1. on behalf of the HCP_AF team
S and X band Hybrid photo-injectors
activities at INFN/LNF
presented by
Bruno Spataro
on behalf of the HCP_AF team

2. This work is made possible by the efforts :
Contributors
This work is made possible by the efforts :
HCP-AF Group, INFN - LNF
University of Roma 1
UCLA (Los Angeles)
CSN5 – November 11,2013,Padova

3. SUMMARY Hybrid photo-injector at 2.856 GHz
Photoinjectors
Gun photoinjector (1.6 cells) at GHz RF
Hybrid photo-injector at GHz
Hybrid photo-injector at GHz
Collaborations : UCLA (Los Angeles)
CSN5 – November 11,2013,Padova

4. A 100Hz RF Gun Operation frequency 2.856 GHz 1.6 cell gun Dual feed
Input RF power
Half Cell
Full Cell z
Operation frequency GHz
1.6 cell gun
Dual feed
Race track geometry
100 Hz repetition rate
Multibunch operation
Numeric codes for simulations:
HFSS and Superfish
CSN5 – November 11,2013,Padova

5. circulators are necessary in order to protect the RF power source
Conventional photoinjector vs. Hybrid photoinjector
The photoinjector design employs a combination of two accelerating structures:
a standing wave gun and an accelerating linac
Conventional photoinjector
RF Gun
Chicane
TW structure
Load
Phase shifter
Circulator
Laser
The SW and TW structures are fed independently
The SW structure reflects nearly all of the input power at the beginning of the RF fill process
circulators are necessary in order to protect the RF power source
1.6-cell SW
TW section
Solenoid magnets
Laser
Hybrid photoinjector
The SW RF gun section is coupled on-axis to the TW section through a coupling cell that serves as feeding system to both SW/TW.
This enables to build a RF gun at higher frequency where no adequate high power circulators yet exist
RF properties of a X band hybrid photoinjector
CSN5 – November 11,2013,Padova

6. X-band hybrid photoinjector: first step
S-Band hybrid + Scaling Laws
While scaling the design from S-Band to X-Band
is conceptually simple
Pratical limits require some changes in both RF and magnetostatic designs:
DIMENSIONS α 1/f
FIELD α f
CHARGE α 1/f
S-Band Main parameters:
-Frequency : GHz
-Total length : cm
-Peak field in SW : 60MV/m
-Charge : n
X-Band Main parameters:
-Frequency : GHz
-Total length : cm
-Peak field in SW : 240MV/m 200MV/m
-Charge : pC
Scaling factor
2.856GHz
GHz *4
It is necessary to evaluate a different design for an appropriate configuration of magnets
We limited the electric field to 200MV/m and so to reach 3.5MeV (energy level nedeed for the applications that we are investigating) the SW section must be expanded to 2.6 cells.
New material is needed (NORCIA’s activities !!)
Charge and Wavelength Scaling of RF Photoinjector Designs", J.B. Rosenzweig and E. Colby, Advanced Accelerator Concepts p. 724 (AIP Conf. Proc. 335, 1995).
CSN5 – November 11,2013,Padova

7. The Hybrid photoinjector: Advantages
Emittance-Compensating
Solenoids
Cathode
Input Port
TW structure
SW 1.6-cell gun
Input
coupler
It strongly mitigates impedance mismatches, and therefore reflected RF power, during and after the RF filling of the SW section--> no circulator needed. Scaling to X-Band enabled
One may use a greatly simplified high power RF system than a split photoinjector
The device is much more compact than the split system
High acceleration field in X-band of 200 MV/m peak, and thus very high brightness beam 
It avoids the bunch lengthening observed during the drift in a split photoinjector and in fact strongly longitudinally focuses, through velocity bunching due to  90°phase shift between SW cell and input coupler 
The emittance compensation dynamics remains manageable even in the presence of strong compression
Wide variety of applications enabled 
Applications: Injector for FELs interdisciplinary scientific research: biology, medicine, materials science, non linear optics.
CSN5 – November 11,2013,Padova

8.  Mode of the gun
For Dc=2/3 the beam is accelerated in both structures SW gun and and TW section;
For Dc=5/12 the bunch is accelerated in the SW section and longitudinally compressed (velocity bunching technique) in the TW one Dc

