Superconducting High BrightnessSuperconducting High BrightnessRF Photoinjector DesignRF Photoinjector Design
M. Ferrario, J. B. Rosenzweig, J. Sekutowicz, M. Ferrario, J. B. Rosenzweig, J. Sekutowicz, G.Travish, W. D. MoellerG.Travish, W. D. Moeller
INFN, UCLA, DESYINFN, UCLA, DESY
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Main Questions/ConcernsMain Questions/Concerns
• Emittance Compensation ? Emittance Compensation ?
• High Peak Field on Cathode ?High Peak Field on Cathode ?
• Cathode Materials and QE ?Cathode Materials and QE ?
• Q degradation due to Magnetic Q degradation due to Magnetic Field ?Field ?
Before Cool-DownBefore Cool-Down
B
After Cool-DownAfter Cool-Down
B
No independent tuning of accelerating field No independent tuning of accelerating field and RF focusing effectsand RF focusing effects
Transverse non linearitiesTransverse non linearities
€
Er ≅ −r
2
∂
∂zEz (z,0) +
r 3
16
∂3
∂z 3Ez (z,0) + .......
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LCLS GUN + SOLENOIDLCLS GUN + SOLENOID
Gun Working PointGun Working Point
00.51
1.52
2.53
3.54
0 0.5 1 1.5 2 2.5
enxT_[um]enxT+_[um]enxCore_[um]enxH-_[um]enxH_[um]enx_[um]enxT_[um]
Z_[m]
these slices also carry the most pronounced energy these slices also carry the most pronounced energy spreadspread
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′ γ = 2
σ w
ˆ Ι
3I0γ
€
γ= 8
3
ˆ I
2Ioε th ′ γ
€
σ ' = 0
Matching Conditions with the Matching Conditions with the LinacLinac
00.51
1.52
2.53
3.54
0 2 4 6 8 10
enxT_[um]enxT+_[um]enxCore_[um]enxH-_[um]enxH_[um]enx_[um]enxT_[um]
Z_[m]
Linac Working PointLinac Working Point
The emittance compensation occuring in the booster The emittance compensation occuring in the booster when the invariant envelope matching conditions are when the invariant envelope matching conditions are satisfied is actually limited by the head and tail slice satisfied is actually limited by the head and tail slice
behavior behavior
Splitting Acceleration and Splitting Acceleration and FocusingFocusing
25 cm10 cm
50 cm
• The Solenoid can be placed downstream the cavity The Solenoid can be placed downstream the cavity
• Switching on the solenoid when the cavity is cold Switching on the solenoid when the cavity is cold prevent any trapped magnetic fieldprevent any trapped magnetic field
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E0 ∝ λ rf−1
120-140 MV/m==> 52-60 MV/m
€
λ1.3 GHz
λ 3 GHz
= 2.33
€
σ i ∝ λ rf
€
Bz ∝ λ rf−1
( )€
Q∝ λ rf 1 nC ==> 2.33 nC
€
Q1 nC
Q2.33 nC
3 = 0.75
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σ i ∝Q1 / 3
Scaling the LCLS design from S-band to L-bandScaling the LCLS design from S-band to L-band
(JBR)(JBR)
L-band SC gun design with coaxial couplerL-band SC gun design with coaxial coupler
J. SekutowiczJ. Sekutowicz
-20
-10
0
10
20
30
40
50
60
-0.05
0
0.05
0.1
0.15
0.2
0 0.2 0.4 0.6 0.8 1
Ez_[MV/m] Bz_[T]
Ez_[MV/m]
Bz_[T]
z_[m]
0
1
2
3
4
5
6
0 5 10 15
HBUNCH.OUT
sigma_x_[mm]enx_[um]
sigma_x_[mm]
z_[m]
Q =1 nCQ =1 nC
R =1.69 mmR =1.69 mm
L =19.8 psL =19.8 ps
thth = 0.45 mm-mrad = 0.45 mm-mrad
EEpeakpeak = 60 MV/m (Gun) = 60 MV/m (Gun)
EEacc acc = 13 MV/m (Cryo1)= 13 MV/m (Cryo1)
B = 3 kG (Solenoid)B = 3 kG (Solenoid)
I = 50 AI = 50 A
