arXiv subjects

Alexander Pukhov

Publications and source records attributed to Alexander Pukhov.

At least 19 recordsLinked to original sources

arXiv2026

First-principles simulation of free-electron lasers with the Galilean electromagnetic particle-in-cell method: even harmonics of planar undulators

Free-electron laser (FEL) codes average the electron motion over the undulator period and add the radiation harmonics through on-axis coupling factors. This is accurate for the fundamental, but the even harmonics of a planar undulator, which vanish on the axis, are not modelled at all. We show that the Galilean electromagnetic particle-in-cell method (GEM-PIC), which resolves the electron motion within the undulator period and the carrier of the forward radiation field without a Courant limit, simulates FELs on two CPU cores at two to three times the cost of a time-dependent run of the averaged code GENESIS. The quasi-cylindrical code GEM-PIC-RZ reproduces the coherent spontaneous emission of a planar undulator in each azimuthal mode $m = 0$--3 to within 2\,\% of the radiation integral, and seeded amplifiers to within a few per cent of GENESIS run with the same physics. For a seeded planar FEL at 10.6\,nm it predicts 12\,MW of second-harmonic power at saturation, $1.1\times10^{-3}$ of the fundamental; this harmonic is absent from the averaged model. The unaveraged code Puffin, which has no space charge, gives a comparable level, 9 to 11\,MW for the space-charge-widened and the initial beam size, and the same transverse structure.

physics.plasm-ph↗

arXiv2026

GEM-PIC-RZ: a Galilean electromagnetic particle-in-cell code in quasi-cylindrical geometry with nonuniform grids

Laser-plasma accelerators span scales from the micrometre wavelength of the driving laser to acceleration lengths of millimetres or metres. Full particle-in-cell (PIC) simulations resolve the laser carrier but are constrained by the Courant condition, whereas quasi-static models are computationally efficient but do not resolve it. We present GEM-PIC-RZ, a quasi-cylindrical implementation of the Galilean electromagnetic PIC scheme. The method retains the slow evolution of the forward field and therefore resolves the laser carrier while allowing a propagation step governed by the evolution of the laser and plasma rather than by the cell size. User-defined nonuniform grids and a charge-conserving regularization of the bubble closure support efficient simulations across these scales. Comparisons with the full PIC code FBPIC show agreement for the laser wake, the depletion and redshift of a few-cycle pulse, and ionization injection; the measured CPU time is about 2\,\% of that for FBPIC in the reported runs; the injected charge carries a model uncertainty of about 15\,\% from the regularization. An envelope model, in contrast, underestimates the depletion of the few-cycle pulse fivefold. Self-injection in a sharp density down-ramp is not yet reproduced and remains an open problem.

physics.plasm-ph↗

arXiv2026

QUARZ: a quasi-static particle-in-cell code with non-uniform radial grids for plasma wakefields

We present a quasi-static particle-in-cell scheme for beam-driven plasma wakefields in cylindrical geometry that works on an arbitrary non-uniform radial grid. The field equations are discretized with hat-function finite elements and conservative finite volumes on any monotone set of radial nodes, and the transverse magnetic field is obtained explicitly, without iterations, following Baxevanis and Stupakov. An optional azimuthal mode $m=1$ describes beam offsets, tilts and hosing. The discretization keeps the discrete Gauss law exact, deposits a uniform plasma exactly on every node including the axis, and remains second-order accurate on smoothly stretched grids. Resolution can therefore be placed where the physics needs it. For a tightly focused witness beam that pulls the plasma ions onto the axis, a grid that is fine only near the axis gives the same near-axis fields as a uniform grid with the same axis resolution, with thirty times fewer cells and in thirty times less computing time. Such a grid also resolves the singular closure of a cold-plasma bubble, which a uniform grid hides by smoothing it over one cell; smoothing the plasma sources over a fixed length, chosen independently of the grid, regularizes the closure and makes the wake behind it converge. The scheme is implemented in the open-source C++/Kokkos code QUARZ (Quasistatic Arbitrary-resolution RZ), which runs on CPUs and GPUs and uses MPI with a pipelined decomposition along the co-moving coordinate. Simulations of the self-modulation of the AWAKE proton bunch agree with the three-dimensional code HiPACE++. QUARZ is, to our knowledge, the first production-grade simulation code in plasma physics that is \emph{zero-touch}: all of its source code, tests and documentation were written by an AI agent directed by a physicist who did not write a single line of the code.