9. Integrated velocity bunching operation on the hybrid photoinjector
Input
coupler
Set for the beam
to go to proper phase in TW
(Dc=5/12 velocity bunching operation)
Output
coupler
Traveling wave structure Dc
Eacc
Eacc
Eacc
bunch z z z
CSN5 – November 11,2013,Padova

10. Ez(z,t)=E0(z)cos(ph(z)+t)
Integrated velocity bunching (1/2)
Amplitude of the E field along the structure (E0(z))
Ez(z,t)=E0(z)cos(ph(z)+t)
Phase (ph(z)) of the electric field along the structure: the sensitivity of the phase with respect to the resonant frequency of the SW structure requires very good stabilization of the temperature or an RF feedback
0.5deg/kHz
CSN5 – November 11,2013,Padova

11. SW/TW Hybrid Photoinjector
RF in
Small Reflection
Low Emittance Beam Generation
RF out
Laser
- Exclude isolator IC
Velocity Bunching OC
- RF gun
- p mode
- Very weak coupling
- 90 deg phase difference
between SW and IC. SW
5/12 l TW
2 p /3 mode
Set for the beam
to go to proper phase in TW
75 cm SW TW
S-band
(LNF- Rome Univ., UCLA)
60 MV/m (peak)
13.5 MV/m (Average)
X-band
(LNF- Rome Univ.,UCLA)
240 MV/m (peak)
54 MV/m (Average)
CSN5 – November 11,2013,Padova

12. Summary (preliminary) of beam dynamics estimations
Hybrid photoinjector gives short bunch with low emittance
In the 0.25 pC case, the beam quality may be extremely good
The emittance will be limited by thermal contribution
S-band
X-band
Charge
1 nC
1 pC
250 pC
0.25 pC
Normalized Emittance (rms)
3.7 mm.mrad
0.095 mm.mrad
2.4 mm.mrad
0.022 mm.mrad
Bunch length (rms)
95 mm
4.6 mm
13 mm
0.9 mm
Kinetic energy
20.8 MeV
20.6 MeV
20.5 MeV
Energy spread (rms)
1.3 %
0.17 %
1.1 %
0.16 %
CSN5 – November 11,2013,Padova
(Thermal emittance not included)

13. 3GHz Hybrid gun Design Parameters
Accelerator Parameters
Resonant frequency
2856 MHz
SW cavity mode
pi mode
TW cavity mode
2pi/3 mode
RF Power
25 MW
Peak field strength in SW
60 MV/m
Average acc. Field strength in TW
13 MV/m
Number of TW cells
6 cells
Cavity length
40 cm
Solenoid Field
1.5 kGauss
The big input energy 25 MW was chosen to have a post accelerator. The gun with lower input power can be designed.
Relatively lower field strength in the standing wave part to restrict the energy gain for velocity bunching.
CSN5 – November 11,2013,Padova

14. Test Beam Line cathode [m] 0.61 2.02 0.93 1.03 1.30 1.45 1.81 1.59
Mirror Box
1.16
Dipole IG
Deflector FC
Slit
Gun GV
screen
IP 100
Ф 6”
Single slit emittance measurement.
Bunch length measurement with 9.6-GHz deflector.
Ф 6” round dipole magnet.
CSN5 – November 11,2013,Padova

15. 3 GHz hybrid gun constructed at LNF and installed at UCLA ( Los Angeles) for commissioning
RF IN
RF Pulse
RF Out
Commissioned up to 13 MW.
- We could run the klystron only at MW max. in stable manner (E = 30 MV/m, SW)
Solenoid+Gun
Test Bench at Pegasus Lab, UCLA.
- The klystron is planed to be exchanged with another one which is expected to give 20 MW in the next of month.
CSN5 – November 11,2013,Padova

16. Field Strength
20 % lower.
RF Design
Frequency
2856 MHz
Input Power
25 MW
E peak in SW
60 MV/m
E acc in TW
(E peak)
13 MV/m
(17 MV/m)
Solenoid Design Bz
1.5 kGauss
Measured field strength in the standing wave part was 20 % lower than the designed.
Degrade of Q value caused it.
The cavity was left in air for months.
Oxidation of the cavity wall could increase the resistivity.
CSN5 – November 11,2013,Padova

