Degradation and Recovery of Cavity Performance in compact-ERL Injector Cryomodule at KEK Eiji Kako (KEK, Japan) 2012, April 2012, Sept. Eiji Kako (KEK, Japan) Dirty SRF WS at HZB, Sept. 14, 2017

Injector-Linac section of compact-ERL CW RF Power Source Injector-Linac Section Injector Cryomodule Eiji Kako (KEK, Japan) Dirty SRF WS at HZB, Sept. 14, 2017

History of cool-down cycles in injector cryomodule ~0.3 mA Stable beam operation ~10 mA ~100 mA Observation of severe x-ray radiation ~1 mA Trial of high RF power pulsed conditioning ~40 pC Eiji Kako (KEK, Japan) Dirty SRF WS at HZB, Sept. 14, 2017

Stable beam operation at Vc = 5 MV in May-July, 2013 In this operation, cryogenic system was stopped during night/week-end.  natural warm-up. Presently, only 2K pumping system was stopped during night/week-end.  keep at 4.2 K. Specification: ave. Eacc = 7 MV/m total Vc = 5 MV Eiji Kako (KEK, Japan) Dirty SRF WS at HZB, Sept. 14, 2017

Change of x-ray radiation levels in 2013 - 2017 Return to previous level after warm-up Severe increase No change after warm-up Gradual increase after pulsed RF conditioning after pulsed RF conditioning Initial level Eiji Kako (KEK, Japan) Dirty SRF WS at HZB, Sept. 14, 2017

Change of x-ray radiation levels in three cavities DC-Gun side (clean environment) (by individual cavity operation) Photo cathode of Gun Faraday Cup Gate Valve Stopped at moving contamination after pulsed RF conditioning Acceleration of F.E. electrons by Cavity-1 moving contamination Acceleration of F.E. electrons by Cavity-2 after pulsed RF conditioning Main sources of field emission locates in Cavity-3 severe degradation Merger side (dirty environment) degradation start after pulsed RF conditioning Eiji Kako (KEK, Japan) Dirty SRF WS at HZB, Sept. 14, 2017

Unexpected vacuum discharge phenomenon (1) Many bursts of vacuum pressure Gradual increase of x-ray radiation level Vacuum discharge caused by charging-up of Faraday-cup due to field emitted electrons come from injector cavities. Eiji Kako (KEK, Japan) Dirty SRF WS at HZB, Sept. 14, 2017

Unexpected vacuum discharge phenomenon (2) Field emitted electrons from Cavity-1 hit the photocathode. Then, secondary electrons extracted by applied DC voltage of 400 kV, and furtherly accelerated by injector cavity. Finally, high energy electrons collided with a screen monitor and radiate x-rays. Eiji Kako (KEK, Japan) Dirty SRF WS at HZB, Sept. 14, 2017

Quench detection : QL-interlock < 0.8 x QL (normal) High RF power pulsed conditioning CW 25 kW Kly. CW 300 kW Klystron Quench detection : QL-interlock < 0.8 x QL (normal) Eiji Kako (KEK, Japan) Dirty SRF WS at HZB, Sept. 14, 2017

Performance recovery by pulsed RF conditioning Vc= 5MV Vc= 5MV Vc= 3MV Vc= 3MV before before after after Vc= 5MV before Vc= 5MV before Vc= 3MV Vc= 3MV after after Heat load at 2 K was dramatically reduced at 5 MV. Eiji Kako (KEK, Japan) Dirty SRF WS at HZB, Sept. 14, 2017