LAWRENCE Livermore has long been recognized as a leader in the world of scientific computer simulations--creating multidimensional models of the dynamic and complex forces unleashed by nuclear explosives, visualizing the processes at work in the birth and death of stars, and studying the effects of greenhouse gases on global climate and of pollutants in our environment. It is not surprising, then, that the Laboratory is also a leading developer of computer codes that simulate propagation and interaction of electromagnetic (EM) fields.|
Livermore's EM field experts study and model wave phenomena covering almost the entire electromagnetic spectrum (Figure 1). Applications are as varied as the wavelengths of interest: particle accelerator components, material science and pulsed power subsystems, photonic and optoelectronic devices, aerospace and radar systems, and microwave and microelectronics devices.
Building from its seminal work on time-domain algorithms1 in the 1960s, the Laboratory has fashioned top-notch resources in electromagnetic and electronics modeling and characterization. Using Laboratory-developed two-dimensional and three-dimensional EM field and propagation modeling codes and EM measurement facilities, Livermore personnel can evaluate, design, fabricate, and test a wide range of accelerator systems and both impulse and continuous-wave RF (radio-frequency) microwave systems.
Center of Developing Technology|
The Laboratory's unique connectivity has kindled progress in many key areas of accelerator design and photonic, opto-electronic, and RF systems. The focal point for these activities is the computational electronics and electromagnetics thrust area, which provides technical support for existing and developing programs. "We are a resource that crosses technical boundaries, yet we are integral to Livermore's major program areas," says thrust area leader Cliff Shang.
Administratively a part of Electronic Engineering's Defense Sciences Engineering Division, the thrust area operates like a technology development center. "With the EM Laboratory and EE personnel, the thrust area champions development of the best electromagnetic modeling capability available," says Richard Twogood, Deputy Associate Director for Electronics Engineering.
The thrust area has created a variety of production computer codes. (See the box below for a summary of Livermore codes.) For example, Figure 2 shows how the NEC code is used in the development of an antenna for micropower impulse radar (MIR). Other codes are also in development.
With the codes, the EM Laboratory personnel fabricate the resulting systems and components and then analyze the prototypes against the codes.
Rick Ratowsky calls it a Virtual Optical Bench, a user-friendly graphics interface for designers of photonic circuits, the optical world's equivalent of electronic circuits. "It is a photonics design tool with broad applicability," says Ratowsky, an electrical engineer.
Some photonics components have very small features, at a single wavelength scale; some features are very large, say thousands of wavelengths. Such devices have been very difficult to design because there is no easy way to put these differently scaled parts together.
A modeling code known as MELD (Multi-Scale Electrodynamics) will allow different length scales to be used concurrently--saving optoelectronics researchers much time and effort. (See figure below.) The MELD code draws on techniques of both the electromagnetics and optics communities and integrates them in a way never used before.
"I can't say that all the techniques themselves are unique to Livermore," observes Ratowsky, "but their implementation and integration are." MELD joins a long list of EM codes developed at Livermore. Following is a brief summary of key modeling codes. All are used to solve equations arising from the fundamental classical electromagnetic field equations enunciated by James Clerk Maxwell in 1873.
Specialized Models for Analysis and Design|
Creating new codes and refining production codes are the thrust area's primary activities, but the work does not occur in a vacuum. "We are not out just to design and develop EM codes," Shang says. "We develop modeling technology that can be directly applied to design problems, and we are adding new EM capabilities to anticipate future requirements of Laboratory programs. To accomplish these tasks effectively, our specialists work closely with program personnel."
Shang points out that "pairing experts from different disciplines--characteristic of the way the Laboratory does business--is essential for code development. This is the case in accelerator physics as well as in photonics and opto-electronics. The communities overlap. When you take the relevant knowledge and best codes from each to solve problems, you can end up with a new and very interesting set of codes."
Such codes are important because photonic devices are central to the growth of high-speed communications and computation. Signals in photonic networks can travel long distances at the speed of light, with very little power loss. These optoelectronic networks are made up of fibers, waveguides, sources, receivers, converters, and a host of other devices--all of which must be carefully designed if they are to work well together. Simulating a device before fabrication saves money and effort.
Many of the specialized codes created at Livermore have been made available to other government laboratories, universities, and industry. "The original NEC code is one of our most well-known codes," says Shang, "from the Department of Defense radar community to ham radio operators who design their own antennas."
In addition to their work in photonics and opto-electronics, EM thrust area personnel support a wide variety of Livermore R&D programs. Major emphases are stockpile stewardship and non-nuclear defense. For example, they are using their expertise and codes to support development of an advanced accelerator, which will help assure the safety and reliability of the nation's stockpiled nuclear weapons. They also support the Department of Defense (DoD) in assessing EM susceptibilities in conventional military systems.
