Professor Roland A Smith

Summary

In 2012 I rejoined Plasma Physics after spending some 25 years working in the Quantum Optics and Laser Science Group at Imperial College, and was Head of the Plasma Group until 2017.  The QOLS and Plasma research groups have strong areas of common interest, particularly in the creation and use of advanced high-power laser systems to study matter under extreme conditions of temperature, pressure and magnetic fields and much of my personal research work spans these two areas.  The Blackett Laboratory Laser Consortium (a multi-group collaborative venture) acts as a UK centre of excellence for this multi-disciplinary activity and is the UK's leading group for the creation and use of ultra-short laser pulses.  I also acted as the Principle Investigator for the UK's Inertial Fusion Energy (IFE) Network which is developing strategies for future large scale National and International Fusion Energy projects.

I am an experimental physicist and currently hold the positions of Professor of Laser Physics at Imperial College.  My main research interests cover the areas of high power and ultra-short pulse laser development, experimental laser matter interaction physics (particularly with atomic cluster and nanoscale targets), laboratory based experimental astrophysics and attoscience.

To support my research programme my team and I are currently built a operate "Cerberus", a large, multi beam high-power laser system at Imperial College.  This system is the largest University based laser in the UK and delivers multi-TW (>10¹²W) laser pulses along with several ns >50J long pulse beams to a range of experiments including the Magpie Z-Pinch, MACH pulsed power facility and a dedicated new laboratory funded by the Royal Society for the study of matter under extreme conditions.  This combination of advanced laser system and large scale pulsed power facility allows us to explore the physics of extreme processes such as the launch of “magnetic tower jets” by young stellar objects and black holes.

 

Current PhD projects

Cerberus High energy Laser System and OPCPA Lasers

 

Cerberus is the UK's largest University based laser system and is used for both development and testing of advanced laser concepts, and for driving a broad range of experimental plasma physics campaigns.  These include both stand-alone laser-plasma interaction and X-ray generation experiments and work in conjunction with the world's largest open access Z-Pinch MAGPIE.  Cerberus is a "hybrid" multi-beam system that utilises large aperture flashlamp pumped Nd:Glass based power amplifiers.  This is a mature and well understood technology able to deliver very high energy, sub-picosecond laser pulses with peak powers of many terawatts using a technique called Chirped Pulse Amplification (CPA).  To extract the best performance from these very powerful amplifiers we couple them to an advanced "optical parametric chirped pulse amplification" (OPCPA) front end.

 

In CPA a short light pulse is stretched out in time to lower its peak power and avoid damage to the laser.  It is then amplified and recompressed back to near its original duration.  In OPCA the "traditional" gain storage amplifiers typically used in a laser are replaced with non-linear crystals.  Here energy can be instantaneously transferred from a high power but long duration "pump" beam into an initially low energy but high-bandwidth "seed" pulse.  OPCPA has a number of key advantages, very high single pass gain and high bandwidth, low thermal loading, high contrast and high repetition rate operation.  These advantages are balanced against the need to have a very high quality, temporally and spatially smooth pump laser.  We have a number of opportunities for projects to work on the Cerberus system itself and its application in creating and probing extreme states of matter.  We are particularly interested in making and characterising very high contrast light pulses where the “optical noise” ahead of the main pulse in time is greatly reduced by using picosecond-pumped OPCPA.  This has important consequences for the ability to deliver and exploit a “clean” ultra-high intensity light pulse in an experiment.

 

Table top electron acceleration with a kHz few cycle optical parametric laser.

 

Particle accelerators able to reach GeV energies have traditionally been huge, expensive devices 100's of meters long, based on "old" and well understood technology that is very hard to improve significantly.  However experimental work over the last 5 years has shown that a new generation of very high power laser can also accelerate electrons to the GeV regime, but over distances of just a few cm.  To date this work has been confined to experiments with very large but low shot rate laser systems based at a small number of National Laboratories.  We would now like to exploit a new generation of ultra-short pulse laser systems to move this exciting technique out of large scale facilities and into a University environment where it can be exploited for advanced imaging applications.

