1. Status and perspectives on X band structures at INFN-LNF
presented by Bruno Spataro
on behalf of DEMETRA experiment at INFN-LNF
HG workshop June 13-16,2017- Valencia, Spain

2. SUMMARY
Status on the X band structure : Molybdenum bulk (Mo), Electroforming and Electroplating structures
Welded X band structures with Electron Beam Welding (EBW) and Tungsten Inert Gas (TIG) methods
S and X Mode launchers for future RF photoinjectors
Some investigations on the Molybdenum (Mo) and Molybdenum trioxide (MoO3) films deposited on a thick Copper (Cu)
Collaborations : SLAC, KEK, UCLA (Los Angeles)
HG workshop June 13-16,2017- Valencia, Spain

3. To gain know-how in vacuum microwave technologies on the X and W band
DEMETRA (Dielectric and Metallic Radiofrequency Accelerator) is the INFN R&D program on ‘multicell resonant structures’ operating at X (10 ÷ 12) GHz and W band (100 GHz)
the MOTIVATION …….
The research project is devoted to the R&D of key components for existing accelerators and for the next generation of accelerators. New technologies are necessary to achieve the multi-TeV energies of future linear colliders, x-ray FELs, etc.
To gain know-how in vacuum microwave technologies on the X and W band
Approach :
To study new materials and manufacturing techniques including single and multi-layer surfaces with precision-controlled properties
HG workshop June 13-16,2017- Valencia, Spain

4. X-band device realization issue: guidelines
How to improve the high power perfomance (e.g. discharge rate) ?
Avoiding the device heating at high temperature as done in conventional brazing (or hard structures)
Using material with a higher fusion temperature
R&D on fabrication techniques
R&D on material
Electroplated and Electroforming
EBW and TIG Bonding
Open structures
Molybdenum coatings
Sintered Molybdenum (Bulk)
All above techniques can be transferred from X-band to W band
HG workshop June 13-16,2017- Valencia, Spain

5. Drawing of the three cells  mode X band structure (SW - 11
Drawing of the three cells  mode X band structure (SW GHz) for the breakdown studies
This structure was designed at SLAC in order to concentrate the RF field in the mid-cell for breakdown studies
Longitudinal field profile of the
operating π mode - SW structure
Mandrel sketch for the fabrication of the ‘hard’ structure using the electroforming technique
In order to facilitate the film deposition in the irises region a proper rounded shape has been adopted
Comeb - MLT Companies
HG workshop June 13-16,2017- Valencia, Spain

6. JINST, March 8, 2016 (COMEB and MLT Companies)
Status on the X band structure…..
Electroformed prototype of the three cells  mode X-Band structure
(COMEB and MLT Companies)
High power test have been carried out at SLAC by giving good results
JINST, March 8, 2016
The mandrel’s model for the electroforming
The structure’s mandrel with a 3 µm thick of Au’s coating
Fitting tests between mandrel
and CF flanges
Final structure after coating of Ni
with a 4 mm thick
Structure installed
for the cold RF tests
HG workshop June 13-16,2017- Valencia, Spain

7. X band Galaxie gun prototype (J. Rosenzweig - UCLA) with cooled irises
Status on the X band structure…..
X band Galaxie gun prototype (J. Rosenzweig - UCLA) with cooled irises
manufactured with the electroforming process
Tuners (2 mm diameter)
cooled irises
The tuners can be machined before electroforming or
with the EDM process
(COMEB & MLT Companies)
HG workshop June 13-16,2017- Valencia, Spain

8. Status on the X band structure…..
Three cells  mode X band electroplated structure
Electroplated structure
Field profile – π mode
A three cells Cu structure, encapsulated by galvanoplastic procedure under vacuum leak test
[NIM-A-657 (2011) ]
Cu encapsulated (electroplating) structure: measured -mode field profiles by bead-pull technique.Q0 = 7700 (measured)
The Cu cells are already machined
to make the electroplating structure
High power test have been carried out at SLAC by giving very promising results. Most likely this structure was improving during 150 ns experiment (V. Dolgashev et al.).
HG workshop June 13-16,2017- Valencia, Spain