physics.plasm-ph↗

arXiv2026

Strongly interacting singlet scalar dark matter during reheating

We revisit the singlet scalar dark matter model in the presence of a non-standard cosmological history prior to radiation domination. We focus on the regime in which the relic abundance is set by 4-to-2 self-annihilations while the dark and visible sectors remain in kinetic equilibrium, i.e. the standard strongly interacting massive particle (SIMP) framework. In the conventional radiation-dominated cosmology, this realization is not viable, as it requires sub-MeV masses and large quartic couplings in tension with bounds on dark matter self-interactions. We show that this conclusion is significantly modified if freeze-out occurs during non-standard cosmological eras. The altered Hubble expansion rate and the possible non-conservation of the standard model entropy change the freeze-out dynamics, allowing the observed relic density to be achieved with perturbative couplings and consistent with astrophysical constraints. We determine the region where SIMP production dominates over the WIMP mechanism and confront the viable parameter space with current and future direct detection and collider bounds.

hep-ph↗

arXiv2026

Radiative Corrections to the Direct Detection of Inelastic Scattering of Higgsino-like Neutralino Dark Matter

The direct detection (DD) of Higgsino-like dark matter (DM) through inelastic scattering processes may provide a promising avenue, along with the elastic scattering, when the mass splitting between the neutral Higgsino pairs is extremely tiny. The mass splitting can be reduced further by adopting the on-shell renormalization for the neutral and charged Higgsinos in the MSSM. Moreover, all the one-loop electroweak (EW) corrections to the three-point vertices for the neutralino(s)-Higgs interactions have been considered, while both elastic and inelastic DM-nucleon scattering cross-sections have been calculated primarily through Higgs exchange. Subsequently, the expected number of scattering events in the latest LUX-ZEPLIN (LZ) DD experiment are also computed. Our results highlight a few scenarios in which the inelastic component may exceed the corresponding elastic component, leading to enhanced direct detection scattering rates.

hep-ph↗

arXiv2026

A plasma photocathode for spin-polarized electron beams via state-selected hydrogen halide photofragments

Spin-polarized electron beams are essential tools for probing fundamental symmetries and for the search beyond the Standard Model. While plasma-based accelerators are a promising pathway towards higher-energy frontiers, they have so far failed to deliver a competitive polarized source: existing proposals are challenging to realize and achievable polarizations remain far below conventional sources. Here, we introduce a photocathode-like scheme, applied to a gas of pre-polarized hydrogen and halogen atoms. A VUV and a visible laser pulse excite the halogen atoms to create a two-component ionization medium, consisting of low-threshold excited halogen atoms and high-threshold polarized hydrogen. Particle-in-cell simulations show witness beams with tens of pC charge retaining up to 97% of the initial polarization, rivaling state-of-the-art conventional sources.

physics.plasm-ph↗

arXiv2026

Galilean Electromagnetic Particle-in-Cell Code

We introduce a Galilean electromagnetic particle-in-cell (GEM-PIC) algorithm, which transforms the full set of Maxwell equations and the Vlasov equation into the boosted coordinates. This approach preserves the electromagnetic structure of the interaction while exploiting scale separation for computational effi ciency. Unlike quasistatic methods, GEM-PIC does not have to distinguish between beam and streaming particles, allowing a self-consistent treatment of particle trapping. The EM-PIC algorithm allows for highly effi cient and accurate simulations of plasma-based wakefield acceleration.

