Upcoming Seminars:

For further information, please contact Dr Matthew Streeter

 

Thursday 27th June 2019 at 2.00 pm, Blackett 539

Laser-driven proton beam profiles in ultra-high fields
Lieselotte Obst-Huebl, HZDR Dresden

"Extreme field gradients intrinsic to relativistic laser-interactions with thin solid targets enable compact multi-MeV proton accelerators with unique bunch characteristics. Protons are accelerated in TV/m fields that are established within the micrometer-scale vicinity of the high- power laser focus. This initial acceleration phase is followed by ballistic proton bunch propagation with negligible space-charge effects over millimeters to hundreds of centimeters to the particle detector or a proton target at a dedicated irradiation site. The detected proton emission distribution can be influenced by the spatio-temporal intensity distribution in the laser focus, electron transport through the target, potential plasma instabilities, as well as local and global target geometry and surface properties.
Substantially extending this picture, our recent results show a critical influence of the milimeter scale vacuum environment on the accelerated proton bunch, where residual gas molecules are ionized by the remnant laser light that is not absorbed into the target plasma but reflected or transmitted. In an experiment with μm-sized hydrogen jet targets, this effect lead to the counter-intuitive observation of laser near-field feature imprints in the detected proton beam profiles. Our results show that the remnant laser pulse induces a quasi-static deflecting field map in the ionized residual background gas that serves as a memorizing medium and allows for asynchronous information transfer to the naturally delayed proton bunch. Occurring under typical experimental laser, target and vacuum conditions, all-optical imprinting needs to be taken into account for sensible interpretation of modulated proton beam profiles.”

Wednesday 5 June 2019 at 2.00 pm, Blacket building room 741 

Deep into the mysteries of disks around stars and how they eject energy into space in the form of magnetic towers

Dr Martín Huarte-Espinosa, University of Houston's Hewlett Packard Enterprise Data Science Institute

The talk is a mix between numerical simulations, high-performance computing, astrophysics and laboratory experiments and is about interdisciplinary research involving Imperial College's MAGPIE lab along with astronomical observations and large-scale computer simulations aimed at understanding the Physics of magnetic towers. Details about the model, motivation, goals and results will be discussed, as well as the difference between magnetic tower-type outflows and the popular magneto-centrifugal launch mechanism in astrophysical plasma dynamics. The simulations allow a clear exploration of the stability conditions of magnetic towers for different conditions. The models of magnetic towers featuring realistic cooling Physics quickly break into magnetized plasma clumps which resemble a good deal of astronomical observations and the behaviour in plasma laboratory experiments too. Clear magnetic field lines layers can be seen in the numerical model, which support the entire magnetic tower structure.

Wednesday 6th March 2019 at 3.00 pm, Blackett building room 741

Acceleration of electrons in the plasma wakefield of a proton bunch at the AWAKE experiment

Prof. Matthew Wing, University College London

Plasma wakefield acceleration is a promising technique to increase the energy or reduce the size of accelerators. Pioneering experiments have shown that a laser pulse or electron bunch drive electric fields in plasma of 10s GV/m and above, orders of magnitude beyond those achieved in conventional RF accelerators. The use of proton bunches is compelling, as they have the potential to drive wakefields and accelerate electrons to high energy in a single accelerating stage. The long proton bunches currectly available can be used, as they undergo self-modulation, which longitudinally splits the bunch into a series of high density microbunches, which then act resonantly to create large wakefields. The AWAKE experiment at CERN uses intense bunches of protons from the SPS to drive wakefields in which bunches of electrons are injected. In this talk, the first measurements of the modulation of the proton bunch and of electrons accelerated up to 2 GeV at the AWAKE experiment are presented. The future AWAKE programme and potential applications in high energy physics are also discussed.

Wednesday 13th February 2019 at 3.00 pm, Blackett building room 741   

Experimental observations and theoretical understanding of boundary plasma transport
Dr. Fulvio Militello, CCFE Culham Science Centre

Plasma transport in the boundary of magnetic fusion experiments has particular features that
make it extremely interesting from a theoretical point of view. Particles are ejected from the well
confined region in structures called filaments as they are elongated along the magnetic field lines.
The filaments are coherent objects that can travel many times their characteristic size
perpendicular to the magnetic field. Their motion is intrinsically non-linear and, despite the
boundary plasma is characterised by large fluctuations, the filaments hardly interact with each
other. Boundary plasma transport is extremely important for reactor relevant machines, where
the filaments can carry a significant number of energetic ions towards the wall, where they can
induce erosion and therefore limit the life time of the machine. Interestingly, understanding the
statistics and the dynamics of the filaments allow to construct average profiles that have
emergent features and that can be predicted only by having a clear theoretical picture of the
fluctuations that generate them. The talk will give an introductory overview of filament physics
and of how the profiles in the boundary plasma are formed. It will cover both theoretical and
experimental observations, leading to a coherent description of the problem.

Wednesday 6th February 2019 at 3.00 pm, Blackett building room 741  

Plasma accelerators: Discharge plasma source for proton-driven acceleration and new developments in laser-driven betatron x-ray imaging.

Nelson C. Lopes, Grupo Lasers e Plasmas, IPFN, Instituto Superior Técnico, Lisboa

In the AWAKE experiment in CERN, 400 GeV proton beams are self-modulated in a plasma to resonantly excite a wakefield that is used to accelerate electrons to close to 1 GeV in meter scale plasmas. Future developments include the demonstration of acceleration scalability and will require a significant acceleration length extension. We proposed to use preformed plasma sections created by direct current discharges in gas filled tubes as the acceleration medium. Several prototypes of plasma tubes and driving circuits have been tested to demonstrate the feasibility of the plasma source. The next prototype, with an initial length of 5 m in under design and aims to demonstrate the scalability of the source to tens of meters, the possibility of combine plasma source to obtain kilometres of acceleration and to test new diagnostics to control the plasma density.

