To use a laser for particle acceleration, an unusual state of matter called plasma is required. Plasma is hotter (and more energetic) than gas, so much so our conventional atoms have their electrons stripped from them to become electrically charged ions. A very intense laser can be fired through a plasma to further separate the electrons from the ions and in doing so create an electric field.
Ideally, this laser will be intense enough to form ‘wakefields’ in the plasma. Strong plasma waves can form highly powerful electric fields over remarkably short distances. As the laws of attraction dictate that any charged particle that enters such a field will feel a force and accelerate, this creates an effective and compact particle accelerator. This can be up to a thousand times stronger than a traditional, radio-frequency accelerator!
However, there are still some challenges when it comes to making laser-driven plasma accelerators as efficient as possible. The Oxford Laser Plasma Accelerators Group (LPAG), led by Prof. Simon Hooker, have recently performed experiments at Gemini to demonstrate how two of these challenges can be overcome with one single solution.
The first challenge is ensuring that the laser pulses used to create the fields within the plasma remain focused as they travel in space. For a successful accelerator they must maintain a width of around 30 micro-metres, equivalent to a human hair! However, light is often inclined to deviate and scatter, and diffraction causes the laser pulses to bend and spread out over the length of the plasma. This reduces the intensity of the beam which will inhibit the accelerator.
The team have developed a type of ‘plasma channel’ to overcome this. An initial auxiliary laser is fired in a cone-shaped lens to form a cylinder of hot plasma, which then explodes outwards. Inwards, a low-density cavity is left in the gap that the cylinder left behind, with the high-density plasma surrounding it. This cavity becomes a tunnel for the main laser pulses to travel through, continually focusing them like an optical fibre. When timed perfectly, this allows the team to significantly reduce losses in intensity and power.
Diagram of the plasma channel being created (Image credit: J. Chappell, University of Oxford).
The second challenge of laser-driven particle accelerators is successfully injecting particles (usually electrons) into the plasma to accelerate and study. These need to be located in the perfect place at the perfect time. The LPAG’s method creates a drop in the plasma’s density at the entrance to the channel. This disrupts the plasma in such a way that it collects and traps a bunch of electrons, which can then be accelerated through the channel. These electrons have reasonably similar energies, making them useful in many investigations.
The LPAG used their technique to accelerate electrons up to energies of 1.2GeV in a plasma channel just 11cm long. This electric field is 100x greater than a conventional accelerator! Their further simulations showed that a feasible 41cm channel would increase the electron’s energies to 3.65 GeV whilst keeping a successful and high-quality beam. A laser-based accelerator like this could drive a new type of X-ray free electron laser, which are currently expensive, complex, and often over a kilometre long! The LPAG’s solutions could be the key to a table-top version of this, making these high-demand machines more accessible for science and engineering.