Optics and Lasers in Engineering 7 (1986/7) 1-13 Plasma Transients during Laser Drilling in Subatmospheric Pressure Atmospheres of Air Bekir Sami Yilbast Mechanical Engineering Department, Birmingham University, Birmingham, UK and Zahide Yilbas Department of Mechanical Engineering, Miihendislik Fakiiltesi, Erciyes Universitesi, Kayseri, Turkey ABSTRACT The present work presents data on the dependence of electron temperature and density as a function of time and of background gas pressure. In addition, it examines the delay time between the start of the laser pulse and the arrival of the plasma at a Langmuir probe, and the dependence of this delay time on the material being drilled and the background gas pressure. NOMENCLATURE aw A e /is /p L KB Absorption coefficient of plasma (l/m) Active probe area (m 2) Electron charge (1.6022 x 10-19 C) Electron current (A) Saturation ion current (A) Probe current (A) Electron current density (A/m 3) Boltzman constant (1.38 x 10-23 J/K) t Present address: Mfihendislik Fakiiltesi, Erciyes Universitesi, Kayseri, Turkey. 1 Optics and Lasers in Engineering 0143-8166/86/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Northern Ireland