Thesis for the degree of Doctor of Philosophy Intense laser-plasma interactions Joel Magnusson Department of Physics Chalmers University of Technology Gothenburg, Sweden, 2019 Intense laser-plasma interactions Joel Magnusson ISBN 978-91-7905-198-3 c Joel Magnusson, 2019 Doktorsavhandlingar vid Chalmers tekniska högskola Ny serie nr 4665 ISSN 0346-718X Department of Physics Chalmers University of Technology SE–412 96 Gothenburg Sweden Telephone +46 (0)31-772 1000 Cover: Illustration of an electric dipole wave showing the electric (red) and magnetic (blue) field lines. Chalmers Reproservice Gothenburg, Sweden, 2019 Intense laser-plasma interactions JOEL MAGNUSSON Department of Physics Chalmers University of Technology Abstract In the interaction of ultra-intense laser fields with matter, the target is rapidly ionized and a plasma is formed. The ability of a plasma to sus- tain acceleration gradients, orders of magnitude larger than achievable with conventional accelerators, has led to a great interest in laser-driven plasma- based particle and radiation sources, with applications in materials science, biology and medicine. In this thesis, two separate, yet highly related, topics are pursued.
The first half of the thesis concerns plasma-based techniques for ion acceleration, through the interaction of intense laser fields with solid density targets. In the most accessible acceleration scheme, the ion acceleration is mediated by a population of suprathermal, hot, electrons produced by the rapid heating of the target surface. We study the effect of adding microstructures to the target surface, show how this affects the distribution of hot electrons and discuss its implications for ion acceleration. We further study a novel acceleration scheme, aimed at achieving controllable ion acceleration using a frequency chirped standing wave.
We analyse the robustness of this scheme, named chirped-standing-wave acceleration, under non-ideal conditions and discuss its prospects and limitations. The second half of the thesis concerns laser-matter interactions where the emission of high-energy photons necessitates a quantum mechanical de- scription of radiation reaction and enables a prolific production of electron- positron pairs. In this regime, we study the interaction of an energetic elec- tron beam with an optimally focused laser field, in the form of a dipole wave, and highlight its capabilities as a multi-GeV photon source. We further dis- cuss the phenomena observed in this setup, in particular investigating the emergence of pair production cascades, and provide a review of previous results.
Finally, we highlight a number of regimes within reach of upcoming laser facilities. Keywords: laser, plasma, ion acceleration, radiation generation, radiation reaction, pair production cascades, particle-in-cell i ii Publications This thesis is based on the following publications: A J. Marklund, Energy partitioning and electron momentum distributions in intense laser-solid interactions, The European Physical Journal D 71, 231 (2017). Gonoskov, Prospects for laser-driven ion acceleration through controlled displace- ment of electrons by standing waves, Physics of Plasmas 25, 053109 (2018).
Bulanov, Laser-particle collider for multi-GeV photon production, Physical Review Letters 122, 254801 (2019). Bulanov, Multiple-colliding laser pulses as a basis for studying high-field high- energy physics, Accepted for publication in Physical Review A.plasm-ph] iii Statement of contribution Paper A: I developed the numerical diagnostics tools used for tracking the par- titioning of energy between different energy channels. I performed all simulations, analysed the data and produced all figures. I prepared the draft of the entire paper, except for parts of the introduction.
I finalized the text for publication together with the co-authors. Paper B: I developed the idea for the paper together with the co-authors and I performed all simulations. I performed all data analysis and I pro- duced all figures. I derived the estimates for when CSWA performs efficiently, leading up to equation 13, and derived the remaining equa- tions together with co-authors.
I prepared the draft of the entire paper, excluding section 2 and part of the introduction. I finalized the text for publication together with the co-authors. Paper C: I developed the concept for the paper together with co-authors. I performed all QED-PIC simulations and the analysis of the related data.
I produced all figures except figure 2 and derived the motivating estimates together with the co-authors. I prepared a majority of the draft, primarily excluding parts of the introduction and the section containing the motivating estimates. I finalized the text for publication together with the co-authors. Paper D: I developed the concept for the paper together with co-authors.
I performed all simulations and data analysis. I produced figures 2, 3 and 5 together with the co-authors and produced all remaining figures. I wrote a majority of the draft in close collaboration with the co- authors. I finalized the text for publication together with the co- authors.
iv Related publications not included in the thesis E L. Korn, Nano and micro structured targets to modulate the spatial profile of laser driven proton beams., Journal of Instrumentation 12, C03040 (2017) F L.Margarone, Manipulation of laser-accelerated proton beam profiles by nanostruc- tured and microstructured targets, Physical Review Accelerators and Beams 20, 081301 (2017). v vi Acknowledgements I would first and foremost like to express my deepest gratitude towards my supervisors Mattias Marklund and Arkady Gonoskov for all of their support, and for continuing to both inspire and encourage me. I would also like to thank Andreas Isacsson for his guidance, as well as for sharing his wisdom whenever I disturb him in his office.
I am grateful to my many colleagues and especially past and present members of my research group for creating a pleasant and comradely environment, and with whom I have enjoyed a great many crappy movies. I would also like to thank Stuart Mangles for his hospitality during my stay at Imperial, Chris Ridgers and Chris Murphy for their hospitality during my visit to York, and Stepan Bulanov for being ever so easy to collaborate with. A special thanks to Tom Blackburn for being the best imaginable office mate and for putting up with me for almost 4 years, as well as to Benjamin Svedung Wettervik for providing daily distractions from the work at hand and for making sure the office sofa is never alone. Finally I would like to express my gratitude to my family and many friends for their support.
