Plasma Accelerators Driven In Waveguides: Training the Next Generation of Facility Users

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics


Particle accelerators are used in many areas of the physical and biological sciences. For example, fundamental studies of the building blocks of matter are carried out with huge accelerators at institutions such as CERN. On a smaller scale, synchrotrons use accelerated electron beams to create light which is widely tunable from the infra-red to x-rays.The conventional accelerators used in these machines employ radio-frequency electric fields to accelerate charged particles. However, the maximum electric field that can be used is limited by electrical breakdown in the beam pipes, so that accelerating particles to high energies requires a very long accelerator (the largest machine at CERN is 27 km in circumference!).Laser-driven plasma accelerators offer a way to make particle accelerators much more compact. In these devices an intense laser pulse propagates through an ionized gas (a plasma). As it does so, the laser pulse pushes the electrons away from it and sets up a plasma wave which follows behind the laser pulse; this behaviour is directly analogous to the water wake which trails a boat crossing a lake. In the case of a plasma wave, at the peaks of the wave there are more electrons than average, and at the troughs there are fewer. As a result of this charge separation, a very large electric field forms between the peaks and troughs of the plasma wave. This field can be about 1000 times larger than the maximum electric field used in conventional accelerators, which means that a plasma accelerator can be 1000 times shorter than a conventional one and still produce particles of the same energy.This idea for making compact accelerators was first proposed over 25 years ago, but until recently the energies they could reach were relatively low. The primary reason for this is that the driving laser pulse naturally defocuses as it propagates through the plasma, reducing its intensity to the extent that acceleration ceases after only a few millimetres.Over the last few years our group has developed a new technique for channelling the intense laser pulses over long distances. This technique involves forming a so-called plasma waveguide by firing an electrical discharge through a narrow, gas-filled capillary. The plasma formed in this way has a lower density on axis, which acts to continually refocus the laser radiation and so prevent it from defocusing. The plasma waveguide is therefore similar to an optical fibre.Very recently we used this channelling technique to extend the length of laser-driven plasma accelerators by more than a factor of 10, and so increase the energy of the accelerated electrons to a billion electron volts - that is, the energy an electron would gain if it were accelerated by two plates with a billion volts between them. This electron energy is about the same as used in conventional synchrotrons - but the plasma accelerator is only 33 mm long, compared the tens of metres required for a conventional accelerator.The present programme of research aims to build on these advances and develop techniques for increasing the energy of the accelerated electrons and providing more control of the acceleration process.
Description This grant provided support for a Facilities Access student to work in the field of laser-driven plasma accelerators. The programme: (i) provided the student with experience of working with a new national laser facility (Astra-Gemini); (ii) provided training in important research techniques, including operation of high-power femtosecond laser systems, femtosecond pulse characterisation, laser guiding, electron beam diagnostics, vacuum handling, high-voltage discharges, and laser safety; (iii) gave a solid grounding in atomic and laser physics and in the skills necessary to undertake research.

The Facilities Access student developed a new, more compact design of plasma channel based on a capillary discharge. This was used to guide very intense laser pulses from the Astra-Gemini laser over several centimetres of plasma, thereby generating beams of electrons with energies of up to 0.9 GeV.
Exploitation Route Laser-driven plasma accelerators of the type investigated in this project may find applications in generating particle beams capable of probing large-scale volumes, and as such may have applications to border control and national security. Plasma accelerators of the type studied by the Facilties Access student can generate energetic particle beams in one-thousandth of the distance required by a conventional, radio-frequency accelerator. They therefore offer the prospect of a new generation of very compact particle accelerator with many potential applications in the physical and biological sciences. For example, the 0.9 GeV electron beams generated in this project could be passed through a magnetic undulator to generate bright, ultrafast x-ray beams. Today this is routinely done in synchrotron facilities, but plasma accelerators could generate similar wavelengths from a much smaller and cheaper system which would be suitable for university or industrial laboratories. Of key importance is the fact that x-ray light sources based on laser-driven accelerators would provide ultrafast and tunable pulses of electrons, x-rays, and visible radiation with inherent synchronization.
Sectors Agriculture, Food and Drink,Education,Environment,Healthcare,Pharmaceuticals and Medical Biotechnology,Security and Diplomacy

Description Further funding
Amount £732,351 (GBP)
Funding ID EP/H011145/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 10/2009 
End 09/2013