Worst-Case X-ray Photon Energies in Ultrashort Pulse Laser Processing

Area of Science:

• Materials Science
• Laser Physics
• Radiation Physics

Background:

• Ultrashort pulse laser processing can generate secondary X-ray emissions when laser irradiance exceeds 10^13 W cm^-2.
• Understanding these X-ray emissions is crucial for safety and process control in laser material processing.

Purpose of the Study:

• To investigate spectral X-ray emissions during ultrashort pulse laser processing of tungsten and steel.
• To determine maximum X-ray photon energies and calculate dose equivalent rates under various laser conditions.

Main Methods:

• Utilized three complementary spectrometers (CdTe and silicon drift detectors) for simultaneous X-ray spectral analysis.
• Employed an ultrashort pulse laser workstation with controlled parameters (274 fs pulse duration, 1030 nm wavelength, 50-200 kHz repetition rates, 33 μm spot diameter).
• Operated in single pulse and burst laser modes, varying pulse energy and repetition rate at constant average power (approx. 20 W).

Main Results:

• Peak irradiances ranged from 7.3 × 10^13 W cm^-2 to 3.0 × 10^14 W cm^-2.
• X-ray dose equivalent rate increased with lower repetition rates and higher pulse energy at constant average power.
• Burst mode laser processing significantly elevated dose rates and X-ray photon energies.
• A maximum X-ray photon energy of approximately 40 keV was recorded during burst mode processing of tungsten at 50 kHz repetition rate and 3 × 10^14 W cm^-2 peak irradiance.

Conclusions:

• Laser irradiance thresholds for X-ray generation in ultrashort pulse laser processing were confirmed.
• Burst mode operation presents a higher risk due to increased X-ray dose rates and energies.
• Characterization of X-ray emissions is essential for safe implementation of high-power ultrashort pulse lasers.