Laser driven wakefield accelerators can be optimised to be high-quality, high brightness, short pulse and small source size hard x-ray sources. The betatron motion of the relativistic electron bunches inside the plasma acceleration structures produced by 100 TW laser pulses are able to produce high quality single images of millimetre to centimetre tick biological or industrial samples not only in the usual absorption contrast but also in phase contrast. In homogenous samples, such as soft tissue slices, phase-contrast allows to visualise details of the tissue architecture with smaller radiation doses. However, further reduction of the imaging dose and quantitative phase contrast reconstruction techniques can be achieved with spectral filtering. In laser driven betatron sources filtering at the interesting energies for biological phase contrast imaging (40-80 KeV) will be strongly limited by the source brightness and filter limitations. We propose to improve betatron imaging 100 TW laser based imaging beamlines by using a multilayer reflecting filter to significantly reduce the imaging dose and image noise as well as to allow quantitative phase contrast sample reconstruction.

Wednesday 30th January 2019 at 3.00 pm, Blackett building room 741 

Promises and challenges of laser-driven sources for high-value manufacturing, nuclear waste management & hadrontherapy

Dr. Ceri Brenner, Central Laser Facility

This seminar will cover the promises that laser-driven sources can deliver with respect to improvements in performance or productivity for a selection of applications, and will outline some interesting challenges that have been posed by potential end-users of this disruptive innovation.

An extreme photon intensity (> 1018 W/cm2) interaction with matter can establish a TV/m electric field in plasma that seeds a micro-particle accelerator, which then drives suprathermal electron transport through matter and gives rise to a bright picosecond-pulse beam of x-rays, ions or neutrons. The MeV-temperature of these beams makes them suitable for a wide range of applications, from imaging and inspection of large and dense objects such as advanced manufactured components in aerospace or nuclear waste containers, to proton and ion beam cancer therapy.

Friday 14th December 2018 3 pm, Huxley building, room 503

Laboratory astrophysics with magnetized laser-produced plasmas

Andrea Ciardi, Sorbonne Université, Observatoire de Paris, Université PSL, École normale supérieure, CNRS, LERMA

Laser produced plasmas interacting with strong, externally applied magnetic fields are key to the diversity of new studies ranging from magnetized ICF to scaled astrophysical plasma phenomena. With the focus on laboratory astrophysics, I will review some of the recent advances made and present new results on topics related to the stability of these magnetized flows, the generation of pulsed jets and particle acceleration in colliding plasma flows. 

Wednesday 12th December 2018 at 3 pm, Blackett 741

X-ray phase contrast imaging applied to laser driven shocks

Dr Luca Antonelli, York Plasma Institute

X-ray phase contrast imaging (XPCI) is an imaging technique based on the phase-shift of an X-ray photon induced by the refractive index. In particular, the phase-shift is related to the real part of the refractive index, while the imaginary part is related to the absorption. A coherent X-ray source such as a synchrotron or X-ray free electron laser are the best choice for XPCI, however, it is possible to use broadband incoherent X-ray sources by limiting the source size and careful positioning of the experiment and detector. The interaction of high power laser with matter produces X-rays according to the intensity, energy and pulse duration. These sources can be used for XPCI. 

A characterization and the application of XPCI using a laser-produced bremsstrahlung source to a shock will be presented. The X-ray source was created by irradiating a 5 μm diameter tungsten wire with a Nd:Glass laser pulse 0.5 ps long and energy of 25 J in first harmonic. This produces a strong bremsstrahlung radiation. We applied this source to XPCI static objects and a laser-driven shock-wave in a plastic target. In both cases the XPCI clearly indicates the presence of density interfaces with 5 μm spatial resolution. This proof-of-principle experiment shows how this technique can be a powerful tool for the study of warm and hot dense matter on large scale high-energy-density facilities. A XPCI simulation tool who is capable to produced synthetic data to compare with the experimental one will be also presented.

Wednesday 5th December 2018 3 pm, Blackett room 741

Shock Ignition Laser Fusion: A promising approach to a robust ignition design

Dr Robbie Scott, Central Laser Facility, RAL

The Shock Ignition approach to Laser Fusion has the potential to achieve thermonuclear ignition with significantly less energy than the indirect drive (x-ray driven) approach. In the long term this may enable high-gain Laser Fusion with driver energies, and hence capital costs, significantly smaller than those of the National Ignition Facility (NIF). In the medium term, Shock Ignition has the potential to harness NIF’s full energy to create robust designs which have a high ignition margin.

Shock Ignition requires higher laser intensities than conventional approaches to laser fusion. As a consequence of these higher intensities, laser plasma interactions (LPIs) such as Stimulated Raman Scatter, Stimulated Brillouin Scatter and Two Plasmon Decay and the resultant hot-electrons play an energetically significant role in the implosion dynamics. In particular, the hot-electrons may deleteriously pre-heat the fuel or conversely may aid ignition by assisting the generation of the required strong shock. In order to accurately model Shock Ignition, it is vital to incorporate models which describe LPIs within a radiation-hydrodynamics code.

In this talk I will describe an ongoing UK project which combines ignition scale experiments on the Omega and NIF lasers with the development of innovative computational models which will enable kinetic laser-plasma interaction instabilities to be modelled within the UK’s ‘Odin’ radiation-hydrodynamics code. Once these models have been extensively benchmarked at ignition-scale, they will be used to develop next-generation ignition designs for NIF.

