Page 1 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 High Gradient Preparation of a 30 GHz HDS-Thick structure 1.Physical arguments 2.High Pressure Rinsing (HPR) process 3.Assembly procedure 4.Future plans

Page 2 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 Physical arguments Breakdown mechanism Geometrical defect on the metallic surface Conductive or insulated contaminated particles Electronic emission by field effect Thermal effect Local desorption J > A/m 2 Explosion of the emitter site Formation of a localised plasma Breakdown Energy M. Luong, « Etude de l’émission électronique par effet de champ », PhD thesis, DAPNIA/SEA-97-01T, 1997 Perry B. Wilson, “A Theory for the RF Surface Field for Various Metals At the Destructive Breakdown Limit”, SLAC–PUB–12354, Feb 2007

Page 3 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 Proposed R&D plan « High gradient » preparation at CEA/Saclay of a copper accelerating structure and its environment (tanks, flanges, waveguides…) Remove the geometrical defects and surface impurities:  Chemical treatment  HPR with ultra pure water (100 bars) Avoid the contamination by metallic or insulated particles  HPR with ultra pure water (100 bars)  Assembly in a clean room  Sealing of the tank by RF window or vacuum valve We first focus our efforts on these points !

Page 4 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 Proposed R&D plan High power tests on the High Gradient Test Stand at CERN 100 MW RF power available during 70 ns / 50 Hz Comparison with an identical structure without cleaning (under test) Cu Mo Objectives: Reach the breakdown limit quicker ? Reduce the breakdown rate at a given gradient ? Increase the gradient limit ? Test P/C low J.A. Rodriguez et al.

Page 5 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 High Pressure Rinsing Nose Rotating plate Hollow rod HDS 30 GHz Structure Support 4 places Intermediate Support Teflon screw 25 bars 50 bars Very well adapted to quadrants!

Page 6 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 After different water pressures (25, 50, 100 bars) After thermal treatment at 790 °C  3D measurements at Saclay (Mitutoyo Euro_C-A9106), 100 points per iris, +/- 2.5 µm resolution No plastic deformation observed plastic deformation of the iris

Page 7 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/ Cleaning with a detergent solution in a ultrasonic cleaner for all the pieces Manual rinsing with pure water (1 MΩ.cm) Manual rinsing with ultra pure water (18 MΩ.cm) HPR of the 4 quadrants only Pressure 50 bars, ultra pure water (18 MΩ.cm) Drying under class 10 flux Assembly and sealing (vacuum or N2 pressure) Class 100 clean room Packing and transport to cern Assembly procedure High Gradient Test Stand

Page 8 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 Assembly of AS with dynamometric screwdriver at 5cm.kg and 2.97/3 mm balls Cooling system waveguides on AS and top plate Assembly in clean room Manipulation of AS and screws with gloves

Page 9 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 Equipped top flange on the tank Faraday cup Assembly in clean room Waveguide ports Last removal of dust and contaminated particles with pressurized air

Page 10 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 Present status The tank is tight and ready to be installed on the HGTS But !  High power tests results from the other HDS structure (same type but without cleaning):  Gradient deterioration after 5 hours of testing  Presence of burrs and melted sharp defects on matching iris We decide to disassembly our structure in order to remove the burrs with plastic wire We also want to improve the assembly technique of the AS (alignment with microscope), add some RF low power window and check the RF transmission at Saclay before sending the tank at CERN (need 30 GHz network analyser)

Page 11 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 Installation on the High Gradient Test Stand The S parameters through low power window will be checked again at CERN The tank will be opened by o The four waveguides to connect the RF high power network -> would need high power window or RF valve for 12 GHz klystron test stand o The bottom flange to connect the pumping system -> would need vacuum valve for 12 GHz klystron test stand The installation under a portable laminar flow hood would be very useful to limit the contamination on the 30 GHz HGTS

Page 12 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 Suggestions for other preparation techniques B. Visentin - M. Bruchon Thermal treatment 800 °C (remove Nb Hydrure from fabrication) HPR for field emission suppression Thermal treatment 1400°C + Ti coating (superconductive properties) Chemical polishing In situ Baking: 120°C / 48h Electro polishing Role of the structure geometry on the electropolishing process (effect of gravity, fluid velocity…)

Page 13 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 Conclusion We have started an R&D activity on surface treatment and preparation for a CLIC structure to try to improve the high gradient performances Some validations have been made like the pressure of the rinsing water and plastic deformation Recent backgrounds must be taken into account (Burrs removal) But we hope that the test of this structure is at a high level of priority (hopefully in March – April 2008 ?) Some propositions for the X-band klystron test stand have been made (window, valve) An overview of further R&D tasks based on superconductive cavities has been given But it needs a huge number of tries and needs almost a dedicated reliable test stand and/or a very high pulse repetition rate source (radar klystrons?)

Page 14 F. Peauger High Gradient preparation - Team Meeting – CERN – 12/12/07 Acknowledgments CEA Saclay P. Carbonnier, M. Bruchon, Y. Gasser, M. Luong, F. Orsini, A. Mosnier, N. Rouvière, B. Visentin Assembly, clean room, experience from superconductive cavity preparation JC. Barriere, A. Bougault 3D measurement of iris and SEM analysis CERN C. Achard, S. Calatroni, R. Fandos, F. Perret, G. Riddone, M. Taborelli, W. Wuensch, R. Zennaro, Procurement of AS, tank and accessories, help, recommendations, experience… Thank you for your attention !