Injection of Tunnel Ionized Electrons into Laser-Produced Wakes

Arthur Pak
Advisor: Chandrasekhar Joshi

Tuesday, August 24, 2010 at 2:00pm
Engr IV Room 57-124

Abstract:
The injection of electrons via tunneling ionization into a laser driven wakefield accelerator has been studied with experiments, theory and simulations. In this work the large difference in ionization potentials between successive ionization states of trace atoms, is used as the mechanism for injecting electrons into a laser driven wakefield. Here a mixture of helium and trace amounts of either nitrogen or methane (CH4) gas was used. Electrons from the K shell of either nitrogen or carbon were tunnel ionized near the peak of the laser pulse and injected directly into the wake. The wake was created by electrons from majority helium atoms with contributions, depending on which gas mixture was used, from the either the L shell of nitrogen or the hydrogen and the L shell of carbon.

Using the helium nitrogen gas mixture, the threshold intensity at which trapping was observed to occur corresponded to the intensity required to ionize the 6th electron from nitrogen(i.e. the first electron within the K shell). This indicates ionization of the K-shell of nitrogen triggers injection, and the subsequent trapping and acceleration of electrons. The electron energy spectrum was observed to be quasi-continuous for a laser ao > 2. This spectral shape is a result of the continuous ionization and injection which occurs as long as the magnitude of the laser field remains above that required to ionize the K-shell of the nitrogen. The relative amount of charge accelerated, the intensity threshold, and spectral shape of accelerated electrons were found to be in good agreement with 3-D particle in cell code simulations which modeled the interaction.

The trapping of electrons into a wakefield was studied theoretically using a 3-D Hamiltonian description of an electron within a electromagnetic field. From this description, a constant of motion can be found which relates the scalar and vector potentials of the wake and laser to the momentum of an electron. In this manner the required potential difference that an electron must experience to become trapped can be solved for. This estimate of the required potential difference for trapping to occur was found to be in good agreement with that observed in 2-D particle in cell code simulations.

Using the 3-D scaling laws for the laser wakefield accelerator in the blowout regime, it was found injecting electrons directly into the wakefield, as they are when injected via ionization, significantly increases the potential difference, or the amount of energy available to the electron, to become trapped. This increase in available potential can in turn be used to lower the absolute wake amplitude necessary for trapping to occur. This reduction in wake amplitude means that electrons can be trapped into wakes with lower amplitudes driven by lasers with lower peak powers.

The acceleration of electrons at reduced laser powers using ionization injection has been confirmed experimentally. Using the helium nitrogen or helium methane gas mixtures, accelerated electrons have been routinely observed using laser powers 2-4 times lower that what has been required to accelerate electrons from plasmas created from pure helium gas at similar densities.

Biography:

Arthur Pak is a graduate student in the department of electrical engineering, and for the last 6 years has preformed research in the field of laser wakefield acceleration.