Qian Qian

We propose and simulate a laboratory platform to study the effects of positrons in magnetic reconnection using laser-driven capacitor coils. Using particle-in-cell simulations, we show that externally injected MeV electron-positron pairs are trapped in the coil current sheet, significantly modifying the reconnection dynamics and particle acceleration. These pairs increase the reconnection rate by a factor of approximately 8, which Ohm's law decomposition reveals to be driven by the divergence of the generalized pressure tensor. Based on their high energy and magnetization, the pairs also substantially broaden the diffusion region. Particle tracking simulations in realistic coil magnetic fields further demonstrate that injected pairs can remain confined for several picoseconds, providing conditions for sustained interaction with the reconnection region. These results establish a near-term pathway to first laboratory studies of positron-influenced reconnection, bridging high-energy-density experiments with pair-dominated astrophysical environments.

Laser and plasma wakefield accelerators are promising for many applications such as future TeV electron–positron colliders and x-ray free electron lasers (XFELs). These applications require high beam quality in terms of energy spread, emittance, and shot-to-shot stability. To achieve high beam quality, one has to precisely characterize the beam phase space. This is difficult owing to the highly nonlinear acceleration process and the sub-micrometer, sub-femtosecond resolutions required for diagnostics. Here, we propose a new single-shot method for reconstructing the longitudinal phase space of electron beams in laser-driven or particle beam-driven plasma wakefield accelerators. This method is enabled by a laser deflector regime, where an independently controllable laser pulse co-propagating with electrons inside a plasma wakefield modulates the electron motion leading to distinct periodic structures in the angularly resolved spectra. A constrained theoretical model, benchmarked by particle-in-cell simulations, was developed to reproduce the modulated electron spectra and fully reconstruct the longitudinal phase-space distribution of electron beams with a temporal resolution of half a laser period. The reconstruction provides the slice energy spread of the electron beam, which plays a critical role in the performance of applications such as XFELs. We also demonstrate that this laser deflector method can be applied to electron beams from traditional accelerators by periodically modulating the electron spectrum via direct-laser acceleration in vacuum.

We demonstrate that quantum and classical radiation effects can become non-negligible for TeV-class beams propagating through plasma channels typical of staged plasma accelerators. Although the quantum nonlinearity parameter χ e remains small under currently envisioned experimental conditions, the cumulative influence of radiation over long acceleration distances can lead to significant modifications to the beam’s energy spread, emittance, and polarization. Our analytic models, validated by particle-in-cell simulations, highlight that for standard Gaussian beams, the orbit-induced energy spread dominates over quantum stochastic effects but can be mitigated by tailoring the beam profile, for example, through ring-shaped transverse distributions. In regimes where the radiation reaction approaches the accelerating force, the emittance may be cooled, forming distinctive ring-shaped phase-space structures. Finally, we analyze the influence of the radiation effect on spin transport inside the wakefield. These findings underscore the importance of considering both classical and quantum radiation dynamics in the design and optimization of future high-intensity plasma accelerators.

Frequency up-shifting of laser light in a beam-driven plasma wakefield has the potential to provide high-intensity sources of short wavelength radiation. Simulations have demonstrated that a laser pulse can undergo large frequency shifts, limited only by the drive beam energy, when the plasma density is tailored to match the accelerating phase of the wake to the group velocity of the pulse. Here, we study the dynamical evolution of photons in the 1D1P phase-space vicinity of the plasma wake-phase matching condition. Numerical calculations using a photon kinetic model are validated by direct comparison with 1D and quasi-3D particle-in-cell simulations. These calculations form the basis of a linear theory of the photon dynamics which reveals several important results, including scalings for the properties of the witness pulse and a self-similar solution for the photon phase-space dynamics. One prediction of the analytic theory is that the pulse can be compressed indefinitely, though the amount of compression would ultimately be limited by practical constraints. These results suggest that photon acceleration can provide a novel source of sub-femtosecond, short wavelength radiation.

