Experience with the 112 MHz 4K SRF Gun for CeC PoP at RHIC Kevin Smith TTC Meeting at TRIUMF February 6, 2019

Presentation Outline A Mercifully Brief Overview of CeC PoP Experiment at RHIC (BNL) A Less Brief Overview of the 112 MHz SRF Gun Structure The Main Operational Challenges and Solutions Thereto Numerous and Persistent Strong Multipacting Barriers Strong Field Emission Extremely Brief (No) Summary in the Interest of Brevity

Overview of the CeC PoP Expermient Bunching RF cavities Low energy transport beam-line with 5 solenoids Dog-leg: 3 dipoles 6 quads 13.5 MeV SRF linac Low power Beam dump 1.05 MV SRF Gun CeC modulator 4 quads Common section with RHIC CeC “kicker” High power beam dump 2 dipoles CeC FEL amplifier 3 helical wigglers The CeC PoP (Coherent electron Cooling Proof of Principle) Experiment at RHIC is a novel, bunched beam cooling experiment at RHIC. PI is V. Litvinenko. Goal: Strong hadron cooling for a high energy Electron-Ion Collider. In essence, an extremely wideband stochastic cooling system using an FEL.

Overview of the CeC PoP Expermient Bunching RF cavities Low energy transport beam-line with 5 solenoids Dog-leg: 3 dipoles 6 quads 13.5 MeV SRF linac Low power Beam dump 1.05 MV SRF Gun CeC modulator 4 quads Common section with RHIC CeC “kicker” High power beam dump 2 dipoles CeC FEL amplifier 3 helical wigglers The CeC PoP (Coherent electron Cooling Proof of Principle) Experiment at RHIC is a novel, bunched beam cooling experiment at RHIC. PI is V. Litvinenko. Goal: Strong hadron cooling for high energy Electron-Ion Collider. In essence, an extremely wideband stochastic cooling system using an FEL. Two SRF systems: 112 MHz ¼ Wave Photocathode Gun (4K), Design: 2 MV, Operation: 1.1 MV Dark Current Limit) LHe tap from a RHIC “DX” magnet with local 4K Quiet LHe source at the cryomodule 704 MHz 5-Cell Elliptical Linac Cavity (2K), Design: 20 MV, Operation: 13.5 MV (Quench Limit) Shares 112 MHz LHe tap from RHIC. Local booster pumps for 2K operation Integrated 2K Lhe heat exchanger design for noise isolation Both systems share local compressor for return of warm He gas to RHIC plant.

CeC PoP 4K and 2K LHe Systems

112 MHz ¼ Wave Photocathode Gun Before I talk about the Cavity behavior and experiences … Excellent results as a Photocathode Gun World record bunch charge for an SRF Gun 10.7 nC per bunch maximum achieved Record low normalized emittance: 0.32 mm mrad at 0.5 nC QE lifetime from one to two months Room temperature water cooled cathode (i.e. not cold) Requires automatic He blowout system in case of water flow failure

112 MHz ¼ Wave Photocathode Gun Field Probe RF coupling to Field Probe is via this coaxial structure. Qext depends on the position of the cathode stalk.

112 MHz ¼ Wave Photocathode Gun RF coupling to Field Probe is via this coaxial structure. Qext depends on the position of the cathode stalk.

112 MHz ¼ Wave Photocathode Gun FPC Structure The coaxial FPC also acts as the cavity tuner, and thus Qext and fres are coupled. BW1/2 at Qext = 1E7 is 5.6 Hz.

