Creating observable QED collective plasma effects*

To observe in laboratory plasmas the QED plasma regime, it is crucial not just to create a pair plasma – but to create a pair plasma such that collective effects can be observed.   Colliding intense laser beams produces pairs, but collective plasma effects are then notoriously hard to observe [1]. The small pair plasma is only micron-scale.  Compounding the difficulty in observation, the plasma is also moving at relativistic speeds, making the pairs heavy, which diminishes the plasma frequency and the associated collective effects. So, even if produced, collective QED effects would be hard to see. The coupled "production-observation" problem can be approached instead by colliding a relativistic electron beam with a less intense laser.  This creates pairs that have larger plasma frequency, made even larger as they slow down by reversing direction due to the laser pressure [2-5].  Signatures of collective pair plasma effects in the QED cascades then appear in exquisite detail through plasma-induced frequency upshifts in the laser spectrum. Further distinctive features include a chirp to the frequency upshift with parametric dependencies. The electron beam and laser technologies are available, so, if ultra-dense electron beams were to be co-located with multi-PW lasers [6,7], this solution to the coupled production-observation problem means that strong-field quantum and collective pair plasma effects can in fact be explored with existing technology. 

[1] H. Chen, F. Fiuza, Phys. Plasmas 30, 020601 (2023). 

[2] K. Qu, S. Meuren, N. J. Fisch, PRL 127, 095001 (2021).

[3] K. Qu, S. Meuren, N. J. Fisch, Phys. Plasmas 29, 042117 (2022).

[4] A. Griffith, K. Qu, N. J. Fisch, Phys. Plasmas 29, 073104 (2022).

[5] K. Qu, S. Meuren, N. J. Fisch, Plasma Phys. Control. Fusion 65, 034007 (2023). 

[6] S. Meuren et al., arXiv:2002.10051 (2020). 

[7] S. Meuren et al., arXiv:2105.11607 (2021).