 

A new kind of laser system based on "optical parametric chirped pulse amplification" (OPCPA) allows the creation of sub-10 femtosecond high-intensity light pulses at kHz repetition rates.  Instead of using a "classical" laser amplifier, these systems transfer energy from a high quality, but long pulse drive laser to an ultra-short seed pulse instantaneously in a non-linear optical crystal.  A key advantage of this is that this process supports a bandwidth up to 5 times greater than "standard" gain storage laser amplifiers, making it far easier to create ultra-short light pulses.  The aim of this project will be to increase the energy and performance of one of our “few-cycle” sub-10-femtosecond lasers and couple it to an electron acceleration experiment operating at ~10-100MeV electron energies.  This will be used to create synchrotron-like short bursts of coherent X-rays for advanced imaging applications, for example with biological samples.

 

 Laboratory Astrophysics and High Energy Density Plasmas

 

We use a combination of high-energy lasers such as the Imperial College Cerberus system and the MAGPIE Z-pinch to create laboratory scale experimental models of some of nature’s most extreme events.  These include the interaction of supernova remnants with the interstellar medium and the launch of light-year scale plasma jets from the accretion disks surrounding young stars.  For laser based experiments we use “atomic cluster” gases, a low density medium composed of many billions of sub-wavelength scale particles, each a few thousand to a few million atoms in size.  Atomic clusters are spectacularly efficient at absorbing intense laser light and allow us to reach energy densities in the laboratory of ~10E9J/gram, some 100,000 times the energy density of a chemical explosive.  The hot plasma filaments we make in this way expand at up to Mach 100 and are often preceded by an intense burst of soft x-rays that modify the propagation of energy and mass through the gas.  We use ultra-high time resolution imaging to probe the behaviour of these systems, which are analogous to supernova remnants ramming into the interstellar medium.

 

Using MAGPIE we can also launch and probe strongly magnetised plasmas that can model astrophysical systems including so-called Herbig Harrow objects, twin plasma jets light years in scale that are flung out into space as young stars condense from an accretion disk.  We use the Cerberus laser to drive optical or x-ray probes that can capture the behaviour of these energetic plasmas against the strong self emission background of the experiments.  We are now extending this to include multi-MeV proton beam probes that will be driven by a sub-picosecond pulse from the 40 terawatt arm of the Cerberus laser.  These will allow us to capture for the first time the internal magnetic and electric field structures of these complex plasma systems.

 

High Brightness X-ray Sources

Irradiating a small, solid target with a high-power multi-terawatt laser can create an extremely bright x-ray source that is very useful for probing objects such as high-energy density plasmas and laser fusion implosions.  However, shooting a “simple” solid target tends to create an extended x-ray source as electrons are heated by the laser and then spread out around the laser focus.  The situation is further complicated by the large electric currents and magnetic fields generated at the same time, which can for example cause the support stalk of a small target to emit as well.

 

To address this we are creating and characterising new types of laser driven x-ray source using a number of techniques.  For some experiments we launch and then optically trap a few micron bead or liquid droplet using a low power continuous laser beam.  Unlike a “standard” optical trap that uses a microscope system to catch and control a small object floating in a liquid medium, we use a large aperture optical system pointed vertically upwards to balance the force of gravity, a so called “levitation trap”.  This works in air as well as liquids and we are now extending it to work in vacuum to provide high precision, isolated microtargets for laser-interaction and x-ray generation experiments.  The geometry of these small targets also results in local boosts to the laser electric field when we irradiate them with a terawatt (10E12 W)or petawatt (10E15 W) heating beam.  This creates much hotter, brighter plasma sources than one would otherwise expect.

 

Creating and Probing Extreme States of Matter.

 

Laser systems can be used to heat and compress matter to the kinds of extreme conditions only found in nature in the cores of Jupiter scale planets, or in challenging laboratory environments such as the initial compression phase of an inertial confinement fusion (ICF) capsule.  Under these conditions fundamental material properties and chemistry change rapidly and a sample may undergo a complex series of phase changes.  Material can be created in the so called "warm dense matter" regime, which even the most advanced computer simulations find particularly challenging to model.

 

The aim of this project is to link the work of the Plasma Physics Group and the Institute of Shock Physics (ISP).  We will use a high-energy laser driver to launch shocks to few mm scale targets and follow the evolution of the system with a range of high-time resolution optical and X-ray probes.  Part of the project will involve the development and exploitation of new probing techniques based on sub-picosecond ultra-broad bandwidth laser pulses that we can use to investigate the motion and reflectivity of the rear surface of a thin foil as a shock breaks through it.

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