Plasma Electrode Pockels Cell Research Articles

Optical Isolation of the Amplifier Cavity in XG-III PW Beamline

Optical isolation with high-quality, large-aperture polarizers is commonly used in high-power laser facilities to suppress retro-reflected pulses. However, it is hard to manufacture these polarizers. We propose an approach of optical isolation with two plasma-electrode Pockels cells instead of large-aperture polarizers. In this approach, Nd:glass slabs placed at the Brewster′s angle are used as polarizers. The analysis results and the application performances in the Xingguang (Star Light) XG-III PW beamline indicate that this approach can supply good protection to optical components in laser facilities.

大口径倒腔式等离子体开关

大口径倒腔式等离子体开关

边界主动加热管理平均功率普克尔盒热效应

边界主动加热管理平均功率普克尔盒热效应

Switching Characteristics of Large-Aperture Electro-Optical Switches With Plasma Electrodes Part II—Comparison With Experiment

The large-aperture optical switches with plasma-electrode Pockels cell (PEPC) are important elements for the multipass laser amplifiers in the inertial-confinement fusion. In this paper, an optical switch of the PEPC driven by one-pulse process (OPP) is simulated by using a fluid model developed in a previous paper. We computationally study the physical processes of the discharge evolvements and discharge fading away in the large-aperture PEPC switches driven by the OPP. Charging on the surfaces of potassium dihydrogen phosphate crystal in the discharge determines the switching characteristics of the PEPC. The simulation results nearly coincide with the experimental measurement.

Switching Characteristics of Large-Aperture Electro-Optical Switches With Plasma Electrodes Part I—Model

Optical switches with plasma-electrode Pockels cells (PEPCs) are important elements for large-aperture multipass laser amplifiers in inertial-confinement fusion. In this paper, we present a fluid model for a one-pulse process of the PEPC, in which the preionization discharge and the stable discharge plasma are achieved in one pulse. Multiple species including electrons, atomic ions, molecular ions, excited atoms and excited molecules and the energy exchange between the electrons and other species are considered in the model. Under the same condition of discharge, the calculation for two-specie and five-species are implemented and the results showed that the density of electrons in the stable glow discharge of the PEPC by the model of five species is 2%~3% higher than that of the two species model.

High-contrast plasma-electrode Pockels cell

A plasma-electrode Pockels cell (PEPC) has been developed for use on the OMEGA extended performance (EP) laser system that can be used in a high-contrast optical switch, as required for isolation of the system from retroreflected pulses. Contrast ratios reliably exceeded 500:1 locally everywhere in the clear aperture. The key to achieving this improvement was the use of circular windows simply supported on compliant O rings, which is shown to produce very low stress-induced birefringence despite vacuum loading. Reliable operation was achieved operating at a relatively high operating pressure, low operating pressures being found to be strongly correlated to occurrences of local loss of plasma density.

Simulation electro-optic switch of plasma-electrode Pockels cells driven by one-pulse process

In this paper, a fluid model of electro-optic switch of plasma-electrode Pockels cells (PEPC) driven by one-pulse process is proposed. The model includes a simplified set of fluid continuity equations, Poisson's equation and the electron average energy equation, and these equations are numerically solved with the exponential difference scheme, the classical successive over relaxation method (SOR) and the classical fourth-order Runge-Kutta method, respectively. The evolving characteristics of density of charged particles, electronic temperature, electric field, discharge current, charging voltage of the KDP crystal and the switching efficiency of PEPC are simulated. The model for PEPC, which reveals the relationship between the microcosmic physical processes of gaseous discharge plasma and macrocosmic parameters of PEPC, is very useful for the design of large aperture electro-optic switches.

One-dimensional model of a plasma-electrode optical switch driven by one-pulse process

In this paper, we present a one-dimensional simplified model for the processes of gaseous discharge and charging on the surfaces of KDP crystal for one-pulse process in Plasma-Electrode Pockels Cell (PEPC). The PEPCs are used as large-aperture optical switch in laser drivers for inertial-confinement fusion. The plasma electrodes of the switch are produced by high discharge current, while a high voltage pulse is applied across a thin KDP crystal plate through the helium discharge plasma. The evolvements of discharge current, charging voltage on KDP crystal and the switch efficiency of PEPC are simulated. The model is very useful for the design of PEPC and prediction the behavior of the optical switch.

A simple operation of a plasma-electrode pockel’s cell for the laser megajoules

We report on a simple way to operate plasma-electrode pockel’s cells (PEPC), which are used as large-aperture optical switches in laser drivers for inertial-confinement fusion facilities. In the basic operation of a PEPC, one needs to produce a plasma in each of two gas-filled cells placed on opposite sides of an appropriate crystal plate. Between the resulting highly conductive electrodes, a high-voltage pulse is applied to produce a uniform electric field in the crystal, which then undergoes the Pockel’s effect. We have developed a simple PEPC design in which the high-voltage pulse can be directly applied between the gascells without prior ionization of the gas (by a high-current plasma discharge). With reasonable operating parameters, this regime meets the PEPC optical switching requirements for the laser megajoules. In this paper, we present experimental results of this simple regime that strongly simplifies the PEPC concept and improves its reliability.

