All-Optical Plasma Channels and Electron Injection with Spatio-temporal Control

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 within the accelerator, so that accelerating particles to high energies requires a very large device: synchrotron machines are about the size of a football stadium, and 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 the laser pulse; this behaviour is 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. In fact, 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 and still produce particles of the same energy. In this research programme we seek to address two challenges for laser-driven plasma accelerators. The first centres on the fact that the driving laser pulse must remain focused over the several centimetre length of the plasma accelerator. A beam of light will naturally spread out ("diffract"), reducing the intensity of the beam to a low value after only a few millimetres. To overcome this tendency the laser pulses must be guided, just as the light used to transmit communications data is guided in an optical fibre. However, an optical fibre would be destroyed by the enormous laser intensity needed to drive a plasma accelerator. To put this into context, the laser intensity needed is equivalent to focusing the output power of all the power stations on Earth to an area smaller than the cross-section of a human hair! We therefore plan to develop a new type of optical "fibre" made from plasma rather than from glass. The research is aimed at creating plasma fibres with properties ideally suited to laser-driven plasma accelerators. The second challenge we will address is to improve the quality of the electron beams generated by laser-plasma accelerators. In most experiments the accelerated electrons are injected and trapped in the plasma wave through a complex sequence of highly nonlinear processes. Although this approach has been very successful at generating electron beams, these beams typically have a spread in energies which is too large for the most challenging applications, and the properties of the electron beam fluctuate to a large degree. To overcome these problems we will investigate methods to control how electrons are injected and trapped in the plasma wave. Both parts of the proposed programme will take advantage of recent advances in methods to control the delivery of pulses of in both space and time, a process known as "spatio-temporal" control.