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At least 991 records · Page 55

Manipulation of the molecular spin crossover transition of Fe(H 2 B(pz) 2 ) 2 (bipy) by addition of polar molecules

In this paper, the addition of various dipolar molecules is shown to affect the temperature dependence of the spin state occupancy of the much studied spin crossover Fe(II) complex, [Fe{H 2 B(pz) 2 } 2 (bipy)] (pz = pyrazol-1-yl, bipy = 2,2'-bipyridine). Specifically, the addition of benzimidazole results in a re-entrant spin crossover transition, i.e. the spin state starts in the mostly low spin state, then high spin state occupancy increases, and finally the high spin state occupancy decreases with increasing temperature. This behavior contrasts with that observed when the highly polar ρ-benzoquinonemonoimine zwitterion C 6 H 2 ( ... NH 2 ) 2 ( ... O) 2 was mixed with [Fe{H 2 B(pz) 2 } 2 (bipy)], which resulted in locking [Fe{H 2 B(pz) 2 } 2 (bipy)] largely into a low spin state while addition of the ethyl derivative C 6 H 2 ( ... NHC 2 H 5 ) 2 ( ... O) 2 did not appear to perturb the spin crossover transition of [Fe{H 2 B(pz) 2 } 2 (bipy)].

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Beneficial influence of Hf and Zr additions to Nb4at%Ta on the vortex pinning of Nb 3 Sn with and without an O source

In this paper we show that addition of Hf to Nb4Ta can significantly improve the high field performance of Nb 3 Sn, making it suitable for dipole magnets for a machine like the 100 TeV Future Circular Collider (FCC). A big challenge of the FCC is that the desired non-Cu critical current density (J c ) target of 1500 A mm –2 (16 T, 4.2 K) is substantially above the best present Nb 3 Sn conductors doped with Ti or Ta (~1300 A mm –2 in the very best sample of the very best commercial wire). Recent success with internal oxidation of Nb–Zr precursor has shown significant improvement in the layer J c of Nb 3 Sn wires, albeit with the complication of providing an internal oxygen diffusion pathway and avoiding degradation of the irreversibility field H Irr . We here extend the Nb1Zr oxidation approach by comparing Zr and Hf additions to the standard Nb4Ta alloy of maximum H c2 and H Irr . Nb4Ta rods with 1Zr or 1Hf were made into monofilament wires with and without SnO 2 and their properties measured over the entire superconducting range at fields up to 31 T. We found that Group IV alloying of Nb4Ta does raise H Irr , though O 2 addition still slightly degrades it. As noted in earlier Nb1Zr work with an O source, the pinning force density F p is strongly enhanced and its peak value shifted to higher field by internal oxidation. A surprising result of this work is that we found better properties in Nb4Ta1Hf without SnO 2 , F pMax achieving 2.35 times that of the standard Nb4Ta alloy, while the oxidized Nb4Ta1Zr alloy achieved 1.54 times that of the Nb4Ta alloy. The highest layer J c (16 T, 4.2 K) of 3700 A mm –2 was found in the SnO 2 -free wire made with Nb4Ta1Hf alloy. Using a standard A15 cross-section fraction of 60% for modern powder-in-tube and rod restack process wires, we estimated that a non-Cu J c of 2200 A mm –2 is obtainable in modern conductors, well above the 1500 A mm –2 FCC specification. Moreover, since the best properties were obtained without SnO 2 , the Nb4Ta1Hf alloy appears to open a straightforward route to enhanced properties in Nb 3 Sn wires manufactured by virtually all the presently used commercial routes employed today.

36 MATERIALS SCIENCE↗

Model-based, in-situ, non-destructive qualification and certification of parts made by autonomous additive manufacturing

