LLE Builds New High Voltage, Large Aperture Pockels Cell Drivers

December 2006

Plasma Electrode Pockels Cell

To meet the demands of its OMEGA EP laser, the Laboratory for Laser Energetics (LLE) added four solid-state high voltage, large-aperture Pockels-cell drivers in 2006.

LLE's OMEGA EP laser system utilizes a large-aperture square beam and multi-pass amplifier design to generate high-energy laser pulses that extend the capabilities of inertial confinement fusion experiments. A large-aperture beam is required to keep the beam fluence below the damage threshold of various optics within the laser system. The beam requires a corresponding large-aperture optical switch to control the number of times the laser pulse traverses the multi-pass amplifier cavity.

Scientist with PEPC

The PEPC (Plasma Electrode Pockels Cell) is an electro-optic switch, originally developed at Lawrence Livermore National Laboratory (LLNL), can support the stringent requirements of high contrast (>1000:1) switching over a clear aperture of nearly 1600 square centimeters. When used in conjunction with a cavity polarizer, the PEPC switches the polarization of the laser pulse briefly to hold the pulse in the laser amplifier cavity for a total of four passes, thereby allowing the pulse to be amplified more efficiently.

To perform this switching function, a drive potential of approximately 17kV must be applied across the electro-optic KDP (KH2PO4) crystal within the PEPC at the appropriate laser operation timing intervals. The drive is produced by a high-voltage, solid-state-switch pulse generator called the SPG (Switch Pulse Generator), also referred to as the PEPC driver. The SPG is required to produce pulses up to 20kV with a duration of 300ns and rise/fall times of 100ns.

History of the SPG

PEPC Pulse

The original OMEGA EP laser design required only one switch pulse to be applied to the PEPC during each laser operation interval. A thyratron electron-tube, based high-voltage switch was chosen for this application because of its high current and high-voltage rapid-switching capabilities. A thyratron is a single-action "closing" switch that can be switched once per laser operation interval. As the design of the EP laser was refined, laser development scientists determined that a second PEPC switching operation was required to prevent target retro-reflections from being amplified and potentially damaging the laser optics. The second or double pulse operation made the thyratron-based driver incompatible with its inherent single pulse mode of operation. A new switch pulse generator technology was needed which incorporated a switch that could open as well as close multiple times during the operation interval.

In meetings with the National Ignition Facility (NIF) PEPC development team at Lawrence Livermore National Laboratory, LLE learned of a Livermore group that had developed solid-state switch-pulse generators for large nuclear collider beam "kickers" based on a multiple power MOSFET (Metal-Oxide-Semiconductor-Field-Effect Transistor) approach.

The MOSFET design is capable of multiple pulses and is inherently more predictable and reliable than the thyratron-based design. Specifications for LLE's PEPC driver were within the capabilities of this technology. As a result, LLE contracted with Lawrence Livermore National Laboratory to construct two solid-state switch pulse generators (SS-SPG's). These generators were successfully developed, tested and delivered in 2006. Optical testing with the PEPC, driven with a double-pulse excitation by the first SS-SPG, provided promising results.

Structure of the SPG

Pulse Generator

The SS-SPG consists of 30 individual and simultaneously triggered 750V, 1.6kA power MOSFET switch stacks with incorporated pulse charge storage. Each switch stack is formed from 28 parallel-connected MOSFET's totaling 840 devices in each SS-SPG. Pulse charge storage is accomplished through 14 parallel-connected 12-microfarad capacitors in each stack for 1.4kJ of total stored pulse energy for 30 stacks. The output of each switch stack drives the primary winding of a dedicated pulse transformer. The secondary windings of the 30 transformers are series connected to sum the voltage of each individual MOSFET stack. This transformer coupling, or inductive adding technique, exposes the MOSFET switching devices to a maximum of 750V while producing the 20kV output pulse at the desired 1.6kA current necessary to drive the PEPC.

LLE Builds Its Own Drivers

LLE was able to secure funding for two additional beam lines during the construction of the EP laser system. In-house engineers were able to reduce costs and gain a better understanding of the SS-SPG's design details by building the pulse generators at LLE. This assured LLE's ability to support and maintain the highly sophisticated electronics.

Pulse Generator Board

Working with Lawrence Livermore National Lab's design, LLE engineers obtained the necessary components. A critical part of the process is verifying that specifications are met. However, some unique components did not have the application specific data available. LLE developed a number of specialized tests for these components. Once characterized, only those that matched the original design guidelines were accepted.

Production of the circuit boards required special skills and equipment. Three different contractors, two in the Rochester area and one in California, were needed to fabricate and assemble 785 individual circuit boards. LLE designed test fixtures and procedures were used to check each board before assembly into the driver chassis.

Fabrication and assembly of the two drivers took 10 months. Diligent checking and testing of all components was rewarded by the two finished driver assemblies working the first time power was applied. Activation of the drivers on the EP laser beam lines is scheduled for January of 2007.

Building these drivers in-house provided LLE engineers with important technological skills and knowledge that can be applied to future high voltage, high current pulse applications utilizing solid-state switching technology.