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

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

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

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 

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).

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

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

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

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)

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

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

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)

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

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

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