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

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

SUMMARY Hybrid photo-injector at 2.856 GHz Photoinjectors Gun photoinjector (1.6 cells) at 2.856 GHz RF Hybrid photo-injector at 2.856 GHz Hybrid photo-injector at 11.424 GHz Collaborations : UCLA (Los Angeles) CSN5 – November 11,2013,Padova

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 2.856 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

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

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 : 2.856GHz -Total length : 300cm -Peak field in SW : 60MV/m -Charge : 1n X-Band Main parameters: -Frequency : 11.424GHz -Total length : 75cm -Peak field in SW : 240MV/m 200MV/m -Charge : 250pC Scaling factor 2.856GHz 11.424 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

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

 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

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

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

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

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)

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

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

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 11.5 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

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

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

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

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

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

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

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

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

11.424 RF Hybrid gun SW TW EpSW / <E>TW = 4 rf power in rf power out 11.424 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

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

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 A/m @ 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

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

CSN5 – November 11,2013,Padova

Thermal analysis for 20MW power input RF and temperature on outer surface 40 ˚C ΔTmax = 0.185 ˚ 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

11.424 GHz hybrid gun Prototype under test (with untuned device and no brazing). Mode frequency is ~7 MHz off !! 11.424 GHz hybrid photoinjector longitudinal electric field profile (with untuned device and no brazing) . Mode frequency is ~7 MHz off 11.424 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

Thank you very much for your attention !!!