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A team led by Professor Simon Hooker's group at Oxford have shown that very compact particle accelerators could be driven by trains of laser pulses  travelling  through a plasma (an ionized gas). This work illustrates the potential for efficient plasma accelerators operating at much higher pulse repetition rates than possible to date.


Principle of the multi-pulse laser wakefield accelerator.

Each pulse in a train of low energy laser pulses (red) drives its own plasma wave (blue). These waves will add coherently if the laser pulses are spaced by the wavelength of the plasma wave. The variation of the electron density within the wave generates large electric fields which can be used to accelerate charged particles. In this cartoon, only four laser pulses are shown, whereas a working MP-LWFA would use a train of 10 – 100 pulses.

In a plasma accelerator an intense laser pulse forces the plasma electrons out of its way to form a trailing electron density wave (or “plasma wave”). The electric fields generated within this density structure are about 1000 times stronger than those within a conventional accelerator, and since they move with the driving laser pulse they can be used to accelerate relativistic charged particles.

Electron beams with particle energies of a few GeV, similar to those used at large-scale synchrotron facilities, have already been generated by laser-plasma accelerator stages only a few centimetres long. However, the lasers used are inefficient (the “wall-plug” efficiency is less than 0.1%) and can only operate a few times per second.

In 2014 the Oxford group, and colleagues, proposed that a large amplitude plasma wakefield could be driven by a train of low-energy laser pulses. In this “multi-pulse laser wakefield accelerator” (MP-LWFA), each laser pulse drives its own wakefield. If the laser pulses are spaced by the wavelength of the plasma waves (determined by the plasma density), these will add coherently so that the amplitude of the plasma wave grows towards the back of the pulse train.

The MP-LWFA approach allows the use of emerging laser technologies which cannot easily generate short, high-energy laser pulses, but which could generate thousands of pulse trains per second, and with a wall-plug efficiency > 10%.


Recent experimental success

In the new work – performed at the Central Laser Facility, Rutherford Appleton Laboratory – these ideas were tested by measuring the amplitude of plasma waves driven by trains of up to 7 laser pulses. As reported in Physical Review Letters, the wakefield amplitude increased sharply when the resonance condition was met.

The team also demonstrated the potential for a future "energy-recovery" plasma accelerator by showing that a trailing, out-of-resonance laser pulse could remove unused wakefield energy. The energy is extracted via blue-shifting of the laser pulse, and in principle could be recycled. This ability could be important in future plasma accelerators generating high average beam currents.

A cartoon showing the concept of multi-pulse laser wakefield acceleration






Analysed data showing a picture of the wakefield driven by the train of pulses





Dr Kevin O'Keeffe was awarded the Cavendish Medal for the excellence of his research at the SET for Britain poster competition held on 12th March in the House of Commons.

Kevin is studying the use of high-intensity laser pulses to generate coherent beams of soft x-rays via highly nonlinear processes in gaseous media. These compact sources of ultrafast x-rays have many potential applications, such as studying the dynamics of chemical and physical processes occurring on timescales as short as a billionth of a nanosecond, and coherent diffractive imaging of biological structures.

However, without taking additional steps, the efficiency of these sources is very low - primarily due to the difference in speed with which the driving and generated radiation propagate, which in turn causes destructive interference between x-rays generated at different points in the medium. To overcome this Kevin, who works in Simon Hooker's group in Atomic and Laser Physics, is developing "quasi-phase-matching" techniques which have the potential to increase the brightness of the x-ray beam by several orders of magnitude.

“I’m surprised and happy," said Kevin. "I really didn’t expect to win. It’s really nice to get the recognition and acknowledgement that the research we are doing is important.”

In April 2017, The Plasma Accelerators and Ultrafast X-rays group hosted the Coherent Imaging with High Harmonic Sources Workshop, held at Merton College, Oxford. The meeting brought together thirty researchers from fifteen different institutions worldwide. The objectives of the meeting were

  • To establish how coherent X-ray microscopy with table-top sources can move from imaging test objects towards characterizing systems of genuine scientific interest.
  • To explore how the unique properties of these beams can be exploited to the full.
  • To discuss how temporally-resolved imaging with high harmonic sources can be achieved.

The workshop provided a timely opportunity for researchers to reflect on the rapid development of microscopy techniques utilising high harmonic generation, while also offering a chance to discuss scientific problems which may be addressed using coherent x-ray imaging.

Invited talks were delivered, respectively, by Christian Spielmann of Friedrich Schiller University Jena and Stefan Witte of the Advanced Research Center for Nanolithography, Netherlands. Charlie Pooley of the University of Southampton was judged the winner of the poster competition for his contribution titled “EUV transmission ptychography using a high harmonic source”.

Feedback from workshop attendees was unanimously positive and it is hoped that a similar meeting will be organised in the next few years.

In April 2017 the 7th Oxford Photonics day was held in the Department of Engineering Science, University of Oxford, bring together photonics researchers across the university for a day of talks and networking. During the poster session of the meeting, Daniel Treacher, graduate student in the Plasma Accelerators and Ultrafast X-rays group, was awarded a prize for his poster on 'Improving the Resolution Obtained in Lensless Imaging with Spatially Shaped High-Order Harmonics'. Congratulations Daniel!

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