We report on a measurement of collective behavior in a relativistic electron-positron pair plasma produced in the laboratory. Using the Fireball platform at CERN's HiRadMat facility, 440 GeV protons were used to generate an ultra-relativistic, charge-neutral electron-positron pair beam that propagated through an ambient RF discharge plasma. Magnetic-field amplification due to a beam-plasma instability was diagnosed using a high-sensitivity Faraday-rotation probe, supported by detailed characterization of the diagnostic impulse response. The measured path-integrated magnetic field agrees quantitatively with predictions from particle-in-cell simulations. The results provide a critical benchmark for models of relativistic beam-plasma interactions in astrophysical contexts such as blazar jets and pulsar-wind nebulae.
Introduction. Electron-positron pair plasmas are expected to be present in a variety of high-energy astrophysical environments, including in the vicinity of black holes and neutron stars [1][2][3][4]. Collective pair-plasma processes are frequently invoked to explain astrophysical phenomena including particle acceleration, magnetic field generation, and to account for the observed nonthermal radiation.
Producing densities of pairs sufficient to access plasma effects in terrestrial settings is challenging. Techniques for high-yield positron production employ a range of drivers, including nuclear reactors [5], particle accelerators [6,7], and high-power lasers [8][9][10][11][12][13]. A comprehensive overview of progress in laser-driven methods is given by Chen and Fiuza [14].
In this article, we build on previous work [15,17] using the novel, accelerator based ‘Fireball’ platform for laboratory studies of relativistic pair plasmas. By improving the Fireball diagnostic suite, we were able to measure signatures of magnetic-field amplification due to a beamplasma instability. To our knowledge, this represents the first measurement of collective behavior in a terrestrial pair-plasma jet.
Experimental Setup. Our experimental setup, depicted in Fig. 1a, relies on ultra-relativistic protons (momentum p = 440 GeV/c) accelerated in the CERN Super Proton Synchrotron (SPS) and sent to the HiRadMat facility [18]. The figure also shows the inductively coupled radio-frequency (RF) plasma discharge (“plasma cell”), which provides the ambient argon plasma into which the pair beam propagates; its properties will be described later in the text. The proton beam, delivered as a single bunch per extraction, contains 3 × 10 11 particles per pulse with a bunch duration of approximately 250 ps, and impinges on a cylindrical target comprising 360 mm of carbon (mass density 1.84 g cm -3 ) followed by 10 mm of tantalum (mass density 16.7 g cm -3 ).
The interaction of protons with the target’s graphite The system was driven by the 440 GeV proton beam from the Super Proton Synchrotron at HiRadMat (CERN), delivering a peak intensity of 3 × 10 11 protons per pulse. The beam had a Gaussian transverse profile with 1 mm standard deviation and a Gaussian temporal profile of approximately 250 ps duration. (b) Spectra / average divergence for the residual primaries and dominant secondary species produced by the target. These data are from FLUKA (Monte Carlo) simulations and were validated against the experimental measurements reported in Ref. [15]. (c) Profile of electron density in the RF discharge (ambient) plasma, based on Langmuirprobe measurements reported in Ref. [16].
section initiates a hadronic cascade, including both charged and neutral pions. The longer-lived charged pions (π ± ) predominantly undergo further hadronic interactions in the target, whereas neutral pions (π 0 ) decay promptly into a directional beam of GeV-energy γ-rays. In the tantalum section, these γ-rays drive an electromagnetic cascade, generating electron-positron pairs. The pair production is further amplified by the Bremsstrahlung of electrons and positrons in the Bethe-Heitler process [19]. A more detailed discussion of the optimization process used to design the target is given elsewhere [20].
The spectrum and energy-resolved average divergence of the electron-positron pairs, as well as a residual population of primary protons, are shown in Fig. 1b. As expected, the proton spectrum peaks at 440 GeV, corresponding to the initial energy of the incident protons. The beam divergence, σ θ , is defined as the standard deviation of θ = p ⊥ /p ∥ , the ratio of transverse to longitudinal momentum components in the laboratory frame. The curves in the plot are based on simulations performed with the Monte Carlo code FLUKA [21,22], and were validated experimentally using data from beamprofile screens and a magnetic spectrometer, as reported in Ref. [15]. The data indicate that electron and positron yields are approximately equal, with pairs having an average Lorentz factor ⟨γ⟩ ∼ 500 and an average divergence ⟨σ θ ⟩ ∼ 25 mrad. The energy spectra follow a power-law distribution, the details of which are given in Ref. [15].
The total positron yield was N + = 2.5 × 10 13 , with a positron-to-electron ratio N + /N -= 0.8, resulting in a peak pair density of n 0 = 5 × 10 11 cm -3 at the entrance to the plasma cell. This estimate is based on FLUKAderived transverse fluence profiles, assumes a pair density equal to twice the local positron density (neglecting the small electron excess), and takes the longitudinal profile of the secondaries to match that of the primary protons.
The electron number density within the RF discharge plasma cell (n 0 ) as a function of position is shown in Fig. 1c. Details of the cell design and plasma density/temperature measurements are provided in Ref. [16], where characteristic electron temperatures are reported to be in the range of 1-10 eV,
This content is AI-processed based on open access ArXiv data.