Brunel-dominated proton acceleration with a few-cycle laser pulse

Area of Science:

• Laser-driven particle acceleration
• Plasma physics
• High-intensity laser-matter interactions

Background:

• Proton acceleration is crucial for applications like cancer therapy and materials science.
• Conventional methods often rely on target normal sheath acceleration (TNSA).
• Understanding novel acceleration mechanisms is key to optimizing proton beam properties.

Purpose of the Study:

• To experimentally investigate the dependence of backward accelerated proton energy on laser parameters.
• To elucidate the underlying electron dynamics driving proton acceleration.
• To compare the observed mechanism with established models like TNSA.

Main Methods:

• Utilizing a kHz laser system delivering ultrashort (5 fs), high-contrast (10^8) laser pulses.
• Interacting laser pulses with thick solid targets to generate proton beams.
• Performing experimental measurements of proton cutoff energy.
• Conducting two-dimensional particle-in-cell (2D-PIC) simulations.
• Developing a theoretical model to explain acceleration mechanisms.

Main Results:

• Proton cutoff energy was found to depend on laser pulse energy, polarization, and duration (5-500 fs).
• Experimental data and simulations showed strong agreement.
• The study identified direct acceleration by Brunel electrons as the dominant mechanism.
• This mechanism was contrasted with the conventional target normal sheath acceleration (TNSA).

Conclusions:

• Brunel electron acceleration is a significant mechanism for generating backward accelerated protons with ultrashort lasers.
• This finding provides a new perspective on laser-driven proton acceleration, distinct from TNSA.
• The results offer a pathway for optimizing laser-driven proton sources by controlling Brunel electron dynamics.