1. Operational Experience with the Cornell ERL Injector SRF Cavities
Bruce Dunham
Cornell University
June 12, 2013

2. Bruce Dunham – TTC Topical Meeting on CW-SRF 2013
Outline
The ERL Injector
RF Power Systems
Operational results at high average current
What’s next?
Bruce Dunham – TTC Topical Meeting on CW-SRF 2013

3. Bruce Dunham – TTC Topical Meeting on CW-SRF 2013
The ERL Injector
Requirements:
5-15 MeV
77 pC per bunch
100 mA average current
0.3 um emittance
(normalized rms)
Status:
Emittance goals met, simulations verified
75 mA maximum average current
63 hour 1/e cathode lifetime at 65 mA
Max energy 13 MeV
Bruce Dunham – TTC Topical Meeting on CW-SRF 2013

4. Bruce Dunham – TTC Topical Meeting on CW-SRF 2013
Injector Cryomodule
Complete cryomodule
5X 2-cell cavities (made in house)
HOM loads with RF absorbing tiles
Couplers (made by industry)
Bruce Dunham – TTC Topical Meeting on CW-SRF 2013

5. Bruce Dunham – TTC Topical Meeting on CW-SRF 2013
RF Power System
RF System:
6-130 kW e2V CW klystrons
(5 cavities plus 1 deflector)
1 CW IOT for the buncher
500 kW RF power for the beam
Bruce Dunham – TTC Topical Meeting on CW-SRF 2013

6. Bruce Dunham – TTC Topical Meeting on CW-SRF 2013
Recent Results
Ran 65 mA average current at 4 MeV for 8 hours – 2000 Coulombs of charge. Had 2 RF coupler trips and 2 spurious trips.
Cathode lifetime (NaK2Sb cathode) of 63 hours (1/e). With the available laser power, 1 week of uninterrupted operation is possible
Bruce Dunham – TTC Topical Meeting on CW-SRF 2013

7. Bruce Dunham – TTC Topical Meeting on CW-SRF 2013
Coupler Adjustments
Couplers are adjusted to provide ~0 reflection at the desired current.
Beam Current (0-65 mA)
Klystron reflected power (kW)
All 5 cavities
Bruce Dunham – TTC Topical Meeting on CW-SRF 2013

8. Bruce Dunham – TTC Topical Meeting on CW-SRF 2013
Do the HOM’s get hot?
Beam current (blue)
0.5 K
HOM temperature (green)
At most 0.5K temperature rise (65 mA, 4 MeV, 2-3 ps rms bunch length)
Bruce Dunham – TTC Topical Meeting on CW-SRF 2013

9. Bruce Dunham – TTC Topical Meeting on CW-SRF 2013
Issues
Coupler conditioning (with beam)
Coupler quadrupole fields –> beam asymmetry
HOM RF absorbing tiles*
Coupler cooling*
Spurious trips, machine protection*
Some of the RF absorbing tiles facing the beam became insulating at 80 K and distorted the beam. We removed half of the tiles, which still provides adequate HOM damping.
Beam distortion due to tile charging
Bruce Dunham – TTC Topical Meeting on CW-SRF 2013

10. Bruce Dunham – TTC Topical Meeting on CW-SRF 2013
Excessive Coupler Heating
We found that the couplers get quite warm during high power operations.
The flow to the HOM’s and couplers are in parallel, but we discovered the coupler tubing is too small. So the HOM’s get more flow, and the couplers less than designed for.
Temperatures reach equilibrium in 4 hours
Bruce Dunham – TTC Topical Meeting on CW-SRF 2013

11. Bruce Dunham – TTC Topical Meeting on CW-SRF 2013
What trips first?
In this case, fast transient recording shows that the RF trips first.
In this case, the beam trips first.
Machine protection is extremely critical for high power operation. We need to detect a problem and block the laser with a few usec to prevent damage to the beamline, cavities and cathode.
We still see spurious trips that we cannot identify
Bruce Dunham – TTC Topical Meeting on CW-SRF 2013

12. Bruce Dunham – TTC Topical Meeting on CW-SRF 2013
What’s Next?
Summer:
Install the 7-cell cavity (horizontal test cryostat) in the injector
Fall-Spring:
Measure higher-order cavity modes using the beam*
Transport high average current (7-cell cavity off) to test new HOM loads
Continue to push current up towards 100 mA
*following N. Baboi, ‘Studies on Higher Order Modes in Accelerating Structures for Linear Colliders”
Bruce Dunham – TTC Topical Meeting on CW-SRF 2013

13. This work is supported by the National Science Foundation grant
DMR
Bruce Dunham – TTC Topical Meeting on CW-SRF 2013