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Hartmut Ruhl

Publications and source records attributed to Hartmut Ruhl.

At least 19 recordsLinked to original sources
arXiv2025

Reply to "Comments to Marvel Fusions Mixed Fuels Reactor Concept"

In "arXiv:2312.13429" Lackner et al. use standard methods to decide if it is possible to ignite mixed fuels. They correctly identify that the increased radiation losses make ignition significantly more challenging than for pure DT fuels, since this leads to higher ignition temperatures. Further, they conclude that at those temperatures the reduced electronic $\alpha$-stopping makes ignition impossible. We show that this conclusion is not correct. The model used for $\alpha$-stopping by Lackner et al. is only approximately correct for low temperatures and hydrogen isotopes. By extending the $\alpha$-stopping model to include ionic $\alpha$-stopping we show in \cite{ruhlkornarXiv5} that the contribution of ionic $\alpha$-particle stopping cannot be neglected. The ionic $\alpha$-stopping together with the neutron stopping, which is also neglected by Lackner et al., lead to elevated ion temperatures implying $kT_i > kT_e$. Those three effects combined lead us to the conclusion, that ignition of mixed fuels is indeed possible with far reaching implications, contrary to the analysis by Lackner.

physics.plasm-ph↗
arXiv2024

Properties of non-cryogenic DTs and their relevance for fusion

In inertial confinement fusion, pure deuterium-tritium (DT) is usually used as a fusion fuel. In their paper \cite{gus2011effect}, Guskov et al. instead propose using low-Z compounds that contain DT and are non-cryogenic at room temperature. They suggest that these fuels (here called non-cryogenic DTs) can be ignited for $\rho_{DT} R \geq 0.35 \, gcm^{-2}$ and $kT_{e} \geq 14 \, keV$, i.e., parameters which are more stringent but still in the same order of magnitude as those for DT. In deriving these results the authors in \cite{gus2011effect} assume that ionic and electronic temperatures are equal and consider only electronic stopping power. Here, we show that at temperatures greater than 10 keV, ionic stopping power is not negligible compared to the electronic one. We demonstrate that this necessarily leads to higher ionic than electronic temperatures. Both factors facilitate ignition compared to the model used in \cite{gus2011effect} showing that non-cryogenic DT compounds are more versatile than previously known. In addition, we find that heavy beryllium borohydride ignites more easily than heavy beryllium hydride, the best-performing fuel found by Guskov et al. Our results are based on an analytical model that incorporates a detailed stopping power analysis, as well as on numerical simulations using an improved version of the community hydro code MULTI-IFE. Alleviating the constraints and costs of cryogenic technology and the fact that non-cryogenic DT fuels are solids at room temperature open up new design options for fusion targets with $Q>100$ and thus contribute to the larger goal of making inertial fusion energy an economically viable source of clean energy. In addition, the discussion presented here generalizes the analysis of fuels for energy production.

physics.plasm-ph↗
arXiv2023

Numerical validation of a volume heated mixed fuel concept

While the underlying physics of the ICF approach to nuclear fusion is well understood and a technological implementation of the indirect drive variant of the ICF paradigm has recently been given at NIF commercially viable ICF concepts for energy production and beyond are still under investigation. In the present paper we propose core elements of a novel fast direct drive mixed fuel ICF concept that might be commercially viable. It makes use of ultra-short, ultra-intense laser pulses interacting with nano-structured accelerators embedded into the mixed fuel context. The embedded accelerator technology promises to be highly efficient and capable of fast fuel heating without fuel pre-compression but is not the focus of the paper. It is the predominant purpose of the mixed fuel concept to avoid cryogenic fuels since specific chemical compounds exist that are capable of chemically binding $\text{DT}$. To which extent mixed fuel concepts can work is investigated in the paper. Under the assumption that the proposed direct drive fast heating concept is capable of rapidly heating the fuel uniformly to sufficiently high temperatures it is found with the help of MULTI, an ICF community code, that a $\text{pBDT}$ mixed fuel design can reach a target yield $Q_T >1$ with $\text{MJ}$ level external isochoric heating. The simulations are used to validate a theoretical scaling model of the mixed fuel reactive hydro flows. The paper does not present a reactor point design.