E = 120 MeVE = 120 MeV
nn = 0.6 mm-mrad = 0.6 mm-mrad
nn
[mm-mrad][mm-mrad]
Z [m]
HOMDYN Simulation
6 MeV6 MeV
3.3 m
PARMELA PARMELA simulationssimulations
J. B. RosenzweigJ. B. Rosenzweig
SCRF GUN
BNL All-Niobium SC GunBNL All-Niobium SC Gun
T. Srinivasan-Rao et al., PAC 2003
1/2 cell, 1.3 GHzMaximum Field: 45 MV/m
Q.E. of Niobium @ 248 nmwith laser cleaningbefore: 2 x 10-7
after: 5 x 10-5
I. Ben-Zvi, Proc. Int. Workshop, Erlangen, 2002
No contamination from cathode particlesNo contamination from cathode particles
2 10-5
4 10-5
6 10-5
8 10-5
0.0001
0.00012
0.00014
0.00016
0.00018
0 10 20 30 40 50 60 70
BNL_SCRF_CAT
QE
QE
G [MV/m]
SCRF GUN
Measured
Limited by the available voltageLimited by the available voltage
Measurements at room T Measurements at room T on a dedicated DC on a dedicated DC
systemsystem
Extrapolation to Extrapolation to Higher Field Higher Field
CONCLUSIONSCONCLUSIONS
• RF focusing is not necessaryRF focusing is not necessary
• 60 MV/m peak field in SC cavity have been 60 MV/m peak field in SC cavity have been already demonstratedalready demonstrated
• Work in progress @ BNL to demonstrate Nb QE Work in progress @ BNL to demonstrate Nb QE ~10~10-4 -4 @ 60 MV/m plus new ideas@ 60 MV/m plus new ideas
• Multibunch effectsMultibunch effects
• The new working point for a Split Photoinjector The new working point for a Split Photoinjector can be easily adopted by a SCRF guncan be easily adopted by a SCRF gun
Simple Case: Transport in a Long Simple Case: Transport in a Long SolenoidSolenoid
σ'' +ks2σ =
Kσ
ks =qB
2mcβγ
σeq ξ( ) =K ξ( )
ks
σ'' =0 ==> Equilibrium solution ? ==>==> Equilibrium solution ? ==>
0 0.0005 0.001 0.0015 0.002 0.0025metri
0.5
0.6
0.7
0.8
0.9
g K =2Ig ξ( )Io βγ( )
3g(
σ =σ eq+δσ δσ'' +2ks2δσ =0
Small perturbations around the Small perturbations around the equilibrium solutionequilibrium solution
Same Plasma Same Plasma FrequenciesFrequencies
σ ξ( )=σ eq ξ( )+ σ ξ( )−σ eq ξ( )( )cos 2ksz( )
σ' ξ( ) =− 2ks σ ξ( ) −σ eq ξ( )( )sin 2ksz( )
Different AmplitudesDifferent Amplitudes
ε z( )= σr2 σr
' 2 − σrσ r' 2
÷sin 2ksz( )
0 1 2 3 4 5metri
-0.5
0
0.5
1
1.5
2
envelopes
0 1 2 3 4 5metri
0
20
40
60
80
emi
σr(z)
(z)
Envelope oscillations drive Emittance oscillations
δγγ
=00.5≤g ξ( )≤1
′ σ =0
Bunch with a Linear Bunch with a Linear Energy SpreadEnergy Spread CorrelationCorrelation
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γ+ =γo 1+δγ( )
€
γ−=γo 1−δγ( )
€
δγ =δγγo
€
σ+ =σeq+ +δσ+ cos 2k+z( )
′ σ + = − 2k+δσ+ sin 2k+z( )
€
σ−=σeq− +δσ − cos 2k−z( )
′ σ − = − 2k−δσ − sin 2k−z( )
€
n =γ
2ko σ eqo 2δσ o +δγ( ) sin
Δk
2z
⎛
⎝ ⎜
⎞
⎠ ⎟cos k z( ) +δσ o
2 sin Δkz( )
€
k =1
2k+ + k−( ) = 2ko
€
Δk = 2 k− − k+( ) = 2 2koδγ
€
δσo = σ c − σ eqo
A Spread in Plasma Frequencies drives A Spread in Plasma Frequencies drives a a BeatingBeating in Emittance Oscillations in Emittance Oscillations
On a longer time scaleOn a longer time scale
increasing the initial envelope offset the emittance evolution is increasing the initial envelope offset the emittance evolution is dominated by the beating term and the original minimum is dominated by the beating term and the original minimum is
recovered only after a longer periodrecovered only after a longer period