physics.plasm-ph↗

arXiv2026

Radiative depolarization of high-energy electron beams in wakefield accelerators

The preservation of witness beam polarization in wakefield accelerators will be crucial for future collider applications. While extensive theoretical studies on the injection and initial acceleration of polarized electrons exist, a study concerning higher-energy regimes has been neglected thus far. Besides the spin precession usually considered in wakefield-related research, radiative effects could become increasingly relevant at higher energies as the witness electrons perform betatron oscillations during which they will emit photons. In the present study, we use particle-in-cell simulations extended with Monte-Carlo routines to study the influence of radiative spin-flips on beam polarization. We find that at high energies, the importance of radiative effects on beam polarization mainly comes down to the alignment of the witness beam with respect to the wakefield.

physics.plasm-ph↗

arXiv2026

Pinching injection in wakefields for spin-polarized electron beams

Pinching of the driver beam in plasma wakefield acceleration is generally considered an unwanted effect that needs to be mitigated. Here, we propose that this effect can be utilized for the injection of spin-polarized electron beams from hydrogen halide targets into wakefields. Particle-in-cell simulations show that the electron spin is preserved on a level of 50% for a wide range of parameters due to the injection geometry. The presented injection scheme provides a possible pathway to alleviate some of the restrictions associated with pre-polarized hydrogen halide targets.

physics.plasm-ph↗

arXiv2025

Electron-positron pair generation using a single kJ-class laser pulse in a foam-reflector setup

We investigate the process of creating electron-positron pairs from laser-matter interaction in pre-ionised foam targets using particle-in-cell simulations. A high-intensity laser pulse drives electrons via direct laser acceleration up to a cone-shaped reflector. The high-energy electrons interact with the reflected laser pulse, generating abundant pairs. The effects of the plasma-channel shape on the propagation of the laser pulse and subsequent pair production is studied. The results show that the number of Compton emission and Breit-Wheeler pair creation events is highly sensitive to the diffraction of the laser due to its interaction with the foam.

physics.plasm-ph↗

arXiv2025

Kination and the Inert Doublet Model

The inert doublet model is a two-Higgs-doublet extension of the standard model that provides a minimal and versatile framework for frozen-out dark matter. Assuming standard cosmology, if the dark matter mass ranges between approximately 120 GeV and 500 GeV then it turns out to be underabundant, as gauge interactions render its annihilation too efficient. In this work, we show that this mass window becomes allowed in cosmological scenarios where dark matter freeze-out occurs during a period with a stiff equation of state, $w > 1/3$, such as kination. This predictive setup satisfies all current experimental constraints while remaining within the reach of upcoming detection efforts.

hep-ph↗

arXiv2025

Efficient Simulation of Electron-Positron Pair Production in Foam Targets in the low χ-Regime

The generation of electron-positron pairs using direct laser-accelerated electrons and a cone-shaped reflector target for the generation of strong electromagnetic fields is investigated using particle-in-cell simulations. A newly implemented sub-sampling routine for the code vlpl is presented which allows for a better description of quantum electrodynamical processes which would otherwise come at a high computational cost.

physics.plasm-ph↗

arXiv2025

Long Living Hot and Dense Plasma from Relativistic Laser-Nanowire Array Interaction

Long-living, hot and dense plasmas generated by ultra-intense laser beams are of critical importance for laser-driven nuclear physics, bright hard X-ray sources, and laboratory astrophysics. We report the experimental observation of plasmas with nanosecond-scale lifetimes, near-solid density, and keV-level temperatures, produced by irradiating periodic arrays of composite nanowires with ultra-high contrast, relativistically intense femtosecond laser pulses. Jet-like plasma structures extending up to 1~mm from the nanowire surface were observed, emitting K-shell radiation from He-like Ti$^{20+}$ ions. High-resolution X-ray spectra were analyzed using 3D Particle-in-Cell (PIC) simulations of the laser-plasma interaction combined with collisional--radiative modeling (FLYCHK). The results indicate that the jets consist of plasma with densities of $10^{20}$-$10^{22}$ cm$^{-3}$ and keV-scale temperatures, persisting for several nanoseconds. We attribute the formation of these jets to the generation of kiloTesla-scale global magnetic fields during the laser interaction, as predicted by PIC simulations. These fields may drive long-timescale current instabilities that sustain magnetic fields of several hundred tesla, sufficient to confine hot, dense plasma over nanosecond durations.

