1. Design and Application of the Hybrid X-Band Photoinjector
A. Valloni (University of Rome, Sapienza)
on behalf of J.B. Rosenzweig (UCLA)
and the Hybrid Photoinjector Collaboration
D. Alesini, N. Bernard, L. Faillace, L. Ficcadenti, A. Fukusawa,
M. Migliorati, A. Mostacci, P. Musumeci, B. O'Shea, L. Palumbo,
B. Spataro, A.Valloni

2. Design and Applications of the hybrid X-band photoinjector
Outline
Design and Applications of the hybrid X-band photoinjector
The hybrid photoinjector:
- RF point of view
- Beam dynamics point of view
2. S and X band applications:
- Plasma Wakefield Reproduction of Space Radiation Environment
- Coherent Cerenkov radiation production
3. Conclusions

3. Hybrid v. Conventional Photoinjectors: Radiofrequency Point-of-view
“Standard” photoinjector design employs two accelerating structures:
a standing wave photocathode RF gun and a post-acceleration linac
Conventional photoinjector
RF Gun
Chicane
TW structure
Load
Phase shifter
Circulator
Laser
SW and TW structures fed independently
SW structure reflects nearly all input power at start of RF fill
Circulators, isolators needed to protect RF power source
1.5-cell SW
TW section
Solenoid magnets
Laser
Hybrid photoinjector
SW RF gun section fed on-axis from coupling cell that also feeds (the majority of the power) to a lower gradient downstream TW section.
One can build an RF gun at higher frequency, higher gradient, where no adequate high power circulators yet exist
See A. Valloni, et al., RF Properties of the X-band Hybrid Photoinjector

4. Hybrid v. Conventional Photoinjectors: Beam Dynamiscs Point-of-view
Emittance-Compensating
Solenoids
Cathode
Input Port
TW structure
SW 1.6-cell gun
Input
coupler
Device is more compact than the split system
High acceleration field in X-band of 200 MV/m peak; very high brightness beam 
Avoids post-gun bunch lengthening; instead strongly longitudinally focuses, from velocity bunching due to 90° phase shift between SW cell and input coupler 
Emittance compensation dynamics remains robust even with strong compression
With long TW section, over 20 MeV obtainable
Short TW section case (with outcoupler feeding standard 3 m linac in S-band case) can be used as stand-alone low energy injector
Wide variety of applications enabled
High energy: THz FEL, Inverse Compton
Low energy (subject of this talk)
Novel plasma wakefield accelerators 

5. To be commissioned in 2011 at UCLA
Starting point for scaling
SLAC S-band hybrid
To be commissioned in 2011 at UCLA
Hybrid with 1.55 cell standing wave
section; Optimized for 60 MV/m peak
field in SW section, avg. 13 MV/m in TW
Hybrid with second focusing solenoid envelope
Note: Short traveling wave section allows laser
injection; low energy applications
To scale to SLAC X-band, need to reach 240 MV/m peak field; restricted to only 200 MV/m
for adequate energy
Charge and Wavelength Scaling of RF Photoinjector Designs", J.B. Rosenzweig and E. Colby, Advanced Accelerator Concepts p. 724 (AIP Conf. Proc. 335, 1995).

6. Acceleration/Chirping
Beam dynamics: low energy section only
(2.5 cell design, 2 nC, S-band case)
Velocity bunching compression
Acceleration/Chirping
Emittance well-compensated
Beam size controlled with solenoids

7. Application I: Plasma Wakefield Reproduction
of Space Radiation Environment for Space
Craft Electronics Simulation/Testing
The rise of “killer electrons”
Horne et al., “Wave acceleration of electrons in the van Allen radiation belts“, Nature 437, 2005
Chen et al., “The energization of relativistic electrons in the outer van Allen radiation belt“, Nature Physics 3, 2007
Horne et al., “Plasma astrophysics: Acceleration of killer electrons“, Nature Phys. 3, 2007
Horne et al., “Gyro-resonant electron acceleration at Jupiter“, Nature Physics 4, 2008

