Lane Carlson

The Science and Technologies for Fusion Energy With Lasers and Direct-Drive Targets

We are carrying out a multidisciplinary multi-institutional program to develop the scientific and technical basis for inertial fusion energy (IFE) based on laser drivers and direct-drive targets. The key components are developed as an integrated system, linking the science, technology, and final application of a 1000-MWe pure-fusion power plant. The science and technologies developed here are flexible enough to be applied to other size systems. The scientific justification for this work is a family of target designs (simulations) that show that direct drive has the potential to provide the high gains needed for a pure-fusion power plant. Two competing lasers are under development: the diode-pumped solid-state laser (DPPSL) and the electron-beam-pumped krypton fluoride (KrF) gas laser. This paper will present the current state of the art in the target designs and lasers, as well as the other IFE technologies required for energy, including final optics (grazing incidence and dielectrics), chambers, and target fabrication, injection, and tracking technologies. All of these are applicable to both laser systems and to other laser IFE-based concepts. However, in some of the higher performance target designs, the DPPSL will require more energy to reach the same yield as with the KrF laser.

A CONTINUOUS, IN-CHAMBER TARGET TRACKING AND ENGAGEMENT APPROACH FOR LASER FUSION

Abstract Target engagement is the process of measuring the target trajectory and directing the driver beams to hit the target at a position that is predicted based on these measurements. New target engagement concepts have been proposed in the last few years to continuously track the targets and to verify that the tracking system is aligned with the driver beams for each shot. For transverse position, a laser beam continuously backlights the target and the position of the Poisson spot in the center of the target’s shadow is measured. Axial target displacement is measured using a laser interferometer and counting interference fringes as the target moves away from the laser source. Final steering corrections use a “glint” reflected off the target ˜1 ms prior to firing the laser beams and collected in a separate Position Sensitive Detector (PSD) for each driver beamlet. The position of the glint on the PSD is compared to the position of an alignment beam that is collinear with the driver beam. Steering corrections are then made based on the difference in position of the two spots reaching the PSD.

TARGET TRACKING AND ENGAGEMENT FOR INERTIAL FUSION ENERGY - A TABLETOP DEMONSTRATION -

Abstract In the High Average Power Laser program, we have developed an integrated target tracking and engagement system designed to track an inertial fusion energy target traveling 50-100 m/s in three dimensions and to steer driver beams so as to engage it with ±20 μm accuracy. The system consists of separate axial and transverse detection techniques to pre-steer individual beamlet mirrors, and a final fine-correction technique using a short-pulse laser “glint” from the target itself. Transverse tracking of the target uses the Poisson spot diffraction phenomenon, which lies exactly on axis to the centroid of the target. The spot is imaged on a digital video camera and its centroid is calculated in ˜10 ms with 5 μm precision. In our tabletop demonstration, we have been able to continuously track a target falling at 5 m/s and provide a fast steering mirror with steering commands. We are on the verge of intercepting the target on-the-fly and of verifying the accuracy of engagement. Future work entails combining transverse tracking, axial tracking, triggering and the final “glint” system. We also will implement a verification technique that confirms successful target engagement with a simulated driver beam. Results and integration progress are reported.

Developing a commercial production process for 500 000 targets per day : A key challenge for inertial fusion energy

As is true for current-day commercial power plants, a reliable and economic fuel supply is essential for the viability of future Inertial Fusion Energy (IFE) [Energy From Inertial Fusion, edited by W. J. Hogan (International Atomic Energy Agency, Vienna, 1995)] power plants. While IFE power plants will utilize deuterium-tritium (DT) bred in-house as the fusion fuel, the “target” is the vehicle by which the fuel is delivered to the reaction chamber. Thus the cost of the target becomes a critical issue in regard to fuel cost. Typically six targets per second, or about 500 000∕day are required for a nominal 1000MW(e) power plant. The electricity value within a typical target is about

Planarization of Isolated Defects on ICF Target Capsule Surfaces by Pulsed Laser Ablation

3, allocating 10% for fuel cost gives only 30 cents per target as-delivered to the chamber center. Complicating this economic goal, the target supply has many significant technical challenges—fabricating the precision fuel-containing capsule, filling it with DT, cooling it to cryogenic temperatures, layering the DT into a unifo...

Target Injection with Electrostatic Acceleration

Abstract Demanding surface-quality requirements for inertial confinement fusion (ICF) capsules motivated the development of a pulsed laser ablation method to reduce or eliminate undesirable surface defects. The pulsed laser ablation technique takes advantage of a full surface (4π) capsule manipulation system working in combination with an optical profiling (confocal) microscope. Based on the defect topography, the material removal rate, and the laser pulse energy and its beam profile, a customized laser raster pattern is derived to remove the defect. The pattern is a table of coordinates and number of pulses that dictate how the defect will be vaporized until its height is level with the capsule surface. This paper explains how the raster patterns are optimized to minimize surface roughness and how surface roughness after laser ablation is simulated. The simulated surfaces are compared with actual ablated surfaces. Large defects are reduced to a size regime where a tumble-finishing process produces very high-quality surfaces devoid of high mode defects. The combined polishing processes of laser ablation and tumble finishing have become routine fabrication steps for National Ignition Facility capsule production.

