Plasma Seminars Series
For further information, please contact Dr Jack Davies Hare
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 21st 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 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.
 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 7th 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 7th 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
15th 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
8th 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
17th 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
23rd 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
20th 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/(μ0nemec2) > 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:
Maximizing implosion performance for inertial confinement fusion science
John Kline, Los Alamos National Laboratory
While the march towards achieving indirectly driven inertial confinement fusion at the NIF has made great progress, the experiments show that multi-dimensional effects still dominate the implosion performance. Low mode implosion symmetry and hydrodynamic instabilities seed by capsule mounting features appear to be two key limiting factors for implosion performance. One reason these factors have a large impact on the performance of ICF implosions is the high convergence required to achieve high fusion gains. To tackle these problems, a predictable implosion platform is needed meaning experiments must trade-off high gain for performance. To this end, LANL has adopted three main approaches to develop a 1D implosion platform where 1D means high yield over 1D clean calculations. Taking advantage of the properties of beryllium capsules, a high adiabat, low convergence platform is being developed. The higher drive efficiency for beryllium enables larger case-to-capsule ratios to improve symmetry at the expense of drive. Smaller capsules with a high adiabat drive are expected to reduce the convergence and thus increase predictability. The second approach is liquid fuel layers using wetted foam targets. With liquid fuel layers, the initial mass in the hot spot can be controlled via the target fielding temperature which changes the liquid vapor pressure. Varying the initial hot spot mass via the vapor pressure controls the implosion convergence and minimizes the need to vaporize the dense fuel layer during the implosion to achieve ignition relevant hot spot densities. The last method is double shell targets. Unlike hot spot ignition, double shells ignite volumetrically. The inner shell houses the DT fuel and the convergence of this cavity is relatively small compared to hot spot ignition. Radiation trapping and the longer confinement times relax the conditions required to ignite the fuel. Key challenges for double shell targets are coupling the momentum of the outer shell to the inner shell and mixing of the mid-Z material from the inner shell into the fuel. The primary theme for each of these approaches is reduced implosion convergence with the goal of achieving a 1D implosion. Once established, a systematic approach to solving limiting issues for ICF can be undertaken. This presentation will discuss the approaches, results, and plans for each of these campaigns.
An Overview of the Los Alamos Inertial Confinement Fusion and High-Energy-Density Physics Research Programs
Steve Batha, Los Alamos National Laboratory
The Los Alamos Inertial Confinement Fusion and Science Programs engage in a vigorous array of experiments, theory, and modeling. We use the three major High Energy Density facilities, NIF, Omega, and Z to perform experiments. These include opacity, radiation transport, hydrodynamics, ignition science, and burn experiments to aid the ICF and Science campaigns in reaching their stewardship goals. The ICF program operates two nuclear diagnostics at NIF, the neutron imaging system and the gamma reaction history instruments. Both systems are being expanded with significant capability enhancements.
|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
|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|
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
We have been developing new paths to achieve the higher hard x-ray fluences necessary for radiation hardness testing of military systems and other applications. The focus of this research has been on development of new ultra-low-density pure metal foams fabricated as self-supporting structures of nanowires, and parallel development of novel metal-nanoparticle-loaded carbon nanotube foams. In this presentation I will discuss the foam development work, review the x-ray spectral emissivity data obtained from Omega and NIF laser experiments with the new foam targets, and compare the experimental results to those predicted by 2D simulations with a radiation-hydrodynamics code incorporating detailed non-LTE atomic models. I will show how this work has opened up two new paths to getting higher x-ray conversion efficiency at high photon energies for the laser-driven x-ray sources needed for radiation hardness testing, and helped to advance our understanding of non-equilibrium atomic physics.
30th March: Non-LTE Modeling of Radiatively-Driven Dense Plasmas
Howard Scott, Lawrence Livermore National Laboratory
741 Blackett Laboratory 3pm
There are now several experimental facilities that use strong X-ray fields to produce plasmas with densities ranging from ~1 to ~103 g/cm3 . Large laser facilities, such as the National Ignition Facility (NIF) and Omega reach high densities with radiatively-driven compression, short-pulse lasers such as XFELs produce solid density plasmas on very short timescales, and the Orion laser facility combines these methods. Despite the high densities, these plasmas can be very far from LTE, due to the large radiation fields and/or short timescales, and simulations mostly use collisional-radiative (CR) modeling which has been adapted to handle these conditions. These dense plasmas present challenges to CR modeling. Ionization potential depression (IPD) has received much attention recently as researchers work to understand experimental results from LCLS and Orion [1,2]. However, incorporating IPD into a CR model is only one challenge presented by these conditions. Electron degeneracy and the extent of the state space can also play important roles in the plasma energetics and radiative properties, with effects evident in recent observations [3,4]. We discuss the computational issues associated with these phenomena and discuss methods for handling them.
 O. Ciricosta, S.M. Vinko, H.-K. Chung, et al, Phys. Rev. Lett. 109, 065002 (2012)
 D.J. Hoarty, P. Allan, S.F. James, et al, Phys. Rev. Lett. 110, 265003 (2013)
 S.P. Regan, R. Epstein, B.A. Hammel, et al, Phys. Rev. Lett. 111, 045001 (2013)
 L.A. Pickworth, B.A. Hammel, V.A. Smalyk, et al, submitted to Phys. Rev. Lett.
This work performed under the auspices of U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
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