To address the significant productivity challenges associated with the qualification and certification (Q&C) tasks of additively manufactured (AM) parts, which have traditionally relied on rigorous post‐build inspection and testing, we propose an integrated framework that combines model‐based qualification and certification (MBQ&C) with autonomous additive manufacturing (AAM). MBQ&C employs high‐fidelity predictive models, developed within the Integrated Computational Materials Engineering (ICME) paradigm, to simulate process–structure–property–performance relationships for assessing a part’s fitness for use. Since predictive models are commonly machine learning (ML)-based or reduced-order surrogates of validated physics models, they run efficiently, enabling timely inference. In parallel, the self-driving AAM utilises ML-based adaptive, closed‐loop control strategies to avoid, mitigate, or repair defects and anomalies during fabrication, thereby increasing the likelihood of producing acceptable parts. A key feature of the combined AAM-MBQ&C framework is that predictive models explicitly incorporate defects or anomalies that persist after the build, using instance-specific data captured via in-situ sensing. This customisation enables a build‐specific assessment of fitness for use, rather than relying on nominal or generic parameters. Such individualised evaluation provides a robust basis for Q&C-related acceptance decisions relating to each build. Additionally, the rapid solution capabilities of ML or reduced-order models enable the determination of a part’s suitability for service shortly after build completion. As the framework matures, it has the potential to substantially reduce reliance on conventional point‐design approaches—such as time‐consuming post‐build computed tomography scanning and costly destructive testing. Thus, the AAM-MBQ&C framework represents a transformative, scalable strategy for quality assurance of AM components, as parts produced within a stable, validated, and certified envelope can be certified with reduced testing. Key benefits include: (1) significant gains in Q&C productivity through efficient, model-centric assessment; (2) performance-based classification of defects into critical and non-critical categories; (3) the ability to predict potential deviations in the performance of parts affected by real-time, adaptive process control interventions relative to those produced under a certified process, and (4) the enabling of virtual Q&C for service environments that are difficult, hazardous, or impractical to access or reproduce experimentally. Collectively, these capabilities strengthen the business case for AM, particularly for high‐consequence and mission‐critical applications. Finally, although this work focuses on powder-based AM, the proposed techniques could be extended to AM processes employing alternative feedstock forms.

Gunasegaram, Dayalan↗

Aboveground resilience to species loss but belowground resistance to nitrogen addition in a montane plant community

Decades of empirical work have demonstrated how dominant plant species and nitrogen fertilization can influence the structure and function of plant communities. More recent studies have examined the interplay between these factors, but few such studies use an explicit trait-based framework. In this study, we use an explicit trait- based approach to identify potential mechanisms for community- level responses and to test ecological niche theory. Methods We experimentally manipulated plant communities (control, -dominant species, -random biomass) and nitrogen (N) inputs (control, +organic N, +inorganic N) in a fully factorial design. We predicted that traits related to plants’ ability to take up different forms of soil N would differ between dominant and subordinate species, resulting in interactive effects of dominant species loss and N fertilization on plant community structure and function. The study took place in a montane meadow in the Rocky Mountains, Colorado, USA. Important Findings After four years, the plant community in removal plots converged toward a species composition whose leaf and root functional traits resembled those of the previously removed dominant species. Ecosystem productivity generally increased with N addition: soil carbon efflux was ~50% greater when either form of N was added, while inorganic N addition increased aboveground biomass production by ~60% relative to controls. The increase in production was mediated by increased average height, leaf mass:area ratio and leaf dry matter content in plant communities to which we added inorganic N. Contrary to our predictions, there were no interactive effects of N fertilization and dominant species loss on plant community structure or ecosystem function. The plant community composition in this study exhibited resistance to soil N addition and, given the functional convergence we observed, was resilient to species loss. Together, our results indicate that the ability of species to compensate functionally for species loss confers resilience and maintains diversity in montane meadow communities.

59 BASIC BIOLOGICAL SCIENCES↗

Effects of an additional conduction band on the singlet-antiferromagnet competition in the periodic Anderson model

The competition between antiferromagnetic (AF) order and singlet formation is a central phenomenon of the Kondo and periodic Anderson Hamiltonians and of the heavy fermion materials they describe. In this paper, we explore the effects of an additional conduction band on magnetism in these models, and, specifically, on changes in the AF-singlet quantum critical point (QCP) and the one particle and spin spectral functions. To understand the magnetic phase transition qualitatively, we first carry out a self-consistent mean field theory (MFT). The basic conclusion is that, at half filling, the coupling to the additional band stabilizes the AF phase to larger f d hybridization V in the PAM. We also explore the possibility of competing ferromagnetic phases when this conduction band is doped away from half filling. Here, we next employ quantum Monte Carlo (QMC) which, in combination with finite size scaling, allows us to evaluate the position of the QCP using an exact treatment of the interactions. This approach confirms the stabilization of AF order, which occurs through an enhancement of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. QMC results for the spectral function A (q,ω) and dynamic spin structure factor χ (q,ω) yield additional insight into the AF-singlet competition and the low temperature phases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Development and experimental study of an automated laser-foil-printing additive manufacturing system

