High-Voltage Performance Testing in LAr of the PMMA Cathode Connection for the DarkSide-20k Experiment

This paper summarizes a comprehensive test campaign conducted at the University of California, Davis, which successfully validated the operation of the DarkSide-20k experiment's custom PMMA cathode high-voltage connection in liquid argon up to -100 kV, ensuring the reliability of the drift field system for the upcoming dark matter search.

The Gist

Imagine you are trying to catch a ghost. Not a spooky sheet-wearing ghost, but a Dark Matter ghost (specifically, a WIMP) that rarely interacts with anything in our universe. To catch it, scientists are building a massive, ultra-sensitive "fish tank" called DarkSide-20k.

This fish tank is filled with Liquid Argon (a super-cold, invisible liquid version of the gas in your air) and is buried deep underground in Italy to shield it from cosmic rays. The goal is to see a tiny flash of light when a dark matter particle bumps into an argon atom.

However, to see that flash, the scientists need to create a perfect, invisible "electric highway" inside the tank to guide the tiny electrical signals (electrons) to the top. This requires a massive amount of electricity—about 75,000 volts (75 kV)—pushed into the bottom of the tank.

The Problem: The "High-Voltage Umbilical Cord"

Think of the high-voltage cable as a giant, super-powerful umbilical cord feeding electricity into the bottom of the tank. But there are two big problems:

1. The Cold: The tank is at -186°C. Most materials get brittle and crack like old ice when it gets that cold.
2. The Spark: If the electricity leaks or jumps the wrong way (a "spark" or discharge), it ruins the experiment. It's like trying to run a high-speed train on a track made of wet spaghetti; if the track isn't perfect, the train crashes.

The connection point where the cable meets the bottom of the tank is the most dangerous spot. It's a complex shape made of plastic (PMMA) and special insulating layers. If the electric field gets too strong at any tiny point, it could cause a short circuit.

The Solution: The "Dry Run" in a Bucket

Before they installed this dangerous, expensive cable into the massive 20-ton tank, the team at the University of California, Davis, decided to test it first.

They built a miniature version of the setup in a small stainless steel bucket (a dewar) filled with about 20 liters of liquid argon.

• The Analogy: Imagine you are building a skyscraper. Before you pour the concrete for the real thing, you build a full-scale model of the foundation in your backyard to see if it holds up. That's what they did.
• The Test: They took the exact same cable and the special "stress cone" (a funnel-shaped plastic piece that smooths out the electric field) and dipped it into the bucket.

The Process: A Slow, Gentle Freeze

You can't just dump liquid nitrogen on a plastic pipe; it would shatter.

• The Metaphor: Think of cooling the system like waking up a hibernating bear. You have to wake it up slowly. If you shock it, it gets angry (or in this case, the plastic cracks).
• The Execution: They cooled the bucket down very slowly over 11 days, using heaters to act as a "thermal buffer" (like a shock absorber) to ensure the temperature dropped evenly. They kept the temperature difference between the top and bottom of the bucket very small to prevent stress.

The Big Moment: Turning Up the Voltage

Once the bucket was full of liquid argon and super cold, they turned on the power.

• The Goal: They wanted to see if the system could handle the normal operating voltage (75 kV) and even go higher to find its breaking point.
• The Result: They slowly cranked the voltage up to -100 kV (which is actually higher than the tank will ever need). They left it running for 14 days.

What happened?

• No Sparks: The cameras saw no flashes of light (which would mean a spark).
• No Leaks: The electricity stayed exactly where it was supposed to go.
• No Cracks: The plastic didn't break from the cold.
• Stable: The current (the flow of electricity) was tiny and steady, like a calm river, not a raging flood.

The Conclusion

The test was a massive success. The "umbilical cord" works perfectly in the freezing cold. It can handle more voltage than the experiment will ever need without breaking or sparking.

Why does this matter?
It means the scientists can now build the real DarkSide-20k detector with confidence. They know their "electric highway" is safe, stable, and ready to catch those elusive dark matter ghosts. It's like passing a rigorous safety inspection before opening a new bridge to the public.

In short: They built a tiny, frozen test tank to prove that their high-voltage cable wouldn't explode or crack in the real, giant tank. It passed with flying colors.

Technical Summary

1. Problem Statement

The DarkSide-20k (DS-20k) experiment is a next-generation dual-phase liquid argon (LAr) time projection chamber (TPC) designed for direct dark matter detection. A critical component of the detector is its high-voltage (HV) system, which establishes a uniform 200 V/cm electric drift field by biasing the cathode at approximately −75 kV.

