Getting the high voltage right

Jeff Nelson (left) and Will Henninger of William & Mary working on a ProtoDUNE-SP high-voltage test setup inside the 35-ton prototype cryostat, located at Fermilab’s PC-4 building. Credit: R.Hahn, Fermilab

As DUNE pioneers one technology after another, each one requires validation. Since high voltage issues in liquid argon are not fully understood, work is advancing on a setup at Fermilab to evaluate the ProtoDUNE-SP high voltage design. A team led by Sarah Lockwitz and Alan Hahn of Fermilab will evaluate the individual components, including the field cage profiles, the resistive plate cathode and the ground planes, as well as the integrated components to make sure that they work together properly.

The cryostat from the 35-ton prototype will provide the vessel for this test.

“We cannot fit the full-sized ProtoDUNE TPC in the 35-ton cryostat,” said Lockwitz, “however this will be a full-field test. The device will have the first ten profiles of a TPC at their planned voltages.”

Sarah Lockwitz

The test itself involves monitoring the current out of the power supply, the voltage near the end of the resistor network, and changes in the current flowing through the HV cable just outside of the cryostat. In addition, cameras and PMTs will be positioned internally to help diagnose any potential problems.

“We plan to start with a low voltage, about 2 kV, on the feedthrough with the cryostat empty, until we have a solid baseline and we feel sure of the HV chain’s health,” said Hahn. “Then we’ll purge and fill the cryostat, keeping the voltage at about 2 kV, then, once it is full, start to purify the LAr while raising the voltage at about 60 V/s to -180 kV over the first week.”

If a breakdown or other instability occurs, the team will try to locate it, and determine what voltage the system can hold stably for a few days and how long a voltage just below the breakdown value can hold. They will increase the voltage as much as possible again in the last week, when the LAr should be highly purified.

Alan Hahn

Once this test is complete, the team will empty the cryostat and install a beamplug, similiar to the one planned for ProtoDUNE-SP, to test how it behaves when high voltage is applied to the detector.

Colleagues from Argonne, Brookhaven, Kansas State and William & Mary arrived at Fermilab just after the holidays to help install the field cage in the cryostat.

“The CPA, anodes, and the top and bottom field cage profiles were mounted by 9 a.m. of day two,” said Lockwitz. “We also fixed the placement of the field cage to properly connect to the high voltage feedthrough and have begun to make the electrical connections. The remaining connections plus a number of field fixes are in progress. We have to complete them before we install the end walls.”

Instrumentation work also continues, new bases for the PMTs are being made, and a few loose ends are still coming together.

“We plan to close up the cryostat soon, probably once we get back from the collaboration meeting at CERN, then begin the purge and fill,” said Lockwitz. “Our schedule is tight since this test needs to inform the ProtoDUNE-SP design, and their review is scheduled for early spring. Getting the high voltage right is critical for the success of the detector.”