Characterization of laser-driven proton acceleration from water microdroplets

Area of Science:

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

Background:

• Water droplets are practical targets for proton acceleration experiments.
• Laser temporal intensity contrast and angle of incidence influence proton energy.

Purpose of the Study:

• Investigate the impact of laser incidence angle on proton acceleration from water droplets.
• Determine optimal conditions for efficient laser energy coupling and hot electron generation.

Main Methods:

• Irradiating 20 μm water droplets with 0.4 μm laser pulses of varying intensity contrast.
• Analyzing proton energy as a function of droplet position relative to the laser polarization axis.
• Utilizing 2D particle-in-cell simulations to validate experimental findings.
• Monitoring plasma expansion with an ultra-short optical probe laser.

Main Results:

• Central laser irradiation resulted in inefficient energy coupling and low proton energies.
• A controlled pre-pulse enhanced hot electron generation and proton energies in central irradiation.
• Grazing laser incidence, without pre-plasma, yielded slightly higher proton energies.
• Hot electron streams along the droplet surface at grazing incidence drive proton acceleration.

Conclusions:

• Laser incidence angle significantly impacts proton acceleration efficiency.
• Grazing incidence is more effective for proton acceleration due to superior hot electron generation.
• Water droplets offer a promising platform for high-repetition-rate proton acceleration studies.