The ability of high-energy laser systems to provide complex laser pulse shapes has growing importance in many research disciplines such as laser fusion[
High Power Laser Science and Engineering, Volume. 8, Issue 1, 010000e8(2020)
Laser-system model for enhanced operational performance and flexibility on OMEGA EP On the Cover
The development of laser performance models having real-time prediction capability for the OMEGA EP laser system has been essential in meeting requests from its user community for increasingly complex pulse shapes that span a wide range of energies. The laser operations model PSOPS provides rapid and accurate predictions of OMEGA EP laser-system performance in both forward and backward directions, a user-friendly interface and rapid optimization capability between shots. We describe the model’s features and show how PSOPS has allowed real-time optimization of the laser-system configuration in order to satisfy the demands of rapidly evolving experimental campaign needs. We also discuss several enhancements to laser-system performance accuracy and flexibility enabled by PSOPS.
1 Introduction
The ability of high-energy laser systems to provide complex laser pulse shapes has growing importance in many research disciplines such as laser fusion[
Essential features of PSOPS are (1) accurate, real-time predictions of expected performance of all four OMEGA EP beamlines within a small fraction of the OMEGA EP shot cycle; (2) an intuitive, easy-to-use interface for laser operators; (3) rapid optimization capability of the code between laser shots to fine-tune predictions based on shot performance; (4) forward and backward prediction capabilities. These features allow laser-system operators to quickly and accurately optimize laser pulse shape, energy and laser diagnostic filtrations prior to each OMEGA EP shot.
The paper is organized as follows. Section
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2 The OMEGA EP laser system
The OMEGA EP laser system is capable of producing either picosecond-scale infrared (IR) pulses (via optical parametric chirped-pulse amplification (OPCPA)) or nanosecond-scale UV pulses. For this paper, we confine our attention to the latter capability and will not consider the OPCPA operation further. Each of the four OMEGA EP beamlines uses a folded architecture based on the National Ignition Facility (NIF)[
An integrated front-end system (IFES)[
Laser pulse shape, energy and near-field beam profile are measured at several locations along the beam path. Diagnostic stages relevant to the PSOPS model and associated measurements are shown in Figure
3 Functional overview of PSOPS
A major consideration in the development of the PSOPS model was to provide an accurate pulse-shape and energy prediction capability in both forward and backward directions that would give real-time guidance to the laser facility to satisfy the demands of rapidly evolving experimental campaign needs. This rapid prediction capability enables several performance enhancements for the facility that provide improved system performance accuracy and flexibility (see Section
The PSOPS architecture allows one to run simulations in both the forward and backward directions, as illustrated in Figure
During shot operations, PSOPS is used in the forward simulation direction to provide rapid predictions of laser-system performance using measured inputs to the amplifier chain. The measured input beam profile and real-time PSM-measured pulse shape are used with the expected beamline injected energy and previously measured beamline SSG to predict the IR and frequency-converted UV performance at the end of the beamline. Pulse-shape distortion through the remainder of the front end after the regen is minimal because front-end Nd:glass laser amplifiers are maintained sufficiently below gain-saturation conditions. A front-end qualification shot is taken at the start of a shot day to confirm the expected injected energy and to measure the injected beam’s near-field distribution that is used as input to the PSOPS model. A graphical user interface (GUI) that displays the predicted and requested UV power allows laser operators to adjust pulse shapes and verify expected on-target UV energy between shots. The predicted stage energies are also displayed on the GUI and compared to the expected values.
4 Description of the PSOPS model
4.1 Analytic solution to coupled equations
In high-pulse-energy laser systems, efficient energy extraction from the laser gain medium requires that the laser fluence should approach the medium’s saturation fluence. Such laser operation depletes the inversion leading to temporally dependent saturation of the gain, which, in a pulsed laser system, causes the output pulse to become temporally distorted. Analytic solutions to the coupled-rate and energy-transport equations for a homogeneously saturating thin slab are used in PSOPS to determine the time-dependent gain within each laser disk at discrete locations across the laser aperture. The four-level equations can be expressed as[
The integrals in Equations (
4.2 Multi-pass beamline amplification
In the absence of losses or higher-order effects contributing to pulse-shape distortion, Equations (
4.3 Saturation fluence considerations
The saturation fluence for Nd:glass laser media given by Equation (
Assuming negligible pumping during the laser pulse width
For subsequent passes of the laser pulse through an amplifier disk, some amount of gain recovery might be expected, owing to drain of the terminal level. In this case, the terminal-level population would be reduced to
4.4 Small-signal gain measurements
The spatially dependent, single-pass SSG for each amplifier disk used in the model is derived from full-system SSG measurements for the seven-disk booster-amplifier configuration and for each main-amplifier configuration to be used during shot operations. The SSG is measured by dividing calorimetrically calibrated beam-fluence measurements at the injection plane and beamline output. The resulting beam ratio for an amplified shot in the small-signal regime is divided by the same ratio with the amplifier disks unpumped, thereby eliminating the contribution from passive loss. The single-disk SSG
4.5 Model calibration and simulation methods
The optimization from which Figure
The first method is an extension of the optimization done in Figure
In the second method, the local saturation fluence for each grid point and amplifier disk is determined per the equation in Figure Backward simulation using requested UV pulse shape, energy, and beam profile to calculate required regen pulse shape, stage energies, system throttles, and laser diagnostic filtrations. Forward simulation using measured inputs to confirm results of backward simulation and refine, if necessary. Take full system UV shot, and check model calibration by comparing measured and post-shot simulated UV energies and pulse shapes. Calibrate model, if necessary.
