UC San Diego

The interaction of a laser pulse with a Sn preplasma formed by a low energy prepulse was investigated for an extreme ultraviolet (EUV) lithography light source. A much lower ion kinetic energy and nearly the same conversion efficiency from laser to in-band (2% bandwidth) 13.5 nm EUV light were simultaneously observed as compared with those from the direct interaction with a solid surface. The reason comes from the interaction of the laser pulse with a smooth preplume induced by the prepulse. The density profile of the preplume was measured with time-resolved shadowgraphy and could be fitted with a Gaussian function. The energy of the ions located at the flux peak E-p scales with the length of the preplume l(s) as E-p proportional to 1/l(s). Laser absorption in the low-density preplume and ion acceleration during plasma expansion are discussed. This result provides a general way to control particle energy from a laser plasma interaction. (c) 2007 American Institute of Physics.