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

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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

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γ= 8

3

ˆ I

2Ioε th ′ γ

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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

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λ1.3 GHz

λ 3 GHz

= 2.33

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σ i ∝ λ rf

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Bz ∝ λ rf−1

( )€

Q∝ λ rf 1 nC ==> 2.33 nC

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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( )

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γ−=γo 1−δγ( )

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δγ =δγγo

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σ+ =σeq+ +δσ+ cos 2k+z( )

′ σ + = − 2k+δσ+ sin 2k+z( )

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σ−=σeq− +δσ − cos 2k−z( )

′ σ − = − 2k−δσ − sin 2k−z( )

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n =γ

2ko σ eqo 2δσ o +δγ( ) sin

Δk

2z

⎛

⎝ ⎜

⎞

⎠ ⎟cos k z( ) +δσ o

2 sin Δkz( )

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k =1

2k+ + k−( ) = 2ko

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Δk = 2 k− − k+( ) = 2 2koδγ

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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