RF power couplers vacuum issues J.M. Jimenez Vacuum, Surfaces and Coatings Group (TE-VSC)

Vacuum, Surfaces and Coatings (VSC) Group Technology Department (TE) J.M. Jimenez, March’10 Main Topics Introduction Vacuum Issues Closing Remarks

Vacuum, Surfaces and Coatings (VSC) Group Technology Department (TE) J.M. Jimenez, March’10 Introduction RF couplers as « seen » by Vacuum Experts (1/3) Shall allow for high power injection in RF cavities –Actively cooled  Demineralized water is commonly used Are installed in RF cavities… –Cleanliness and dust free assemblies  to avoid electron field emission and performance degradations (quenches, multipactor) …after being successfully tested in dedicated test benches –Design shall provide reliability of vacuum connections and components  Several mounting and dismounting

Vacuum, Surfaces and Coatings (VSC) Group Technology Department (TE) J.M. Jimenez, March’10 Introduction RF couplers as « seen » by Vacuum Experts (2/3) Shall provide good static vacuum –Design shall integrate the state-of-the-art in Vacuum engineering  Outgassing, diffusion, permeation, virtual leaks, leaks tightness, etc. are important issues … and good dynamic vacuum e.g. in presence of RF power –Needs to be bake-out  Mechanical design shall integrates this huge constraint Is often operated at cryogenic temperature on SC-RF cavities –Mechanical design shall allow for differential contraction of materials  Is often a source of complexity …and often needs to allow for an electrical polarization –Complex electrical insulation using ceramics  Fragility in particular at the connection and during bake outs

Vacuum, Surfaces and Coatings (VSC) Group Technology Department (TE) J.M. Jimenez, March’10 Introduction RF couplers as « seen » by Vacuum Experts (3/3) RF couplers are definitely complex and delicate systems from the Design point of view –Many technical reasons to “release” vacuum aspects to the second order during the design stage but –Wrong design will impact the testing / assembly phase Leaks, weaknesses appearing at the different stages of the assemblies will cost lot of time / money Can put the project into critical path –Defaults will be difficult to correct and will impact the machine reliability Don’t underestimate vacuum issues !

Vacuum, Surfaces and Coatings (VSC) Group Technology Department (TE) J.M. Jimenez, March’10 Vacuum Issues Performances & Instrumentation Vacuum performances –Outgassing, degassing Shall be minimized using appropriate treatments –Virtual leaks and real leaks Not acceptable –Surface treatments To prevent dynamic effects Vacuum instrumentation –Required to interlock multipacting during the RF commissioning and operation Use of reliable gauges (Penning) since can stand long cables, radiations, RF induced EMC noise and bake out Installed to be shielded from the coupler (electron collection)

Vacuum, Surfaces and Coatings (VSC) Group Technology Department (TE) J.M. Jimenez, March’10 Vacuum Issues Mechanical Design (1/2) Welds according to state-of-the-art for Vacuum applications –Post-welding cleaning is mandatory –Full penetrant or internal welds shall be used Porous and cracked welds are not allowed –Double welds shall have a continuous weld at the vacuum side and discontinuous at the atmosphere side Avoid trapped volumes and allow for future leak detections –Crossing welds shall be avoided completely Brazing is allowed if –Made under vacuum (if inside the vacuum enveloppe) –Use of brazing fluxes is strictly forbidden Avoid later corrosion leading to leaks –Brazing material shall stay compatible with bake out

Vacuum, Surfaces and Coatings (VSC) Group Technology Department (TE) J.M. Jimenez, March’10 Vacuum Issues Mechanical Design (2/2) Vacuum connections using flanges –Conflat type flanges are the most reliable Recommend to use forged 316 LN to avoid the smoothing of the knife after several bake-outs Grooves for leak detection are mandatory –Take care to the knife integrity during machining of the grooves! –Helicoflex gaskets are an alternative Surface preparation is essential –Shall not be used to cope with a design weakness Coated gaskets shall be used –Avoid sticking during bake outs

Vacuum, Surfaces and Coatings (VSC) Group Technology Department (TE) J.M. Jimenez, March’10 Vacuum Issues Reliability and future operation Avoid coolant/Vacuum “interfaces” –Cooling circuits shall not be in contact with Vacuum trough brazing and welds  Often lead to leaks which are Difficult to localize (continuous opening and closing of the leak leading to long cuts in the operation) Strongly impacting the operation of machines, in particular for the SC-RF systems were a warm-up is time consuming –Cooling circuits shall be “independent” to allow its drying for leak detections Design shall be optimized in view of cleaning and coatings –Make easier the removal and drying of the elements after the Vacuum cleaning or brazing preparation Traces of cleaning detergents degrade the coating performances –Allow for the better coating if no “screened” surfaces / configurations

Vacuum, Surfaces and Coatings (VSC) Group Technology Department (TE) J.M. Jimenez, March’10 Closing Remarks Recalls (1/2) Operating pressure defines the « type » of Vacuum Design

Vacuum, Surfaces and Coatings (VSC) Group Technology Department (TE) J.M. Jimenez, March’10 Closing Remarks Recalls (2/2) Performance depends more on the design and on material choices than on the pumping scheme –Pumping speed range from 1 l/s to few 1000 l/s –Outgassing of materials expand by 7-8 orders of magnitude –Vacuum design often predefine the ultimate pressure Increasing the pumping speed does not impact as expected on the pressure Selecting the Vacuum instrumentation depends on: –Pressure range –Local configuration –External constraints Radiation: only passive gauges can stand radiations Magnetic field and EMC noises: study in a case by case Cable length (case of most of the accelerators)

Vacuum, Surfaces and Coatings (VSC) Group Technology Department (TE) J.M. Jimenez, March’10 Closing Remarks Vacuum acceptance tests are mandatory At Design stage –Selection of materials, gaskets, assembly configurations –Integration issues to favor leak detection and bake out (space available…) During the production and assembly –Systematic checks of the production Vacuum leak tightness Vacuum cleaning issues Before installation or cryostating –Final check is strongly recommended – Always made in the past Procedures and travelers are mandatory –To prevent damages to the couplers: over heating, ensure a temperature gradients compatible with the dilatation coefficients of the different materials

Vacuum, Surfaces and Coatings (VSC) Group Technology Department (TE) J.M. Jimenez, March’10 Closing Remarks Already successful in the past… –Mechanical design –Reliability during operation …take that experience into consideration –Optimization is always desirable –Changes shall be discussed with Vacuum experts “We are here to help you to make your best choice…”