Energy Threshold for Laser Induced Breakdown on a Metal Surface Under High and Ultra High Vacuum Conditions

Unipolar arcing is the primary breakdown process when a powerful laser pulse interacts with a target surface. The unipolar arc model assumes that the initial ionization occurs in desorbed gas layers. To check this experimentally a metal surface was illuminated under different vacuum conditions. The experiments were conducted at 10-4, 10-6 and 10-8 torr vacuum. A neodymium:glass laser of wavelength 1.06 um in the Q-switched mode was utilized. Type 304, polished, stainless steel plates were used as targets. Results confirmed that higher laser energy was needed to trigger the laser induced breakdown at lower pressures.