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Demonstration of 255-kV high-voltage generation with a Cavallo multiplier system

This paper reports a room-temperature demonstration of a Cavallo electrostatic multiplier system that successfully generated approximately 255 kV from a 25 kV input in SF $_6$ gas, validating its potential as a viable low-current, in situ high-voltage source for cryogenic precision measurements while highlighting electrode surface preparation and alignment as critical factors for future reliability.

Original authors: S. M. Clayton, T. M. Ito, A. Jacobs, A-T. Le, M. F. Makela, C. M. O'Shaughnessy, N. S. Phan, E. Renner, T. A. Sandborn, T. J. Schaub, I. L. Smythe, J. Surbrook, M. A. Blatnik, B. W. Filippone

Published 2026-07-29

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CC BY 4.0

Original authors: S. M. Clayton, T. M. Ito, A. Jacobs, A-T. Le, M. F. Makela, C. M. O'Shaughnessy, N. S. Phan, E. Renner, T. A. Sandborn, T. J. Schaub, I. L. Smythe, J. Surbrook, M. A. Blatnik, B. W. Filippone

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to build a super-powerful battery, but you are working inside a giant, frozen box filled with liquid helium. The problem is that the wires you usually use to plug in electricity would melt the ice or let too much heat in, ruining the experiment. Scientists need a way to create massive electric forces inside this frozen box without any wires connecting to the outside world. To do this, they use a clever trick called "electrostatic induction." Think of it like a magic trick where you rub a balloon on your hair to make it stick to a wall; you aren't creating new electricity, you are just shuffling existing charges around to build up a strong push. This paper is about a team of scientists who built a machine to do exactly that: a mechanical "charge shuffler" that can generate huge voltages right where they are needed, without any wires.

The scientists in this paper built a prototype of a machine called a Cavallo multiplier. You can picture it as a high-tech, mechanical version of a "bucket brigade" for electricity. Instead of people passing buckets of water, this machine uses a moving metal arm (let's call it the "shuttle") to pass tiny packets of electric charge from one place to another. The machine has three main parts: a "source" electrode that has a small, steady voltage (like a 25,000-volt battery), a "shuttle" that moves up and down, and a "collector" bowl that catches the charge.

Here is how the magic happens: The shuttle starts near the source. Because of the source's electric field, the shuttle gets a little bit of charge induced on it. Then, the shuttle is grounded (connected to the earth) to let the opposite charge flow away, leaving it with a specific charge. Next, the shuttle is lifted up, isolated from the ground, and carried over to the collector bowl. When it touches the bowl, it dumps its charge. The machine repeats this cycle hundreds of times. With every trip, the collector bowl gets a little more charged, and the voltage climbs higher and higher, like a snowball rolling down a hill getting bigger and bigger.

The team tested this machine at room temperature inside a tank filled with a special gas called sulfur hexafluoride (SF6), which is often used to stop electricity from jumping across gaps (like in high-voltage power lines). They started with an input voltage of 25,000 volts (25 kV). After running the machine for a while, they managed to boost that voltage all the way up to approximately 255,000 volts (255 kV). That is a tenfold increase, and it was done without any wires connecting the high-voltage part to the outside world.

However, the journey wasn't perfectly smooth. The scientists found that the machine didn't just stop because it ran out of "oomph" or because the gas broke down. Instead, the machine hit a ceiling because of tiny, invisible bumps on the metal surfaces. When the voltage got very high, the electricity started to leak away in tiny bursts, kind of like how water might spray out of a hose if there's a tiny crack in the nozzle. These leaks were caused by microscopic rough spots on the metal electrodes. The team discovered that if they polished the metal surfaces to make them super smooth, the machine could hold the voltage much better. They also found that the machine was incredibly stable over long periods, with only tiny amounts of charge leaking away (in the range of picoamperes, which is one-trillionth of an ampere), proving that once the voltage is built up, it can stay there for a long time.

The paper concludes that this "bucket brigade" machine works exactly as the math predicted, as long as you account for the fact that the moving parts aren't perfectly aligned and that the metal surfaces need to be perfectly smooth. The main takeaway is that the machine itself is a success. The reason it didn't reach even higher voltages in this test wasn't a flaw in the design, but rather the limitations of the gas and the tiny imperfections on the metal. This is great news for the future, because the scientists plan to use this machine in a super-cold environment with liquid helium. Since liquid helium is much better at stopping electrical leaks than gas, and because the machine doesn't need wires, this "Cavallo multiplier" could be the key to unlocking new physics experiments that require massive electric fields in the coldest places in the universe.

