Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - PLASMA (Running away and radiating)

Teaser

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 build a flexible ensemble of...

Summary

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 build a flexible ensemble of 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: (1) how are the particles raised from the thermal level to higher energies, (2) what are the dynamics of and the radiation emitted by these particles and (3) how do the superthermal particles affect the rest of the plasma. Radiation here is used in a broad sense: it includes electromagnetic waves and particle generation (e.g. neutron or positron production).

The project is focused on questions relevant to magnetic fusion plasmas and laser-produced plasmas. The solutions, however, are constructed in a general way, which allows extension to other types of plasma (e.g. magnetospheric plasmas, solar flares). The motivation to study charged particle beam formation is different in the two focus areas: in magnetic fusion plasmas an important concern is the generation of runaways and in laser-produced plasmas the interest concerns advanced radiation sources. It is from these that the title of the project, ``Running away and Radiating\'\', stems.

Work performed

Building a generalized model for collisions is of paramount importance for the project and it required new theoretical and numerical developments. We have implemented a relativistic nonlinear Fokker-Planck collision operator, which will be a considerable improvement compared with the current state-of-the-art. Using this numerical tool we showed that transition to electron slide-away will happens at field strengths significantly lower than previously predicted. Considerable work has been devoted also to implementing the full Boltzmann operator that includes both small-angle and large-angle collisions.

We performed kinetic modelling of scenarios with time-dependent plasma parameters; in particular, we investigated hot-tail runaway generation during a rapid drop in plasma temperature. With the goal of studying runaway-electron generation with a self-consistent electric field evolution, we also described the implementation of a collision operator that conserves momentum and energy, and demonstrated its properties. An operator for avalanche runaway-electron generation, which takes the energy dependence of the scattering cross section and the runaway distribution into account, was investigated. We showed that the simplified avalanche model used in previous work can give inaccurate results for the avalanche growth rate (either lower or higher) for many parameters, especially when the average runaway energy is modest, such as during the initial phase of the avalanche multiplication. The developments pave the way for improved modelling of runaway-electron dynamics during disruptions or other dynamic events.

Bremsstrahlung radiation emission is an important energy loss mechanism for energetic electrons in plasmas. We investigated the effect of spontaneous bremsstrahlung emission on the momentum-space structure of the electron distribution, fully accounting for the emission of finite-energy photons. We found that the electrons, accelerated by electric fields, can reach significantly higher energies than expected from simple energy-loss considerations. Furthermore, we showed that the emission of soft photons can contribute significantly to the dynamics of electrons with an anisotropic distribution.

For energetic electrons in magnetized plasmas, the Abraham-Lorentz-Dirac (ALD) radiation force, in reaction to the synchrotron emission, is significant and can be the dominant process limiting the electron acceleration. We investigated the effect of the ALD-force on runaway electron dynamics in a homogeneous plasma using relativistic finite-difference Fokker-Planck codes. Under the action of the ALD force, we found that a non-monotonic feature (a “bump”) is formed in the tail of the electron distribution function if the electric field is sufficiently large. We also observed that the energy of runaway electrons in the bump increases with the electric field amplitude, while the population increases with the bulk electron temperature. The presence of the bump divides the electron distribution into a runaway beam and a bulk population. This mechanism may give rise to beam-plasma type instabilities that could, in turn, pump energy from runaway electrons and alter their confinement.

One of the most common ways for studying runaway electrons in experiments is to take images and measure the spectrum of the synchrotron radiation emitted by them. We developed a synthetic synchrotron diagnostic tool, SOFT, to model the synchrotron cameras used in experiments, and to simulate the images resulting from runaway electron synchrotron emission. The angular and spectral distribution of synchrotron radiation is derived from first-principles, and by utilizing characteristic properties of synchrotron radiation, a simplified model can be derived. Using SOFT, the dependence of the synchrotron image on electron energy, pitch angle, radial distribution and camera location has been investigated, allowing general conclusions to be drawn about the synchrotron

Final results

Predictions show that a major part of the initial plasma current in large tokamaks, such as ITER, can be converted to runaway electron current in a disruption. The subsequent uncontrolled loss of a runaway beam could lead to melting of the plasma facing components. Thus, studies are urgently needed to find ways to mitigate the detrimental effects of runaway electron beams. As the plasma current in present devices cannot be increased above a few megaamperes, experimental simulation of reactor-scale 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 PLASMA proect and through this the project 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, large particle number and high degree of laminarity (low transverse emittance). 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, production of short lived isotopes for medical diagnostics, non-destructive inspection of materials, injectors for conventional accelerators, study of cosmic radiation damage to spacecraft components etc Some applications employ the unique properties of laser-driven beams. In other applications, where short beam pulse duration is not essential, and conventional accelerators can be employed, plasma-based accelerators can still be advantageous if they provide similar beam characteristics with smaller installation size and operation cost. 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.