Purpose This paper aims to present the development and experimental study of a fully automated system using a novel laser additive manufacturing technology called laser foil printing (LFP), to fabricate metal parts layer by layer. The mechanical properties of parts fabricated with this novel system are compared with those of comparable methodologies to emphasize the suitability of this process. Design/methodology/approach Test specimens and parts with different geometries were fabricated from 304L stainless steel foil using an automated LFP system. The dimensions of the fabricated parts were measured, and the mechanical properties of the test specimens were characterized in terms of mechanical strength and elongation. Findings The properties of parts fabricated with the automated LFP system were compared with those of parts fabricated with the powder bed fusion additive manufacturing methods. The mechanical strength is higher than those of parts fabricated by the laser powder bed fusion and directed energy deposition technologies. Originality/value To the best knowledge of authors, this is the first time a fully automated LFP system has been developed and the properties of its fabricated parts were compared with other additive manufacturing methods for evaluation.

Engineering↗

Electromagnetic Modeling of Lattice Structures in Additively Manufactured Electric Machines

Recent advancements in metal additive manufacturing (AM) show great potential to revolutionize the design and manufacturing of electromagnetic components used in the field of electrical engineering. Lattice structures directly printed by AM processes typically offer better structural performance with reduced weight, such as high stiffness, surface area, elongation, energy absorption, and porosity, than the solid counterpart. This paper aims to study the electromagnetic modeling of lattice structures used in additively manufactured magnetic cores or windings for electric machines. Three dimensional (3D) electromagnetic finite element (FE) analysis with high performance computing (HPC) shows the highest fidelity in predicting the electromagnetic performance of designs with lattice structures by preserving complex geometry details. FE-based homogenization methods have also been explored to potentially speed up concept design. A case study based on an additively manufactured axial-flux permanent magnet machine with a Hilbert pattern stator validates the discussed electromagnetic modeling approaches.

Chung, Shi-Uk [ANSYS, Inc.]↗

Surface electron modulation of metal oxide‐based electrochemical devices by surface additives—linking sensors and fuel cells

Abstract The interaction between ambient oxygen and the metal oxide surface is central to various electrochemical devices. Solid oxide fuel cell cathodes and semiconducting metal oxide gas sensors are two prominent examples. Parallels between these two systems are highlighted in this perspective. In both cases, the presence of foreign surface species has been found to significantly alter the interactions of the metal oxides’ surfaces with oxygen. On the one hand, interactions are hindered due to the presence of certain impurities, such as the degradation of fuel cell cathode by Si‐species that originate from processing or from operating environments. On the other hand, electrochemically active noble metal additives have been intentionally used to tune the sensor response of metal oxides. In the case of electrochemically active additives, the need for operando spectroscopies to elucidate the mechanism is discussed. Electronic coupling of the surface additive with the metal oxide is found to play a central role in both cases. We use these results to demonstrate that insights gained from either field can be effectively applied to the other.

Materials Science↗

Identification of xylan arabinosyl 2- O -xylosyltransferases catalyzing the addition of 2- O -xylosyl residue onto arabinosyl side chains of xylan in grass species

Grass xylan, the major hemicellulose in both primary and secondary cell walls, is heavily decorated with α-1,3-linked arabinofuranosyl (Araf) residues that may be further substituted at O-2 with xylosyl (Xyl) or Araf residues. Although xylan 3-O-arabinosyltransferases (XATs) catalyzing 3-O-Araf addition onto xylan have been characterized, glycosyltransferases responsible for the transfer of 2-O-Xyl or 2-O-Araf onto 3-O-Araf residues of xylan to produce the Xyl-Araf and Araf-Araf disaccharide side chains remain to be identified. In this report, we showed that a rice GT61 member, named OsXAXT1 (xylan arabinosyl 2-O-xylosyltransferase 1) herein, was able to mediate the addition of Xyl-Araf disaccharide side chains onto xylan when heterologously co-expressed with OsXAT2 in the Arabidopsis gux1/2/3 (glucuronic acid substitution of xylan 1/2/3) triple mutant that lacks any glycosyl substitutions. Recombinant OsXAXT1 protein expressed in human embryonic kidney 293 cells exhibited a xylosyltransferase activity catalyzing the addition of Xyl from UDP-Xyl onto arabinosylated xylooligomers. Further, consistent with its function as a xylan arabinosyl 2-O-xylosyltransferase, CRISPR-Cas9-mediated mutations of the OsXAXT1 gene in transgenic rice plants resulted in a reduction in the level of Xyl-Araf disaccharide side chains in xylan. Furthermore, we revealed that XAXT1 close homologs from several other grass species, including switchgrass, maize, and Brachypodium, possessed the same functions as OsXAXT1, indicating functional conservation of XAXTs in grass species. Together, our findings establish that grass XAXTs are xylosyltransferases catalyzing Xyl transfer onto O-2 of Araf residues of xylan to form the Xyl-Araf disaccharide side chains, which furthers our understanding of genes involved in xylan biosynthesis.

