Lawrence Livermore National Laboratory
Optical Switch
The NIF laser uses the largest-ever plasma electrode Pockels cell (PEPC) optical switches, enabled by LLNL-developed crystal rapid-growth processes. Slabs cut from the crystals are used in the PEPCs and in the final optics assembly frequency conversion units.
View Video. A key component in the laser chain, an optical switch called a plasma electrode Pockels cell (PEPC), was invented and developed at LLNL (see The Seven Wonders of NIF). A Pockels cell is a device that rotates the polarization of a beam when a voltage is applied across an electro-optic crystal. Depending on the voltage applied, the Pockels cell either allows light to pass through or to reflect off a polarizer, creating an optical switch. In the NIF beamline, the PEPC allows the laser pulse to make four passes through the main amplifier, building up its energy with each pass. Without this multi-pass configuration, NIF's beamlines would have to be much longer than they are.
What makes the PEPC unique are its plasma electrodes. A conventional Pockels cell applies the switching voltage to metal electrodes that are beside the crystal as the beam passes through it. In the PEPC, two plasma cells are created and used as electrodes, the crystal is sandwiched between the electrode cells, and the beam passes through both electrodes and the crystal. The plasma, an ionized gas like that found in a fluorescent light bulb, is electrically conductive yet transparent to the NIF laser beam. Compared to a conventional Pockels cell, this configuration allows for uniform rotation (switching) across a large beam with a much thinner crystal, therefore requiring a much smaller pulsed power system to apply the switching voltage pulse.
A plasma electrode Pockels cell is assembled in the Optics Assembly Building.
The NIF PEPCs are built in modular units called line replaceable units, or LRUs. Each PEPC LRU contains four 40-centimeter potassium dihydrogen phosphate (KDP) switch crystals, servicing four beams in a one-beam wide by four-beam high configuration. The plasma electrodes cells each service two beams in a one-by-two configuration, but since a plasma is required on both sides of the crystals, there are four plasma cells in each LRU.
A plasma pulse generator (PPG) chassis supplies a 20-microsecond, five-kilovolt voltage pulse to each cell to generate plasma before the beam arrives. After the beam has reflected off the polarizer and passed the LRU, a switch pulse generator (SPG) applies a 400-nanosecond, 17-kilovolt pulse to the cells across the crystal. This pulse activates the switch and traps the beam in the main amplifier cavity, allowing each beam to make four passes through the main amplifier. Two SPGs are required for each LRU. The entire 192-beam PEPC system contains 48 four-beam PEPC LRUs, 192 PPGs, and 96 SPGs, along with control, diagnostic, vacuum, gas, and timing systems.
The NIF Pockels cells LRU was designed at LLNL and fabricated in the NIF Optics Assembly Building. The system has excellent switching uniformity and an aperture-average extinction ratio of better than 125 to one.
The plasma pulse generators are mounted in racks located above the NIF beamlines in the laser bays.
More Information
"An update on NIF pulsed power" Pulsed Power Conference, 2009. PPC '09. IEEE (subscription required)
"Development of Solid-State Drivers for the NIF Plasma Electrode Pockels Cell"IEEE International Power Modulators and High Voltage Conference, Proceedings of the 2008 (subscription required)
"Plasma electrode Pockels Cell subsystem performance in the National Ignition Facility" Pulsed Power Conference, 2007 16th IEEE International (subscription required)
