Exploring the Use of Graphene as a Target Material for Laser Plasma Ion Acceleration

Abstract

The interaction of a solid target with an ultra-high intensity laser pulse can result in the laser plasma acceleration of ions. The recent proposal of a new laser plasma ion acceleration scheme, named chirped standing wave acceleration, has created interest in a new class of ultra-thin solid target materials, either freestanding or as a part of novel compound targets. In this thesis, it is assessed, if it is feasible to use targets made of freestanding graphene, a carbon allotrope consisting of only one or a few atomic layers.
For this purpose, a target system was developed to mount freestanding graphene in the ion acceleration experiment at the Lund Laser Centre, and commercially available graphene targets were put to a number of tests. Using the Lund Terawatt Laser, the threshold for laser induced damage of the targets was determined. Further, freestanding graphene targets were exposed to ultra-high intensity laser pulses, in order to evaluate the effect on these ultra-thin samples and the supporting structure. The conditions were similar to those in conventional laser plasma acceleration experiments. For analysis and alignment purposes, the targets were imaged using an existing on-line microscope system, and the steps required to extend the imaging system with a Raman spectroscopy setup were explored.
The specially designed and constructed target mounting system was found to work reliably, and the graphene targets used in this project were found to be robust enough to be handled in the experimental environment. While the Raman spectroscopy was not fully implemented, the microscope system was extensively used and found capable to reveal occasional imperfections of the freestanding graphene samples. The damage threshold fluence was found to be approximately 0.1 J/cm^2 for the graphene targets. In the ultra-high intensity shots, small damage was inflicted to the frame supporting the graphene, and particle acceleration was observed. Accelerated ions were recorded with nuclear track detectors. They show traces of protons with energies above 1 MeV, and some signals also of heavier ions. The acceleration is attributed to a target normal sheath acceleration-like process, possibly involving the graphene-supporting copper grid, but the limited data does not allow a definite interpretation.
The results of this thesis show that freestanding graphene is robust enough to be used in future studies of laser plasma interaction. Adjustments need to be made to the existing target geometry to prevent an ionisation of the graphene-supporting structure. Based on the measured damage threshold, it is concluded, that, for future studies, the temporal laser pulse contrast needs to be improved, by reducing the amplified spontaneous emission of the laser.

Popular Abstract

Particle accelerators are machines that generate beams of very fast moving particles. The particles can approach the speed of light and have large kinetic energy. Such particle beams are important for research in physics, biology, chemistry, and materials science. Industry also makes use of particle accelerators, and there are even applications in modern medicine for diagnostic and therapeutic purposes, for example for treating cancer.
The particles used in an accelerator are usually negatively charged electrons or positively charged ions. Together they form uncharged atoms, the building blocks of all matter around us. When isolated, however, the particles' charge allows them to be accelerated in electric fields. This is the mechanism all particle accelerators rely on.
The kinetic energy a particle can gain over a given distance is determined by the strength of the electric field. In conventional accelerators, the electric fields are confined in hollow metal structures, so called cavities. This limits the applicable field strengths, because extremely strong fields would damage the structure of the cavities themselves. The only way to increase the maximum energy of the particles is to build a longer accelerator. That is the reason why high energy particle accelerators are very big and expensive machines. The most famous example is the Large Hadron Collider (LHC), used for fundamental physics research at CERN close to Geneva, which has a circumference of 27 km and has cost several billion euros.
A technique called laser plasma acceleration is a new approach to this challenge. This mechanism eliminates the need for cavities, and hence also the technological limitation of the accelerating field strength. Instead the electric fields are generated in the interaction of a powerful laser pulse with a small amount of matter.
Modern high power laser systems can generate flashes of laser light with mind-boggling properties. The duration of these laser pulses can be as short as a few tens of femtoseconds. A femtosecond is a billionth of a millionth of a second, a timescale so short that even light can only travel a distance shorter than the diameter of a human hair. Such short pulses can reach a power of tens to thousands of terawatts. For comparison, the average power consumption of the entire world is roughly fifteen terawatts.
For laser plasma acceleration, these laser pulses are focused to a spot with a size of a few micrometres, comparable to the size of a bacterium. Concentrating such a high power on such a small area creates an ultra high intensity, exceeding 10^18 watt per square centimetre. Since light is an electromagnetic wave, this produces extremely strong electric fields. When such a focused pulse hits matter, it rips the constituting atoms apart. This creates a mixture of free electrons and ions, a so called plasma.
Under the right conditions, the laser pulse pushes the free electrons forward as an ensemble, while the ions essentially stay at rest. Since the ions and electrons have opposite charges, there is a strong attractive force between them. This force accelerates the ions and makes them follow the electrons in their movement. The electric field between the electrons and the ions is several thousand times stronger than any field that can be generated in a conventional accelerator cavity. Therefore, the required acceleration length in a laser plasma accelerator is very short.
Laser plasma acceleration is a young research field and a lot of improvements are required, before it can be used for the applications mentioned above. One such optimisation concerns the target. If we want to accelerate ions, the target material for the laser pulse is usually a thin solid foil, and recently, it was motivated to consider new super-thin target materials.
In this thesis, it is investigated if it would be possible to use graphene as a target material. Graphene is the thinnest known material in the world. In its purest form it consists of a single layer of carbon atoms. Graphene has plenty of fascinating properties which have made it an extremely popular research subject during the last years. The thesis describes how it is possible to integrate graphene into laser plasma acceleration experiments. It also contains some interesting measurements that yield important information and leads for the next steps that should be taken in the development and research of graphene targets. One of these experiments, for example, served to determine at which laser intensity the graphene starts to show signs of damage, which helps to determine the required laser parameters. In this manner the stage is set for upcoming dedicated studies of this special material. It will be interesting to see where these investigations will lead, because there is hope that graphene targets may one day be used for the laser plasma acceleration of carbon ions, which would be immensely interesting for the particle therapy of difficult to reach tumours.

• author: Pahl, Hannes
• supervisor: Claes-Göran Wahlström
• organization: Department of Physics; Atomic Physics
• course: FYSM30 20162
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Laser Plasma Acceleration, Target Material, Graphene
• publication/series: Lund Reports on Atomic Physics
• report number: LRAP 533 (2017)
• language: English
• id: 8900167
• date added to LUP: 2017-01-31 10:34:15
• date last changed: 2017-01-31 10:34:15