RadiaBeam Technologies, Santa Monica CA 2nd European Advanced Accelerator Workshop (EAAC) 13-19 September 2015, Elba, Italy High-Gradient Normal-Conducting Radio-Frequency Photo-injector System for the STAR Project Luigi Faillace RadiaBeam Technologies, Santa Monica CA Monday, 14 September 2015

OUTLINE STAR project layout Radio-Frequency design of the photoinjector system Basic beam dynamics simulations Thermal/Stress Analysis Machining, Cold Test, Brazing and Tuning Modal Analysis of vibrations Conclusions 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

STAR LAYOUT Southern Europe Thomson Source for Applied Research at University of Calabria in Southern Italy. e-beam source RF Photoinjector Laser e-beam * from http://www.cnism.it 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

RF GUN DESIGN Input RF waveguide Operation frequency: 2.856 GHz, -mode 1.6 cell gun Single feed Race track geometry “z-coupling” and “fat-lip” coupling slots Elliptical coupling irises RF Pulse Length ~ 4μs and 100 Hz repetition rate Numeric codes for simulations: HFSS, Superfish. Laser ports simpler RF power system than the case of dual feed Avoid phase shift between the two input waves in the case of dual feed dummy waveguide to diminish dipole field To minimize quadrupole field To reduce H field, i.e. RF pulsed heating RF probe Dummy waveguide Beam axis To decrease surface electric field HFSS Model 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

MAIN RF PARAMETERS Resonant frequency f 2.856 GHz Mode separation f=f-f0 15.3 MHz Shunt Impedance Rshunt 30 MΩ/m (avg.) Unloaded Quality Factor Q0 14,350 Coupling factor β 2.0 Probe coupling -73 dB Peak E-field @ cathode 120MV/m (PRF=10.5 MW) Peak surface E-field 106MV/m 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

BASIC BEAM DYNAMICS SIMULATIONS Laser Reference particle E-field seen by the ref particle 5.5 MeV Energy gain of the ref particle 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

RF PULSED HEATING T = 46.8°C RF pulsed heating, due to surface magnetic field, causes a temperature gradient ΔT on the metal. ΔT is independent of the surface thickness and the cooling system. “Safe limit” in case of copper is about 110°C. Crucial areas are the laser ports and the coupling slots! “rounded irises” are used (1.5mm and 9mm radius). The peak surface magnetic field is nearly H||= 3.3*105 A/m @ input RF power = 10.5 MW Laser port assuming 4μs pulse T = 46.8°C Coupling Slot below the safety upper limit! (50° C, SLAC) tRF : pulse length  : electrical conductivity  : skin depth ’ : density c :specific heat k : thermal conductivity *D.P. Pritzkau, “RF Pulsed Heating”, SLAC-Report-577, Ph.D. Dissertation, Stanford University, 2001 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

MAGNETIC FIELD COMPONENTS DIPOLE AND QUADRUPOLE MAGNETIC FIELD COMPONENTS A dummy waveguide (higher cut-off frequency), symmetric to the RF input waveguide, allows to erase the field dipole component. The quadrupole component is eliminated by using a “race track” geometry. Higher order modes are considered negligible. RF input power D Cross section of the full cell. The field is calculated along circumferences with different radii r and for different values of the offset D, by which the two cell arcs are drifted apart. r Dummy waveguide 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

MAGNETIC FIELD COMPONENTS Fourier Series of the azimuthal component Hφ of the H-field nth component n=0 Hϕ0 (r) H0 (r)=Monopole n=1 Hϕ1 (r) H1(r)=Dipole n=2 Hϕ2 (r) H2(r)=Quadrupole The monopole component H0 is unaltered for all values of D The dipole component H1 ~ 0 is unaltered for all values of D The quadrupole component H2  0 for D = 7.92 mm D r 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

THERMAL ANALYSIS Thermal analysis of the RF Gun has been carried out by using Ansys13 (Coupling with HFSS) 53.3 °C RF Pulse length, FWHM 4 s Duty factor 4*10-4 Ppeak 10.5 MW Pavg ~4.2 kW Water flux < 3 l/min Water temperature 35 °C Ambient Lab temperature 22 °C Cooling channels Temperature distribution (°C)

STRESS ANALYSIS <50μm <30 MPa The deformation produces a frequency shift. In order to keep the gun on resonance at full average power (100 Hz and 4s), the water temperature will need to be decreased at max by -8°C (estimation and previous experience with the Fermi II gun) <50μm <30 MPa Total deformation (m) The Equivalent (von Mises) Stress is a measure of the yield strength (in terms of pressure Pa) of the metal, above which the material starts to deform plastically, i.e. non-reversible change of the shape. Max value is <30 MPa (yield strength of soft copper is 70MPa). Equivalent Stress distribution (Pa) 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

STAR PHOTOINJECTOR SYSTEM FULL ASSEMBLY CAD MODEL Emittance-compensation Solenoid RF Gun RF Gun Stand Photoinjector Stand 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

MODAL ANALYSIS OF THE STAR GUN AND STAND SUB-ASSEMBLY Modal analysis of vibrational sources performed with Ansys No natural frequencies below 20 Hz (this was a requirement) First and second resonance at 40.7 Hz and 68.5 Hz, respectively. 40.7 Hz 68.5 Hz 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

RF GUN MACHINING, COLD TEST AND BRAZING All the gun parts were machined in-house. Cold test measurements (scattering parameters, resonant frequency, other main RF parameters) carried out at SLAC by using a clamping setup. Hydrogen Brazing (followed by high temp bake-out) was performed by the brazing team at the SLAC klystron department. Machining Cold-Test Tuning Brazing 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

FREQUENCY/FIELD TUNING SYSTEM Ridge Bi-directional deformation tuners on half cell (2x) Bi-directional deformation tuners on full cell (2x)

RF GUN TUNING The tuning of the gun resonant frequency (≈500kHz after final brazing and high temperature bake-out) was carried out by using the ridge inside half cell and the bi-directional deformation tuners on full cell. 15.5 MHz Measurement HFSS Frequency (goal) 2.856 GHz (35 °C, vacuum) Frequency (tuning) 2.855735 GHz (24.6 °C, 0% humidity) Mode separation 15.5 MHz 15.3 MHz Q0 14,250 14,350 Coupling beta 1.95 2 RF probe coupling = -74 dB 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy

FIELD MEASUREMENTS On-axis electric field measured in a bead drop setup by using a 2mm diameter metallic bead Guiding plug (teflon disc with small hole in the center) Field balance, Ecathode:Efull cell = 1:1.01 (>99%) Bead-drop field measurement after final tuning Bead-drop setup during pre-tuning cold-test

CONCLUSIONS The design, machining, brazing and tuning of the whole photoinjector system for the STAR project was successfully carried out by RadiaBeam. Ready to be shipped to the University of Calabria for installation and high-power tests. Acknowledgements for the brazing of the RF gun: SLAC klystron group (Andrew Haase, John Van Pelt, Andy Nguyen, Lisa Bonetti and Michael Gonzales)

Thanks for your attention! 2nd European Advanced Accelerator Workshop (EAAC), 13-19 September 2015, Elba, Italy