RF power couplers vacuum issues

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

2. Main Topics • Introduction • Vacuum Issues • ClosingRemarks

3. IntroductionRF 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

4. IntroductionRF 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

5. IntroductionRF 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 !

6. Vacuum IssuesPerformances & 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 multipactingduring 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)

7. Vacuum IssuesMechanical 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

8. Vacuum IssuesMechanical 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

9. Vacuum IssuesReliability 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

10. Closing RemarksRecalls (1/2) Operating pressure defines the « type » of Vacuum Design

11. Closing RemarksRecalls (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)

12. Closing RemarksVacuum 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

13. 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…”