Dependence of Pit Formation in Hard Tissue of Human Teeth on Free Electron Laser Pulse Structures

The purpose of this study was to compare and clarify the differences in pit formation between two modes of free-electron laser（FEL）irradiation with different beam cur rents and pulse structures and how the effects of these modes vary with the tissue structure of human teeth. FEL irradiation using the full-bunch mode（Fm）and burst mode（Bm）. The beam current was carried out at 200 mA for Fm and 2 A for Bm. The micro-pulse interval within the macro-pulse structure was 350 ps for Fm and 22.4 ns for Bm. The wavelength of the FEL was adjusted to 2.94 µm, and macro-pulse energy was adjusted to 6.0 mJ/macro-pulse by polarizing plate. The macro-pulse irradiation was set to 1 or 5 times for both modes. As the number of macro-pulse irradiations increased, the pits became deeper in Fm than in Bm, regardless of the site on the tooth. On the other hand, the bulge height was lower for Bm than for Fm, regardless of the site on the tooth or the number of macro-pulse irradiations. No scorch marks were observed in the pits for either mode. It was concluded that the differences in the pit including bulge formation were the macro-pulse structure between Fm and Bm. The intensity of the FEL irradiation caused plasma evaporation in the tooth hard tissue, and the tooth substance was ablated before thermal conduction occurred. The relation with the thermal relaxation time also confirmed that pits were formed without visible scorch marks.