presented by Bruno Spataro on behalf of the SALAF team. Overview of X-Band Activities at INFN X-Band Structures, Beam Dynamics and Sources Workshop Cockcroft Institute November 30th –3rd December 2010

Contributors This work is made possible by the efforts : SALAF Group, INFN - LNF V. Dolgaschev, S. Tantawi, SLAC Y. Yigashi, KEK A. Mostacci, L. Palumbo, University of Roma 1 J. Rosenzweig et. al., UCLA R. Parodi, INFN-Genova M.G. Grimaldi et. al., University of Catania

SUMMARY Design and characterization of a  mode section at GHz Design and characterization of a  /2 mode section at GHz Technological activity status on electroforming, molybdenum sputtering and soft bonding

SALAF (L inear A cceleranting S tructures at H igh F requency ) is the INFN r&d programm on “ multicell resonating structures” operating at X-band (10 ÷ 12 GHz). To use in high brilliance photo-injectors (SPARC-phase-2) to compensate for the beam longitudinal phase-space distorsion, enhanced by the bunch compression of the acceleration process To gain know-how in vacuum microwave technologies the MOTIVATION ……. RF GUN RF compressor Traveling Wave accelerating structures X-band structure X-Band Structures, Beam Dynamics and Sources Workshop Cockcroft Institute November 30th –3rd December 2010 Basic layout of the SPARC Linac

r 1 = 1mm p h r2r2 p = mm h= 2 mm r 2 = 4 mm Structure with coupling tubes r = mm (End Cell) r = mm (Central Cells) r = mm (End Cell) Study and simulation of a 9-cell SW π-mode X-band structure p h r2r2 Symmetry planes p = mm h = 2 mm r 2 = 4 mm Structure with no coupling tubes r = mm  r 2 / = 0.15 X-Band Structures, Beam Dynamics and Sources Workshop Cockcroft Institute November 30th –3rd December 2010

With beam-tubes and reduced end-cells radius flatness on-axis of the longitudinal E-field  … simulation of 9-cell  -mode …. With beam-tubes and constant cavity radius no flatness on-axis of the longitudinal E-field  Z (cm) Ez (MV/m) Ez

structure with mirrors Frequency [MHz] Mode [  ] / / / / / / / Frequency [MHz] Mode [  ] / / / / / / / / structure with tubes DISPERSION CURVE with and without beam-tubes h = 2 mm … simulation of 9-cell  -mode …. K = 2.42 % Coupling coefficient X-Band Structures, Beam Dynamics and Sources Workshop Cockcroft Institute November 30th –3rd December 2010 ×10 6

SINGLE ELEMENTS ASSEMBLED X-BAND MODEL TAPERED COUPLER RF INPUT SLOT TIGHTENING BARS TUNING RODS Construction of a π-mode Standing-Wave 11.4 GHz copper model NIM A 554 (2005) ; LNF (2003) X-Band Structures, Beam Dynamics and Sources Workshop Cockcroft Institute November 30th –3rd December 2010 π-mode Cu model after brazing cavity Ag-Cu alloy location vacuum tight electrical contact

DETECTION OF THE FUNDAMENTAL MODE RESONANCES BY THE INPUT COUPLER X-Band Structures, Beam Dynamics and Sources Workshop Cockcroft Institute November 30th –3rd December 2010 HFSSSuperfish Before brazing After brazing f0f Q0Q LONGITUDINAL INDUCED MODES  -mode GHz Network Analyzer s21 Transmission coefficient lateral probe-lateral probe Frequency (Hz)  -mode GHz INPUT COUPLER INDUCED MODES 22 MHz Network Analyzer s11 Frequency (Hz) Dispersion Curve Before-After Brazing

FIELD FLATNESS ± 1% π-mode Cu model RF measurements NIM A 554 (2005) 1-12  -mode ACCELERATING ELECTRIC FIELD BEHAVIOR AFTER the 9-CELL TUNING E 2 /E M Normalized longitudinal field profile Length (arb. Units)