59 BASIC BIOLOGICAL SCIENCES↗

Effects of CO2 Addition on the Turbulent Flame Front Dynamics and Propagation Speeds of Methane/Air Mixtures

Exhaust gas recirculation (EGR) is one of the most promising methods of improving the performance of power-generating gas turbines. CO2 is known to have the largest impact on flame behavior of any major exhaust species, but few studies have specified its thermal, kinetic, and transport effects on turbulent flames. Therefore, in this study, methane/air mixtures diluted with CO2 are experimentally investigated in a reactor-assisted turbulent slot (RATS) burner using OH planar laser-induced fluorescence (PLIF) measurements. CO2 addition is tested under both constant adiabatic flame temperature and variable adiabatic flame temperature conditions in order to elucidate its thermal, kinetic, and transport effects. Particular attention is paid to CO2's effects on the flame surface density, progress variable, turbulent burning velocity, and flame wrinkling. The experimental measurements reveal that CO2's thermal effects are the dominant factor in elongating the turbulent flame brush and decreasing the turbulent burning velocity. When thermal effects are removed by holding the adiabatic flame temperature constant, CO2's kinetic effects are the next most important factor, producing an approximately 5% decrease in the global consumption speed for each 5% of CO2 addition. The transport effects of CO2, however, tend to increase the global consumption speed, counteracting 30–50% of the kinetic effects when the adiabatic flame temperature is fixed. It is also seen that CO2 addition increases the normalized global consumption speed primarily through an enhancement of the stretch factor.

Engineering↗

Tailoring Surface Roughness Using Additive Manufacturing to Improve Internal Cooling

Abstract Surface roughness present on internal cooling channels produced with additive manufacturing has been previously shown to augment heat transfer and pressure loss to levels similar to traditionally cast turbulators. Given the ability of the surface roughness to improve the cooling performance of small cooling channels, the question arises on whether there is an optimal combination of random roughness features to maximize internal cooling performance. To investigate this question, test coupons with different surface roughness morphologies and magnitudes were manufactured by manipulating the parameters in the laser powder bed fusion additive manufacturing process. The coupons were tested to characterize the friction factor and Nusselt number of the cooling channels over a range of Reynolds numbers. Results showed that certain roughness combinations outperformed others, increasing the internal cooling performance of the channels. Additionally, manipulation of the performance using the process parameters allowed for reductions in build time, which could be useful for controlling component cost.

Engineering↗

Development of a Method for Shape Optimization for a Gas Turbine Fuel Injector Design Using Metal-Additive Manufacturing

Adjoint shape optimization has enabled physics-based optimal designs for aerodynamic surfaces. Additive manufacturing (AM) makes it possible to manufacture complex shapes. However, there has been a gap between optimal and manufacturable surfaces due to the inherent limitations of commercial computational fluid dynamics (CFD) codes to implement geometric constraints during adjoint computation. In such cases, the design sensitivities are exported and used to perform constrained shape modifications using parametric information stored in computer aided design (CAD) files to satisfy manufacturability constraints. However, modifying the design using adjoint methods in CFD solvers and performing constrained shape modification in CAD can lead to inconsistencies due to different shape parameterization schemes. This paper describes a method to enable the simultaneous optimization of the fluid domain and impose AM manufacturability constraints, resolving one of the key issues of geometry definition for isogeometric analysis. Similar to a grid convergence study, the proposed method verifies the consistencies between shape parameterization techniques present within commercial CAD and CFD software during mesh movement as a part of the adjoint shape optimization routine. By identifying the appropriate parameters essential to a shape optimization study, the error metric between the different parameterization techniques converges to demonstrate sufficient consistencies for justifiable exchange of data between CAD and CFD. For the identified shape optimization parameters, the error metric to measure the deviation between the two parameterization schemes lies within the AM laser-powder bed fusion (L-PBF) process tolerance. Additionally, comparison for subsequent objective function calculations between iterations of the optimization loop showed acceptable differences within 1% variation between the modified geometries obtained using the two parameterization schemes. This method provides justification for the use of multiphysics guided adjoint design sensitivities computed in CFD software to perform shape modifications in CAD to incorporate AM manufacturability constraints during the shape optimization loop such that optimal designs are also additively manufacturable.

