Laboratory observation of collective beam-plasma instabilities in a relativistic pair jet

Using CERN's HiRadMat facility, researchers experimentally demonstrated and quantitatively validated magnetic-field amplification caused by collective beam-plasma instabilities in a relativistic electron-positron pair jet, providing a critical benchmark for astrophysical models of blazar jets and pulsar-wind nebulae.

The Gist

The Big Idea: Recreating a Cosmic Monster in a Lab

Imagine a Blazar. It's a super-massive black hole at the center of a distant galaxy, shooting out a jet of particles so fast they are almost moving at the speed of light. This jet is made of "pair plasma"—a ghostly soup of electrons and their antimatter twins, positrons.

For decades, scientists have wondered: What happens when this super-fast jet crashes into the empty space around it? Does it just fly through? Or does it create a massive explosion of magnetic energy, like a cosmic lightning storm?

Until now, we could only guess using math. But this team of scientists decided to build a miniature version of a Blazar jet inside a laboratory at CERN (the home of the Large Hadron Collider) to see what actually happens.

The Setup: The "Fireball" Machine

Think of the experiment as a high-speed collision course.

1. The Bullet: They fired a beam of protons (the building blocks of atoms) at 440 GeV. That's like a bullet moving at 99.999999% the speed of light.
2. The Target: This bullet hit a block of graphite and tantalum. When the protons smashed into it, they didn't just stop; they exploded into a shower of new particles, creating a massive cloud of electrons and positrons.
3. The Ocean: This new cloud of particles was shot into a tube filled with a special, glowing gas (plasma). Think of this gas as the "ocean" the jet is swimming through.

The Mystery: The Invisible Storm

When a fast-moving jet of charged particles (the pair beam) enters a stationary gas (the plasma), physics predicts a beam-plasma instability.

The Analogy: Imagine a school of fish (the beam) swimming rapidly through a calm pond (the plasma). If the fish swim too fast and too close together, they create a wake. In this cosmic version, the "wake" isn't just water; it's a magnetic field.

The scientists wanted to know: Does this wake get strong enough to create a massive magnetic storm, or does it fizzle out?

The Detective Work: The "Faraday" Flashlight

To see this invisible magnetic storm, they used a clever trick called Faraday Rotation.

• The Metaphor: Imagine shining a laser pointer through a glass of water. If you put a strong magnet near the glass, the light doesn't bend, but its "twist" (polarization) changes. The stronger the magnet, the more the light twists.
• The Tool: They shot a green laser beam through the center of their plasma tube. If the beam-plasma instability created a magnetic field, the laser's twist would change.
• The Challenge: The event happened incredibly fast (in a quarter of a billionth of a second). It was like trying to take a photo of a hummingbird's wing with a camera that had a slow shutter speed. They had to build a super-sensitive camera (a Faraday probe) and figure out exactly how their camera "blurred" the image so they could correct for it.

The Results: The Storm is Real!

When they turned on the plasma gas, the laser twist changed dramatically.

• Without Plasma: The laser went straight through with no change. No storm.
• With Plasma: The laser twisted wildly. This proved that the beam of electrons and positrons had created a massive, amplified magnetic field just by interacting with the gas.

They compared their real-world data to supercomputer simulations (digital twins of the experiment). The real data matched the computer predictions perfectly.

Why Does This Matter?

This is a "Holy Grail" moment for astrophysics.

1. Solving a Cosmic Puzzle: Astronomers see gamma rays from Blazars, but they don't see the "secondary" light that should be there if these magnetic storms were happening in space. This experiment proves that the storms do happen, but they might be weaker or behave differently than we thought. This helps us understand why the universe looks the way it does.
2. The Benchmark: Before this, scientists were guessing how these magnetic fields grew. Now, they have a measured number. It's like finally having a ruler to measure a storm instead of just guessing its size.
3. Future Tech: Understanding how to control these high-energy particle beams and magnetic fields could one day help us build better particle accelerators or even new ways to generate energy.

The Bottom Line

Scientists took a beam of antimatter, shot it through a gas, and watched it create a magnetic storm. They proved that the math they use to understand the most violent objects in the universe (black holes) is actually correct. They didn't just simulate a cosmic event; they captured a piece of the universe's violence in a bottle.

Technical Summary

1. Problem Statement

Electron-positron ($e^+e^-$) pair plasmas are ubiquitous in high-energy astrophysical environments, such as the vicinity of black holes, neutron stars, blazar jets, and pulsar wind nebulae. Collective processes in these plasmas are believed to drive particle acceleration, magnetic field generation, and non-thermal radiation. However, reproducing the necessary densities of pair plasmas in terrestrial laboratories to study these collective effects has been a significant challenge.

Previous experiments using the "Fireball" platform at CERN (specifically Ref. [17]) failed to detect magnetic field amplification, leading only to an upper limit on instability growth rates. This null result left open questions regarding the validity of models predicting that beam-plasma instabilities could dissipate ultra-relativistic pairs in blazar jets, a mechanism crucial for explaining the absence of reprocessed GeV $\gamma$-ray emission and inferring intergalactic magnetic fields.

2. Methodology

The study utilized the Fireball platform at CERN's HiRadMat facility to create a relativistic, charge-neutral electron-positron pair beam propagating through an ambient plasma.

