Jlab CMTF Fast Cooldown Naeem Huque Jefferson Lab

Introduction • Rapid cooling of cavities through the Nb transition temperature (9. 25 K) has been shown to reduce remnant magnetic fields and improve Q 0 • The Cryogenic Test Facility (CTF) which supplies the Cryomodule Test Facility (CMTF) is temporarily reconfigured to increase helium flow to LCLS-II cryomodules • Flux gate jumps are used to determine the points at which the cavities go through the transition temperature • Diodes on the HOM Couplers are used to determine cooling rate at this point. The cooling rate of the cavities should be higher • Diodes on the helium vessels are not reliable at the moment

2 K Return 5 K, 3. 0 ATM LCLS-II Cryo Lines in CMTF 2 K – 4 K Heat Exchanger LCLS-II Cryomodule 2 K Supply 5 K Shield 50 K Shield

CTF Limitations • CTF and CMTF designed for CEBAF cryomodules and adapted for use with LCLS-II cryomodules • CEBAF Cryomodules only have 50 K shield and 2 K line; LCLS-II Cryomodules have additional 5 K Shield • Adaptation pushes the limits of the facility • Total CTF capacity is 100 g/s of helium gas, not all of which is useable • Capacity is reduced by pressure drop in subcoolers, heat exchangers and valve box flow meter • Cold Box 3 capacity is 20 g/s steady state, which is split between the CMTF and the Vertical Test Area (VTA) • Recovery system is limited to 13 g/s

CTF Reconfiguration Flex U-Tube Recovery Cold Box 3 Dewar Valve Box Compressors Flex U-Tube Cryomodule 2 K – 4 K Hx Junction Box • • Bypassing the Valve Box avoids the pressure drop associated with the subcooler within, allowing more flow into the module Pressure was originally increased to 5. 0 atm, and then to 6. 0 atm on the final run

Magnetic Flux Gates • Bartington® Instruments Mag-F fluxgates are fitted between the magnetic shields on cavities 1, 2, 5, 7, and 8 • Flux gates are parallel to the cavity axis and close to the vertical plane containing the cavity axis S. K. Chandrasekaran et. al, MAGNETIC FIELD MANAGEMENT IN LCLS-II 1. 3 GHZ CRYOMODULES, LINAC 2016, East Lansing, MI, USA

Fast Cooldown Results Inlet Pressure Cavity 5 Field Delta (m. G) Cooling Rate of Cavity 5 HOM (K/min) CM 04 5 atm 0 1 CM 05 Run 1 5 atm 0. 03 1 CM 05 Run 2 5 atm 0. 03 2 CM 05 Run 3 6 atm 0. 04 2. 7 Notes CTF stalled at 10 K

CM 05 Run 3 Fast Cooldown Transition Cooling Rates: 2. 5 – 3. 0 K/min

Summary • A maximum cooling rate of 3. 0 K/min (Cavity 1, CM 05 Run 3) was achieved from the fast cooldown • No change in Q 0 values were observed after fast cooldown. However, Q 0 values were originally close to VTA values. • The cavity cooling rate should be higher, as the HOM Couplers are conductively cooled • Fastest cooldown was achieved using 6. 0 atm at the inlet of the 4. 5 K line, and without using the vaporizer bypass • Flow rates are unreliable, but it is estimated that the rate was 30 – 40 g/s (from CTF venturi and CMTF flow meter) • Future work includes obtaining helium vessel temperatures and accurate flow rates