Modeling of plasma behavior in a plasma electrode Pockels cell

We present three interrelated models of plasma behavior in a plasma electrode Pockels cell (PEPC). In a PEPC, plasma discharges are formed on both sides of a thin, large-aperture electro-optic crystal (typically KDP). The plasmas act as optically transparent, highly conductive electrodes, allowing uniform application of a longitudinal field to induce birefringence in the crystal. First, we model the plasma in the thin direction, perpendicular to the crystal, via a one-dimensional fluid model. This yields the electron temperature and the density and velocity profiles in this direction as functions of the neutral pressure, the plasma channel width, and the discharge current density. Next, me model the temporal response of the crystal to the charging process, combining a circuit model with a model of the sheath which forms near the crystal boundary. This model gives the time-dependent voltage drop across the sheath as a function of electron density at the sheath entrance. Finally, we develop a two dimensional MHD model of the planar plasma, in order to calculate the response of the plasma to magnetic fields. We show how the plasma uniformity is affected by the design of the current return, by the longitudinal field from the cathode magnetron, and by fields from other sources. This model also gives the plasma sensitivity to the boundary potential at which the top and bottom of the discharge are held. We validate these models by showing how they explain observations in three large Pockels cells built at Lawrence Livermore National Laboratory.

Plasma Electrode Pockels Cell for the National Ignition Facility

The National Ignition Facility (NIF), now under construction at Lawrence Livermore National Laboratory, will be the largest laser fusion facility ever built. The NIF laser architecture is based on a multi-pass power amplifier to reduce cost and maximize performance. A key component in this laser design is an optical switch that closes to trap the optical pulse in the cavity for four gain passes and then opens to divert the optical pulse out of the amplifier cavity. The switch is comprised of a Pockels cell and a polarizer and is unique because it handles a beam that is 40 cm × 40 cm square and allows close horizontal and vertical beam spacing. Conventional Pockels cells do not scale to such large apertures or the square shape required for close packing. Our switch is based on a Plasma-Electrode Pockels Cell (PEPC).In a PEPC, low-pressure helium discharges (1–2 kA) are formed on both sides of a thin slab of electro-optic material. Typically, we use KH2PO4 crystals (KDP). The discharges form highly conductive, transparent sheets that allow uniform application of a high-voltage pulse (17 kV) across the crystal. A 37 cm × 37 cm PEPC has been in routine operation for two years on the 6 kJ Beamlet laser at LLNL. For the NIF, a module four apertures high by one wide (4×1) is required. However, this 4×1 mechanical module will be comprised electrically of a pair of 2×1 sub-modules.Last year (FY 97), we demonstrated full operation of a prototype 2×1 PEPC. In this PEPC, the plasma spans two KDP crystals. A major advance in the 2×1 PEPC over the Beamlet PEPC is the use of anodized aluminum construction that still provides sufficient insulation to allow formation of the planar plasmas. In this paper, we discuss full 4×1 NIF prototypes.

Subnanosecond switching, plasma electrode Pockels cell for high-contrast imaging with partially coherent light

A high-contrast optical switch for imaging partially coherent light (~150 times the diffraction limit) requires that it have a large angular acceptance. We describe the development of a high-speed Pockels cell that uses a thin crystal to simultaneously achieve high-contrast (greater than 1800:1) and large-angular acceptance (greater than 7 mrad for a 5-mm aperture). A KD*P crystal was used in a longitudinal-mode configuration with plasma discharges forming low-resistance, high optical transmission electrodes to couple the switching voltage. Rise times of the switched optical pulse of the order of 500 ps were observed. Characterization of the device in the near and far fields was also performed.

Performance of a prototype for a large-aperture multipass Nd:glass laser for inertial confinement fusion

The Beamlet is a single-beam prototype of future multibeam megajoule-class Nd:glass laser drivers for inertial confinement fusion. It uses a multipass main amplifier, adaptive optics, and efficient, high-fluence frequency conversion to the third harmonic. The Beamlet amplifier contains Brewster-angle glass slabs with a clear aperture of 39 cm x 39 cm and a full-aperture plasma-electrode Pockels cell switch. It has been successfully tested over a range of pulse lengths from 1-10 ns up to energies at 1.053 mum of 5.8 kJ at 1 ns and 17.3 kJ at 10 ns. A 39-actuator deformable mirror corrects the beam quality to a Strehl ratio of as much as 0.4. The 1.053-mum output has been converted to the third harmonic at efficiencies as high as 80% and fluences as high as 8.7 J/cm(2) for 3-ns pulses.

Performance of large-aperture optical switches for high-energy inertial-confinement fusion lasers

We describe the design and performance of large-aperture (>30 cm × 30 cm) optical switches that have demonstrated, for the first time to our knowledge, active switching of a high-energy (>5 kJ) optical pulse in an inertial-confinement fusion laser. These optical switches, which consist of a plasma-electrode Pockels cell (PEPC) and a passive polarizer, permit the design of efficient, multipass laser amplifiers. In a PEPC, plasma discharges on the faces of a thin (1-cm) electro-optic crystal (KDP or KD*P) act as highly conductive and transparent electrodes. These plasma electrodes facilitate rapid (<100 ns) and uniform charging of the crystal to the half-wave voltage and discharging back to 0 V. We discuss the operating principles, design, optical performance, and technical issues of a 32 cm × 32 cm prototype PEPC with both KDP and KD*P crystals, and a 37 cm × 37 cm PEPC with a KDP crystal for the Beamlet laser. This PEPC recently switched a 6-kJ, 3-ns pulse in a four-pass cavity.

Plasma Electrode Pockels Cells for the Beamlet and NIF Lasers

We describe Plasma Electrode Pockels Cells (PEPC) for the Beamlet laser and the proposed National Ignition Facility (NIF) laser. These PEPCs, together with passive polarizers, function as large ape...