A  /2 biperiodic cavity: technical design Accelerating cell Axial Coupling cell Tuners RF probe location The structure is designed for brazing X-Band Structures, Beam Dynamics and Sources Workshop Cockcroft Institute November 30th –3rd December 2010 A  /2 biperiodic cavity: 17 cells copper prototype NIM: A 586 (2008 p = mm h = 2 mm r 2 = 4 mm gap (coupling cell ) = 1 mm

Real structure with coupling tubes l c.c. = 1 mm P = mm r 1 = 1 mm r = mm (End Cell) r = mm (End Cell) r = mm (Central Cells) 4 mm t = 2 mm Simmetry planes r c.c. = mm Structure with closed stop-band r = mm  X-Band Structures, Beam Dynamics and Sources Workshop Cockcroft Institute November 30th –3rd December 2010

Frequency (GHz) Mode (rad) dispersion curve with and without beam tubes The frequencies separation between the operating  /2 frequency and the adjacent ones frequencies is about given by ΔF= 39MHz and ΔF= 36MHz against the operating mode bandwidth ΔF= 1.6 MHz. From the spacing of the lower and upper cut-off frequencies, the coupling coefficient is given by K = 3.6%.

Copper prototype (  /2 mode) HFSSSuperfish Meas. (R sh /L)/Q 0 [  /m] (200) X-Band Structures, Beam Dynamics and Sources Workshop Cockcroft Institute November 30th –3rd December 2010 Field Flatness  2.5% Field profile measurement Field profile simulation vs measurements z axis (cm)  Dispersion curve Coupler feeding End cells antennas

Bi-Periodic StopBand Copper prototype: dispersion diagram  /2 mode Stop Band ~ 1.6 GHz (meas.) Other

 -MODE COPPER PROTOTYPE MAIN PARAMETERS  - mode frequency GHz Form factor r/Q (  /m) 9400 ( 9165 ) Unloaded Q8000 ( 8413 ) External Q7900 E-Field flatness ± 1 % Number of cells9 Structure length110 mm  /2 - MODE COPPER PROTOTYPE MAIN PARAMETERS  2  - mode frequency GHz Form factor r/Q (  /m) 9150 ( 9452 ) Unloaded Q6850 ( 7100 ) External Q6910 E-Field flatness ± 2.5 % Number of cells (acc.)9 Structure length110 mm X-Band Structures, Beam Dynamics and Sources Workshop Cockcroft Institute November 30th –3rd December 2010 In red, the theoretical values In red, the theoretical values Average accel.field = 42 3MW peak power Peak surface electric field, E sur = 105 MV/m Peak surface electric field, E sur (MV/m) = 102 Power dissipation, P d = 2.45 KW/m Power dissipation, P d = 2.68 KW/m (assuming a duty cycle of ) (assuming and duty cycle of )

SW 1.6 Cell Gun Input Cell Emittance-Compensating Solenoids Cathode TW structure Input Port THE SW/TW HYBRID STRUCTURE  mode TW SW RF gun  mode Integrated accelerating section Integrated velocity bunching See talk and posters

1)Eliminate transient reflection associated with SW structures (especially needed for X-band); 2)Compactness: -simplicity (RF distribution system, etc.) -energy efficiency from TW section 3)Promising good beam dynamics in term of beam emittance and reachable bunch length (velocity bunching) 1) THE HYBRID STRUCTURE: ADVANTAGES

X-band device realisation issue How to improve the high power performance (e.g. discharge rate) ? R&D on materials R&D on fabrication techniques using materials with higher fusion temperature; Guidelines: avoiding the device heating at high temperature as done in conventional brazing R&D on material R&D on fabrication techniques Sintered Molybdenum (Bulk) Electroforming Soft Bonding Molybdenum sputtering on Copper

Copper and Molybdenum prototypes for the breakdown studies Cu brazed (the reference case) Photographs of the two X band cavities LNF Molybdenum brazed

Results of high-power test of the 3-cell standing wave structure performed by … “V.A. Dolgashev, SLAC” 30 October 2008 X-Band Structures, Beam Dynamics and Sources Workshop Cockcroft Institute November 30th –3rd December 2010 The COPPER model has been tested to SLAC for power testing. …….. 3-cell Copper –  mode - SW structure The model was designed to concentrate the RF field in the mid-cell to achieve high-gradient field, to investigate the discharge limits (V.A.Dolgaschev, SLAC) The Palladium-Copper-Silver (PALCUSIL) alloys were used with different composition (different melting points). The reference case...

