Record accumulation of antiprotons in a Penning-Malmberg Trap and their preparation for improved production of antihydrogen beams

This paper reports the commissioning of a new Penning-Malmberg trap at CERN's GBAR experiment that achieves a record accumulation of over $6.4 \times 10^7$ antiprotons in under 35 minutes and delivers $6.4 \times 10^6$ antiprotons per shot with reduced emittance, significantly enhancing the production rate of antihydrogen beams for gravitational studies.

The Gist

The Big Picture: Catching Ghosts to Test Gravity

Imagine you are trying to catch a ghost. Not a spooky one, but an antiproton—a particle of "anti-matter" that is the exact opposite of a proton. These ghosts are incredibly rare, incredibly fast, and if they touch normal matter, they vanish in a flash of energy.

The GBAR experiment at CERN wants to catch these ghosts, slow them down until they are almost frozen, and then combine them with other particles to make antihydrogen. Why? To see how gravity affects anti-matter. Does it fall down like a rock, or does it float up like a helium balloon?

The problem is that the "ghosts" coming from the CERN factory (called ELENA) are moving way too fast and are too spread out to catch easily. This paper describes a new, record-breaking machine built to catch them, cool them down, and pack them tightly together.

---

The Problem: A High-Speed Bullet vs. A Tiny Hole

Think of the antiproton beam coming from CERN as a high-speed bullet train (traveling at 100 keV). The target they need to hit is a tiny, narrow tunnel (the Positronium cavity) where the magic happens.

If you try to shoot a bullet train directly into a narrow tunnel, two things go wrong:

1. Speed: It's moving too fast to stop and combine with other things.
2. Spread: The train is too wide and messy. It's like trying to pour a bucket of water through a drinking straw; most of it spills out.

In the past, scientists tried to slow these bullets down by shooting them through a piece of foil (like a brake pad). But this was messy, damaged the particles, and lost a lot of them.

The Solution: The "Magnetic Parking Garage"

The GBAR team built a new system that acts like a high-tech parking garage for these anti-matter particles. Here is how it works, step-by-step:

1. The Soft Landing (The Decelerator)

Instead of slamming the brakes with a foil, they use a pulsed drift tube.

• The Analogy: Imagine a car driving down a hill. Instead of hitting a wall, the road suddenly drops away, and the car glides down a ramp, losing speed gently.
• What they did: They use a fast-switching electrical field to "drop the floor" under the antiprotons, slowing them from 100 keV down to a gentle 3 keV. This is like turning a speeding bullet into a rolling marble.

2. The Parking Garage (The Trap)

Once the particles are slow, they are guided into a Penning-Malmberg Trap.

• The Analogy: Think of this as a magnetic bowl. The walls are made of invisible magnetic and electric fields that keep the particles from touching the sides.
• The Cooling: Inside the garage, there is a cloud of cold electrons (like a swarm of tiny, cold bees). The hot antiprotons fly into this cloud and bump into the cold bees. Through these bumps, the antiprotons lose their heat and slow down even more. This is called sympathetic cooling.
• The Result: The particles go from a chaotic, hot mess to a calm, cold, organized cloud.

3. The Packing Machine (Rotating Wall)

Now that the particles are calm, they are still spread out. The team needs to pack them tighter.

• The Analogy: Imagine a crowd of people in a large room. If you spin the room, the people are pushed toward the center by centrifugal force, packing them into a tight circle.
• What they did: They use a "Rotating Wall" technique, applying a spinning electric field that pushes the antiproton cloud into a tiny, dense ball. This increases the density, making it much easier to use them later.

4. The Launch (Re-acceleration)

Once the cloud is packed and cold, they need to send it to the target.

• The Analogy: Think of a slingshot. They pull the packed cloud back, then release it with a precise burst of energy to shoot it out at the perfect speed (around 6–10 keV).
• The Buncher: They also use a "buncher" to make sure all the particles leave at the exact same time, like a synchronized swim team diving in together, rather than a messy splash.

The Record-Breaking Results

The team tested this new system and achieved some amazing numbers:

• Efficiency: They managed to catch 56% of the antiprotons coming from the factory. Before, they were losing most of them. It's like catching more than half the rain in a bucket, whereas before you only caught a few drops.
• The Stacking Record: They didn't just catch one batch; they kept catching more and more, stacking them on top of each other like layers of a cake. They managed to hold 64 million antiprotons in the trap at once. This is a world record.
• Speed: They reached this record amount in less than 35 minutes.

Why Does This Matter?

By catching and packing so many antiprotons, the GBAR experiment can now produce antihydrogen much faster and more efficiently.

• The Goal: They want to create antihydrogen ions (charged anti-atoms), cool them down to near absolute zero, and then drop them to see how gravity pulls on them.
• The Impact: If they can measure this precisely, it could change our understanding of the universe. Does anti-matter obey the same laws of gravity as normal matter? This experiment is the first step to answering that question with high precision.

Summary

The GBAR team built a magnetic parking garage that catches fast anti-matter particles, cools them down with a cloud of electrons, packs them tightly like sardines, and launches them perfectly into a tiny target. They broke a world record by catching more anti-matter than ever before, paving the way for a new era of gravity experiments.

Technical Summary

1. Problem Statement

The GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN aims to measure the gravitational acceleration of antimatter with high precision ($10^{-6}$). To achieve this, the experiment requires the production of ultracold antihydrogen ions ($\bar{H}^+$), which are subsequently photo-detached to form neutral antihydrogen.

The primary bottleneck in this process is the production rate of antihydrogen, which scales quadratically with the density of the positronium (Ps) target. To maximize this density, a compact cavity target is used, which imposes stringent requirements on the transverse emittance of the incoming antiproton beam.