physics.plasm-ph↗
arXiv2023

Uniform volume heating of mixed fuels within the ICF paradigm

The paper investigates the feasibility of achieving uniform high-power volume heating for a fusion reactor concept employing a mixed fuel composition involving $\text{pBDT}$. The realm of mixed fuel fusion concepts remains relatively unexplored. The pursuit of uniform high-power volume heating presents a technological challenge, yet it bears ramifications for fusion reactor designs. In this study, we introduce the proposition of employing embedded nano-structures that represent structured foams. These structured foams interact with short-pulse lasers, thereby achieving ultra-high power volume heating both within the fuel and the adjacent hohlraums. Notably, structured foams exhibit superior efficiency compared to unstructured foams, plasma or surfaces when it comes to absorbing high-power, short-pulse lasers. The suggested incorporation of these embedded structured foams interacting with an array of ultra-short laser pulses offers a high laser absorption power density, along with meticulous control over energy and power distribution within the fuel, both in spatial and temporal dimensions. This holds the potential for the realization of fusion reactors characterized by straight-forward designs and low complexity, where $Q_F \approx Q_T > 1$ is expected for the fuel and target gains. Depending on the fuel composition they can be strong neutron sources.

physics.plasm-ph↗
arXiv2022

High current ionic flows via ultra-fast lasers for fusion applications

In the present paper we introduce a new accelerator concept for ions. The accelerator is nano-structured and can consist of a range of materials. It is capable of generating large ionic currents at moderate ion energies. The nano-structures can be tailored towards the accelerator thus being capable of driving ion beams with very high efficiency. The accelerator is powered by laser arrays consisting of many repetitive and efficient lasers in the $100 \, \text{J}$ range with ultra-short intense laser pulses. Combining nano-structures and the proposed ultra-short pulse lasers can lead to new levels of spatio-temporal control and energy efficiency for fusion applications.

physics.acc-ph↗
arXiv2022

A first numerical investigation of a recent radiation reaction model and comparison to the Landau-Lifschitz model

In recent work we presented an explicit and non-perturbative derivation of the classical radiation reaction force for a cut-off modelled by a special choice of tubes of finite radius around the charge trajectories. In this paper, we provide a further, simpler and so-called reduced radiation reaction model together with a systematic numerical comparison between both the respective radiation reaction forces and the Landau-Lifschitz force as a reference. We explicitly construct the numerical flow for the new forces and present the numerical integrator used in the simulations, a Gauss-Legendre method adapted for delay equations. For the comparison, we consider the cases of a constant electric field, a constant magnetic field, and a plane wave. In all these cases, the deviations between the three force laws are shown to be small. This excellent agreement is an argument for plausibility of both new equations but also means that an experimental differentiation remains hard. Furthermore, we discuss the effect of the tube radius on the trajectories, which turns out to be small in the regarded regimes. We conclude with a comparison of the numerical cost of the corresponding integrators and find that the integrator of the reduced radiation reaction to be numerically most and the integrator of Landau-Lifschitz least efficient.

physics.comp-ph↗
arXiv2022

High-charge 10 GeV electron acceleration in a 10 cm nanoparticle-assisted hybrid wakefield accelerator

In an electron wakefield accelerator, an intense laser pulse or charged particle beam excites plasma waves. Under proper conditions, electrons from the background plasma are trapped in the plasma wave and accelerated to ultra-relativistic velocities. We present recent results from a proof-of-principle wakefield acceleration experiment that reveal a unique synergy between a laser-driven and particle-driven accelerator: a high-charge laser-wakefield accelerated electron bunch can drive its own wakefield while simultaneously drawing energy from the laser pulse via direct laser acceleration. This process continues to accelerate electrons beyond the usual decelerating phase of the wakefield, thus reaching much higher energies. We find that the 10-centimeter-long nanoparticle-assisted wakefield accelerator can generate 340 pC, 10.4+-0.6 GeV electron bunches with 3.4 GeV RMS convolved energy spread and 0.9 mrad RMS divergence. It can also produce bunches with lower energy, a few percent energy spread, and a higher charge. This synergistic mechanism and the simplicity of the experimental setup represent a step closer to compact tabletop particle accelerators suitable for applications requiring high charge at high energies, such as free electron lasers or radiation sources producing muon beams.

