Lawrence Livermore National Laboratory

NIF’s master oscillator generates a very small, low-energy laser pulse (see Injection Laser System). The pulse may range from 100 trillionths to 25 billionths of a second long and has a specific temporal shape as requested by NIF experimenters. The low-energy pulse is split and carried on optical fibers to 48 preamplifier modules (PAMs) for initial amplification and beam conditioning and shaping.

The PAMs increase the energy by a factor of 10 billion to about 10 joules. Each PAM is a line replaceable unit containing two different amplification stages. The first is a regenerative amplifier that amplifies the pulse energy from about 750 picojoules (trillionths of a joule) to about 15 millijoules (mJ) (thousandths of a joule), an amplification factor of 20 million. The injected Master Oscillator Room (MOR) pulses travel through a laser diode-pumped neodymium-doped glass rod 116 times. The pumped light derives from a 4-kilowatt array of laser diodes whose output is channeled into one end of the rod via a hollow-lens duct concentrator. The output beam from the amplifier is then shaped from a round 3-millimeter (mm) Gaussian beam to a square 18x18-mm beam.

The second amplification stage amplifies the shaped beam by a factor of up to 10,000 using a four-pass relay imaging laser amplifier. The amplifier is a neodymium glass rod pumped by 12 flashlamps. The output energy of the PAM can be varied from about 10 mJ to about 10 joules.

PAMs perform three kinds of precision beam shaping:

  • spatial shaping to make the square beam more intense around the edges to compensate for the higher gain profile in the center of the large amplifiers;
  • spectral shaping and beam smoothing to eliminate both hot spots and dark spots at the focus by manipulating the focal beam pattern with fast changes in wavelengths; and
  • temporal shaping to ensure that the laser pulse delivers energy to the target with a prescribed time-history for efficient ignition.

The 48 beams from the 48 PAMs are then split into four beams each for injection into the 192 main laser amplifier beamlines.

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