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. 2024 Jun 12;15(1):5029.
doi: 10.1038/s41467-024-49346-2.

Laboratory realization of relativistic pair-plasma beams

C D Arrowsmith  1 , P Simon  2   3 , P J Bilbao  4 , A F A Bott  5 , S Burger  2 , H Chen  6 , F D Cruz  4 , T Davenne  7 , I Efthymiopoulos  2 , D H Froula  8 , A Goillot  2 , J T Gudmundsson  9   10 , D Haberberger  8 , J W D Halliday  5 , T Hodge  5   11 , B T Huffman  5 , S Iaquinta  5 , F Miniati  5 , B Reville  12 , S Sarkar  5 , A A Schekochihin  5 , L O Silva  4 , R Simpson  6 , V Stergiou  5   2   13 , R M G M Trines  7 , T Vieu  12 , N Charitonidis  2 , R Bingham  7   14 , G Gregori  5
Affiliations

Laboratory realization of relativistic pair-plasma beams

C D Arrowsmith et al. Nat Commun. .

Abstract

Relativistic electron-positron plasmas are ubiquitous in extreme astrophysical environments such as black-hole and neutron-star magnetospheres, where accretion-powered jets and pulsar winds are expected to be enriched with electron-positron pairs. Their role in the dynamics of such environments is in many cases believed to be fundamental, but their behavior differs significantly from typical electron-ion plasmas due to the matter-antimatter symmetry of the charged components. So far, our experimental inability to produce large yields of positrons in quasi-neutral beams has restricted the understanding of electron-positron pair plasmas to simple numerical and analytical studies, which are rather limited. We present the first experimental results confirming the generation of high-density, quasi-neutral, relativistic electron-positron pair beams using the 440 GeV/c beam at CERN's Super Proton Synchrotron (SPS) accelerator. Monte Carlo simulations agree well with the experimental data and show that the characteristic scales necessary for collective plasma behavior, such as the Debye length and the collisionless skin depth, are exceeded by the measured size of the produced pair beams. Our work opens up the possibility of directly probing the microphysics of pair plasmas beyond quasi-linear evolution into regimes that are challenging to simulate or measure via astronomical observations.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Experimental setup.
Protons with 440 GeV/c momentum are extracted from the SPS ring with maximum intensity of 3 × 1011 protons in a single bunch of duration 250 ps (1-σ), and transverse size σr = 1 mm. The transverse beam profile of the secondary beam is imaged using a 70 mm × 50 mm × 0.25 mm chromium-doped (Chromox) luminescence screen positioned 10 cm downstream of the target, and a blocker foil (50 μm aluminum) is used to minimize stray optical light. The Chromox screen is oriented at 45° to the beam path and viewed by a digital camera which has an exposure time of 24 ms to capture the entire scintillation of the screen. The 3.8 m standoff distance of the digital camera leads to image resolution of 50 μm, however the actual resolution is 100 μm due to the translucence of the Chromox. At a distance 2 m downstream of the target, electrons, and positrons are separated from the secondary beam and spectrally resolved using a magnetic spectrometer comprised of an electromagnet and a pair of luminescence screens (200 mm × 50 mm × 1 mm) centered at a distance 240 mm off-axis. 20-cm thick bricks of concrete (not shown in the diagram) are placed at the entrance of the electromagnet, leaving a 40 mm-wide aperture. Concrete is also placed to block the target from the direct view of the cameras to minimize speckle background arising on the camera images from the impact of high-energy hadrons scattered around the experimental area.
Fig. 2
Fig. 2. Transverse beam profile imaged using a luminescence screen.
a Direct comparison of FLUKA Monte-Carlo simulations with raw image data obtained when the target is irradiated and the secondary beam is produced ('Target in'), versus when the target is removed, and only the primary proton beam irradiates the screen ('No target'). An absolute fluence calibration is obtained using the known density profile of the primary proton beam. b Integrated image intensity (total intensity) from 68 shots is converted to an absolute particle number, showing the case where the target is irradiated (red circles, 46 shots), and when it is removed (black diamonds, 22 shots). The error bars reflect the standard errors of the fitted parameters for each shot. FLUKA Monte-Carlo simulations of the predicted light yield are shown for both cases (black-dashed and red-dashed lines), showing good agreement with the experimental data. The blue dot-dashed line indicates the contribution from e± in the FLUKA simulation, highlighting that this is the dominant contribution to the enhanced signal.
Fig. 3
Fig. 3. Magnetic particle spectrometer.
The energy spectra of e− (blue) and e+ (red) are obtained from images of luminescence screens (dimensions 200 mm × 50 mm × 1 mm) centered 240 mm from the beam axis on either side (see Fig. 1). Electrons and positrons are deflected onto the screens by the vertically-oriented dipole magnetic field of the electromagnet, whilst hadrons with a higher momentum and uncharged γ rays are mostly absorbed by the beam dump behind the electromagnet. The spectrum in the energy range 30 ≤ E [MeV] ≤ 220 is constructed by piecing together images from multiple shots using different magnetic field strengths (B = 0.1−0.34 T). The measured e± spectra are characterized by a power-law with spectral index dN/dE ∝ E−1.0. The shaded regions correspond to the error associated with the absolute calibration. FLUKA simulations (histograms) are able to accurately predict the experimentally obtained spectra.
Fig. 4
Fig. 4. Comparison of laboratory-produced, high-density pair beams.
The peak number and density of pairs reported in this study (red square) compared with previous experiments performed at high-power laser facilities (black squares): Orion/OMEGA-EP, Texas-PW, Astra-Gemini, OMEGA-EP('21), OMEGA-EP('14). The data are labeled by the facility where the experiment was performed, and the fill fraction of each marker corresponds to the fraction of positrons to electrons in the experiment, Ne+/Ne− (■ = 100%, □ = 0%, also see the key). The blue-shaded region corresponds to when the beam volume is smaller than the corresponding Debye screening volume (N±/ND < 1, assuming the screening length used in this work).
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