3-cell Sintered Molybdenum Bulk  mode - SW structure The model was designed to concentrate the RF field in the mid-cell Higher Power tests of the brazed model have been carried out at SLAC (V. Dolgashev et al.) The PALCUSIL alloys were used for brazing Molybdenum-Molybdenum and Molybdenum-Stainless Steel joints. Machining with the ‘tungsten carbide’ tools Q 0 = 4800 (measured) Jim Lewandowski, SLAC, 1/14/09 Bad results !

Sintered Molybdenum (bulk) issue long time for machining the cavity 300 nm roughness using ‘tungsten carbide’ tools It is not easy to braze. It is likely to have a gas contamination and an uneven loading stress in the braze region (joints are not completely filled with alloy ).

“ Electroforming ” is a galvanotechnical process to fabricate a metal structure using electro-deposition of a metal (usually Copper) over a mandrel (usually Aluminum) in a plating bath of Cu-SO 4 + H 2 SO 4 (copper sulphate + sulphuric acid ) The Al-core is afterward chemically eliminated with NaOH (sodium hydroxide) treatment (for Al cores). Electroforming is a very attractive process, alternative to the brazing technology Mixed processes, like electroforming after cell manufacturing with standard techniques (Electroplating process), are under development, too. copper metal Electroforming R&D and Test Electroforming properties : The speed of plating process is ≈ 0.6 mm/day Dimensional tolerances: ± 2.5 µm Surface finishing: 150 ÷ 200 nm (to be improved, studies are in progress); High device reproducibility. B. Spataro, R&D on X-band Structures at LNF Basic scheme for the electroforming

B. Spataro, R&D on X-band Structures at LNF …… Electroforming R&D and Test Aluminium mandrel of the RF coupler and cell ready for the electroforming Electroformed RF coupler and cell 5 cells mandrel of a Mo-Cu structure Mo discs are already machined to be the iris of the electroformed cell Another view of the coupler mandrel is shown

…… Electroforming R&D and Test π mode Q 0 = 5406 Fundamental mode response of Cu-Mo electroformed structure RF cells after removing the Aluminium core with alkaline solution (sodium hydroxide NaOH). Cross section of a Mo-Cu electroformed structure. The Mo discs with an external ribs improve the mechanical properties. Next step: to improve the quality of the Cu surface altered by the alkaline solution by depositing silver on the core or using other methods.…to be investigated)

Q 0 = 5788 Electroformed First Cu-Zr Electroformed model after baking Electroforming: other materials The color is due to Oxidation effect

A 3 cell Cu OFHC structure, encapsulated by galvanoplastic procedure under vacuum leak test. From electroforming to elecroplating Cu encapsulated (electroplating) structure: measured  -mode field profiles by bead-pull technique. Q 0 = 7700 (measured) Higher Power tests of the model have been carried out at SLAC (V. Dolgashev et al.)

Experimental set up for the RF Magnetron Sputtering Power 60 W Vacuum level 4*10 -2 mbar Sputtering activities ongoing at LNF ….. Deposition rate about 0.5 nm/sec HUNZIGER COMPANY DEVICE Schematic diagram of a DC magnetron plasma source e

Aluminium dish treated with copperTwo euro cents covered with aluminium A titanium-steel screw covered with copper film Aluminium cylinder covered with gold Sputtering activities ongoing at LNF …..

AFM (Atomic Force Microscopy) shows the surface of a copper sample before molybdenum sputtering Roughness behaviour of the Sputtered molybdenum on a Copper sample AFM (Atomic Force Microscopy) image of deposited Molybdenum (100nm) on a copper sample by sputtering technique. The roughness of the film is comparable to that of the substrate. This indicates that the roughness is determined by the substrate.