Technical Summary: Demonstration of 255-kV High-Voltage Generation with a Cavallo Multiplier System

Problem Statement
Many fundamental physics experiments, such as searches for electric dipole moments (EDMs) and rare-event detection using time projection chambers (TPCs), require intense electric fields within cryogenic environments. Conventional high-voltage feedthroughs are often impractical in these settings due to significant heat loads from leakage currents, material compatibility issues (e.g., magnetic contamination or radiopurity constraints), and physical design constraints. The specific motivation for this work was the neutron electric dipole moment (nEDM) search at the Spallation Neutron Source, which required a potential of 635 kV (75 kV/cm) in a liquid-helium cryostat below 0.5 K. To circumvent the limitations of external feedthroughs, the authors propose an internal, low-current electrostatic generator: the Cavallo multiplier.

Methodology and Apparatus
The authors constructed a room-temperature demonstrator of a Cavallo multiplier designed for eventual cryogenic deployment. The system consists of three primary electrodes:

Electrode A (Bias): Connected to an external DC high-voltage supply (up to 25 kV).
Electrode B (Transfer): A mechanically actuated electrode mounted on an insulating rod, capable of moving between a grounded position near A and a contact position with C.
Electrode C (Collector): A bowl-shaped high-voltage electrode supported by a PMMA insulating ring, acting as the charge accumulator.
Electrode D: A grounded electrode supporting the PMMA ring and defining the capacitive load.

The system operated in approximately 600 Torr of sulfur hexafluoride (SF6) to simulate high-voltage insulation conditions. A custom rotary field mill, mounted beneath electrode D, was used for non-contact voltage measurement of electrode C to preserve the high-impedance nature of the system. The charging cycle involves inducing charge on B while grounded near A, transporting it to C, and depositing it, repeating this process to build up voltage.

Key Results

Voltage Generation: The system successfully generated high voltages from a 25 kV DC-biased input.
- For the polarity directly calibrated by the field mill (negative bias on A), the maximum measured collector voltage was 237.7 ± 1.7 kV at a bias of -19 kV.
- For the opposite polarity (positive bias on A), the system reached approximately 255 kV at a bias of 25 kV. This value carries an additional sign-dependent systematic uncertainty of ~5.8 kV due to calibration limitations.
Model Validation: A capacitance-based model, incorporating realistic electrode misalignments (specifically a 17% reduction in the induction capacitance $C_{AB}$ and a 24% increase in the ground coupling $C_{BG}$ ), quantitatively described the charging curves up to approximately 200 kV.
Leakage and Dissipation:
- Long-term stability: Voltage-hold measurements at ~245 kV showed stable, quasi-steady leakage currents on the order of 7.7 pA over 19 hours.
- Transient discharge: Near the maximum voltage, the system exhibited transient discharge processes with peak currents reaching ~1 nA. These events were linked to electrode surface conditions rather than the intrinsic dielectric strength of the SF6 gas.
Surface Sensitivity: A major discharge event at 25 kV bias caused surface degradation (burn marks) on the ground ring and electrode C, limiting subsequent runs to ~110–120 kV. Mechanical repolishing of these surfaces restored high-voltage performance, confirming that surface condition and local field enhancement are critical limiting factors.

Key Contributions

Experimental Demonstration: This work provides the first room-temperature demonstration of a Cavallo multiplier designed for cryogenic applications, validating its ability to generate voltages exceeding 200 kV in a controlled gas environment.
Quantitative Modeling: The study establishes that the charging dynamics are accurately described by a capacitance model once realistic mechanical misalignments and parasitic couplings are included.
Performance Characterization: The authors characterize the transition from ideal charge accumulation to voltage saturation, identifying that the limit in SF6 is not the gas's dielectric strength but rather field-activated dissipation processes at electrode surfaces and interfaces.
Non-Contact Measurement: The successful implementation and calibration of a custom rotary field mill for high-voltage monitoring in a high-impedance system are detailed.

Significance and Claims
The paper claims that the Cavallo multiplier is a viable, low-current, in situ high-voltage source for experiments where conventional feedthroughs are unsuitable. The authors emphasize that the observed voltage limitations in the room-temperature SF6 environment are not intrinsic to the multiplier's geometry or the electrostatic principle. Instead, the performance ceiling is dictated by electrode surface preparation, alignment tolerances, and the control of insulation-related charge dissipation.

The authors conclude that these results validate the concept for future cryogenic implementations (specifically in liquid nitrogen and superfluid helium), where the denser dielectric medium and improved surface conditions (e.g., electropolishing) are expected to allow operation at significantly higher voltages, potentially reaching the target 635 kV required for nEDM experiments. The study underscores that successful cryogenic operation will depend primarily on managing interfacial breakdown phenomena rather than the bulk dielectric strength alone.

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