An electric current is discharged through a gas-filled quartz tube, generating a hot plasma. This plasma radiates intense light which can be coupled into a laser crystal, optically exciting the laser medium. For Nd:YAG lasers, lamps filled with the noble gases Xenon and Krypton are frequently selected as they are most efficient at radiating wavelengths of light which are in the excitation spectrum of the Nd atoms and are readily absorbed by the Nd:YAG crystal. Flash lamps are operated in a pulsed mode; they discharge high current pulses (hundreds of amps) ranging in duration from .1 to 20.0 milliseconds. They are filled with either Krypton or Xenon: Krypton is more efficient at low currents and Xenon at high currents. Fill pressures are usually less than one atmosphere to help prevent explosions during the highly energetic current pulses.