Wednesday 28th November 2018 at 3 pm, Blackett 741

Structure, cascades and structures in simulations of plasma turbulence

Prof. David Burgess, Queen Mary University of London

Natural collisionless plasmas, such as the solar wind, can be characterized in terms of their turbulence properties, which are often taken from a framework focused on universal properties such as power spectra with power law slopes. Kinetic simulations remind us that turbulence in collisionless plasma has a complex network of processes involving particle energization and magnetic field topology, such as magnetic reconnection, kinetic instabilities and wave-particle interactions, as well as nonlinear wave-wave processes. We present results from kinetic plasma simulations that illustrate various aspects of this complex system, concentrating on the role of plasma structures. We discuss two different kinds of system: the relaxation of an ordered system of multiple current sheets towards turbulent-like behaviour; and the formation of electron scale coherent structures within plasma turbulence.

Wednesday 21st November 2018, 3 pm, Blackett 741

Modification of Transport in Burning Plasmas due to alpha-electron Collisions

Dr Brian Appelbe

Ignition in ICF & MIF requires alpha particles to transfer energy to the deuterium-tritium plasma via Coulomb collisions. For plasma temperatures  < ~25 keV, the alphas transfer energy predominantly to the plasma electrons. The electron-electron collision time is much shorter than the alpha-electron collision time since the alpha number density is a small fraction of the electron number density. Therefore, it is usually assumed that the electron distribution function remains Maxwellian when alpha particles are present.

In this work it is shown that a net flux of alpha particles can perturb the electron distribution function from Maxwellian. The electron kinetic equation is solved in the presence of arbitrary populations of alpha particles and an external magnetic field to quantify this perturbation. This is used to derive a set of modified transport coefficients for unmagnetized and magnetized burning plasmas. It is shown that a flux of alpha particles can increase the heat flow from hot regions of the plasma. Transport of the magnetic field is also affected by this process.

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The Interaction of a Magnetized Plasma Flow with Magnetized and Unmagnetized Obstacles

Dr Lee Suttle

The interactions of fast-streaming, magnetized plasmas can result in a wide range of fundamental plasma physics processes such as the formation of MHD shocks, magnetic turbulence, magnetic reconnection and wave-particle interactions. 

Here we present MAGPIE experiments where a plasma flow, generated by the ablation of a pulsed-power driven inverse wire array, interacts with strongly magnetized obstacles. The plasma flow is super-Alfvénic (Vflow>2VA) and contains an embedded magnetic field (B~2T, ReM~100). The level of magnetization of the obstacles, as well as the magnetic field geometry and orientation of the obstacle can be controlled, to study a range of interaction types and topologies. The detailed structure of the interactions is measured using laser interferometry and optical fast-frame imaging.

Wednesday 24th October 2018 at 3 pm, Blackett 741

Dynamics of laser-driven solids - How does condensed matter respond on sub-nanosecond timescales?

Andy Higginbotham, York Plasma Institute, University of York

Dynamic compression of systems via laser ablation allows us to reach some of the most extreme conditions currently attainable in condensed matter.  By tailoring pulse shapes, pressures of 10’s of megabar (multiple TPa) can be achieved at only a few eV temperature.  These conditions correspond to those found in terrestrial or giant planetary cores, and as such, are of considerable importance to our understanding of exoplanetary systems.

However, in comparing these laser driven samples (with nanosecond lifetimes) to celestial bodies we face significant challenges in terms of understanding the effects of timescale and kinetics.  In this talk we will present work conducted at X-ray Free Electron Lasers aimed at understanding how matter rearranges on nanosecond timescales, and begin to address questions of the applicability of these techniques to astrophysical regimes.  We will discuss the mechanisms by which planar driven samples approach their free energy minima on the hydrostat, and investigate whether these minima are indeed reached via such extreme loading paths.

Wednesday 17th October 2-18, 3 pm in Blackett 741

Life After Academia

Ed Hill, Co-founder at Count & Visiting Scientist at Imperial Plasma Group

Almost exactly two years ago I left my job here in the Plasma Group to be a co-founder of a tech start-up, Count. I’ll firstly explain what we’re up to, then I’ll describe what that move was like - I’ll compare and contrast the two jobs, through a potted history of the company and my role in it up to now. Even if you’re not thinking of moving, it’ll hopefully be an interesting window on that world.

I’ll then show you where we’ve got to with a demo of the product, describe what we think it solves and why we think it’s valuable, and try out a few general interest and physics use cases. This will be a joining-in demo (it’s a web app so just through your browser - there’s no installing stuff or anything like that), so please bring your laptops if you’d like to have a go!

 

Wednesday 19th September 2018, 1:30 pm in Huxley 503 

Recent studies regarding laser-plasma interaction and electron transport at CEA

Pascal LOISEAU

CEA, DAM, DIF, F-91297 Arpajon, France

It is well known that, in the context of inertial confinement fusion, laser-plasma interaction (LPI) has deleterious effects and still represents a major risk of failure for ignition. In this talk, after a short reminder of LPI in the context of indirect drive ICF, I will describe different tools that we have developed at CEA for studying the laser propagation and plasma heating. Finally, I will present the first LPI LMJ experiment that is planned next year.

 

Tuesday 26th June 2018,  3:00 - 4:00 pm Huxley 711C

Laser plasma acceleration at CLAPA

Dr Haiyang Lu

State Key Lab of Nuclear Physics and Technology, School of Physics, Peking University

In 2013 Peking University built a new lab, which is named CLAPA (Compact Laser Plasma Accelerator) for laser plasma acceleration research, which includes a 200TW laser system and three target chambers for different research purpose. The laser system was in operation in June 2016. The talk will cover most of the progress which has been achieved recently. Protons with highest energy of 15 MeV has been achieved in this system and proton beam transportation lines has been installed and successfully used for applications. Electrons with energy up to 500 MeV has been achieved recently as well. Some application attempts both on protons and electrons, as well as x-rays were also presented. This talk will also include a little about CLAPA at PKU, and future plans.

