NERSC Initiative for Scientific Exploration (NISE) 2011 Awards

Laser Acceleration of Ions Using Few Times Critical Density Gas Targets

Michael Helle, Naval Research Lab

Associated NERSC Project: High Energy Laser-Driven Acceleration Based on the Laser Wakefield Accelerator (mp343)
Principal Investigator: Phillip Sprangle, Naval Research Lab

Our research will focus on the generation of high energy ion beams using a high intensity laser hitting a gas target. The high energy ion beams produced by this interaction can be applied to such diverse fields as ion beam fusion, the detection of special nuclear materials, and ion beam therapy for various types of cancer. Gas target are particularly attractive since they can be operated at high repetition rates. The high repeatability of our scheme will allow for real time scanning, a feature that is currently lacking. Another key advantage to this type of acceleration is that it can dramatically reduce the size and cost of producing the beams necessary for these applications. The compactness and reduction in cost of such systems provides a unique opportunity to introduce national laboratory like relativistic ion beams to private organizations, universities, and medical facilities.

The generation of high energy ions by means of high intensity laser irradiation of solid targets has been a subject of active research for over a decade. More recently, experimental groups at both Brookhaven National Laboratory and UCLA have shown ion acceleration using CO_2 lasers interacting with gas jets that, when ionized, yield plasma densities that are a few times critical density. The advantages of such targets are that they are relatively simple and can be easily operated at high repetition rates.

The physics that drive this type of acceleration is not yet well understood. Of particular interest is the scaling of such acceleration to various laser pulse parameters (including multiple pulses) and the effect of the longitudinal plasma density profile on the acceleration process. Additionally, since the plasma is only a few times critical density, frequency upshifted radiation is able to propagate deeper into the target which could lead to interesting new physics in itself.

Preliminary simulations using the PIC code TurboWAVE have been made in 2D. These have already proven to be very insightful. Specifically the simulations show the effect of the ponderomotive force on acceleration when moving from 1D to 2D and the generation of unique forward scattered radiation patterns. The ponderomotive force acts to push electrons away from the laser axis, creating a charge separation between the ions and electrons. The space charge fields setup by this separation leads to the acceleration of the ions. In 1D, the electrons can be pushed only in the direction of the laser pulse which tends to over estimate the amount of charge separation. By moving into 2D, the electrons are able to be pushed in the laser propagation direction as well as the direction transverse to that. This is due to the finiteness of the laser pulse's transverse profile. Moving to 2D yields a more realistic picture of the physical process and hints that it maybe advantageous to use a shorter pulse to produce greater longitudinal charge separation before the electrons are able to move transversely off the laser axis.

We wish to further explore this type of acceleration by moving to 3D. Here we will be able to examine the full effects of the ponderomotive force, particle slippage, and other nonlinear effects. To do this we plan to continue to use the code TurboWAVE. TurboWAVE is a particularly strong code since it exhibits strong scaling up to 10^4 processors (demonstrated) for similar problems and has been well benchmarked in 3D against Laser Wakefield Acceleration experimental results.