• The Challenge: When the 100 keV antiproton beam from CERN's ELENA facility is decelerated to the required low energies (below 10 keV) using a pulsed drift tube, the transverse emittance increases by a factor of $\sqrt{E_i/E_e}$ (approximately 6-fold for deceleration to 3 keV). This emittance growth makes it difficult to focus the beam into the narrow (1.5 mm $\times$ 2 mm) Ps target cavity.
• Previous Limitations: Existing methods using degrader foils or simple deceleration without trapping suffer from low trapping efficiencies and insufficient beam quality (high emittance) for the compact target.

2. Methodology

The authors developed and commissioned a specialized beamline and trapping system designed to decelerate, capture, cool, compress, and re-accelerate antiprotons while minimizing emittance growth.

A. Beamline and Deceleration

• Source: 100 keV antiproton bunches ($\sim 1.1 \times 10^7$ particles) from ELENA, delivered every 110 seconds.
• Decelerator: A pulsed drift-tube system reduces the beam energy from 100 keV to 3 keV. The system uses fast high-voltage switching to ground the electrode potential while particles are inside the tube, achieving nearly 100% transport efficiency.
• Optics: Einzel lenses and steerers guide the beam, compensating for magnetic field gradients.

B. The Penning-Malmberg Trap (PT)

The core of the system is a nested Penning-Malmberg trap housed in a 5 T superconducting magnet.

• Structure: Consists of a Multi-Ring Electrode (MRE) assembly with High-Voltage (HV) electrodes for capture/acceleration and Ring Electrodes (RE) for confinement.
• Cooling Mechanism:
  1. Pre-loading: A cold electron cloud ($\sim 3 \times 10^8$ electrons) is trapped and cooled via synchrotron radiation to $\sim 15$ K.
  2. Capture: The 3 keV antiproton beam is injected and captured between HV electrodes.
  3. Sympathetic Cooling: Antiprotons interact with the cold electron plasma, losing energy until they are confined in a harmonic potential well (depth 140 eV).
• Compression: A Rotating Wall (RW) technique is applied using segmented electrodes. By applying rotating dipole electric fields (1–14 MHz), the electron plasma is compressed, which sympathetically compresses the antiproton cloud, reducing its radial size and emittance.
• Extraction and Re-acceleration: The trapped cloud is extracted and re-accelerated to 1–10 keV using a double-gap buncher system (HV2) to compress the bunch length (time focus).

C. Diagnostics

• Detectors: Microchannel Plate (MCP) assemblies with CMOS cameras for spatial profiling and plastic scintillation detectors (flux monitors) for intensity measurement.
• Calibration: Flux monitors were calibrated against the ELENA non-destructive pick-up system, showing a linear response with $\sim 3\%$ deviation.

3. Key Contributions

1. Novel Trapping Scheme: Implementation of a high-efficiency trapping system that combines pulsed drift-tube deceleration with a Penning-Malmberg trap for sympathetic cooling and rotating wall compression.
2. Emittance Control: Successful reduction of the antiproton beam emittance post-deceleration, enabling the beam to be focused into the compact Ps target cavity required for high-density antihydrogen production.
3. Record Accumulation: Demonstration of a stacking mode where the trap acts as a reservoir, accumulating antiprotons from multiple ELENA cycles.
4. Beam Optimization: Integration of a double-gap buncher to reduce the temporal bunch length from 190 ns to 80 ns, improving the time focus for the reaction.

4. Key Results

• Trapping Efficiency: The system achieved a trapping efficiency of 56(3)% relative to the incoming ELENA beam intensity.
• Single-Shot Yield: Each extraction delivers $6.4(0.4) \times 10^6$ antiprotons at energies up to 10 keV.
• Record Accumulation: By stacking 18 consecutive ELENA injections, the team accumulated a total of $6.4(0.4) \times 10^7$ antiprotons in under 35 minutes. This is a new world record, significantly exceeding the previous best of $1 \times 10^7$ (ASACUSA experiment).
• Beam Quality:
  • Cooling: Antiprotons were cooled to below 140 eV within 1.5 seconds.
  • Compression: Rotating wall compression at 8 MHz maximized the areal density.
  • Lifetime: The antiproton plasma lifetime was estimated to exceed 10,000 seconds.
• Simulation Validation: Particle-in-Cell (WARP) simulations matched experimental beam profiles, estimating an initial plasma radius of 0.5 mm and a final beam emittance of 3 mm·mrad. Simulations predict that $3.9 \times 10^6$ antiprotons will pass through the narrow target cavity, a significant improvement over previous non-trapped configurations.

5. Significance

• Enhanced Antihydrogen Production: The ability to deliver a high-intensity, low-emittance antiproton beam directly to a compact Ps target will increase the antihydrogen production rate quadratically. This is critical for the GBAR experiment's goal of measuring gravity on antimatter.
• Scalability: The demonstrated accumulation of $>6 \times 10^7$ antiprotons proves the feasibility of building large reservoirs of antimatter, a prerequisite for producing the positively charged antihydrogen ions ($\bar{H}^+$) needed for laser cooling.
• Technological Benchmark: The successful commissioning of this pulsed-drift-tube and Penning-trap combination sets a new standard for low-energy antimatter experiments, offering a blueprint for future facilities aiming to manipulate antiprotons with high efficiency and precision.

In conclusion, this work represents a major technological leap in antimatter physics, solving the critical bottleneck of beam quality and intensity required to advance precision tests of the Weak Equivalence Principle using antihydrogen.