 

Friday 6th April 2018, 3:00 - 4:00 pm Blackett 741

Vacuum Electron Heating and Protons Acceleration by Surface Plasma Wave

Dr Pawan Kumar

Raj Kumar Goel Institute of Technology, Ghaziabad, IP, India

Surface plasma wave (SPW) is a guided electromagnetic mode that propagates along the interface between a conductor and a dielectric. Its field decays exponentially away from the interface, the dielectric as well as into the conductor. SPW has numerous applciations including electron heating and proton acceleration in target normal sheath acceleration. The normal component of SPW electric field pulls the electrons away from the plasma during the half cycle. Each electron sees, besides the normal component of SPW, a static space charge field produced by the preceding electrons. As the electron returns back to interface, it possesses finite kinetic energy that is deposited into the plasma. The large amplitude SPW also has capability to accelerate the protons when it is resonantly excited by a p-polarized laser obliquely impinged on a rippled surface.

Wednesday 21^st March, 3:00-4:00 pm Blackett 741

Guiding laser-produced fast electrons using super-strong magnetic fields

Dr Kate Lancaster, York Plasma Institute

 Currently we are able produce with laser plasma interactions some of the most extreme conditions on earth. When ultra-intense lasers are focused on to solid material, the fields associated with the laser are so strong that electrons can easily escape the atoms in the material. Absorption of the laser pulse results in the generation of a population of relativistic electrons, with currents on the order of Mega Amperes. The physics associated with how the electrons are produced and subsequently transported in plasma is complex and proves challenging to diagnose and study. Importantly, these fast electrons are the driver for much of the subsequent physics during these interactions including generation of energetic particles/photon sources, unique atomic physics states such as hollow atoms, hydrodynamic phenomena, production of warm/hot dense matter relevant to stellar interiors, heating of matter relevant to alternative laser driven fusion schemes such as fast ignition, and conditions relevant for understanding of nuclear astrophysics in the most extreme objects in our universe.

This talk will illustrate some of the experiments happening on petawatt-class lasers concerning how to control important fast electron beam parameters (such as divergence) using novel structured targets. Alex Robinson et al[1] first proposed using targets incorporating a resistivity gradient to confine fast electrons. At the material interface of a high resistivity feature, e.g. a wire, surrounded by a lower resistivity material a strong magnetic field is generated which confines electrons to areas of higher resistivity and higher current density. In this talk experiments using targets with novel silicon embedded features created by Scitech Precision Ltd using MEMS technology will be presented. A novel duel channel front surface imaging system was created in order to enable both pre-shot alignment and on-shot focal spot position, information critical for performing these types of complex experiments.

[1] A. P. L. Robinson and M. Sherlock, PoP, 14 083105 (2007) 

 

Wednesday 14th March 2018, 3-4pm, Blackett 741

'Magnetic reconnection approaching the relativistic regime’

Dr Charlotte Palmer, Lancaster University

Magnetic reconnection is a process that contributes significantly to plasma dynamics and energy transfer in a wide range of situations, including inertial confinement fusion experiments, stellar coronae and compact, highly magnetised objects like neutron stars.   There are many different models to describe this phenomena and laboratory experiments are used to refine these models and assess their applicability. Magnetic fields can be generated using high power lasers through several mechanisms, most famously the Biermann battery associated with the formation of azimuthal magnetic fields around a laser focus due to non-parallel gradients in electron temperature and density. At high laser intensities, relativistic surface currents play a significant role in the generation of the azimuthal magnetic fields.  Experiments exploring magnetic reconnection at moderate intensities have been performed at numerous international facilities.  This seminar will introduce laser-driven reconnection moving from regimes driven by the mentioned moderate intensity, long pulse lasers to relativistic, high intensity laser pulses before presenting on-going analysis of reconnection fields measured during a recent experiment that approached the relativistic regime. 

Wednesday 7^th March, 3:00-4:00 pm Blackett 741

Optimal transport in proton radiography and shadowgraphy

Dr Muhammad Kasim, University of Oxford

Proton radiography and shadowgraphy are common diagnostics employed in plasma physics to measure magnetic/electric field and plasma density variation in plasma. Despite their wide use in the plasma physics community for years, methods to accurately retrieve quantitative information (e.g. magnetic field strength) from the measurable information were just introduced recently by Kasim, et al. (2016), and Bott, et al. (2017). These methods are based on optimal transport, a field of study in mathematics and economics.

In this talk, I will present the methods to infer quantitative information from proton radiograph and shadowgraph images. Introductions to the proton radiography, shadowgraphy, and optimal transport will be presented as well as the results on experiments. Some applications of optimal transport in other fields, such as machine learning, will also be presented.

Wednesday 28th February, 3:00-4:00 pm Blackett 741
The First Light Fusion concept and related physics challenges
Dr Nick Hawker, CTO of First Light Fusion

First Light Fusion is a private company, based in Oxfordshire, working on a novel approach to inertial confinement fusion. The basis of the concept includes both a novel target design, using shock-driven cavity collapse and other similar phenomena, and a novel driver design, using a high-velocity projectile rather than a laser. This has the potential to create a technology that is much quicker and cheaper to reach a power plant, if the physics challenges can be solved. This seminar will introduce the First Light concept and will explore some of these physics challenges, before concluding with a brief discussion of the reactor design and the energy landscape as a whole.

Wednesday 7^th February, 3:00-4:00 pm Blackett 741

Magnetic reconnection: recent progress from Magnetospheric Multiscale satellite observations

Dr Jonathan Eastwood, Imperial College London

Magnetic reconnection is a process of fundamental importance to plasma physics in both space and the laboratory. It is also increasingly cited as a process at work in astrophysical systems. This for two reasons: it controls plasma connectivity, and it can rapidly release and convert stored magnetic energy. Although the consequences of reconnection are large-scale – for example in the context of the Earth’s magnetosphere, collisionless reconnection controls the circulation of plasma and energy transport leading to geomagnetic substorms and storms – the physics that enables reconnection operates on small scales down to the electron scale. Thus, progress in understanding reconnection ultimately requires detailed electron-scale measurements of reconnection, and the magnetic reconnection diffusion region in particular.

