ionisation appears to have no influence on the formation of hot spots and the stability of the discharge as long as the preionisation electron density is above a level where the avalanche heads overlap during the breakdown phase.3 With a pre-ionisation below this threshold level, the whole discharge volume is filled with micro-arcs, which strongly reduce the optical output. 'Department of Applied Physics, Twente University, P.O. Box 217, 7500AE Enschede, The Netherlands 1. W. J. Witteman, et al., in Conference on Lasers and Electro-Optics, Vol. 11, Technical Digest Series (Optical Society of America, Washington, DC, 1993), paper CThI3. J. Couts, C. E. Webb, J. Appl. Phys. B59, 704 (1986). A. J. Palmer, Appl. Phys. Lett. 25, 138 (1974). 2.

J. Fousek

Papers

Photorefractive grating fixing and enhancement in multidomain ferroe[ectric crystals