physics.plasm-ph↗
arXiv2022

A laser-driven mixed fuel nuclear fusion micro-reactor concept

We propose a laser-driven near-solid density nano-structured micro-reactor concept operating with mixed nuclear fusion fuels. The micro-reactor is capable of making use of a range of neutronic and aneutronic fuels. Its core parts consists of an embedded nanoscopic nuclear fuel based laser-driven nano-accelerator that is capable of producing non-thermal fuel distributions almost instantly.

physics.plasm-ph↗
arXiv2022

HEWES: Heisenberg-Euler Weak-Field Expansion Simulator

Vacuum polarization, a key prediction of quantum theory, can cause a variety of intriguing phenomena that can be triggered by high-intensity laser pulses. The Heisenberg-Euler theory of the quantum vacuum supplements Maxwell's theory of electromagnetism with nonlinear photon-photon interactions mediated by vacuum fluctuations. This work presents a numerical solver for the leading weak-field Heisenberg-Euler corrections. The present code implementation reaches an accuracy of order thirteen in the numerical scheme and takes into account up to six-photon interactions. Since theoretical approaches are limited to approximations and the experimental requirements for signal detection are high, the need for support from the numerical side is apparent.

hep-ph↗
arXiv2021

Numerical Simulations of the Nonlinear Quantum Vacuum in the Heisenberg-Euler Weak-Field Expansion

The Heisenberg-Euler theory of the quantum vacuum supplements Maxwell's theory of electromagnetism with nonlinear light-light interactions. These originate in vacuum fluctuations, a key prediction of quantum theory, and can be triggered by high-intensity laser pulses, causing a variety of intriguing phenomena. A highly accurate numerical scheme for solving the nonlinear equations due to the leading orders of the Heisenberg-Euler weak-field expansion is presented. The algorithm possesses an almost linear vacuum dispersion relation even for comparably small wavelengths and incorporates a nonphysical modes filter. The implemented solver is tested in one spatial dimension against a set of known analytical results for vacuum birefringence and harmonic generation. More complex scenarios for harmonic generation are demonstrated in two and three spatial dimensions.

physics.comp-ph↗
arXiv2021

On Radiation Reaction in Classical Electrodynamics

The Lorentz-Abraham-Dirac equations (LAD) may be the most commonly accepted equation describing the motion of a classical charged particle in its electromagnetic field. However, it is well known that they bare several problems. In particular, almost all solutions are dynamically unstable, and therefore, highly questionable. The question remains whether better equations of motion than LAD can be found to describe the dynamics of charges in the electromagnetic fields. In this paper we present an approach to derive such equations of motions, taking as input the Maxwell equations and a particular charge model only, similar to the model suggested by Dirac in his original derivation of LAD in 1938. We present a candidate for new equations of motion for the case of a single charge. Our approach is motivated by the observation that Dirac's derivation relies on an unjustified application of Stokes' theorem and an equally unjustified Taylor expansion of terms in his evolution equations. For this purpose, Dirac's calculation is repeated using an extended charge model that does allow for the application of Stokes' theorem and enables us to find an explicit equation of motion by adapting Parrott's derivation, thus avoiding a Taylor expansion. The result are second order differential delay equations, which describe the radiation reaction force for the charge model at hand. Their informal Taylor expansion in the radius of the charge model used in the paper reveals again the famous triple dot term of LAD but provokes the mentioned dynamical instability by a mechanism we discuss and, as the derived equations of motion are explicit, is unnecessary.

physics.class-ph↗
arXiv2020

Linear Breit-Wheeler pair production by high-energy bremsstrahlung photons colliding with an intense X-ray laser pulse

A possible setup for the experimental verification of linear Breit-Wheeler pair creation of electrons and positrons in photon-photon collisions is studied theoretically. It combines highly energetic bremsstrahlung photons, which are assumed to be generated by an incident beam of GeV electrons penetrating through a high-$Z$ target, with keV photons from an X-ray laser field, which is described as a focused Gaussian pulse. We discuss the dependencies of the pair yields on the incident electron energy, target thickness, laser parameters, and collision geometry. It is shown that, for suitable conditions which are nowadays in reach at X-ray laser facilities, the resulting number of created particles seems to be well accessible for enabling the first experimental observation of the linear Breit-Wheeler process $γγ\to e^+e^-$.