In 2015 NASA launched the four spacecraft Magnetospheric Multiscale (MMS) mission with the primary goal of studying magnetic reconnection in the Earth’s magnetosphere. Its state-of-the-art payload is capable of measuring the 3D electron plasma distribution on 30 ms timescales, 100 times faster than previous missions and finally able to resolve the electron scale physics that controls reconnection. In this seminar I will discuss why reconnection is important in space and astrophysical phenomena, the MMS mission and its instrumentation, and then review some of the exciting results that have already been discovered using MMS. These include measurements of the electron dissipation region, ion-scale flux ropes, and how the presence of a guide field in 3D reconnection modifies plasma heating.

Wednesday 31st January, 3:00-4:00 pm Blackett 741

Verification and validation procedures with applications to plasma-edge turbulence simulations

Dr Fabio Riva, Culham Centre for Fusion Energy

The methodology used to assess the reliability of numerical simulation codes constitutes the Verification and Validation (V&V) procedure. V&V is composed by three separate tasks: the code verification, which is a mathematical issue targeted to assess that the physical model is correctly implemented in a simulation code; the solution verification, which evaluates the numerical errors affecting a simulation; and the validation, which determines the consistency of the code results, and therefore of the physical model, with experimental data.

To perform a code verification, we propose to use the method of manufactured solutions, a methodology that we have generalized to PIC codes, overcoming the difficulty of dealing with a numerical method intrinsically affected by statistical noise. The solution verification procedure we put forward is based on the Richardson extrapolation, used as higher order estimate of the exact solution. These verification procedures were applied to GBS, a three-dimensional fluid code for SOL plasma turbulence simulation based on a finite difference scheme, and to a unidimensional, electrostatic, collisionless PIC code. Finally, in order to increase the reliability of our SOL modelling, several plasma turbulence simulation codes are validated against measurements taken in a number of experimental devices.

 

29th November 2017 3:00-4:00 pm Blackett 741  

Laser-based QED in intense backgrounds: ethos and recent developments

Dr Ben King, University of Plymouth

The textbook approach to QED typically considers the interaction of charges with low numbers of photons, using the weakness of the charge-field coupling to neglect the contribution from higher orders. However, when charges are subject to a strong enough EM background, higher order contributions cannot be neglected, and one must include arbitrarily-high orders of charge-field interaction.

The next generation of high intensity laser facilities are of great interest in studying and testing these all-order solutions to so-called "strong-field QED", through measurement of effects such as nonlinear Compton scattering, photon-seeded pair-creation, photon-photon scattering and EM cascades. After reviewing the standard approach to calculating these effects (the "plane wave model") and their inclusion in numerical simulations, I will briefly survey recent developments, and current challenges in laser-based strong-field QED.

Talbot-Lau X-ray Deflectometry: An Electron Density Diagnostic for Laser and Pulsed Experiments

 22nd November 2017 3:00-4:00 pm Blackett 741

 Dr Maria Pia Valdivia, Johns Hopkins University, Baltimore, USA

 A new technique called Talbot-Lau X-ray Deflectometry (TXD) is being developed as an electron density diagnostic of High Energy Density (HED) plasma experiments. The method is based on measurements of refraction angle deviation due to refraction index gradients. TXD simultaneously detects x-ray refraction, attenuation, and scatter images of low-Z objects from a single Moiré fringe image. Moreover, the refraction and attenuation images can also provide an elemental composition diagnostic. High magnification TXD systems have been developed for 8 and 17 keV x-ray energy using conventional x-ray tubes as sources. In order to test the method in an HED environment, an 8 keV Talbot-Lau interferometer employing free standing, ultrathin gratings was deployed at a laser and at a pulsed power facility. The steps from operation with the conventional x-ray source to operation with laser and pulsed power driven backlighters are reported. Grating survival and electron density diagnostic was demonstrated at the Multi-TeraWatt facility at LLE for 30 J, 8 ps laser pulses, using K-shell emission from Cu foil and wire micro-targets. Moiré images obtained under laser backlighting were compared to laboratory data acquired with the x-ray tube as well with the theoretic predictions. In addition, grating survival and Moiré pattern formation was observed using 8 keV illumination from an x-pinch backlighter driven by a 350 kA, 1 kA/ns pulsed power driver. These studies show that TXD can detect both sharp and smooth density gradients with source-limited spatial resolution, thus allowing implementation of the TXD electron density technique as a HED plasma diagnostic.

 

Cosmological magnetic fields and particle acceleration in the laboratory

 15^th November 2017 3:00-4:00 pm Blackett 741

 Prof. Gianluca Gregori, University of Oxford

 Turbulence and magnetic fields are ubiquitous in the Universe. While magnetic fields are believed to play an important role during the evolution of the Universe and the generation of cosmic rays, their origin and present-day values still remain a mystery. Here we present the results of laboratory experiments using high power lasers that have shed lights on the generation, amplification of the magnetic fields and the production of cosmic rays. In particular we will show that in a turbulent and magnetized plasma, magnetic fields can reach near-equipartition values, a condition which is needed in order to see sizeable stochastic particle diffusion and acceleration. We conclude the talk by discussing an hypothetical mechanisms of magnetic field generation  (born from a fruitful collaboration between plasma physics and particle theory) that could potentially explain the observed magnetic fields in voids.

