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Running away and radiating

Periodic Reporting for period 4 - PLASMA (Running away and radiating)

Reporting period: 2020-04-01 to 2020-09-30

Particle acceleration and radiation in plasmas has a wide variety of applications ranging from cancer therapy and lightning initiation, to the improved design of fusion devices for large scale energy production. The goal of this project is to develop theoretical and numerical models that describes the acceleration processes and the resulting fast particle dynamics in two focus areas: magnetic fusion plasmas and laser-produced plasmas. The fundamental questions that are addressed in this project are: how are the particles raised from the thermal level to higher energies, what are the dynamics of and the radiation emitted by these particles and how do they affect the rest of the plasma.

Understanding runaway electron dynamics in suddenly cooling impure plasmas is vital for developing schemes aimed at mitigating their effects in future fusion devices. As the plasma current in present devices cannot be increased above a few mega-amperes, experimental simulation of high-current tokamak disruptions is not possible. Experimental data must be used for validation of theoretical and numerical models, that will be exploited to predict runaway generation and suggest mitigation strategies in next-step fusion devices. This is at the focus of the project and through this it has great impact on the safety and reliability of next generation magnetic fusion devices.

The electron and ion beams produced in plasma-based accelerators have very attractive properties, e.g. ultra-short duration at the source and large particle number. Plasma-based particle and radiation sources could therefore be suitable for a wide range of novel applications, such as ultrafast studies of condensed matter, radiotherapy, injectors for conventional accelerators. The PLASMA project provides new theoretical tools in the field of plasma-based acceleration, and suggests methods for optimizing plasma-based acceleration regarding stability and efficiency. This will bring plasma-based radiation sources closer to applications.
The accurate treatment of collisions and radiation is of paramount importance for modelling fast electron dynamics in plasmas. We analyzed the effect of collisional nonlinearities in the Fokker-Planck description, bremsstrahlung and synchrotron radiation emission, as well as the full Boltzmann operator including both small-angle and large-angle collisions. We demonstrated that interaction with partially ionized atoms greatly affects fast-electron dynamics by enhancing the rates of angular deflection and energy loss. We determined the effective critical electric field for runaway generation, and analyzed the avalanche growth rate. Based on kinetic simulations we built a neural network for rapid evaluation of runaway generation rates.

We investigated the dynamics of positrons in the presence of strong electric fields, and derived the rate of created positrons that become runaway accelerated or thermalized. We have assessed the energy dependence of the transport of relativistic electrons in perturbed magnetic fields and investigated the effect of magnetic perturbations on the runaway avalanche.

We developed a synthetic diagnostic tool to simulate the images produced by runaway electron radiation emission and used it to analyze experimental data from several tokamaks. We performed self-consistent modelling of runaway electron dynamics, in particular we investigated the effect of massive material injection, which is pivotal in the design of safety systems for magnetic fusion devices.

We discovered novel aspects of particle acceleration and radiation generation in laser-produced plasmas, by investigating the effect of colliding pulses, structured pulses with orbital angular momentum, and the interaction of lasers with nanostructured targets, micro-scale foils and waveguides. We found a new proton acceleration mechanism that relies on a laser-induced relativistic electron vortex in a near-critical-density plasma with a density gradient normal to the target surface. We showed that a strong enhancement of proton acceleration can be achieved by splitting the laser pulse into two parts of equal energy and opposite incidence angles, leading to a standing wave pattern at the front side of the target. Furthermore, we found that setups with colliding pulses could be effective in increasing the proton number if we employ thin foils.

We investigated collisional effects on electrostatic shock dynamics, and fast collisional electron heating in thin-foil targets driven by an ultraintense short pulse laser. We showed that multimillijoule terahertz radiation can be produced from laser interactions with microplasma-waveguides and laser-foil interactions. We suggested a new mechanism for generation of frequency tunable isolated relativistic sub-cycle pulses and proposed an experimental setup for generating a train of sub-femtosecond X-ray pulses.

We proposed an experimental setup which can be used to trigger ultrafast relativistic magnetic reconnection in a magnetically dominated regime, by a terawatt-millijoule-class laser interacting with a micro-scale plasma slab. This allows access to an unprecedented regime of reconnection in the laboratory.
We have derived sophisticated models for collisional nonlinearities, effect of bremsstrahlung and synchrotron radiation, magnetic perturbations and partially ionized atoms. We developed synthetic radiation diagnostic tools which are essential for experimental comparisons. Our numerical tools provide the means to simulate runaway dynamics self-consistently and allow for systematic comparisons with experimentally measurable quantities. We have discovered several novel aspects of particle acceleration and radiation generation in laser-produced plasmas, using a wide range of methods, e.g. colliding pulses, shock waves, structured pulses with orbital angular momentum, nanostructured targets, micro-scale foils and waveguides.
Synchrotron radiation from a beam of relativistic runaway electrons