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

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

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

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

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

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

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

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

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

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

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

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

This work is supported by the National Science Foundation grant DMR-0807731 Bruce Dunham – TTC Topical Meeting on CW-SRF 2013