Feeding the voltage through to the cage
Barbara Warmbein (DESY), 13/04/2017
Giant time projection chambers (TPC) filled with liquid argon are the technology of choice for future neutrino experiments like DUNE. Liquid-argon (LAr) TPCs can generate complete 3-D images of the tracks of ionising particles interacting with the medium and reconstruct the particles’ energies well while distinguishing signal events from background.
But there is no such thing as a free lunch (or a simple event reconstruction): researchers must put in a lot of effort to make the challenging detector work. One such challenge is the development of the part that delivers the high voltage needed for the TPC’s drift field. A first prototype of this feedthrough – a giant insulating plug – was successfully tested at CERN under AIDA-2020 WP8.
Future neutrino experiments aiming at drifting charges over distances of several metres require bias voltages at their cathode in the range of hundreds of kilovolts. Achieving this is not straightforward, as the voltage must be fed from air into a hermetically sealed cryogenic system, through a one-metre insulation layer and subsequently into the liquid argon at -186 degrees Celsius. Breakdown can occur in argon more easily than in air, so the design of the feedthrough must avoid sharp edges. On top of this, the materials used in the construction of the high-voltage feedthrough (HVFT) must satisfy the purity requirements of LAr.
Dr. Franco Sergiampietri designed the feedthrough at CERN based on a similar, albeit smaller and lower-voltage version used for the former neutrino experiment ICARUS. An Italian company manufactured the piece of equipment in very close communication with the designer.
“This is a one-off product, which must satisfy a number of unconventional requirements,” explained Laura Molina Bueno, a physicist from ETH Zurich. “It needed a lot of iteration.”
The hard work seems to have paid off: first tests at CERN show that the feedthrough can both transport the voltage into the detector and sustain it there.
The feedthrough consists of a rod of High Molecular Density Polyethylene (HMDPE) measuring 2m long and more than 10cm in diameter, which contains an inner and an outer conductor. HMDPE is an excellent insulator and can withstand cryogenic temperatures without compromising the purity.
For the inner conductor a straight hole must be drilled through the two metres of material – quite a challenge given that the material is relatively soft. The team also moved away from the classic ring shape for the ground terminator, the area where the outer conductor terminates, and adopted an elliptical shape instead.
“Based on our electrostatic simulation we expected better performance of the elliptical shape, and we found that it indeed gave a 30% improvement of the electrical field along the feedthrough compared to previous designs,” said Laura Molina Bueno.
After the first successful tests at full power, the plan is now to use the feedthrough in the 3x1x1 prototype as part of the R&D for the future DUNE detector and test it in long-term operation at different voltages. Another feedthrough is currently being developed for the ProtoDUNE-DP prototype that will follow the 3x1x1 detector.