Design of a high voltage delivery system for noble liquid time projection chambers

This paper presents a novel high voltage delivery system design for the nEXO noble liquid time projection chamber, addressing stability challenges and radiopurity requirements to prevent electrical discharges while offering guidance for future experiments in similar environments.

The Gist

Imagine you are trying to build a giant, ultra-sensitive camera inside a tank of liquid gold (or in this case, liquid xenon) to take a picture of a ghost. This "ghost" is a rare particle event called neutrinoless double beta decay. If you can catch this ghost, it proves that neutrinos are their own antiparticles, which would rewrite the laws of physics.

To take this picture, you need a Time Projection Chamber (TPC). Think of this as a giant, 3D cloud chamber. When a particle zips through the liquid, it knocks electrons loose. You need to catch these electrons and guide them to a camera at the other end.

To do this, you need a massive, invisible "wind" to push the electrons. This wind is created by a High Voltage (HV) system—essentially a giant battery pushing electricity through the liquid.

The Problem:
Putting a massive electrical charge into a liquid is like trying to balance a house of cards on a vibrating table while wearing oven mitts.

1. The Liquid is Picky: The liquid (xenon) is incredibly pure. If you put in the wrong kind of plastic or metal to hold the wires, it might release tiny radioactive particles that look like the ghost you are trying to find.
2. The Spark Risk: If the electric field gets too strong in one tiny spot, the liquid breaks down. It sparks, like a lightning bolt inside a bottle. This ruins the experiment and can destroy the delicate electronics.
3. The Size: The bigger the tank, the harder it is to keep the electricity stable.

This paper is the instruction manual for building the "power cord" and the "insulation" for the nEXO experiment, the next-generation version of this ghost-hunting camera.

Here is how they solved the puzzle, explained with everyday analogies:

1. Smoothing the Rough Spots (The "Sandpaper" Analogy)

Imagine you have a high-pressure water hose. If the nozzle has a tiny scratch or a sharp edge, the water sprays out violently and might burst the hose.

• The Science: Electricity behaves similarly. If a metal surface has a sharp edge, a screw head, or a rough spot, the electric field "piles up" there, creating a super-strong spot that causes a spark.
• The Fix: The team made sure every metal part was polished to a mirror finish. They even used special "stress cones" (think of them as smooth, rounded ramps) to guide the electricity gently from the cable into the tank, so it doesn't hit a sharp corner.

2. The "Triple Point" Danger (The "Three-Way Intersection" Analogy)

Imagine a road where a metal road, a rubber tire, and the air all meet at a single point. That intersection is a chaotic mess.

• The Science: In physics, a "triple point" is where a conductor (metal), an insulator (plastic), and the liquid (xenon) all touch. This is the most dangerous place for a spark to start.
• The Fix: They designed the system so these three materials never touch at a sharp angle. They recessed the connections (hid them deep inside) and added "ribs" (like the ridges on a ceramic insulator on a power line) to force the electricity to travel a longer, safer path along the surface.

3. The "Clean Room" Cable (The "Sterile IV Drip" Analogy)

The cable bringing the electricity into the tank has to be made of materials that are "radioactively clean." If the cable is made of cheap plastic with trace amounts of uranium, it will fog up the camera.

• The Science: They used a special cable made entirely of polyethylene (a type of plastic). It's flexible, doesn't crack in the cold, and is very pure.
• The Twist: Even pure plastic can hold onto gas bubbles. They had to "bake" the cable (like drying out a sponge) to remove any trapped air or gas that could turn into bubbles and cause a spark.

4. The "Sphere-in-Sphere" Connector (The "Russian Doll" Analogy)

How do you connect the cable to the big metal ball (the cathode) at the bottom of the tank without creating a spark?

• The Science: They used a sphere-in-sphere design. Imagine a small metal ball inside a larger metal bowl. This shape spreads the electric charge out perfectly evenly, like water spreading out in a round bowl, rather than piling up at the edges.
• The Result: This allows them to push a massive 50,000 volts of electricity through the system without the liquid breaking down.

5. The "Glitch Detector" (The "Smoke Alarm" Analogy)

Before a house burns down, you might smell smoke or hear a crackle.

• The Science: Before a big electrical explosion happens, there are tiny, invisible "glitches"—microscopic sparks that happen a few times a second.
• The Fix: They built a super-sensitive "smoke alarm" (a glitch detector) that listens for these tiny sparks. If it hears them, it tells the computer to turn down the voltage before a big explosion happens. This saved the previous experiment (EXO-200) and will protect the new one.

The Big Picture

The authors of this paper are essentially saying: "We learned a lot from our previous attempt (EXO-200). We know where the sparks happen, we know which materials are too dirty, and we know how to smooth out the edges. Here is our new, super-safe, ultra-clean design for the nEXO experiment."

They are building a high-voltage delivery system that is:

• Smooth: No sharp edges to cause sparks.
• Clean: No radioactive dust to fake the results.
• Smart: It listens for warning signs and shuts down before disaster strikes.

This design ensures that when nEXO turns on, it can hunt for the rarest particle events in the universe without the machine itself blowing a fuse.

Technical Summary

1. Problem Statement

Noble liquid Time Projection Chambers (TPCs) are critical for detecting rare events like neutrinoless double beta decay, dark matter, and neutrino interactions. These detectors require stable high voltage (HV) systems to create uniform electric fields (typically 100–500 V/cm) over large drift volumes (1–6 meters).