5 Comparison between simulations and experiment
Spatial and temporal simulations in both forward and backward directions are in excellent agreement with measurements, as shown in Figures
Since the calibrated loss term
|
These results highlight the following important considerations. First, the measured single-disk SSG map for each unique main-amplifier configuration must be used to achieve good agreement with measurements (see Figure
For 100-ps pulses, we found that forward simulations were in good agreement with beamline output IR energy measurements, but simulated UV energy and pulse width were too low and too wide, respectively (Figure
6 OMEGA EP laser-system enhancements enabled by PSOPS
The unique features of PSOPS have provided greater performance accuracy and flexibility by enabling rapid optimization in key areas.Determination of front-end throttle and pulse-shape adjustments required to compensate for such issues as changes in passive loss through a beamline, loss of gain from amplifier flashlamp degradation, spatial variations in saturated gain resulting from changes in injected beam profile and spatiotemporal variations in regen performance. This has improved OMEGA EP’s ability to accurately produce users’ requested UV energies and pulse shapes.Adjustments to on-target energy and pulse shape within predetermined allowances based on a user’s real-time analysis of experimental data.Increased effective pulse-duration range through precise concatenation of pulses across multiple beams.Improved system alignment. As a post-shot analysis and diagnostic tool, PSOPS has been used to guide alignment of beam-shaping apodizers in the front end of OMEGA EP and to understand the effects of beamline-centering errors in order to optimize the fill factor of the amplified beam, reduce near-field modulation and help elucidate causes of beamline gain changes.
These improvements are described in detail below.
6.1 Improvements to UV energy and pulse-shape accuracy
Drifts in system performance can lead to noticeable deviations between simulated and achieved pulse shapes and energies, which can be minimized with an agile system model such as PSOPS. For example, Figure
Although the AWG adjustment is not currently a closed loop, the regen performance is typically sufficiently stable so that only minor adjustments are required. Closed-loop AWG waveform optimization will be implemented in the near future. In addition to changes in regen performance, small changes in beamline gain and losses may result in approximately 5% discrepancy between the measured and simulated UV energy on the first shot of the day, in which case the model can be calibrated before the second shot based on the measured pulse power, as described in Section
6.2 Improvements to experimental flexibility
PSOPS has also enhanced laser facility flexibility by enabling users to adjust requested UV pulse shapes and energies between laser shots within a predefined range that is determined uniquely for each experimental campaign. The allowed range of energy and pulse-shape modification is assessed with respect to the laser system’s fluence limits, the range of energy and pulse shapes planned for the day and the likelihood of maintaining each beamline’s 90-minute shot cycle. In the example shown in Figure
Most of the discrepancies between the requested and measured pulse shapes in Figure
6.3 Increased effective pulse-duration range
Currently, OMEGA EP’s regens can accommodate single beamline pulse widths of up to 10 ns. However, improved system modeling in conjunction with precision timing allows the technique of pulse stitching to achieve up to a
We note that all four long-pulse beams are derived from the same single-frequency oscillator, and the focusing optic assemblies on the OMEGA EP target chamber are mounted such that the beams form a cone of approximately
6.4 Improved system alignment
PSOPS has been used as a tool to optimize the alignment of beam-shaping apodizers in the Sources front end (see Figure
PSOPS has also been used to perform iterative, multi-axis optimization of the apodizer alignment to reduce the peak fluence of the frequency-converted UV beam. Figure
7 Summary
PSOPS is a semi-analytic model that is used on the UV beamlines of OMEGA EP to rapidly predict pulse shapes, stage energies and near-field beam distributions in both forward and backward directions and has enabled several enhancements to laser-system performance accuracy and flexibility. The use of analytic solutions to the coupled-rate and energy-transport equations, with incorporation of the measured SSG and appropriate modifications to the saturation fluence, has enabled accurate and rapid optimization of laser-system performance within a small fraction of the OMEGA EP 90-minute shot cycle. PSOPS is the key enabler of an automated capability to compute and specify the laser system’s stage energies and corresponding diagnostic filtrations prior to each OMEGA EP shot based on evolving on-target pulse-shape and energy requirements. In conjunction with precision timing, the model has allowed the technique of pulse stitching to achieve up to a 4
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M. J. Guardalben, M. Barczys, B. E. Kruschwitz, M. Spilatro, L. J. Waxer, E. M. Hill. Laser-system model for enhanced operational performance and flexibility on OMEGA EP[J]. High Power Laser Science and Engineering, 2020, 8(1): 010000e8
Category: Research Articles
Received: Nov. 23, 2019
Accepted: Feb. 7, 2020
Published Online: Mar. 27, 2020
The Author Email: M. J. Guardalben (mgua@lle.rochester.edu)