TESLA Technology Collaboration TTC Meeting Milano 6 9

TESLA Technology Collaboration (TTC) Meeting, Milano, 6 -9 February 2018. Hot-topic: Performance recovery: Cavity, Modules, Linacs Multipacting and conditioning cases for EU-XFEL Denis Kostin, DESY February 7 th, 2018

Multipacting and conditioning cases for EU-XFEL Denis Kostin, DESY. Outline Cavity Multipacting and conditioning cases for EU-XFEL Conclusion TTC meeting, Milano, February 7 th, 2018 2

Multipacting and conditioning cases for EU-XFEL Denis Kostin, DESY. Cavity Multipacting resonant RF discharge and breakdown TTC meeting, Milano, February 7 th, 2018 Qload normal RF pulse breakdown MP occurs in a TESLA-type cavity at accelerating gradients of 18. . 25 MV/m. MP depends heavily on cavity surface (SEY). 3

Multipacting and conditioning cases for EU-XFEL Denis Kostin, DESY. TTC meeting, Milano, February 7 th, 2018 4 Multipacting and conditioning cases for EU-XFEL [1] EU-XFEL RF Station (4 modules) MP conditioning log example (A 22). MP conditioning starts with first cavity to reach 18. . 19 MV/m and having MP. Conditioning takes from 15 min. up to 1. . 2 hours (30 min. in this case). Conditioning is done with shorter RF pulse to decrease the cryogenic load. MP conditioning is a sequence of cavities breakdowns with increasing gradient and different cavities in an RF station. Final limit is given by not conditionable breakdown in a certain cavity, usually defined by waveguide RF power distribution.

Multipacting and conditioning cases for EU-XFEL Denis Kostin, DESY. TTC meeting, Milano, February 7 th, 2018 Multipacting and conditioning cases for EU-XFEL [2] MP statistics for EU-XFEL 5

Multipacting and conditioning cases for EU-XFEL Denis Kostin, DESY. TTC meeting, Milano, February 7 th, 2018 Multipacting and conditioning cases for EU-XFEL [3] MP statistics for EU-XFEL MP cavities additionally sorted by module number in RF station (1. . 4) and by cavity number in a module (1. . 8). 6

Multipacting and conditioning cases for EU-XFEL Denis Kostin, DESY. TTC meeting, Milano, February 7 th, 2018 Conclusions Multipacting (MP) effect, a resonant discharge causing bursts of field emission and a cavity breakdown is well known and studied for the TESLA-type (or EU-XFEL type) SRF cavity; MP depends on cavity surface properties, mostly on the Secondary Electron Yield (SEY) and it can be conditioned by a sequence of a controlled cavity breakdowns (MP discharges) with increasing accelerating gradient value. RF conditioning changes cavity surface properties (SEY); Our experience shows, that venting a cavity vacuum system (with a dry nitrogen gas) mostly resets the conditioning and it must be repeated upon pumping and cooling down; 16. 5% out of 704 EU-XFEL cavities (88 modules) tested and taken into operation with the RF stations A 2. . A 23 in the EU-XFEL linac tunnel did show a MP effect and did need the RF conditioning (on-resonance). All 116 cavities MP was successfully conditioned reaching the cavities final breakdown limits (given by RF power distribution). MP conditioning took 0. 5. . 2 hours per RF station (4 modules with 32 SRF cavities). 7