Ultra-short X-ray flashes combined with High-Z nanoparticles for enhanced radiotherapy

10th November 2017 3.00 - 4.00 pm, Huxley 503,

 Dr. Daniel Adjei, Laboratoire d’Optique Appliquée, Palaiseau, France

Ionizing radiations are routinely used in science and technology for many applications. In particular, these radiations are used in radiotherapy to treat cancer and benign tumours. After surgery, radiotherapy is the main treatment used in the management of patients with cancer. Radiotherapy, today, forms about 60% of all cancer treatments. The search for procedures to eradicate tumours while sparing normal tissues remains one of the most important challenges for radiation therapy. About two years ago, V. Favaudon et al published a breakthrough article with the conclusion that “FLASH radiotherapy might allow complete eradication of lung tumours and reduce the occurrence and severity of early and late complications affecting normal tissue”. FLASH irradiation is the conjunction of very high instantaneous dose (>40 Gy/s) and short pulse (500 ms).

Femtosecond X-ray sources (1 fs=10^-15 s) yielding ultra-high instantaneous dose (up to 10¹⁴ Gy/s) while keeping integrated dose at normal level (few Gy) could push the FLASH therapy to its extreme, opening new paradigm in cancer treatment. So far, no experiment has been performed to test the therapeutic capacity of these femtosecond (fs) ultra-intense X-ray sources. In this presentation, I will present to you a research scheme setup at the Laboratoire d’Optique Appliquée (LOA), France focused on exploring ultrafast X-ray sources for biomedical applications in particular for radiotherapy. The aim of the project is to push further the flash therapy by irradiating tumour cells, in vitro, loaded with high-atomic number (Z) nanoparticles (NPs) with ultrafast X-rays to enhance and localise the radiation effect. The concept is that ultrafast X-rays would generate subsequent ultrafast shower of Auger electrons like a nano ‘bomb’. This Auger electron will induce highly ionized spurs leading to complex molecular damages to surrounding biological media and therefore increased cell death.

 

Ion acceleration with high power lasers: recent developments and perspectives

8^th November 2017 3:00-4:00 pm Blackett 741

Prof. Marco Borghesi, Centre for Plasma Physics, The Queen’s University of Belfast

Ion acceleration driven by high intensity laser pulses is attracting an impressive and steadily increasing research effort. Experiments over the past 10-15 years have demonstrated, over a wide range of laser and target parameters, the generation of multi-MeV proton and ion beams with unique properties such as ultrashort burst emission, high brilliance, and low emittance. The talk will provide an overview of the state of the art in this field by discussing both the established sheath acceleration mechanism (or TNSA), and emerging mechanisms (e.g. Radiation Pressure Acceleration), which hold the promise for acceleration to GeV energies with next generation laser facilities.

In particular, recent developments obtained by the QUB group and collaborators in the framework of the EPSRC-funded UK-wide A-SAIL (Advanced Strategies for Accelerating Ions with Lasers) project will be discussed. This is a UK-wide consortium aimed to the development of ion acceleration towards medical applications, which carries out experimental research at a range of laser facilities, including the PW-class VULCAN and GEMINI lasers at the Rutherford Appleton Laboratory.

Signatures of strong-field QED effects in high-intensity laser-matter interactions 

18 October 2017, 3.00 - 4.00 pm Blackett 741

Dr Chris Ridgers, University of York

Strong-field quantum electrodynamic (QED) processes are predicted to play a role in the interaction of next-generation high-intensity (> 10^23W/cm^2) laser pulses with matter. In particular quantum radiation reaction will play a major role in the motion of the electrons and positrons in the plasma created in the laser focus. The emitted hard-photons resulting in this radiation reaction can also generate pairs, resulting in a cascade and so the creation of dense pair plasmas. We will discuss several signatures of these effects in next-generation laser-matter interactions: the pair plasma can absorb the laser pulse, quenching radiation pressure ion acceleration; the asymmetry in the rate of spin flip transitions could cause the electrons and positrons in the plasma to spin polarize. We will also investigate experiments possible with current high-intensity ( 10^21W/cm^2) lasers. Signatures of quantum radiation reaction on a counter-propagating energetic (1GeV) electron beam will be discussed. In particular we will quantify the degree of broadening of the energy spectrum of the beam due to quantum stochasticity.  Signatures of strong-field QED effects in high-intensity laser-matter interactions

 

Type Ia Supernova: The Single Degenerate Scenario

17^th July 2017, 3.00 – 4.00 pm, room Huxley 502

Prof. Domingo García-Senz, Physics Department of the Polytechnic University of Catalonia, Spain

Historically, the so called Single Degenerate (SD) scenario was the first invoked to explain
the gross observational features of Type Ia supernova explosions. Nowadays this scenario has become much more sophisticated and is also competing with other different explosion models. In this talk I'll review the main features of the SD scenario and its status within the current zoo of models. Some emphasis will be put in the multi-D modelization of these explosions. The possibility of conducting a Laboratory Astrophysics experiment related to the SD paradigma will be commented
at the end of the talk.

Control of electron injection and acceleration in laser plasma accelerators

6th March 2017, 3.00 - 4.00 pm room 539

Olle Lundh,  Department of Physics, Lund Laser Centre, Lund University, Sweden

Laser wakefield accelerators appear promising as compact sources of highly relativistic electrons and ultrashort pulses of X-rays. However, improving the control of the electron beam parameters is crucially important in order to enable laser plasma accelerators to be efficiently used in applications. We report on our recent experimental studies of controlled trapping of electrons in the accelerating phase of the plasma wave. The experiments are performed using the Ti:Sapphire-based multi-terawatt laser at the Lund Laser Centre. A variety of techniques to achieve stable and tunable electron beams are explored, compared and combined, including density down-ramp injection, ionization-induced injection and colliding pulse injection.