8. Application I: Plasma Wakefield Reproduction
of Space Radiation Environment for Space
Craft Electronics Simulation/Testing
Examples of electron flux in the radiation belts of Earth and Jupiter:
electrons have ~ exponential energy spectra
driver
witness
Example of electron flux generated by laser plasma wakefield accelerators
Proposal: substitute PWFA
much higher average power
ideal for testing
- easy obtain monochromatic
beam for calibration

9. Applying hybrid beam to PWFA-generation of Exponential Energy Spectrum
PWFA physical picture
Bulk of beam is decelerated, tail
(falling distribution) is accelerated
OOPIC simulation parameters
(4.3 MeV reached with 60 MV/m peak)
driver
witness
Plasma source (appropriate for this case) from previous UCLA-FNAL experiments (energy spectrum, right)

10. Quasi-Exponential Momentum Spectrum Obtained From PWFA Interaction
>300 MV/m wakefields acting on beam
Beam final longitudinal phase space
driver
witness
Scaled experiment foreseen for X-band case
0.5 nC, 0.5 psec rms, 1 mm-mrad emittance
16 times the plasma density (6E16/cc);
use capillary source from ATF/USC
Momentum spectrum with exponential fit

11. X-band beam dynamics: Scaled Charge (6.7 pC) and Wavelength (11.4 GHz)
Acceleration / Chirping
Velocity bunching compression
@z=30cm σz< 10 mm (90 A)
Beam size controlled with solenoids
@z=30cm σx< 80 mm
@z=30cm εn,x< 0.08 mm-mrad
Emittance evolution
Extremely bright beam Be=2.8E16 A/m2 (100x LCLS injector)
Low charge enables two compelling applications:
Coherent Cerenkov radiation, relativistic electron diffraction

12. X-band application II: production of Coherent Cerenokov radiation
Background: the dielectric wakefield interaction *
Electron bunch ( ≈ 1) drives wake in cylindrical dielectric structure
Dependent on structure properties
Generally multi-mode excitation
Wakefields accelerate trailing bunch
Mode wavelengths (quasi-optical)
Design Parameters
Peak decelerating field
Extremely good
beam needed
(X-band hybrid)
Fields above 5 GV/m (!) observed
“Breakdown limits on Gigavolt-per-Meter Dielectric Wakefields”,
M.C. Thompson, Phys. Rev. Lett., 100, 21 (2008)
Ez on-axis showing multimode wakes (OOPIC)

13. Observation of coherent THz Cerenkov Radiation (CCR) at UCLA, BNL
UCLA Neptune: chicane-compressed (200 mm Q=0.3 nC beam)
PMQ triplet gives sr~100 mm (a=250 mm)
Relatively low energy (10.5 MeV)
Single mode operation demonstrated
Autocorrelation of THz wave train
Two tubes, different b, THz frequencies
BNL ATF: multi-bunch resonant wakes
Neptune CCR frequency spectra
BNL ATF wavelength spectrum
Single bunch wakes give fundamental
l ~ 490 mm, per prediction
Resonant wake excitation, CCR spectrum measured
Excited with 190 mm spacing (2nd harmonic)
Misalignments yield l~300 mm, 1st deflecting mode (important for transverse BBU in wakefield acceleration scenario)
2nd harmonic (accel.)
1st deflecting mode

14. Multi-mode excitation with X-band hybrid
Challenge: beam depth of focus at low energy
Geometric emittance high
Space-charge (dominant)
Use same tubes as BNL expt., with PMQ focusing
13 MV/m fields, >MW peak power
Multi-mode, can choose mode by filtering
Very high average power with high rep rate
photoinjector
Longitudinal wake at exit of 1 cm tube
driver
On-axis wake shows multiple modes

15. Conclusions A INFN-LNF/UCLA/SAPIENZA
collaboration is developing a hybrid photoinjector in X-band
There are several advantages in RF and beam performance of the hybrid system compared to conventional systems and in particular the facility to realize it in X-Band
Wide variety of applications enabled with this device
High energy: THz FEL, Inverse Compton
Low energy
Novel plasma wakefield accelerators