A cryogenic buffer gas beam source with in-situ ablation target replacement

This paper presents the design and performance of a cryogenic buffer gas beam source featuring an in-situ ablation target replacement system that maintains vacuum and cryogenic conditions, thereby achieving a 40% long-term yield improvement for the ACME III electron electric dipole moment search while producing cold thorium monoxide molecules with parameters comparable to traditional sources.

The Gist

The Big Picture: A High-Tech "Molecular Factory"

Imagine you are trying to study a very specific, tiny molecule (Thorium Monoxide) to solve a mystery about the universe (specifically, why matter and antimatter behave differently). To do this, you need a steady, high-speed stream of these molecules, but they have to be extremely cold and very slow so you can measure them accurately.

The scientists built a machine called a Cryogenic Buffer Gas Beam (CBGB) source. Think of this machine as a high-tech snow globe factory.

• The Snow: Instead of snow, it uses super-cold Neon gas (at about -250°C).
• The Flakes: Inside this cold gas, they blast a solid block of ceramic (Thorium Dioxide) with a laser. This turns the ceramic into a mist of molecules.
• The Chill: The cold neon gas acts like a giant freezer, instantly cooling these new molecules down and slowing them down so they form a neat, directed beam.

The Problem: The "Worn-Out Chalkboard"

In previous experiments (like ACME II), there was a major annoyance. The ceramic block used to make the molecules is like a piece of chalk.

• Every time the laser hits it, it chips off a tiny bit of the surface.
• After a few days of blasting, the surface gets rough, pitted, and uneven (like a chalkboard that has been erased too many times).
• When the surface gets rough, the laser doesn't work as well, and the factory stops producing enough molecules.

The Old Fix: To get a fresh piece of chalk, you had to:

1. Turn off the machine.
2. Let the whole thing warm up to room temperature (like letting a freezer thaw).
3. Open the door, swap the chalk, close the door.
4. Wait days for it to freeze again.

This took about 24 hours every time you needed a new target. For a machine that runs 24/7, losing a whole day is a huge waste of time and data.

The New Solution: The "Magic Loading Dock"

This paper introduces a brilliant new invention for the ACME III experiment: a Load-Lock System.

Imagine your house has a special airlock door (like on a submarine or a spaceship).

1. You have a new piece of chalk waiting in a small side chamber.
2. You pump the air out of that side chamber so it matches the vacuum of the main room.
3. You slide the new chalk through the airlock into the main room without ever opening the main door to the outside world.
4. A robotic arm inside grabs the old chalk, swaps it for the new one, and bolts it in place.

The Magic: The main factory never stops. The temperature never changes. The vacuum is never broken. The scientists can swap the "chalk" in about 20 minutes while the machine keeps running.

What Did They Find?

The team tested this new system and found three amazing things:

1. It works just as well as the old way: Even though they swapped the target quickly without warming up the machine, the beam of molecules was just as cold, just as fast, and just as bright as before. They are getting 130 billion molecules per second (per laser pulse).
2. The "Dusty" Bonus: They noticed that as they used more and more targets, the inside of the machine got covered in a layer of dust from the laser blasts. Surprisingly, this dust acted like a blanket, changing how the heat moved. This actually made the molecules fly faster (from 200 m/s to 215 m/s). It's like a car engine running slightly hotter and faster because the radiator is covered in a thin layer of dust.
3. The Big Win (40% More Data): Because they didn't have to stop the machine for a whole day to change the target, they could keep running continuously. By swapping targets every two weeks, they increased their total data collection by 40% over the long term.

Why Does This Matter?

Think of the ACME experiment as a scientist trying to catch a rare butterfly.

• Before: Every time the butterfly net got worn out, the scientist had to go home, sleep for a night, build a new net, and come back. They missed a lot of butterflies.
• Now: The scientist has a machine that lets them swap the net instantly while standing right there. They catch 40% more butterflies in the same amount of time.

This new "Load-Lock" system isn't just for Thorium; it can be used for any experiment that needs to swap materials inside a vacuum or a freezer without breaking the seal. It's a game-changer for precision physics, allowing scientists to run their experiments longer, faster, and with more data than ever before.

Technical Summary

1. Problem Statement

Cryogenic Buffer Gas Beam (CBGB) sources are essential for producing cold, slow molecular beams used in precision measurements, such as the search for the electron electric dipole moment (eEDM). However, a critical bottleneck in these systems is the degradation of the ablation target surface.