9. Status on the X band structure…..
Three cells π mode - SW structures : Sintered Molybdenum bulk (brazed structure)
Three cells Sintered Molybdenum bulk brazed structure issue
Sintered Mo bulk
brazed structure
[NIM-A-657 (2011) ]
Field profile – π mode
Higher power tests of the brazed model have been carried out at SLAC (V. Dolgashev et al.). As a result:
~ ( ) nm roughness using
‘tungsten carbide’ tools; 3 tools have been used for machining the structure ( hard material )
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)
long time for machining the cavity (hard material)
HG workshop June 13-16,2017- Valencia, Spain

10. Assembled before welding
Perspectives on X band structures…..
Manufacturing Single Cell Standing Wave Accelerating cavity joined by Electron Beam Welding (EBW) for high power test at SLAC, 1C-SW-A2.75-T2.0-EBW-Cu-Frascati-#1
Before assembly
3D model
Assembled before
welding
Pre-welding measurements:
Excellent vacuum tightness
SLAC-INFN/LNF-Comeb
HG workshop June 13-16,2017- Valencia, Spain

11. Perspectives on X band structures…..
The first Electron Beam Welded (EBW) hard copper accelerating cavity 1C-SW-A2.75-T2.0-EBW-Cu-Frascati-#1: ready for high power for RF tests:
SLAC-INFN/LNF-Comeb
HG workshop June 13-16,2017- Valencia, Spain

12. High power tests will begin shortly at SLAC
Cold RF tests (after welding) carried out at SLAC
Perspectives on X band structures…..
S11 measurement of 1C-SW-A2.75-T2.0-EBW-Cu-Frascati-#1. The small resonance just larger in frequency of the  mode is from the mode launcher
High power tests will begin shortly at SLAC
RF parameters for 1C-SW-A2.75-T2.0-EBW-Cu-Frascati-#1
Bead pull results. Left : bead pull measured from center  mode Right : Bead pull measured from 50 KHz lower
Conclusion: the center frequency and field profile are close to the design values that tuning
was deemed unnecessary.

13. Next: tungsten inert gas (TIG) welded cavity prototype
Perspectives on X band structures…..
Next: tungsten inert gas (TIG) welded cavity prototype
TIG welded cavity prototype
The machining of the final structure for RF high power test is in progress, too
SLAC-INFN/LNF-Comeb
HG workshop June 13-16,2017- Valencia, Spain

14. Split structure ( A. Grudiev, V. Dolgashev, H. Zha)
Perspectives on X band structures…..
The authors propose a structure made out of two halves bonded together (as an example with Electron Beam Welding (EBW) or TIG (Tungsten Inert Gas) performed along the outside edges
Bonding surface
Beam axis
Cavity
Gap
Gap
Bonding surface
One half piece of the model
A sketch of a three-cell model
The main motivations for this work is to study the high gradient performance of accelerating structures made with novel manufacturing methods
At Frascati (INFN-LNF ) we scheduled to fabricate an accelerating structure milled out of two halves and welded together
HG workshop June 13-16,2017- Valencia, Spain

15. Split structure : dimensions and results of the preliminary design
Perspectives on X band structures…..
Split structure : dimensions and results of the preliminary design
0.5 mm gap
Longitudinal field profile of the operating π mode – SW structure
Simulations with a 1 mm gap have been carried out, too.
0.5 mm gap
Frequencies spectrum
G. Castorina INFN-LNF
HG workshop June 13-16,2017- Valencia, Spain

16. Perspectives on X band structures…..
Open structure acceleranting cavity at 110 GHz based on ‘closed’ version designed at SLAC : first studies (G. Torrisi, O. Leonardi, G. Sorbello et al.)
Tuning - Fields
Q=3002
freq= GHz
The machining of the structure is in progress at INFN-LNS (Catania- I). RF characterizations will be carry out at Rome (University La Sapienza)

