Materialisation of electrons and positrons

Both energetic electrons and energetic positrons bombard the Earth’s upper atmosphere. Electrons can be taken from any atom by ionising it. But positrons are not so ubiquitous. A few of them are created in the decay of radioactive nuclei, but it’s thought that the vast majority simply materialise together with an electron out of the vacuum. This is predicted to happen spontaneously when the electric field exceeds the critical or “Schwinger field” Ec=1.3×1018 V m1, but the process has never been observed. It can also happen when two gamma-ray photons collide, or when one gamma-ray photon enters a very intense magnetic field. Both processes are thought to occur in pulsar and black-hole magnetospheres, where the photons can be provided by electrons accelerated in a strong electromagnetic wave.

The physical parameters (frequency in Herz on the x-axis, magnetic field units of the critical field, 4×109 T on the y-axis) of strong electromagnetic waves in various objects. Assuming counter-propagating vacuum waves, the regime of strong field QED, where the rest-frame electric field exceeds Ec is within reach of next generation optical lasers. In pulsar winds and jets from active galaxies, particle trajectories are strongly influenced by radiation reaction.

Ultra-intense, optical laser pulses from facilities such as the European project “Extreme Light Infrastructure” are expected to reach 1024 W cm2 in the next few years. This is powerful enough to reproduce the conditions for electron-positron creation found in pulsars and black-hole magnetospheres. Detailed calculations suggest that the energy in the laser beams can be dumped into electrons, positrons and gamma-rays once the intensity exceeds a few times 1023 W cm2.

Monte-Carlo simulation of electron-positron pair-creation in counter-propagating laser beams. The number of pairs created by a single electron trajectory is plotted against beam intensity, with the colour-code giving the fraction of electrons that follow that trajectory. When the number of created pairs exceeds unity, aself-sustaining avalanche process is expected.


Recent contributions

Kirk, J. G., Bell, A. R., Ridgers, C. P., 2013, PPCF
Pair plasma cushions in the hole-boring scenario

Ridgers, C. P.; Brady, C. S.; Duclous, R.; Kirk, J. G.; Bennett, K.; Arber, T. D.; Bell, A. R., 2013, Physics of Plasmas
Dense electron-positron plasmas and bursts of gamma-rays from laser-generated QED plasmas

Brady, C. S.; Ridgers, C. P.; Arber, T. D.; Bell, A. R.; Kirk, J. G. 2012, Physical Review Letters
Laser Absorption in Relativistically Underdense Plasmas by Synchrotron Radiation

Kirk, J.G., Bell, A.R. & Ridgers, C.P. 2012 EPS Conference on Plasma Physics
Electron-Positron Pair Modification of the Hole-boring Scenario in Intense Laser-Solid Interactions

Ridgers, C. P., Brady, C. S., Duclous, R., Kirk, J. G., Bennett, K., Arber, T. D., Robinson, A. P. L., & Bell, A. R. 2012, Physical Review Letters
Dense Electron-Positron Plasmas and Ultraintense γ rays from Laser-Irradiated Solids

I. Arka, PhD thesis, 2011
Non-linear waves in the laboratory and in astrophysics: Pair production in counter-propagating laser beams and strong waves in pulsar winds

Duclous, R., Kirk, J. G., & Bell, A. R. 2011, Plasma Physics and Controlled Fusion
Monte Carlo calculations of pair production in high-intensity laser-plasma interactions

Kirk, J. G., Bell, A. R., & Arka, I. 2009, Plasma Physics and Controlled Fusion Pair production in counter-propagating laser beams

Bell, A. R. & Kirk, John G. 2008, Physical Review Letters Possibility of Prolific Pair Production with High-Power Lasers