physics.plasm-ph↗

arXiv2025

Plasma acceleration of polarized particle beams

Spin-polarized particle beams are of interest for applications like deep-inelastic scattering, e.g. to gain further understanding of the proton's nuclear structure. With the advent of high-intensity laser facilities, laser-plasma-based accelerators offer a promising alternative to standard radiofrequency-based accelerators, as they can shorten the required acceleration length significantly. However, in the scope of spin-polarized particles, they bring unique challenges. This paper reviews the developments in the field of spin-polarized particles, focusing on the interaction of laser pulses and high-energy particle beams with plasma. The relevant scaling laws for spin-dependent effects in laser-plasma interaction, as well as acceleration schemes for polarized leptons, ions, and gamma quanta, are discussed.

physics.plasm-ph↗

arXiv2025

Laguerre-Gaussian pulses for spin-polarized ion beam acceleration

Polarized particle sources have a plethora of applications, ranging from deep-inelastic scattering to nuclear fusion. One crucial challenge in laser-plasma interaction is maintaining the initial polarization of the target. Here, we propose the acceleration of spin-polarized Helium-3 from near-critical density targets using high-intensity Laguerre-Gaussian laser pulses. Three-dimensional particle-in-cell simulations show that Magnetic Vortex Acceleration with these modes yields higher polarization on the 90%-level compared to conventional Gaussian laser pulses, while also providing low-divergence beams.

physics.plasm-ph↗

arXiv2025

Interaction of Laguerre-Gaussian laser pulses with borane targets of different hydrogen-boron ratio

We study the interaction of high-intensity Laguerre Gaussian laser pulses with hydrogen-boron compounds targets using 3D particle-in-cell simulations. The ratio of hydrogen to boron is varied throughout different simulation runs as a proxy model for various borane molecules that can be synthesized. We show that the strength of the axial magnetic fields generated via the Inverse Faraday effect depends on the specific ratio of target components, making boranes and the option to tune their composition of interest for proton-boron fusion.

physics.plasm-ph↗

arXiv2025

High-energy polarized electron beams from the ionization of isolated spin polarized hydrogen atoms

We propose a laser-based method for the preparation of high-energy polarized electrons, from the ionization of isolated spin-polarized hydrogen (SPH) atoms. The SPH atoms are prepared from the photodissociation of HCl, using two consecutive UV pulses of ps duration. By appropriately timing and focusing the pulses, we can spatially separate the highly polarized SPH from other unwanted photoproducts, which then act as the target for the acceleration lasers. We show how elastic collisions define number density $n$ and polarization P regimes ($10^{16}\leq$ $n$ $\leq 10^{18}$ cm$^{-3}$, 0.99 $\geq$ P $\geq$ 0.40) for the pre-polarized targets, and use particle-in-cell simulations to demonstrate the method's feasibility.

physics.atom-ph↗

arXiv2025

Proton-Driven Plasma Wakefield Acceleration for Future HEP Colliders

We discuss the main elements of a collider facility based on proton-driven plasma wakefield acceleration. We show that very competitive luminosities could be reached for high energy $e^+e^-$ colliders. A first set of parameters was developed for a Higgs Factory indicating that such a scheme is indeed potentially feasible. There are clearly many challenges to the development of this scheme, including novel RF acceleration modules and high precision and strong magnets for the proton driver. Challenges in the plasma acceleration stage include the ability to accelerate positrons while maintaining necessary emittance and the energy transfer efficiency from the driver to the witness. Since many exciting applications would become available from our approach, its development should be pursued.

physics.acc-ph↗