The XPS (X ray Photoelectronic Spettroscopy) Depth Profiling technique using the PHI 5600Ci system is available at the unit of Genova of the INFN. The sputtering parameters are 1µA Argon Ion at 4Kev energy on a raster covering an area of 5x5mm centered on the monochromatic X Ray spot on the sample. Actually, measurement of the carbon concentration is affected by a strong error ( up to ~ 30% of the measured value) XPS (X ray Photoelectronic Spettroscopy) depth sensitivity is ~ 5-10 nm (depending by the analyzed materials). Chemical composition as function of the Depth Profile of the 300nm molybdenum film on a copper with a thermal treatment carried out by R. Parodi (INFN-Genova) Results are in good agreement with RBS measurements carried out at the Catania University (G.M. Grimaldi et al.) except for the carbon (much less) Mo Cu O2O2 C

Mo and O surface scattering contributions are reported as green labels. The Mo film contains oxygen. The deposited film is characterized by a Mo concentration lower than a pure Mo film with a 100 nm thick (~20 % reduction with respect to a pure Mo film). The electrical measurement using the Van der Pauw configuration, gives a resistivity of Ω cm by about two orders of magnitude higher compared to a pure Mo film with a 100nm thickness, a difference compatible with the presence of oxides in the Mo. RBS (Rutherford backscattering spectrometry) spectrum (black line) and simulation (red line) obtained on Mo film 130 nm thickness deposited on a 2 μm SiO 2 layer on top of a Si substrate (University of Catania). Mo film on a SiO 2 layer roughness is in the range of 1- 2 nm

Grain of powder zoom The study of the sputtering approach as function of the deposited material depth, thermal treatment, chemical composition, morphological properties is in progress. Micro-cracks investigations carried out with the Scanning Electron Microscope (SEM) on Copper dish machined at very low roughness (70 nm) sputtered with 600 nm of Molybdenum after a thermal treatment of 2 hours at 600 °C

Some SEM RESULTS as fuction of the temperature and depth profile Fig. 1 : Micro crack on Copper dish machined at very low roughness sputtered with 600nm of Molybdenum after a thermal treatment of 2 hours at 600 ° C. Fig. 2 : Copper dish machined at very low roughness sputtered with 300nm of Molybdenum after a thermal treatment of 2 hours at 600 °C. Fig. 3 : Copper dish machined at very low roughness sputtered with 600nm of Molybdenum after a thermal treatment of 2 hours at 300 °C. Fig. 4 : Copper dish machined at very low roughness sputtered with 300nm of Molybdenum after a thermal treatment of 2 hours at 300 °C.

Soft bonding 3 cells Cu prototype A Cu OFHC structure under vacuum leak test If contact surfaces are machined at a very low roughness (70nm), the thermal treatment after Sn deposition could be unnecessary. Vacuum tight very good has been obtained with a proper pressure applied to the structure with three bars By brazing like at temperature a little less than 230°C ( Sn melting point) we obtain a good mechanical structure stability. Some tests with copper OFHC remade with different shapes among the contact surfaces gave good results in term of helium vacuum leak Standard model should be realized with the soft bonding plus electroplating technique

Status of the R&D and future programs Two X-band structures (  and  /2 modes) have been characterized at low power RF; One  -mode 9-cells Cu section has been manufactured for higher power tests; Hybrid photo-injector at GHz (A. Valloni: talk on Thursday and poster in this workshop) Technological activity : a) R&D on sputtering method, soft bonding and new alloys with the SLAC, KEK, INFN/Genova, University of Catania collaboration; b) Production of a 3-cell standard prototype (combination of the soft brazing- electroforming- molybdenum sputtering) with the SLAC-KEK collaboration; c) Electron Beam Welding (EBW) activity with the SLAC-KEK collaboration; d) Power tests at SLAC (have been already carried out) in the frame of a M.O.U with INFN, on design, fabrication and test of X-band devices and high gradient power tests of innovative structures. X-Band Structures, Beam Dynamics and Sources Workshop Cockcroft Institute November 30th –3rd December 2010

Thank you very much for your attention !!!