 deg (on crest)
-87deg
Beam Energy 147.5 MeV 101 MeVTotal energy spread
0.11% 1.1%
Bunch length 3.01ps RMS 0.97ps RMSTW Solenoid field 0 450 Gauss
0 800 1600 2400 3200 4000 48000
20
40
60
80
100
120
140
Slice
Cur
rent
(Am
ps)
Z (um)
Slice Current compressed Slice current uncompressed
D. Filippetto HBEB-MAUI_09
Beam after compression @C3
156 158 1601
1.5
2
2.5
3
Gun solenoid current (A)
Emitta
nce
(mm
mra
d)
H emittanceV emittance
Bsl=1.1x1014 A/m2
Emittance without TW solenoids(Gun solenoid current=157Amps):Ex=6.2 mm mradEy=4.0 mm mrad
For a compression factor C=3:
Gain of a factor 3.7 on the maximum slice current (30 Vs 110)
Loss of a factor 1.15 on the minimum slice emittance (1.2 Vs 1.4)
Gain of a factor 2.7 on the slice Max Brigthness (0.41 Vs 1.1x1014)
ΔB/C=0.9
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• Low charge/max Compression Case:Bunch Charge= 60pCBunch length rms= 1.95 psLongitudinal emittance= 54.2 keV*mmLaser spot size rms= 250um
-95 -90 -85 -800
5
10
15
20
Phase (deg)
Com
pres
sion
fact
or
Extreme compression WP
D. Filippetto HBEB-MAUI_09
TW solenoids OFFGun sol Current(151Amps):
Ex=4.1 mm mrad Ey=3.4 mm mrad
Beam @ C-17 (TW sol 45Amps):
Energy=97.6 MeVDE/Erms =1% Ipeak=217.5 Amps
Ex=1.52 mm mradEy=1.62 mm mrad
Proj. emittance
B ≈ 2x1014 Amps/m2
Preli
minary
D. Filippetto HBEB-MAUI_09
• 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
Induced longitudinal-transverse correlation, proj. emittance increase
Lower Energies
higher Energies
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Example: 1mm solenoid misalignment (H)
Out linac2
Out linac1
On crest
VB conditions
VB conditions
Effect on transverse beam shape along the Linac:
PARMELA runs simulatingthe two TW solenoids 1mm off axis respect to the rf cavity, on crest and in the VB conditions
measurements
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X-phi Y-phi
Simulated X e Y vs phi at linac output
QS for slice emittanceRFD on
QS for projected emittanceRFD off
same quadcurrents
-60 -40 -20 0 20 40 60
0
0.5
1
1.5
2
2.5
x 104
Quad strengthBeam
dim
ensi
ons
higher emittance value
Effect on emittance measurement:
tim
e
XY
X
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Slice centroid spread exclusion:Projected emittance from slice
αn, βn, γn, εn twiss parameters of slice n
156 158 1601
1.5
2
2.5
3
Gun solenoid current (A)
Emitta
nce
(mm
mra
d)
H. proj. emitt.V. proj. emitt.H. emitt from slice
D. Filippetto HBEB-MAUI_09
M.Ferrario, V.Fusco, M.Migliorati, L. Palumbo,Int. Journal of Modern Physics A ,Vol 22, No. 3 (2007) 4214-4234
222
',,
'2'2,
2',
2
2',,
2',
2,
2'2'2
2
crossn
centn
envn
Totaln
scentscentsssscentscentscrossn
scentscentscentscentcentn
ssssenvn
;
;[]1[]1[]
1
1 11
s
s
s sp
s
s
p
N
spp
N
s
N
pp
p
N
np
p
NN
NN 2'2'2, xxxxtotxn
uses the slice centroid
different from 0 only if slice centroids do not lie on the same axis
correlation between slice centroid spread and single slice dimension in ph.sp.
εxenv=0 εx
cent=0 εxcross≠0
;ss xxx
Slice centroid contribution to the emittance:
D. Filippetto HBEB-MAUI_09
•From the slice emittance with the quad scan, the values of alpha beta and emittance for each slice are calculated at
one precise position
•From the QS measurements also the system for slice centroids (both in X and X’) can be written and solved (first
order system)
•All the 3 emittance terms can be calculated
156 158 1601
1.5
2
2.5
3
Gun solenoid current (A)
Emitta
nce
(mm
mra
d)
H. proj. emitt.V. proj. emitt.H. emitt from slice
Measured H. Projected emittance @157A (red dot)= 2.3um
εxenv=1.5 um
εxcent=0.52 um
εxcross=1.72 um
εxtot=√(εx
env)2+ (εxenv)2 +(εx
env)2=2.34um
0 200 400 600 800 1000 1200 1400 1600-6
-4
-2
0
2
4
6 x 10-4
Z position (um)
X ce
ntro
id +
sig
ma
(m)
0 200 400 600 800 1000 1200 1400 1600-6
-4
-2
0
2
4
6
8
10
12x 10-5
Z position (um)
Mea
n +
RM
S di
verg
ence
(rad
)Slice centroids Vs Z Slice mean divergence Vs Z
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-0.08-0.06-0.04-0.020.000.020.040.060.080.100.120.14di
verg
ence
(mra
d)
X (mm)
Transverse phase space distorsiondue to beam misalignment
D. Filippetto HBEB-MAUI_09
Conclusions:
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
• 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