1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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