Ultra-intense vortex light for particle acceleration in the plasma

23^rd February 2017 3.00 – 4.00 pm room 539

Dr Jorge Vieira, Group for Lasers and Plasmas, Instituto Superior Técnico, Portugal 

Positron acceleration remains a key challenge for a future plasma based linear collider. Here, we show three-dimensional simulations results that suggest how ultra-intense structured light with orbital angular momentum (vortex light) could be used as a driver for plasma accelerators that could accelerate positrons in strongly nonlinear regimes. We also investigate innovative pathways to generate and amplify vortex laser pulses to ultra-high intensities, required to realise this positron acceleration scheme. We show that some of these pathways can also provide mechanisms to produce high orbital angular momentum harmonics.

Full scale modelling simulations of plasma based accelerators

20^th February 2017 3.00 – 4.00 pm room 539 Blackett.

Dr Jorge Vieira, Group for Lasers and Plasmas, Instituto Superior Técnico, Portugal; 

Laser wakefield accelerators have the potential to deliver relativistic electron bunches for several applications. While in the plasma, electrons are subject to strong focusing forces that lead to intense betatron x-ray radiation generation. Using particle-in-cell simulations and radiation post-processing tools, we show that the laser driver polarisation can control the polarisation of betatron x-rays in direct laser acceleration setups. We show that fast electrons flowing out of the plasma can trigger large-scale magnetic fields around the entire plasma accelerator. If the electron beam collides with a high Z target, an electron-positron fireball bunch forms. We show that these beams can lead to magnetic field amplification through the current filamentation instability, relevant in astrophysical conditions. When only a single filament is present, the resulting plasma wave could be used to accelerate positrons to high energies.

Past Seminars 2016: 

 

6th July: Capturing Structural Dynamics in Crystalline Silicon Using Chirped Electrons from a Laser Wakefield Accelerator

Alec Thomas, Lancaster University and Cockcroft Institute

741 Blackett Laboratory 2pm

Recent progress in laser wakefield acceleration has led to the emergence of a new generation of electron and X-ray sources that may have significant benefits for ultrafast science. In this talk, I will describe the use of laser-wakefield-accelerated electron bunches for time-resolved electron diffraction measurements of the structural dynamics of single-crystal silicon nano-membranes pumped by an ultrafast laser pulse. In our proof-of-concept study, we resolved the silicon lattice dynamics on a picosecond time scale by deflecting the momentum-time correlated electrons in the diffraction peaks with a static magnetic field to obtain the time-dependent diffraction efficiency. Our measurements were enabled by the development of a laser wakefield accelerator operating at kHz repetition rate with good stability, delivering electrons in the 100 keV range with the transverse coherence required for electron diffraction. Necessary beam quality improvements were enabled by  feedback-optimized wavefront manipulation using a deformable mirror, which steered the plasma dynamics and resulted in improvement to the charge and transverse emittance of the electron beam. Further improvements to this scheme may lead to femtosecond temporal resolution, with negligible pump-probe jitter. In addition, the feedback techniques may be applied to the next generation of high repetition-rate, high power lasers for laser wakefield acceleration.

7th July: Relativistic Magnetic Reconnection in the Laboratory

Louise Willingale, Lancaster University and Cockcroft Institute

741 Blackett Laboratory 2pm

Magnetic reconnection is a fundamental plasma process involving an exchange of magnetic energy and plasma kinetic energy through changes in the magnetic field topology. In many astrophysical plasmas magnetic reconnection plays a key role in the dynamics, although making direct measurements is obviously challenging. Therefore, laboratory studies of magnetic reconnection provide an important platform for testing theories and characterising different regimes. Until now, the extremely energetic class of astrophysical phenomena - including high-energy pulsar winds, gamma ray bursts, and jets from galactic nuclei - where the energy density of the reconnecting fields exceeds the rest mass energy density (σ ≡ B²/(μ₀n_em_ec²) > 1), i.e. relativistic reconnection, have been inaccessible in the laboratory. I will present experimental measurements along with numerical modelling of relativistic magnetic reconnection driven by short-pulse, high-intensity lasers that produce extremely strong magnetic fields. Evidence of magnetic reconnection was identified by the plasma’s X-ray emission patterns, changes to the electron spectrum, and by measuring the time over which reconnection occurs.  Three-dimensional particle-in-cell simulations show the plasma density and magnetic field characteristics in the reconnection region satisfy σ > 1, indicating these experiments are in the relativistic reconnection regime. Accessing these relativistic conditions in the laboratory allows for further investigation that may provide insight into unresolved areas in space and astro-physics.

 

15th July: Two seminars:

Date	Name	Title
15/10	Robert Riedel	High power wavelength tunable femtosecond OPCPA
25/11	Ben King	Laser-based strong-field QED (under construction)
2/12	Ceri Brenner	Laser-driven x-ray and neutron source development for industrial applications of solid-foil plasma accelerators
26/1	Mike Dunne	'A billion times brighter': An overview of the revolution underway in X-ray science
29/1	Vasily Kharin	Temporal laser pulse shape effects in non-linear Thomson scattering
17/2	Charlie Ryan	Small and cheap plasma thrusters for the nano and micro-satellite market
30/3	Howard Scott	Non-LTE Modeling of Radiatively-Driven Dense Plasmas
20/4	Jeff Colvin	Advances in Non-Equilibrium Atomic Physics with Novel Laser Targets
6/7	Alec Thomas	Capturing Structural Dynamics in Crystalline Silicon Using Chirped Electrons from a Laser Wakefield Accelerator
7/7	Louise Willingale	Relativistic Magnetic Reconnection in the Laboratory
15/7	John Kline	Maximizing implosion performance for inertial confinement fusion science
	Steve Batha	An Overview of the Los Alamos Inertial Confinement Fusion and High-Energy-Density Physics Research Programs

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Past Seminars 2015:

 