hep-ph↗
arXiv2018

Jet Observable for Photons from High-Intensity Laser-Plasma Interactions

The goals of discovering quantum radiation dynamics in high-intensity laser-plasma interactions and engineering new laser-driven high-energy particle sources both require accurate and robust predictions. Experiments rely on particle-in-cell simulations to predict and interpret outcomes, but unknowns in modeling the interaction limit the simulations to qualitative predictions, too uncertain to test the quantum theory. To establish a basis for quantitative prediction, we introduce a `jet' observable that parameterizes the emitted photon distribution and quantifies a highly directional flux of high-energy photon emission. Jets are identified by the observable under a variety of physical conditions and shown to be most prominent when the laser pulse forms a wavelength-scale channel through the target. The highest energy photons are generally emitted in the direction of the jet. The observable is compatible with characteristics of photon emission from quantum theory. This work offers quantitative guidance for the design of experiments and detectors, offering a foundation to use photon emission to interpret dynamics during high-intensity laser-plasma experiments and validate quantum radiation theory in strong fields.

physics.plasm-ph↗
arXiv2017

An implicit ODE-based numerical solver for the simulation of the Heisenberg-Euler equations in 3+1 dimensions

With the advent of ultra-high power lasers the nonlinear nature of the vacuum of quantum electrodynamics (QED) can be probed. Due to the highly nonlinear structure of the underlying equations new numerical algorithms are required. A numerical scheme for simulating the nonlinear optical effects of the QED vacuum in up to 3 spatial dimensions plus time is derived. Its properties are discussed. The validity of the numerical approach is verified with the help of known analytic results. The algorithm is used to explore nonlinear all optical effects of the nonlinear vacuum for which analytic methods are inefficient or impossible.

physics.comp-ph↗
arXiv2017

Generation of controllable plasma wakefield noise in particle-in-cell simulations

Numerical simulations of beam-plasma instabilities may produce quantitatively incorrect results because of unrealistically high initial noise from which the instabilities develop. Of particular importance is the wakefield noise, the potential perturbations that have a phase velocity which is equal to the beam velocity. Controlling the noise level in simulations may offer the possibility of extrapolating simulation results to the more realistic low-noise case. We propose a novel method for generating wakefield noise with a controllable amplitude by randomly located charged rods propagating ahead of the beam. We also illustrate the method with particle-in-cell simulations. The generation of this noise is not accompanied by parasitic Cherenkov radiation waves.

physics.plasm-ph↗
arXiv2016

Equations of motion for charged particles in strong laser fields

Starting from the Dirac equation coupled to a classical radiation field a set of equations of motion for charged quasi-particles in the classical limit for slowly varying radiation and matter fields is derived. The radiation reaction term derived in the paper is the Abraham-Lorentz-Dirac term.

physics.class-ph↗
arXiv2016

Extreme Scale-out SuperMUC Phase 2 - lessons learned

In spring 2015, the Leibniz Supercomputing Centre (Leibniz-Rechenzentrum, LRZ), installed their new Peta-Scale System SuperMUC Phase2. Selected users were invited for a 28 day extreme scale-out block operation during which they were allowed to use the full system for their applications. The following projects participated in the extreme scale-out workshop: BQCD (Quantum Physics), SeisSol (Geophysics, Seismics), GPI-2/GASPI (Toolkit for HPC), Seven-League Hydro (Astrophysics), ILBDC (Lattice Boltzmann CFD), Iphigenie (Molecular Dynamic), FLASH (Astrophysics), GADGET (Cosmological Dynamics), PSC (Plasma Physics), waLBerla (Lattice Boltzmann CFD), Musubi (Lattice Boltzmann CFD), Vertex3D (Stellar Astrophysics), CIAO (Combustion CFD), and LS1-Mardyn (Material Science). The projects were allowed to use the machine exclusively during the 28 day period, which corresponds to a total of 63.4 million core-hours, of which 43.8 million core-hours were used by the applications, resulting in a utilization of 69%. The top 3 users were using 15.2, 6.4, and 4.7 million core-hours, respectively.

cs.DC↗