33 ADVANCED PROPULSION SYSTEMS↗

Design and Cooling Performance of Additively Manufactured Ceramic Turbine Vanes

Ceramic materials are of significant interest in aviation and power generation gas turbine engines due to their low density and ability to withstand high temperatures. Increased cycle thermal efficiency and higher specific power output is possible by incorporating ceramic components that enable high turbine inlet temperatures and lower required cooling airflow levels. However, ceramics can be difficult and costly to form into the complex shapes used in gas turbine components, often requiring specialized multi-step processes. Furthermore, ceramic components in the hottest areas of a gas turbine, such as vanes or blade shroud seals, will still likely require cooling which is challenging to implement in conventional ceramic manufacturing approaches. Therefore, this study presents a multidisciplinary approach that investigates the design, fabrication and overall cooling effectiveness evaluation of additively manufactured (AM), polymer derived ceramic (PDC) turbine vanes. A thermo-mechanically optimized vane design was generated, ceramic additive manufacturing of the complex cooling configuration was developed, and quantification of the increase in overall cooling effectiveness was performed in a 1X scale, high-speed facility using infrared thermography. This study produced a PDC AM process, capable of printing complex internal cooling schemes in 1X scale turbine vanes. It was found that the optimized vane more than doubled the overall cooling effectiveness observed in the baseline design, which reasonably agreed with thermomechanical optimization model predictions. Additionally, the optimized ceramic vane outperformed an identical metal vane, in terms of area averaged cooling effectiveness, suggesting that the ceramic vane could operate at reduced coolant flow rates to achieve comparable levels of cooling performance.

Fox, Andrew↗

Torch End-Effector and TIG Electrode Changeout Design for a TIG Welding Robot Used in Metal Big Area Additive Manufacturing

Metal Big Area Additive Manufacturing (mBAAM) is a promising approach to large-scale metal additive manufacturing (AM) or 3D printing. The mBAAM system uses an arc-based wire-fed welding robot to build metal parts. A multi-degree-of-freedom robotic arm is known for its extensive range of motion and reliable tool handling. Attaching a torch end-effector to a robotic arm gives it welding capabilities; however, this decreases the motion range and dynamics of the robot. As a result, build volume and printing accuracy are decreased. Additionally, only a portion of time is spent printing in an arc-based process. Maintenance leads to downtime on the system. In a tungsten inert gas (TIG)-based process, the torch electrode wears out over time and must be changed to avoid defective deposition. This paper proposes an approach for a compact torch end-effector to improve the robot’s build volume. This paper also proposes an approach to reducing non-printing process time by designing and implementing a semi-automated electrode changing system.

Masuo, Chris↗

Analysis of Conduction Cooling Strategies for Wire Arc Additive Manufacturing

Metal additive manufacturing (AM) processing consists of numerous parameters which take time to optimize for various geometries. One aspect of the metal AM process that continues to be explored is the control of thermal energy accumulation during component manufacturing due to the melting and solidification of the feedstock. Excessive energy accumulation causes thermal failure of the component while minimal energy accumulation causes lack of fusion with the build plate or previous layer. The ability to simulate the thermal response of an AM component can increase research efficiency by reducing the time to optimize thermal energy accumulation. This paper presents an effective implementation of finite element analysis to determine the thermal response of a wire arc additive manufactured component with various build plate sizes and cooling methods including, integral build plate cooling, oversized build plates with passive cooling, and non-integral build plate cooling. The use of integral build plate cooling channels was shown to decrease the interpass temperature at the conclusion of the build process by 55% and build plate temperature by 96% compared to the conventionally deposited sample with 20 second dwell time. The use of a tall build plate with passive cooling was shown to reduce the interpass temperature by 32% as compared to the conventionally deposited sample with 20 second dwell time. Each cooling strategy evaluated decreased the interpass temperature within a range of 20–55% which enables higher deposition rates and decreased dwell times during depositions. The cooling strategies are designed to be implemented in a hybrid or retrofit AM platform to mitigate concerns of the thermal input from the additive process having detrimental effects on the precision of the machining process. This paper shows that accurate simulations of all strategies can be used to accurately predict the thermal response of the various strategies discussed. These cooling strategies will allow for increased deposition rates with comparable interpass temperature and decreased dwell time, increasing deposition efficiency. This model and these simulations are verified by experimental results. It is concluded that passive strategies, such as the over-sized tall build plate, can be used when liquid coolant in the AM environment could negatively affect the deposition process. Active cooling strategies, such as the integral build plate cooling could be used if low thermal conductivity materials are deposited or higher material deposition rates are desired. This paper discusses the use of active and passive cooling used during AM and shows how a simulation model can be used to make design choices for cooling strategies. The model also enables verification of select critical process parameters such as dwell times for a desired interpass temperature.