Creating a plasma dominated by strong-field QED (SFQED) effects is a major goal of new multi-PW laser facilities. This is motivated by the fact that the fundamental dynamics of such plasmas are poorly understood and play an important role in the electrodynamics of extreme astrophysical environments such as pulsar magnetospheres. The most obvious observable for which such a regime has been reached is the production of a bright flash of X-rays, but distinguishing this from hard X-rays induced by bremsstrahlung radiation is a major challenge. Here we show that the photons from the X-ray flash are highly polarised relative to the unpolarised background, indicating that the SFQED plasma has indeed been produced. For a laser of intensity >10^{21} Wcm^{-2} impinging on a solid Al target, the photons of the flash with energy >10 keV are >65% polarised. ‡ Note that the hard photons are often referred to as gamma-rays for photon energies > 1 MeV. We will refer to all hard photons as X-rays for simplicity. This is because we consider hard photons with 1-100 keV energies as well as those above 1 MeV.

The Gérard Mourou Center for Ultrafast Optical Science (CUOS) at the University of Michigan has constructed a new high-power laser user facility called ZEUS (the Zettawatt Equivalent Ultrashort pulse laser System), which is funded by the National Science Foundation (NSF). The ZEUS currently operates at a power exceeding 2 PW with a pulse duration of 25 fs. The facility has three shielded target areas mainly dedicated to external user experiments. The development of the laser facility, specifically designed to enable frontier plasma physics experiments, is described.

We report on a single-shot longitudinal phase-space reconstruction diagnostic for electron beams in a laser wakefield accelerator via the experimental observation of distinct periodic modulations in the angularly resolved spectra. Such modulated angular spectra arise as a result of the direct interaction between the ultrarelativistic electron beam and the laser driver in the presence of the wakefield. A constrained theoretical model for the coupled oscillator, assisted by a genetic algorithm, can recreate the experimental electron spectra and, thus, fully reconstructs the longitudinal phase-space distribution of the electron beam with a temporal resolution of approximately 1.3 fs. In particular, it reveals the slice energy spread of the electron beam, which is important to measure for applications such as x-ray free electron lasers. In our experiment, the root-mean-square energy spread retrieved is bounded at 9.9 MeV, corresponding to a 0.9%–3.0% relative spread, despite the overall GeV energy beam having approximately 100% relative energy spread.

The Zettawatt-Equivalent Ultrashort pulse laser System (ZEUS) at the University of Michigan is a user facility supported by the National Science Foundation. The name ZEUS refers to the collision of a petawatt (PW) laser pulse with a GeV energy electron beam, producing the equivalent of a “Zettawatt” power laser interaction (1021 W) in the rest frame of the electron beam. The facility aims to enable systematic exploration of strong-field quantum electrodynamics processes by producing synchronized 2.5 PW and 0.5 PW pulses. The ZEUS target area has been designed to enable two collision geometries: one in which the 2.5 PW pulse drives laser wakefield acceleration (LWFA) to generate high-energy electron beams that collide with the 0.5 PW pulse, and another in which the 0.5 PW pulse drives LWFA and electrons collide with the 2.5 PW pulse focused to ultrahigh intensity. This paper describes initial designs and preparations for zettawatt-equivalent experiments at ZEUS.

We have designed a new filter pack array to measure angular variations in x-ray spectra during a single shot. The filter pack was composed of repeating identical columns of aluminum and copper filters of varying thicknesses. These columns were located at different positions to measure the spectrum at each corresponding angle. This array was utilized in an experiment to measure the energy evolution of betatron x rays in a laser wakefield accelerator by curving the wakefield with a transverse density gradient, streaking the x rays across the array in front of an x-ray charge-coupled device (CCD) camera. After subtracting the background and “flattening” the image to remove spatial nonuniformities, a critical energy was calculated for each position that produced the best agreement with the measured signal. There was a clear change in critical energy with angle, shedding light on the dynamics of the electrons that traveled through the accelerator. These angles correspond to distinct emission times, covering a timescale of tens of picoseconds. The filter pack was capable of recovering these angular details without the impact of errors introduced by shot-to-shot variability.