• Beam Generation:

  • Driver: A 440 GeV proton beam from the Super Proton Synchrotron (SPS), delivered as single bunches ($3 \times 10^{11}$ protons/pulse, ~250 ps duration).
  • Target: A cylindrical target consisting of 360 mm of carbon followed by 10 mm of tantalum.
  • Mechanism: Protons interact with the carbon to produce charged and neutral pions. Neutral pions ($\pi^0$) decay into GeV $\gamma$-rays, which enter the tantalum section. There, they drive an electromagnetic cascade via the Bethe-Heitler process, generating a dense, ultra-relativistic ($\langle \gamma \rangle \sim 500$) electron-positron pair beam.
  • Beam Properties: The resulting pair beam has a peak density of $n_0 \approx 5 \times 10^{11} \text{ cm}^{-3}$, an average divergence of $\sim 25$ mrad, and a positron-to-electron ratio of 0.8.

• Ambient Plasma:

  • The pair beam propagates through an inductively coupled radio-frequency (RF) discharge plasma (argon) contained in a "plasma cell."
  • The ambient electron density ranges from $10^{10}$ to $10^{11} \text{ cm}^{-3}$ with temperatures of 1–10 eV.

• Diagnostic (Faraday Rotation):

  • A high-sensitivity Faraday rotation probe was employed to measure the path-integrated magnetic field ($\int B \cdot dl$).
  • Setup: A 532 nm linearly polarized probe laser passes through a Terbium Gallium Garnet (TGG) crystal immersed in the plasma. The magnetic field rotates the polarization angle ($\alpha = V \int B \cdot dl$).
  • Sensitivity: The system used balanced photodiodes (Thorlabs PDB435A) and was characterized for its impulse response function (IRF) to account for the short duration of the proton bunch (~250 ps).
  • Calibration: The probe was calibrated offline using a solenoid to establish a proportionality between voltage and magnetic field.

• Simulations:

  • FLUKA: Used to model the hadronic and electromagnetic cascades in the target to predict the initial beam spectrum and divergence.
  • OSIRIS (PIC): 3D Particle-in-Cell simulations were performed to model the collective interaction between the pair beam and the ambient plasma, predicting magnetic field growth due to instabilities.

3. Key Contributions

• First Terrestrial Measurement: This work reports the first measurement of collective behavior (specifically magnetic field amplification) in a terrestrial relativistic pair-plasma jet.
• Diagnostic Advancement: The implementation of a high-sensitivity Faraday rotation probe, coupled with a rigorous characterization of the detector's impulse response, allowed for the detection of transient magnetic fields on the nanosecond timescale, overcoming the limitations of previous experiments.
• Quantitative Benchmark: The study provides a critical quantitative benchmark for kinetic models of relativistic beam-plasma interactions, bridging the gap between laboratory experiments and astrophysical phenomena.

4. Results

• Magnetic Field Amplification:
  • Plasma-Off: When the ambient plasma was off, no magnetic field signal above the noise floor was observed. This matched FLUKA-based simulations of free propagation (Ampère's law reconstruction), confirming no collective effects occurred.
  • Plasma-On: A clear Faraday rotation signal was detected. The measured path-integrated magnetic field peaked at $A_{exp} = 14.8 \pm 0.6$ mT mm.
• Agreement with Simulations:
  • The experimental results showed excellent quantitative agreement with OSIRIS PIC simulations, which predicted a peak of $A_{PIC} = 18.8 \pm 0.3$ mT mm.
  • The simulations confirmed that the field amplification is driven by the Weibel-like instability (specifically the oblique mode) arising from the interaction between the pair beam and the ambient plasma.
• Growth Rate Analysis:
  • Using the ratio of the measured field to the background field, the experimental growth rate was estimated as $\langle \Gamma_{exp} \rangle \approx 0.40 \text{ ns}^{-1}$.
  • This aligns well with the PIC simulation result of $\langle \Gamma_{PIC} \rangle \approx 0.50 \text{ ns}^{-1}$.
  • Theoretical linear estimates (Eq. 4) suggested a much higher rate ($\sim 2.5 \text{ ns}^{-1}$), indicating that kinetic effects (finite beam radius, energy spread, and divergence) significantly suppress the instability, a finding consistent with the PIC results.

5. Significance

• Astrophysical Validation: The results validate the scaling arguments used to model blazar jets. The observation that instabilities do grow (albeit at a rate suppressed by beam divergence) supports the interpretation that beam-plasma interactions play a role in energy dissipation in astrophysical jets.
• Intergalactic Magnetic Fields: The findings reinforce the robustness of lower limits on intergalactic magnetic fields derived from the absence of GeV emission in blazar spectra. The laboratory data confirms that while instabilities exist, they may not be efficient enough to completely suppress secondary emission without specific magnetic field conditions.
• Future Research: This experiment establishes a controlled laboratory environment for studying the nonlinear evolution, mode structure, and magnetic topology of relativistic pair jets. It opens the door to systematic studies of plasma-mediated interactions that were previously only accessible through theoretical modeling or astronomical observation.

In conclusion, the paper successfully demonstrates that collective beam-plasma instabilities can be generated and measured in a laboratory setting using a relativistic pair beam, providing a crucial experimental foundation for understanding high-energy astrophysical phenomena.