The primary engineering challenges identified were:

• Electrical Discharge Risk: Delivering high voltage in a cryogenic LAr environment without triggering electrical breakdowns.
• Thermal Stress: Ensuring the stability of the connection materials (Polyethylene and PMMA) under extreme temperature gradients (cooling from room temperature to ~87 K).
• Geometric Constraints: Achieving a reliable electrical connection within the confined geometry of the cathode plug.
• Validation Gap: There was a need to validate the custom HV cable and stress-cone assembly design under realistic local electric field conditions before full-scale deployment.

2. Methodology

To address these challenges, a dedicated test campaign was conducted at the University of California, Davis (UC Davis) using a scaled-down but electrically representative setup.

• Test Setup:

  • A closed LAr system housed in a stainless steel (SS) dewar containing ~20 liters of LAr.
  • The actual DS-20k HV cable (rated to 150 kV, featuring a three-layer polyethylene structure) and the custom stress-cone assembly (PE-cone plug bonded to a PMMA cone) were installed.
  • The assembly was connected to a 5 cm aluminum sphere inside a PMMA cylinder to reproduce the local electric field conditions of the actual detector cathode.
  • Simulation: 2D COMSOL simulations were used to verify that the test setup's electric field distribution (specifically at the PE/PMMA interface) matched the DS-20k design at −75 kV.

• Instrumentation & Monitoring:

  • Thermal: PT100 RTDs monitored temperature at two heights to ensure uniform cooling.
  • Optical: Two cryogenic cameras (Jinjiean B19) monitored for electrical discharges (light emission) and bubble formation.
  • Electrical: A Heinzinger PNChp 100000–neg power supply provided HV, interfaced with a National Instruments DAQ system for continuous voltage and current readout.

• Procedure:

  • Cool-down: A controlled, slow cool-down was performed over ~11 days using bottom heaters and a recirculation pump to maintain a temperature gradient of <1.8 K/cm and a rate of ~−0.5 K/h, minimizing thermal stress on polymers.
  • Filling: The system was filled with industrial-grade argon (99.997% purity) to a level just above the stress cone.
  • HV Testing: Voltage was ramped up to −100 kV (33% above the nominal −75 kV) in 10 kV steps at 5 V/s. The system was held at this voltage for 14 days for long-term stability testing.

3. Key Contributions

• First Full-Scale Validation: This work presents the first comprehensive validation of the DS-20k cathode HV connection system in a liquid argon environment.
• Stress-Cone Design Verification: It confirms the efficacy of the custom PE-stress-cone assembly in managing electric field gradients and preventing breakdowns at the interface between the cable and the PMMA cathode window.
• Operational Protocol Development: The paper establishes reliable procedures for cryogenic cool-down, LAr filling, and HV ramp-up/ramp-down specific to large-scale polymer-based TPCs.
• Safety Margin Demonstration: The test successfully operated at voltages significantly exceeding the nominal operating point, proving a robust safety margin.

4. Results

• High-Voltage Stability: The system operated successfully up to −100 kV for 14 days with no electrical discharges.
  • Current: Remained stable at the order of O(100 nA) during ramp-up, ramp-down, and steady-state operation. No current spikes were observed.
  • Voltage: No voltage oscillations were detected within the ~1.5 V readout resolution.
• Optical Monitoring: Cryogenic cameras detected no light emission attributable to discharges. While bubble formation was observed, it was attributed to multiple sources in the dewar rather than the HV connection itself.
• Post-Test Integrity:
  • Electrical Properties: Post-test capacitance ($376 \pm 4$ pF) and resistance ($185 \pm 1$ k$\Omega$) were consistent with pre-test values, indicating no degradation of the cable insulation.
  • Mechanical Integrity: Visual inspection revealed no mechanical damage or cracking in the PMMA or PE components, confirming the success of the controlled thermal stress management.

5. Significance

This successful test campaign provides critical technical assurance for the DarkSide-20k experiment. By demonstrating that the HV system can operate stably at −100 kV (exceeding the required −75 kV) without discharge or material failure, the results:

1. Validate the Design: Confirm that the custom cable and stress-cone assembly are suitable for the harsh cryogenic and high-field environment of the DS-20k detector.
2. Ensure Reliability: Demonstrate the long-term reliability of the connection, which is essential for a planned 10-year operational lifetime.
3. Enable Science Goals: A stable HV system is a prerequisite for the detector to achieve its goal of detecting WIMP-nucleon interactions with a sensitivity down to $10^{-48}$ cm$^2$.

The work concludes that the DS-20k cathode HV supply and plug system is ready for integration, with future tests planned to include an argon purity monitor.