20th April: Advances in Non-Equilibrium Atomic Physics with Novel Laser Targets

Jeff Colvin, Lawrence Livermore National Laboratory

741 Blackett Laboratory 2pm

30th March: Non-LTE Modeling of Radiatively-Driven Dense Plasmas

Howard Scott, Lawrence Livermore National Laboratory

741 Blackett Laboratory 3pm

29th January: Temporal laser pulse shape effects in nonlinear Thomson scattering

Vasily Kharin, GSI Helmholtzzentrum für Schwerionenforschung GmbH

630 Blackett Laboratory 2pm

Scattering of the light on high-energy electron beams has become an indispensable tool in providing tunable wide range X-; and gamma-radiation and found applications in various areas of research. In order to obtain higher photon yield, the intensity of the incident pulse can be increased. High values of field lead to non-negligible ponderomotive shift in the emitted frequency varying with the pulse envelope. The present study is devoted to the influence of the pulse shape on the emitted photon spectrum during the non-linear Thomson scattering. We present analytical results on on-axis spectrum of the scattered radiation in the case of symmetric relativistically intense incident pulse and discuss some properties of angular distribution of the emitted radiation. The possibility of incident pulse reconstruction from the scattered spectrum is also discussed.

17th February: Small and cheap plasma thrusters for the nano and micro-satellite market

Charlie Ryan, University of Southampton

741 Blackett Laboratory 3pm

Currently the propulsion of satellites is undergoing a revolution, with a move away from chemical propulsion to electric propulsion; i.e. propulsion using electrical energy to accelerate propellant. Many types of electric propulsion techniques rely upon the creation of low-density plasma, and the acceleration of the ions created to km/s velocities. This talk will introduce the various different types of plasma thrusters and their operational characteristics. The electric propulsion concepts being experimentally investigated at the University of Southampton will be introduced. Further the drive to smaller and cheaper electric propulsion options will be outlined, motivated by the large growth in nano and micro-satellites, and the need for electric propulsion to be applicable within the smaller constraints of these platforms. The work being completed at the University of Southampton to reduce the cost, size and mass of electric propulsion for spacecraft will be demonstrated, putting it into the context of the significant growth in the microsatellite market.

26th January: 'A billion times brighter': An overview of the revolution underway in X-ray science

Mike Dunne, Director of LCLS, SLAC National Accelerator Laboratory

539 Blackett Laboratory 3pm

This talk will provide an introductory overview of the world’s first “hard x-ray free electron laser facility”, known as LCLS, operated by Stanford University on behalf of the US Department of Energy.  The x-rays produced by LCLS are a billion times brighter than can be produced by conventional sources, such as a synchrotron, and are delivered in ultrafast bursts - typically a few tens of femtoseconds (10-15 seconds). This opens up revolutionary opportunities for the study of novel states of matter, quantum materials, ultrafast chemistry, and structural biology. 

Since its initial operation in 2009, LCLS has enabled a remarkable series of studies, via its ability to provide atomic resolution information, with freeze-frame ‘movies’ of how atomic, chemical and biological systems evolve on ultrafast timescales.  Based on this success, a major upgrade project is now underway that will increase the repetition rate by 4 orders of magnitude and open up entirely new scientific opportunities. Access to LCLS is open to everyone, based purely on the scientific merit of the proposed experiments. Hopefully this talk will help engender further ideas and opportunities for future use of this remarkable new science facility.

2nd December: Laser-driven X-ray and neutron source development for industrial applications of solid-foil plasma accelerators 

Ceri Brenner, Application Development Scientist, STFC Central Laser Facility 

741 Blackett Laboratory 3pm

Pulsed beams of energetic X-rays, electrons and neutrons from intense laser interactions with solid foils are promising for applications where bright, small emission area sources are ideal. Possible end users of laser-driven sources are those requiring advanced non-destructive and non-invasive inspection techniques in industry sectors of high value commerce such as aerospace, nuclear, defence and advanced manufacturing. 

In collaboration with the UK’s Defence Science Technology Laboratory the CLF’s Gemini laser was used to generate an electron beam, via the Wakefield acceleration method in a high pressure gas jet, to drive a source of backscattered X-rays at the test object plane. An X-ray backscatter image of an array of different density and atomic number items is demonstrated for the first time from a laser-generated electron beam. 

Results from an experiment with the Vulcan laser in March 2015 demonstrate the key features of laser-driven Bremsstrahlung beams for imaging applications and show neutron yield enhancement during pitcher-catcher generation. Active detector radiographic imaging of industrially relevant sample objects with a 10 ps drive pulse is also presented, demonstrating that features of 200 micron size are resolved when projected at high magnification.

25th November: Laser-based strong-field QED (under construction)

Ben King, University of Plymouth 
741 Blackett Laboratory 3pm

Since the advent of the laser in the early 1960s, quantum electrodynamics (QED) has been used to predict a variety of new phenomena when charged particles interact with intense laser pulses.

Recent progress in high-intensity laser technology has put several of these phenomena in reach of near-future experiments. Unlike the technology, the progress of theory understanding beyond what was calculated by the 1970s has been limited. In this talk, I will outline some of the theory successes of laser-based strong-field QED and discuss the main challenges and potential benefits of solving them.

15th October: High power wavelength tunable femtosecond OPCPA

Robert Riedel, Class 5 Photonics GmbH/DESY 
1004 Blackett Laboratory 11am

Optical parametric chirped-pulse amplication (OPCPA) is the most promising method for providing compact, wavelength-tunable, high power, femtosecond lasers. We have recently achieved a 112 W OPCPA with wavelength-tunability around 800 nm and 30 fs pulse duration in burst mode (100 kHz in a 800 µs burst at 10 Hz). In this seminar, we discuss the various laser architectures and the critical parameters in achieving similar laser parameters but in continuous operation. Further, energy scaling to the terawatt level is discussed.

For further information on this seminar, contact Christos Kamperidis