• Target Degradation: In experiments using laser ablation (e.g., of Thorium Dioxide, $ThO_2$), repeated laser shots create a porous, uneven surface on the target. This leads to a gradual decline in molecular yield (up to 50% loss) as the "fresh surface" is depleted.
• Operational Downtime: In conventional CBGB systems, replacing a degraded target requires venting the vacuum chamber to air and performing a full thermal cycle (warming up and re-cooling the cryogenic components). This process interrupts data collection for approximately one day.
• Impact on Precision: For long-duration precision experiments like ACME (Atomic, Molecular, and Optical physics), this downtime significantly reduces the total integrated signal and statistical sensitivity. This issue is exacerbated when handling radioactive materials like $ThO_2$, which require additional safety protocols.

2. Methodology: The Load-Lock System

The authors designed and implemented a novel load-lock system for the ACME III experiment that allows for the in-situ replacement of ablation targets without breaking the vacuum or warming the cryogenic environment.

• Mechanical Design:

  • Vertical Assembly: A magnetically coupled feedthrough transfers a target plate from a load-lock chamber into the main beam box.
  • Horizontal Assembly: A ferro-magnetic fluid rotary feedthrough and a bellows-sealed linear actuator manipulate the target plate inside the beam box.
  • Installation Process: The system uses a two-rod mechanism. The vertical rod lowers the target plate into the beam box. A horizontal rod then threads onto the plate, detaches the vertical rod, and translates the plate into position against the buffer gas cell. A 2-axis tilt stage allows for precise alignment and screw tightening despite minor misalignments.
  • Thermal Management: The system includes a sliding door to block black-body radiation from the load-lock access holes, maintaining the cryopumping shields at 5 K. Indium gaskets are used to minimize gas leakage, though no seal is placed between the target plate and the cell (relying on precision machining to keep gaps $<0.1$ mm).

• Experimental Setup:

  • Source: A copper buffer gas cell cooled to 18 K using neon buffer gas.
  • Ablation: A 1064 nm Nd:YAG laser (20 mJ, 50 Hz) ablates the $ThO_2$ target.
  • Detection: Molecular yield is characterized via absorption spectroscopy (measuring optical density) and spin precession measurements (to determine forward velocity).

3. Key Contributions

• First Implementation: This is the first demonstration of a load-lock system for a CBGB source capable of in-situ target replacement under vacuum and cryogenic conditions.
• Continuous Operation: The system eliminates the need for thermal cycling and vacuum venting during target changes, reducing replacement time from ~1 day to ~20 minutes.
• System Characterization: The authors provide a comprehensive characterization of the beam properties (yield, temperature, velocity, divergence) using the new load-lock method and compare them against traditional methods.

4. Key Results

• Beam Performance: The load-lock system produces a beam with performance metrics consistent with traditional sources:
  • Yield: $\sim 1.3 \times 10^{11}$ ground-state $ThO$ molecules per pulse.
  • Rotational Temperature: 4.8 K.
  • Forward Velocity: $\sim 200$ m/s (initially).
  • Extraction Fraction: 11%.
  • Solid Angle: 0.31 sr.
• Thermal Stability: During target replacement, the cryogenic system temperature remains stable. The cell temperature briefly rises when a room-temperature target is installed but stabilizes back to 10 K (cell cryocooler stage) within 20 minutes. The system has sufficient cooling power ($\sim 7$ W) to handle the heat load of the new target.
• Long-Term Yield Improvement:
  • ACME II (No Load-Lock): Over a run of $1.2 \times 10^7$ pulses, the yield dropped by $\sim 50\%$ after the first $2 \times 10^6$ pulses due to surface degradation. The average yield over the run was only $\sim 67\%$ of the initial fresh-target yield.
  • ACME III (With Load-Lock): By replacing targets every two weeks (approx. every $2 \times 10^6$ pulses), the yield remained stable. Even after using six targets, the yield only dropped by $\sim 25\%$ (attributed to dust accumulation in the cell, not target surface degradation).
  • Net Gain: The long-term average yield improved by $\sim 40\%$ compared to the non-load-lock approach.
• Dust Accumulation Effect: The authors observed that as dust accumulated in the cell (after 4 targets), the forward velocity increased from 200 m/s to 215 m/s. They attribute this to dust reducing thermal conductivity between the cell and neon gas, leading to a higher instantaneous gas temperature and faster expansion.

5. Significance

• Enhanced Sensitivity for eEDM Search: The 40% increase in long-term average yield directly translates to a higher statistical sensitivity for the ACME III electron EDM search, allowing for more precise measurements of fundamental physics parameters.
• Operational Efficiency: The system drastically reduces downtime, enabling more continuous data collection. This is crucial for experiments requiring long integration times.
• Broad Applicability: While developed for $ThO_2$ ablation, the load-lock design is applicable to other CBGB sources requiring in-situ precursor replacement, particularly those involving radioactive or hazardous materials where safety protocols make frequent venting undesirable.
• Validation of Cryogenic Robustness: The study proves that complex mechanical manipulations can be performed inside a cryogenic vacuum environment without compromising the thermal stability or vacuum integrity of the system.