Joel Magnusson, 2019 vii viii Contents Abstract i Publications iii Acknowledgements vii 1 Introduction 1 1.1 The road to high-intensity lasers .2 Laser-plasma based acceleration. 8 2 Introduction to plasma physics 11 2. 16 3 Laser-plasma interactions 19 3.1 Single-particle motion in intense fields .1 Non-relativistic motion in a plane wave .2 Relativistic motion in a plane wave .2 Plasma-based acceleration. 26 4 Chirped-standing-wave acceleration 33 4.1 Motivating the need for a different scheme .2 Modelling the chirped laser pulse .3 Chirped standing wave .4 Relativistic self-induced transparency.
38 ix 5 Introduction to radiation reaction 41 5.2 Classical radiation reaction .3 Quantum radiation reaction .1 The Schwinger effect .2 Nonlinear Compton scattering .3 Multi-photon Breit-Wheeler .1 The dipole wave .2 The role of pair production .1 The importance of the field shape .2 Pair production cascades. 62 7 Particle-in-cell scheme 65 7.1 Classical particle-in-cell scheme .2 Extended particle-in-cell scheme (QED-PIC) .1 Assumptions, validity and omitted physics. 72 Bibliography 75 Summary of papers 89 x Chapter 1 Introduction Physics is arguably one of the oldest of sciences and generally concerns the nature of energy and matter. As a science it has expanded tremendously over the centuries, not least compared to the beginning of the 20th century when the theories of quantum mechanics and general relativity revolutionized the science.
Today, physics covers a very broad range of subjects, making it practically impossible for physicists to be experts within every field. Never- theless, physicists often draw inspiration from a common toolbox, applying it in seemingly wildly different areas. Regardless of the field of application, the essence of physics lies in describing the world as accurately as possi- ble using mathematical models. The purpose of this is not only to be able to explain and reproduce what has been observed in experiments, but also to predict new phenomena.
Given enough predictive power the models we create can then also be used as a guide forward and, for this reason, of- ten serves as a strong motivator in the planning and construction of new research infrastructure. However, not all experiments are easily explained and not all phenomena are easy to model. Most physics research today is therefore supported by numerics, one way or another. Not necessarily because the underlying equa- tions are difficult to write down, but because in order to obtain a verifiable or predictive result, the equations must often be solved under realistic con- ditions.
Some problems require a combination of different models, perhaps only valid in different, yet overlapping, regimes. Some problems contain multiple scales and are highly dependent on effects from all of them. Some have unknown initial conditions and some contain all of the above issues. The difficulty then lies in determining what is essential and what is not, and to keep only what is necessary to obtain a valuable result.
For any given problem, this naturally leads to a hierarchy of different models where generality and scope is often traded for tractability. While useful results are regularly obtained from simpler models, even using analytics alone, some 1 2 CHAPTER 1. INTRODUCTION questions can only be answered using more comprehensive ones, often re- quiring large-scale simulations. In this thesis we generally concern ourselves with the interaction of strong electromagnetic fields with matter.
This subject naturally falls into the cat- egory of plasma physics, which itself is mainly a combination of classical electrodynamics and statistical physics. Depending on the problem state- ment, it can also contain significant elements of quantum mechanics, in particular in relation to atomic and nuclear physics. While the contribu- tion from these two topics will remain largely unexplored, given the field strengths considered in parts of this thesis we will discuss effects due to quantum electrodynamics. As may already have become apparent, the topic of this thesis carries many of the issues detailed earlier.
On its own, plasma physics contains a rich set of complex phenomena and is characterized by the collective motion of its constituents. Coupled with the influence of strong electromagnetic fields, hereafter assumed to be generated using a powerful laser, the interac- tion becomes highly nonlinear. Accurate description of the physics therefore require large-scale simulations, not seldom utilizing thousands of processors for several hours, and constitutes the main tool in writing this thesis. This is supported by simplified analytical and numerical models, predominantly based on single-particle dynamics in a given field, in order to gain further insights where applicable.
Even so, the computer models used to simulate the physics are only accurate to a certain extent and important aspects are left out for the benefit of computability. In particular, the process of ioniza- tion is commonly omitted and the initial stages of the interaction is often replaced by an experienced guess on the density and ionization level of the plasma. This is not due to laziness, but reflects our lack of knowledge in key features of the experiments, on which a more accurate description de- pends. It is a consequence of the initial plasma formation occurring over a much larger time scale, and at a much lower intensity, than suggested by the parameters of the main pulse.
Getting back to the subject of this thesis, it more specifically concerns the science of laser-based particle and light sources and is centred around two related topics (i) acceleration of protons through laser-solid interaction and (ii) generation of high-energy photons through laser-beam interaction. Central to both of these topics is the transfer of energy to the particles of interest, which can be either transferred from the field itself or simply me- diated by it, while simultaneously considering other source properties, such as particle numbers and collimation. In relation to this, the thesis covers the basics of laser-plasma and laser-particle interactions, and discusses the principal mechanisms of transferring energy before addressing the papers on which the thesis is based.