Focusing on Stockpile Stewardship
Like the proposed AHF, the FXR allows scientists to assess issues related to safety, reliability, and performance of a nuclear weapon's "primary," its fission trigger. In FXR hydrodynamic experiments, high explosives are detonated to produce pressures so high that solid materials, even when not melted, flow like fluids. X rays are created when charged particles generated by the FXR slam into a target made of a dense metal such as tantalum and provide images that are later analyzed. The FXR, however, offers only a single line-of-sight x-ray record. Stockpile weapons analysis requires simultaneous x-ray images from multiple angles, which the AHF will provide.|
Ringing the high-explosive chamber with several linear accelerators to produce x rays for multiple imaging is very expensive. A Lawrence Livermore option proposes using a single linear induction accelerator that exploits a new and novel beamline component called the "kicker." The kicker (Figure 4) would displace a single electron beam pulse, effectively creating multiple pulses. Each pulse would travel down separate curved beamlines to encounter additional kickers for further splitting. Splitting would occur as many times as necessary to produce a specified number of beamlines for AHF diagnostics. An advanced kicker is also being developed by Livermore that could steer the beam horizontally as well as vertically, thus providing four or more different output beamlines from a single device. The challenge is to maintain beam quality in each subsidiary line. Proof-of-concept experiments for the AHF will be performed this year when Livermore's Experimental Test Accelerator-II (ETA-II) is refurbished and fitted with a kicker unit.
As part of the Laboratory's AHF design team, the EM experts are doing electromagnetic modeling of the beam-line components. Of particular interest are electromagnetic wakefields that could have an adverse impact on beam quality (see box below and Figures 3 and 4).|
"Our approach is to design an accelerator beamline component and simulate the design with our model. This iterative process refines the design before machinists begin fabricating accelerator components," explains electrical engineer Brian Poole. "We use those scattered wakefields to calculate the forces on charged particles that are injected into the beamline at subsequent times and to see how the beam quality is maintained."
Understanding AHF electromagnetic dynamics requires the utmost in computer resources. Solutions to some wakefield problems take a month or more of continuous supercomputer time, so some models are run on a smaller scale because of computer resources. For really large models that simulate the accelerator system being designed, the DOE's ASCI (Accelerated Strategic Computing Initiative) platform will be used. ASCI will allow more than a thousandfold increase in computational speed and data storage. Shang noted that thrust area personnel are developing three-dimensional, massively parallel, time-domain EM production codes that will exploit the new high-performance computing capabilities when they become available.
A charged particle beam traveling through an accelerator transport system, or beamline, has an associated electromagnetic (EM) field.
If the beamline is free of perturbations, the beam's EM field is not disturbed. However, a perturbation can modify the local electrodynamic properties of the structure.
Perturbations can consist of changes in cross section, apertures in the transport system wall, curved beamlines, or the introduction of different materials into the beam transport line.
As the charged particle beam streams past these perturbations in the structure, the beam's EM field is scattered from the structure. This EM field is called a wakefield (Figure 3) because the scattering occurs in the wake of the very-high-velocity particle.
This wakefield can interact with other particles traveling down the beamline behind the exciting particle, sometimes in an undesirable way, leading to the beam's degradation or, worse, breakup.
In addition to their work on radiographic systems such as FXR and AHF, thrust area personnel are involved in several other stockpile stewardship initiatives. They include:|
Supporting Conventional Defense
This summer, high-power tests (4 kilovolts per meter at a distance of 1,300 meters) will be performed by using a pulser supplied by Phillips Laboratory, an Air Force contractor, and the Naval Air Warfare Center's own 10-meter dish reflector to illuminate the target. Using the complete suite of Cobra EM modeling and comparing it with the field test data, Livermore will model Comanche EM susceptibilities.|
The Laboratory EM team also will participate in a separate series of DoD high-power EM exercises at China Lake. Designed as a "shake-out" study of EM testing methodology, the exercises eventually could be used for full assessments of DoD weapons delivery systems.
Of the Livermore team's role in the shake-out tests, Nelson said, "We will serve the DoD as an unbiased participant, verifying that the contractor's equipment works as expected, that the RF source delivers the expected fluence on target, that characterization and measurements of the test object use known and trusted test methodologies, and that EM assessments represent realistic or anticipated threats. It is a role we know well."
Key Words: electromagnetic field, electromagnetic susceptibility, kicker, modeling, opto-electronics, photonics, wakefield.
1. K. S. Yee, "Numerical Solution of Initial Boundary Value Problems Involving Maxwell's Equations in Isotopic Media," IEEE Transactions on Antennas and Propagation AP-14, 302-307 (May 1966).
For additional information contact Clifford C. Shang (510) 422-6174 (firstname.lastname@example.org).
Electrical engineers who collaborate in the Laboratory's computational electronics and electromagnetic thrust area include: (first row) DAVID STEICH, STEVE SAMPAYAN, JEFF KALLMAN, CLIFF SHANG, and BRIAN POOLE; (second row) RICK RATOWSKY, TOM ROSENBURY, and SCOTT D. NELSON. For more information about their work, visit their Internet home page on EM codes at http://www.llnl.gov/eng/ee/documents/ceeta.html and EM facilities at http://www-dsed.llnl.gov/documents/facilities.html. The group is pictured in front of the Experimental Test Accelerator-II (ETA-II). The ETA is a testbed for beam experiments n advanced hydrodynamics testing to characterize the accelerator and design key components. The accelerator ultimately will be a part of the Laboratory's Advanced Hydrotest Facility.