Laser Wakefield Acceleration

The aim of the project is to gain insight into the requirements and control over the injection of electrons into a laser wakefield accelerator. The goal is to produce a stable output of ultra-short (femtosecond) electron bunches in the 100 MeV energy range.

Laser Wakefield Acceleration

When a high-power (Terawatt level) laser pulse is fired into a plasma, it creates a density modulation in its wake, the laser wakefield. This plasma wave propagates at the group velocity of the laser pulse through the plasma, i.e. close to the speed of light. The (longitudinal) electric fields inside this plasma wave can be of the order Teravolt per meter, 4 orders of magnitude larger than those in conventional accelerators. Using these fields to accelerate electrons have great potential for the development of compact high-energy accelerators.

Injection

Although the plasma wave is travelling at nearly the speed of light behind the laser pulse, the plasma electrons only oscillate around their original position and do not experience a net acceleration. In order to ‘trap’ electrons in the wave they need to be pre-accelerated (like a surfer trying to catch a wave). In present experiments, this is mostly done by increasing the laser power to a point where the amplitude of the oscillations of the electrons in the plasma exceeds the wavelength of the plasma wave. At that point, the wave ‘breaks’ and some electrons will be trapped. Using this scheme, it is possible to accelerate electrons to high energies (100 MeV- 1 GeV) in only a few millimeters to a few centimeters of plasma. The main drawback is that the process is difficult to control, because it is very sensitive to the exact laser power and plasma density. To avoid the need for wave-breaking, other schemes have been developed, such as injection by a counter-propagating laser pulse (which interferes with the laser pulse driving the wakefield and thereby providing an extra ‘kick’ at the position in the plasma where the two laser pulses overlap), or by creating a density ramp in the plasma which modifies the velocity of the plasma wave locally. Although these schemes have shown a big improvement in the stability of the output of the accelerator, they still need to be operated close to the wave-breaking limit of the wakefield, which limits the range of parameters and tunability at which the accelerator can be operated.

External injection

The Laser Wakefield Acceleration project at TU/e is aimed at injecting electrons from an external, conventional accelerator into a laser wakefield. The electrons can be pre-accelerated to the energy needed to be trapped by the plasma wave. If successful, this scheme will lead to better control over the injection and a more stable output. It will greatly expand the possible parameters (mainly plasma density and laser power) at which laser wakefield acceleration can be achieved. The energy of the electrons and, to some extent, the duration of the electron bunches, can be continuously tuned. This level of control is essential to develop laser wakefield acceleration for applications.

The Laser Wakefield Accelerator setup at TU/e: The 3-GHz photogun pre-accelerates an electron bunch. The electrons are focused into a capillary discharge plasma channel, together with the high-intensity laser pulse. The pre-accelerated electrons are trapped in the wakefield of the laser pulse.

Background information: (in dutch)

http://www.kennislink.nl/publicaties/surfende-elektronen

http://web.tue.nl/cursor/internet/jaargang53/cursor01/onderzoek/onderzoek.php?page=o1

People involved: The following people are or have been involved in this project:

PhD studentXavier Stragier
Former PhD studentWalter van Dijk
Undergraduate studentsHans Marée, Thijs de Kruif
Former studentsDon van der Drift, Michiel de Moor, Felix Hommersom
TechniciansEddy Rietman, Jolanda van de Ven, Ad Kemper, Harry van Doorn, and Iman Koole
Staff membersSeth Brussaard