17. Four Ports TM01 Mode Launcher for future RF photoinjectors
Perspectives on X band structures…..
Four Ports TM01 Mode Launcher for future RF photoinjectors
On axis coupling in order to reduce multipole modes of the gun and avoid couplers on cells
Four ports TM01 mode launcher to cancel dipolar and quadrupolar modes in the feeding structure itself.
Input Power
Input Power
Input Power
Beam output.
Circular waveguide at cut-off
Output Power to
RF Gun
Input Power
Input Power
G. Castorina INFN-LNF
HG workshop June 13-16,2017- Valencia, Spain

18. Perspectives on X band structures…..
Quadrupole free compact mode launchers – Power couplers for next generation of RF photoguns
S - Band
X - Band
300
600 (mm)
100
200 (mm)
Fields normalized to 200 MW input power
Peak Electric field: 21 MV/m
Peak Magnetic field: 80 kA/m
Fields normalized to 200 MW input power
Peak Electric field: 75 MV/m
Peak Magnetic field: 240 kA/m
Vector plot
of electric field
to cavity
G. Castorina INFN-LNF
HG workshop June 13-16,2017- Valencia, Spain

19. Perspectives on X band structures…..
TM01 mode launcher with 4 Ports J-Splitter - S11 Parameter – S and X Band
S Band
Power transmission
X Band
Power transmission
G. Castorina INFN-LNF
HG workshop June 13-16,2017- Valencia, Spain

20. Sputtering activities ongoing at LNF …..
Schematic diagram of a DC magnetron plasma source
Sputtering activities ongoing at LNF …..
HUNZIGER COMPANY DEVICE
Experimental set up for the RF Magnetron Sputtering
Power 60 W ; F =13.56 MHz Vacuum level 4*10-3 Torr
Deposition rate about
0.5 nm/sec
HG workshop June 13-16,2017- Valencia, Spain

21. AFM (Atomic Force Microscopy) shows the surface
From the Sintered Molybdenum (Bulk) to sputtering molybdenum studies at LNF...… for the roughness reduction
AFM (Atomic Force Microscopy) shows the surface
of a copper sample (70 nm thickness) before molybdenum sputtering
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
Mo coatings with an electrical conductivity comparable to that of copper can be obtained optimizing the thickness and the post-deposition annealing process : Chinese Physics C 36, (2012) and Chinese Physics C 37, (2013)], Surface&Coating Technology 261 (2015);
HG workshop June 13-16,2017- Valencia, Spain

22. Preliminary studies for the dark current reduction….
Perspectives on X band structures…..
First transport experiments performed on a series of thin films of Molybdenum trioxide (MoO3) grown on a thick Cu substrate (with a 20 nm roughness)
Electrical resistance measurements have been performed for different film thickness as function of temperature down to ~20 K.
Layout of the sample with the electrodes on the Molybdenum trioxide (MoO3 ) coated film
Photograph showing some evaporated samples of MoO3 on a thick Cu substrate. Due to the different thickness these films are characterized by different colours
HG workshop June 13-16,2017- Valencia, Spain

23. Molybdenum trioxide (MoO3) investigations
Perspectives on X band structures…..
Comparison of the electrical resistance vs. temperature for MoO3 films on copper with thickness of 300 nm and 500 nm (left); the same using a semilogaritmic scale for the 750 nm thick sample (right) that shows a semiconducting behavior with a very high resistance at low temperature (hundreds of MΩ)
It is noted that the MoO3 films with thickness of 500nm and 750nm show a semiconducting trend while in the 300nm MoO3 film the behaviour is even metallic
Additional investigations for comparing the electrical resistance vs. temperature for the Cu substrate, and the MoO3 films of 30 nm, 100 nm and 500 nm thickness
From the data comparison, the films with thickness below 500 nm show a metallic trend
Reflectivity measurements and Scanning Tunnelling Microscope confirm the metallic and semiconducting trend, too
Next steps:
To identify the chemical status of MoO3 film to probe the presence of different oxides and control the multiphase nature of these metallic films (by synchrotron radiation X -ray diffraction and X-ray absorption spectroscopy)
Laser ablation approach in order to deposit a 3-4 µm film
thickness
Work function measurements as function of the temperature
HG workshop June 13-16,2017- Valencia, Spain

24. Thank you very much for your attention
HG workshop June 13-16,2017- Valencia, Spain