IMPROVING THE ACCURACY OF A TARGET ENGAGEMENT DEMONSTRATION

Various methods for accelerating targets to be injected into an Inertial Fusion Energy (IFE) power plant have been considered such as gas gun, rail gun and electromagnetic induction. One method that could also be used for direct drive targets is electrostatic acceleration. We have been using electrostatic steering to improve target placement accuracy. We optically track the motion of a charged target, and feed back appropriate steering voltage to four steering electrodes. We have also completed fabrication and begun testing of an electrostatic accelerator that advances the electric field each time the charged target passes one of the 96 accelerating electrodes. Many of the accelerating electrodes are segmented to allow transverse position correction based on transverse position measurements during the acceleration process. Calculations indicate that this “first step” accelerator will achieve 10-15 m/s target velocity in 0.9 m with ±4 kV accelerating voltage. Updated target steering results as well as the accelerator design, fabrication, and early experimental results are presented.

TARGET INJECTION PLACEMENT ACCURACY IMPROVEMENT WITH ELECTROSTATIC STEERING

In the High Average Power Laser (HAPL) program, we have developed an integrated target tracking and engagement system designed to track an inertial fusion energy target traveling 50-100m/s in three dimensions and to steer laser driver beams so as to engage it with ±20 μm accuracy from a stand off distance of ˜20 meters. The system consists of separate axial and transverse detection techniques to pre-steer individual beamlet mirrors, and a final fine-correction technique using a short-pulse “glint” laser to interrogate the target’s position 1-2 ms before the target reaches chamber center. We are working to demonstrate the viability of this concept by conducting a table top engagement demonstration at reduced speeds and distances. Integration of the various components has been completed and hit-on-the-fly experiments are now being conducted. Initial engagement efforts from a simulated driver beam overfilling a falling target yielded a 150-μm standard deviation for targets placed ±1.5mm from chamber center. Since then, our efforts have focused on systematically defining and eliminating all sources of error in each component and subsystem. Current engagement accuracy is 42μm RMS. The engagement effort and the step-wise improvements realized are reported, as well as the path toward our goal.

Development, Visualization, and Application of the ARIES Systems Code

Abstract To achieve high gain in an Inertial Fusion Energy (IFE) power plant, driver beams must hit direct drive targets with ±20 μm accuracy. For driver beams to arrive at the target with sufficient simultaneity, the targets must be placed to ±5 mm from chamber center. Better placement accuracy simplifies driver beam steering by reducing the distance that steering mirrors must reposition the beam aim point in the last few ms. Current best target placement experimental accuracy is 0.22 mrad standard deviation which corresponds to 3 mm at 13 m. A factor of two improvement is required to achieve 3 σ accuracy in ±5 mm, and even greater accuracy is desired. General Atomics has recently embarked on a program to improve target placement accuracy through electrostatic steering. Preliminary experiments have improved accuracy of falling charged spheres. We optically track the motion, and feed back appropriate voltage to steering electrodes. A steering algorithm was prepared to steer targets with placement accuracy limited primarily by rate and accuracy of target tracking. Substantial accuracy improvement is expected with higher-frequency tracking and voltage amplification equipment. The results will be reported.

Development of a Visualization Tool for the ARIES Systems Code

The ARIES research program has utilized its comprehensive ARIES systems code (ASC) and a new graphical user interface for visualizing the parameter space as important tools in its analysis of fusion power plant designs. Recently, the ASC has undergone modifications to accommodate different divertor designs, each having unique pumping powers, helium and liquid-metal pump thermal heat recovery, and the latest material, fabrication, and costing algorithms. The modifications and changes made to the code have been documented and verified by members of the ARIES team to ensure accuracy of implementation and self-consistency of design. The code has also been modified to display a wider range of input and output files, formulas, and algorithms for a greater degree of transparency and verification. After the changes to the code were completed and the version was locked, the ASC was employed to scan the physics and technology operating space for relevant power plant designs. Four corners of aggressiveness and conservativeness in both physics and technology serve as the boundaries for the scans within which a range of possible tokamaks exist. The Visual ARIES Systems Scanning Tool (VASST) has been used in parallel with the ASC scans to visualize the tremendous amounts of data resulting from these detailed systems scans. Displaying the data in a colorful and intuitive visual environment and giving the user explorative and visual interaction have helped extract meaningful relationships and trends from the data. Initially, broad scans from the ASC and VASST indicated areas of interest where additional detail was needed. Further scans of higher fidelity helped enhance and further refine the database. After the final scans were completed, VASST facilitated in displaying and filtering the large database to choose two “strawmen” data points at two of the four corners of the aggressive/conservative operating space. These points now serve as reference designs, so more detailed design and calculations can be done. The results of the in-depth designs assist the ASC by feeding back information into the code that can then be generalized for a wider range of operating scenarios relevant to the scanning range. This substantiates the ASC and helps mesh simple formulas with detailed design.