A year on from the previous nus2surf report on the High Voltage (HV) tests conducted in the DUNE 35-ton cryostat at Fermilab, scientists Alan Hahn and Sarah Lockwitz of Fermilab – along with dedicated collaborators from other DUNE institutions – have nearly completed the planned suite of three HV tests leading up to ProtoDUNE.
The first run in early 2016 highlighted a design problem near the cathode plane and led to a modification to the field cage for the second HV run in 2017. The second run also featured a beam plug between the field cage and a ground plane, designed for ProtoDUNE-SP to provide a less dense area for the particle beam to traverse as it enters the detector.
In this second run the team evaluated the individual components of the modified HV system (the field cage profiles, the resistive plate cathode and the ground planes), verified that the integrated components work together properly, and examined how the beam plug behaves when HV is applied to the detector.
LBL scientist Cheng-Ju Lin, Professor Jeff Nelson of the College of William and Mary, and Kansas State postdoc Heng-Ye Liao each spent considerable time at Fermilab over the past few months contributing to the third test. After another cycle of emptying the liquid argon, Nelson, Lin and Liao removed the beam plug and prepared the 35-ton cryostat for its final test prior to ProtoDUNE operation: testing the bare field cage under HV.
This test is nearly finished. Always subject to schedule constraints, the team has already determined that the performance without the beam plug is very similar to the performance with it, and that installation of the beam plug does not affect the maximum voltage that can be applied to the FC. They sent the beam plug to CERN – on schedule – where Lin will install it again, this time in the ProtoDUNE-SP cryostat.
“The 35-ton cryostat is not the easiest environment to work in,” said Lockwitz, “but if a meter-type scale is needed, it’s what we have. Plus, the setup now consistently gets 6-plus millisecond lifetimes, thanks to some clever work by Alan. Showing that there’s not an inherent cap of 3 ms in membrane cryostats is a huge achievement — any gain in purity means you can afford to run at a lower voltage.”
The team is still exploring different currents and studying features in the signal as they raise the voltage to understand where boundaries might be – where little peaks may grow into big ones. They are gaining valuable operational experience, as well.
“We have much more freedom in exploring the high voltage parameter phase space in the 35-ton setup than we would in a real TPC,“ said Hahn. “The experience gained in these last three runs will be invaluable in setting up the operational HV system for ProtoDUNE-SP.”
This is in fact the fifth liquid argon run for the 35-ton cryostat, originally designed to demonstrate that a membrane cryostat can achieve TPC-quality liquid argon purity.
“This just demonstrates that if you build it, they will come,” quipped Hahn.