Around the Lab
Solid-State Pockels-Cell Driver Development at LLE
August, 2012A vital component in the OMEGA laser, a Pockels cell is an electro-optic crystal that, when a voltage is applied to it, rotates the polarization of laser light coming through the device. Depending on the voltage employed, the Pockels cell, combined with a polarizer, can either let light pass through unabated or completely reflect light, effectively establishing an optical switch. The operation of a Pockels cell requires timed rectangular electrical pulses from a special pulse generator, or pulser, with amplitudes ranging from hundreds of volts to tens of kilovolts and pulse widths from tens to hundreds of nanoseconds.
To meet the demands of both the OMEGA and OMEGA EP lasers, the Laboratory for Laser Energetics (LLE) is now in the process of testing and refining its own design of a solid-state Pockels-cell driver. The motivation behind this new design is to replace the currently used pulsers that suffer from the degradation and ultimate failure of a thyratron electron tube—the single-action “closing” switch that performs the basic pulse-generation function within the current design. Several operational issues with the thyratron-based pulser design are indicators of thyratron degradation and failure, including excessive jitter, pulse drift, extended rise/fall time, and excessive pre-fires. Each of these issues individually can cause improper laser system operation and, in the worst case, lead to costly optical component damage.
First engineering prototype of the solid-state Pockels-cell driver
Inductive adder transformer cell
The specific thyratron used within the current Pockels-cell driver recently became obsolete and no direct substitute exists. Retrofitting this thyratron, if a substitute can be found, will require both extensive time and substantive changes within the pulser chassis, while not providing a long-term solution to the issues of thyratron degradation and failure. The best solution is for engineers and technicians working in the LLE Electronics and Controls Group to design and build a new solid-state pulser based on an inductive adder-based approach. This decision is supported by LLE’s successful experience with the inductive adder plasma-electrode Pockels cell (PEPC) solid-state switch-pulse generators (SS-SPG’s) utilized successfully for over six years within the OMEGA EP Laser System.
The basis for the OMEGA EP SS-SPG inductive adder design was originally developed at Lawrence Livermore National Laboratory (LLNL) for solid-state switch-pulse generators for large nuclear collider beam “kickers.” This technology was adapted to the PEPC switch-pulse generator needs. The inductive adder design is capable of variable-width repetitive pulses and is inherently more predictable and reliable than a thyratron-based design.
MOSFET-cell driver circuit board
The prototype of the new solid-state Pockels-cell driver at LLE uses a scaled-down design both in switching voltage and current of the same proven technology as in the OMEGA EP SS-SPG developed at LLNL. The benefits include:
- a reliable solid-state switching metal–oxide–semiconductor field-effect transistor (MOSFET) switch that can open and close in direct response to the drive circuit;
- an internal pulser high-voltage supply limited to 750 V, inductively “added” to attain 10-kV output;
- each inductive adder stack level ground referenced to increase reliability and maintainability;
- 15 identical inductive adder stacks provide replication of circuitry to reduce costs and improve troubleshooting and repair.
An inductive adder single-stack breadboard—a design base for evaluating the basic adder building block—has been successfully assembled and tested. The results from this single-stack breadboard justified building two complete 15-stack layer prototypes. One prototype will be evaluated by the Laser Sources Group within the OMEGA EP Laser System and the other will remain within the Electronics and Controls Group for design refinements. Presently, refinements are centered on improved switching rate performance. The overall benefits to the Laser System when this upgrade takes place include a reliable solid-state solution, which will provide longer service with reduced maintenance and lower operational costs, the ability to test and determine performance issues more efficiently, and an in-house design using commercially available components and locally fabricated parts to ensure availability for critical Omega Laser Facility applications.