Unwieldy yet delicate, the HV system comes together

Francesca Stocker of CERN attaching a ground plane onto a field cage module. Credit: J. Meier, UMN

Francesca Stocker of CERN and Kevin Wood of Stony Brook University are both beginning their PhD research, building and installing the high voltage system for ProtoDUNE-SP that will provide a uniform electric field inside the LArTPC.

The main components of this system, the cathode plane (CPA) and field cage (FC) assembly, are both fabricated in the U.S., the CPAs at ANL and the FC assemblies at Stony Brook University. The CPA/FC units arrive at CERN as partially assembled panels that require both mechanical and electrical interconnection.

Stocker and Wood emphasized that a uniform electric field is crucial in order to avoid significant distortions of the ionization electron trajectories as they drift towards the wire grids, which makes it particularly important to ensure that the mechanical alignment is precise and the electrical properties are nominal for all high voltage components.

The first step the CERN team takes is to pre-assemble the field cage modules.

Kevin Wood of Stony Brook University installing an end wall field cage into position inside the cryostat. Credit: J. Meier, UMN

“This takes place in a clean room to avoid contaminating the liquid argon once the detector is complete,” said Wood. “We wear these clean room suits that make us look like characters out of Breaking Bad.”

Wood had worked on LArTPC simulation efforts for DUNE and ProtoDUNE before coming to CERN. But working inside the cryostat and imagining it filled with liquid argon has impressed upon him the sheer size and sensitivity of the detector in a way that the running simulations just couldn’t do.

“It’s very cool to actually build these huge detector components and maneuver them into position,” he said. “We use cranes and scissor lifts to push them around a specially designed rail system on trolleys. It’s a delicate procedure and you have to be focused and alert, but it’s also good fun.”

While completing her undergrad studies in Bern, and even as a Masters student building a small LArTPC demonstrator, Stocker hadn’t imagined that she’d be doing her PhD research at CERN.

“Now I find myself building a detector and working with people from all parts of the world. I’ve had the craziest six months of my life at CERN! It is a very busy place to work as a student, but the scientists, engineers and technicians will always stop their work to explain what they are doing, and how and why. It is a great opportunity.”

Click to watch video of CPA/FC assembly. Credit: F. Stocker

Stocker underscored the importance of taking great care while working inside the cryostat.

“Even though the components are very big, they remain delicate, and maneuvering them with the man lift requires concentration. We take care to keep everything as clean as possible and are always aware of our surroundings. When you’re working several meters above very sensitive detector elements, even a job as mundane as tightening a bolt requires focus — that bolt can turn into a wrecking ball with one slip of the hand.”

“Francesca and Kevin have only been here since last summer, but already the ProtoDUNE hardware teams consider them the experts on these essential detector components,” said CERN’s Francesco Pietropaolo, convener of the High Voltage consortium. “As we move into the commissioning phase of the detector and its operation in the charged-particle beam, Kevin and Francesca will participate in validating the HV system performance.”