Multipacting was the Most Persistent Issue The cavity exhibits several hard MP levels at 2kV, 22 kV, 30 kV, and 40 kV. Others at higher voltages in FPC and Cathode Stalk structures. This is life in a system with a multiplicity of coaxial structures. These proved to be difficult to condition in general, and impossible to condition with a live (CsK2Sb) photocathode inserted. Cathode QE will be annihilated by MP conditioning – extremely sensitive to vacuum excursions. Once MP occurs, it often “hardens” the MP barrier, requiring some “rest” period prior to attempting turn on again. 10 min to overnight, depending on “integrated” MP activity. Jumping over (through) MP requires strong coupling (low Qext) and maximum RF power. But if the MP barrier holds, strong vacuum activity and “hardening” of the barrier. Operating voltage (~ 1.1 MV CW) requires med-high Qext due to RF power limitation (4kW – originally 1kW) and FPC power dissipation. Susceptible to dropping into MP at or on way up to operating voltage. Solution LLRF system employs a combination of PLL (for turn on) and GDR modes (for regulated operation). Provides a “straight forward” way to accommodate the coupling between Qext and fres. Add LLRF “trap” to detect MP and cut drive to avoid prolonged MP. Automated script to coordinate turn on through to operating point.

Example of Cavity Turn On Attempt with Strong MP Four repeated attempts to turn on result in getting stuck at 22 kV MP barrier. Attempts last only 20ms, controlled by LLRF MP trap code. Prevents significant energy deposition => vacuum activity which would kill cathode QE. Successful jump through 22 kV MP barrier. Lengthen period between attempts from ~ 20 min to ~ 40 min => 5th attempt = successful turn on. Cathode QE not impacted by turn on attempts as MP related vacuum activity is kept minimal. Failure to achieve voltage in 20 ms results in turn off of drive. 1 kV turn on (2.3 kV MP level just above) to allow PLL to lock on to cavity resonance.

Multipacting Well Studied and Understood Now Irina Petrushina (PhD student of V. Litvinenko) did a very thorough study of multipacting in the structure, with modeling and simulations that matched very well with observed barriers. Employed CST, MultP-M, ACE3P – Track3P (best approach). Also developed an equivalent circuit model integrating the resonant multipacting avalanche buildup, which provides reasonably quantitative predictions of RF power and coupling required to punch through a given MP barrier. 28 kV MP 40 kV MP

Multipacting Well Studied and Understood Now

Field Emission Limits Operating Voltage due to Dark Current Strong field emission which has required multiple (successful) rounds of pulsed conditioning and He conditioning. Why? Cavity was unquestionably contaminated prior to and/or during installation. General cavity and beamline installations were not up modern “SRF Clean” standard. Multiple insertions and retractions of a cathode structure which makes metal-metal contact “near” the SRF active surfaces likely do not help maintain clean conditions. It is not clear (to me at least) whether any active cathode material has contaminated the Nb surface(s). It is hypothesized that it has contributed to MP behavior. Result: Can’t operationally run cavity higher than about 1.1 MV CW (design is 2.0 MV). Fortunately the He return compressor can handle ~ 10 g/s, so we can push hard when attempting to condition (we’ve pegged the mass flow gauge at 10 g/s). In general, if field emission becomes excessive at 1.0MV, we can recondition and gain substantial improvement within a shift or two of effort. System has a permanent He processing system attached and available when needed. Operation at 1.0 MV – 1.1 MV has been sufficient to produce very high charge, high quality bunches. ? ?

Field Emission Change from End of Run to Start of Next Run Typical x-ray level at end of Run 17 (June 2017) was about 300 mRem/hr at 1.05 MV. Typical x-ray level at start of Run 18 (Feb 2018) was about 2000 mRem/hr at 0.9 MV. Also, lots of cathode stalk activity during the shutdown. Doesn’t help to vent beamline just downstream. Component replacement. We lose LHe when RHIC cryo shuts down. Warmup redistributes adsorbed gases.

Field Emission Processing – 40ms, 25ms, 1Hz pulsing w/ He 40 ms pulses 20 ms pulses ~ 1.2 Rem/hr at 1.7 MVpk, roughly 2-3 % duty cycle (w.r.t. peak voltage, peak x-ray) 40 ms pulse

Field Emission HPPP w/ Helium Processing Results After processing Before processing

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