However, maintaining stable HV in noble liquids (Liquid Xenon, Argon, Helium) is historically difficult due to:

• Field Enhancements: Sharp edges, surface asperities, and "triple points" (junctions of conductor, insulator, and liquid) create localized electric fields that exceed the bulk dielectric strength of the liquid, leading to discharges.
• Scaling Effects: As electrode areas and volumes increase, the breakdown field strength decreases due to statistical "weakest link" effects and increased stored electrostatic energy.
• Material Constraints: Rare-event searches demand extreme radiopurity, limiting the use of standard insulating materials. Additionally, electronegative impurities must be minimized to prevent electron attachment.
• Thermodynamics: Localized heating can cause bubble formation, which lowers the dielectric strength and triggers breakdowns.

Previous experiments (e.g., EXO-200) often operated below their target electric fields due to instability, compromising detector performance.

2. Methodology

The authors developed a comprehensive HV delivery system design for the nEXO experiment, integrating lessons from the EXO-200 R&D program with general HV engineering principles. The methodology involved:

• Design Optimization: Utilizing Finite Element Analysis (FEA) to model electric fields and minimize enhancements at critical junctions.
• Material Selection: Choosing materials that balance HV performance with strict radiopurity requirements (e.g., electroformed copper, specific polyethylene grades).
• Component Engineering:
  • HV Cable: Selection and modification of an all-polyethylene coaxial cable to ensure cryogenic flexibility and low outgassing.
  • Stress Cones: Designing geometric field-grading structures to manage the electric field at the cable ground termination.
  • Connection Geometry: Implementing a "sphere-in-sphere" geometry for the cable-to-cathode connection to minimize field concentration.
  • Filtering: Designing multi-stage RC filters and "glitch" detectors to monitor and suppress voltage noise.
• Prototyping and Testing:
  • Fabricating full-scale prototypes of the HV connection and stress cones.
  • Conducting tests in air (to verify breakdown voltages and assembly) and planning for liquid xenon validation.
  • Applying rigorous surface finishing (polishing) and cleaning protocols to remove field-enhancing asperities.

3. Key Contributions

The paper presents a novel, radiopure HV delivery system tailored for the nEXO TPC, featuring several key innovations:

• Sphere-in-Sphere Connection: A removable, sphere-in-sphere geometry connects the HV cable to the cathode. This design eliminates the need for solid insulators in the high-field region (relying on liquid xenon) and minimizes field enhancement. The inner sphere is spring-loaded to accommodate thermal contraction.
• Advanced Stress Cone: A custom-machined stress cone made of Ultra-High Molecular Weight Polyethylene (UHMWPE) and semi-conductive UHMWPE. It features machined ribs to increase creepage length and reduce surface flashover risks. It is thermally bonded to the cable to prevent xenon ingress.
• Radiopure Material Strategy: The system uses electroformed copper and specific polyethylene grades (LDPE, UHMWPE) to meet the stringent background budget (allocating only ~4% of the total background to the HV system).
• Noise Suppression & Monitoring:
  • A 3-stage RC filter reduces power supply noise to <50 µV RMS, preventing charge readout interference.
  • A "glitch" detector (capacitive coupling to an oscilloscope) monitors for micro-discharges (millivolt fluctuations) that precede full breakdown, acting as an early warning system.
• Room Temperature Gas Seal: A modified Swagelok® Ultra-Torr fitting with double O-rings seals the xenon volume at room temperature, avoiding cryogenic seal failures and allowing for leak monitoring.

4. Results

• Simulation: 3D electrostatic FEA simulations confirmed that the sphere-in-sphere design keeps the maximum electric field in the liquid xenon below the target of 50 kV/cm. The design successfully manages the "stressed area" (surface area with >90% of max field) to levels comparable to or better than previous stainless steel electrode tests.
• Air Testing: Full-scale prototypes were tested in air up to -46 kV.
  • Breakdown occurred at -42.5 kV, consistent with Paschen's Law predictions for the geometry.
  • Discharges were distributed across the sphere surfaces, confirming no localized field enhancements from manufacturing defects.
  • The stress cone and cable termination withstood the testing without failure, validating the geometric field grading.
• Noise Performance: The filtering system successfully attenuated the power supply noise by a factor of >10,000, meeting the requirement of <30 electrons charge-equivalent noise.
• Radiopurity: Material assays confirmed that the selected components (copper, polyethylene, phosphor bronze) meet the ultra-low background requirements for nEXO.

5. Significance

This work provides a critical blueprint for the next generation of large-scale noble liquid detectors.

• Enabling Higher Sensitivity: By enabling stable operation at the target drift field of ~400 V/cm (approx. -50 kV cathode voltage), the nEXO experiment can achieve the energy resolution and sensitivity required to probe neutrinoless double beta decay half-lives beyond $10^{28}$ years.
• General Applicability: The design principles (triple-point shielding, stress cone geometry, creepage management, and radiopure material selection) are transferable to other noble liquid experiments (e.g., DUNE, DarkSide-20k, LZ).
• Risk Mitigation: The inclusion of a glitch detector and the rigorous pre-deployment testing strategy offer a robust framework for preventing catastrophic discharges that could damage sensitive readout electronics.

In summary, the paper bridges the gap between theoretical HV engineering and the practical constraints of rare-event physics, delivering a validated, high-performance HV system design for the nEXO experiment.