17. Phase Advance 90-deg phase shift
- The phase advance agreed well with the design.
CSN5 – November 11,2013,Padova

18. Solenoid
SW Coil
Backing Coil
TW Coil
Measurement was done without optimize the current to each coil.
CSN5 – November 11,2013,Padova

19. Power dependance There are errors in the gun phase by +/- 5 deg.
There are errors in the gun phase by +/- 5 deg.
As the input power increases, bunching was getting better.
CSN5 – November 11,2013,Padova

20. Temperature scan measurements at 11.5 MW
There are errors in the gun phase by +/- 5 deg.
Higher temperature, more bunching.
CSN5 – November 11,2013,Padova

21. Charge, Energy, and Energy Spread as a function of the gun phase.
There are errors in the gun phase by +/- 5 deg.
The charge measurement shows Schottky effect.
The positive slope in the energy measurement means the bunch gets the energy modulation for bunching.
CSN5 – November 11,2013,Padova

22. Single Slit Emittance Measurement.
0.35 mm.mrad
Parmela said 0.18 mm.mrad was possible.
There could be a problem on resolution at the screen monitor.
CSN5 – November 11,2013,Padova

23. 11.424 GHz Hybrid gun constructed at INFN-LNF
The hybrid gun has been designed at UCLA [J. Rosenzweig and
Ph.D. thesis of A. Valloni ]
CSN5 – November 11,2013,Padova

24. 11.424 RF Hybrid gun SW TW EpSW / <E>TW = 4
rf power in
rf power out
RF Hybrid gun
Electric Field inside the Photoinjector SW TW
asse
EpSW
EpSW / <E>TW = 4
<E>TW
EpSW = 120MV/m <E>TW=30MV/m
for an input power of 10MW
CSN5 – November 11,2013,Padova

25. Ez(z,t)=E0(z)cos(ph(z)+t)
Integrated velocity bunching (1/2)
SW Gun at resonance
SW Gun -200kHz
SW Gun +200kHz
Amplitude of the E field along the structure (E0(z))
Ez(z,t)=E0(z)cos(ph(z)+t)
Phase (ph(z)) of the electric field along the structure: the sensitivity of the phase with respect to the resonant frequency of the SW structure requires very good stabilization of the temperature or an RF feedback
SW Gun at resonance
SW Gun -200kHz
SW Gun +200kHz
0.06 deg/kHz

26. RF pulsed heating T =20° below the upper limit of 60°
RF pulsed heating, due to surface magnetic field,
causes a temperature gradient on the metal.
Input RF power
Crucial areas are the waveguides-to-coupling-cells irises.
Therefore, a “rounded iris” is used.
The peak surface magnetic field is nearly
H||= 1.5*105 input RF power = 10MW
T =20°
t: pulse length
σ: electrical conductivity
δ: skin depth
ρ’ : density
cε:specific heat
k : thermal conductivity
below the upper limit of 60°
D.P.Pritzkau,“RFPulsedHeating”,SLAC-Report-577,Ph.D.Dissertation,StanfordUniversity,2001

27. Full HFSS model Dual feed input coupler Dual feed output coupler
9 Travelling Wave cells
2.5 Standing Wave cells
Section
Splitters
CSN5 – November 11,2013,Padova

28. CSN5 – November 11,2013,Padova

29. Thermal analysis for 20MW power input RF and temperature on outer surface 40 ˚C
ΔTmax = ˚ C
Duty Cycle =5 E-6
ΔTmax = 20.8 ˚ C
Duty Cycle = 5 E-4
Detuned cavities in the SW region are shown !!
CSN5 – November 11,2013,Padova

30. GHz hybrid gun Prototype under test (with untuned device and no brazing). Mode frequency is ~7 MHz off !!
GHz hybrid photoinjector longitudinal
electric field profile (with untuned device and
no brazing) . Mode frequency is ~7 MHz off
GHz hybrid photoinjector longitudinal phase advance (with untuned device with no tuners and no brazing). Mode frequency is ~7 MH off
Machining precision is by ± 2.5 μm
while the roughness is about 60 nm.
The surface finishing was obtained
directly by mechanical machining
with custom cutting tools (diamond
mono-crystal)
CSN5 – November 11,2013,Padova

33. Thank you very much for your attention !!!