Heinrich, Lauren↗

Orientation control of high-χ triblock copolymer for sub-10 nm patterning using fluorine-containing polymeric additives

Directed self-assembly (DSA) of block copolymers (BCPs) Is one of the most promising techniques to tackle the ever-increasing demand for subllthographic features In semiconductor industries. BCPs with high Flory-Huggins parameter (chi) are of particular Interest due to their ability to self-assemble at the length scale of sub-10 nm. However, such high-chi BCPs typically have imbalanced surface energies between respective blocks, making it a challenge to achieve desired perpendicular orientation. To address this challenge, we mixed a fluorine-containing polymeric additive with poly(2-vlnylpyrldine)-block-polystyrene-block-poly(2-vlnylpyrldine) (P2VP-b-PS-b-P2VP) and successfully controlled the orientation of the high-chi triblock copolymer. The additive selectively mixes with P2VP block through hydrogen bonding and can reduce the dissimilarity of surface energies between PS and P2VP blocks. After optimizing additive dose and annealing conditions, desired perpendicular orientation formed upon simple thermal annealing. We further demonstrated DSA of this material system with five times density multiplication and a half-pitch as small as 8.5 nm. This material system Is also amenable to sequential Infiltration synthesis treatment to selectively grow metal oxide In P2VP domains, which can facilitate the subsequent pattern transfer. We believe that this integration-friendly DSA platform using simple thermal annealing holds the great potential for sub-10 nm nanopatterning applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

A Survey of Thermal Sensing Application in Additive Manufacturing

Industry 4.0 represents a significant change in manufacturing with the inclusion of such technologies as additive manufacturing and data analytics. In support of these changes, infrared thermal imaging (IR) and very-near infrared (VNIR) have proven invaluable in numerous applications related to additive manufacturing. In this review, these applications will be presented as they relate to polymer and metal-based additive manufacturing processes. Applications including in-situ characterization, post-build analysis, experimental setups, modeling, and machine learning will be covered. With a general overview complete, the review will conclude with a discussion of challenges identified throughout the literature and thoughts on future trends.

Borish, Michael↗

Beam geometry selection using sequential beam addition

Purpose: The selection of optimal beam geometry has been of interest since the inception of conformal radiotherapy. The authors report on sequential beam addition, a simple beam geometry selection method, for intensity modulated radiation therapy. Methods: The sequential beam addition algorithm (SBA) requires definition of an objective function (score) and a set of candidate beam geometries (pool). In the first iteration, the optimal score is determined for each beam in the pool and the beam with the best score selected. In the next iteration, the optimal score is calculated for each beam remaining in the pool combined with the beam selected in the first iteration, and the best scoring beam is selected. The process is repeated until the desired number of beams is reached. The authors selected three treatment sites, breast, lung, and brain, and determined beam arrangements for up to 11 beams from a pool comprised of 25 equiangular transverse beams. For the brain, arrangements were additionally selected from a pool of 22 noncoplanar beams. Scores were determined for geometries comprised equiangular transverse beams (EQA), as well as two tangential beams for the breast case. Results: In all cases, SBA resulted in scores superior to EQA. The breast case had the strongest dependence on beam geometry, for which only the 7-beam EQA geometry had a score better than the two tangential beams, whereas all SBA geometries with more than two beams were superior. In the lung case, EQA and SBA scores monotonically improved with increasing number of beams; however, SBA required fewer beams to achieve scores equivalent to EQA. For the brain case, SBA with a coplanar pool was equivalent to EQA, while the noncoplanar pool resulted in slightly better scores; however, the dose-volume histograms demonstrated that the differences were not clinically significant. Conclusions: For situations in which beam geometry has a significant effect on the objective function, SBA can identify arrangements equivalent to equiangular geometries but using fewer beams. Furthermore, SBA provides the value of the objective function as the number of beams is increased, allowing the planner to select the minimal beam number that achieves the clinical goals. The method is simple to implement and could readily be incorporated into an existing optimization system.

62 RADIOLOGY AND NUCLEAR MEDICINE↗