http://ilp-www.mit.edu/ List of 40 most recently entered publications, as compiled by the Industrial Liaison Office. en-us Copyright 2009 MIT ILP Sat, 7 Nov 2009 09:14:56 GMT http://ilp-www.mit.edu/favicon.ico http://ilp-www.mit.edu/display_page.a4d?key=P5d http://ilp-www.mit.edu/display_paper.a4d?paperId=84223 Abstract—In this paper, we consider the problem of functional compression for an arbitrary tree network. Suppose we have k possibly correlated source processes in a tree network, and a receiver in its root wishes to compute a deterministic function of these processes. Other nodes of this tree (called intermediate nodes) are allowed to perform some computations to satisfy the node’s demand. Our objective is to find a lower bound on feasible rates for different links of this tree network (called a rate lower bound) and propose a coding scheme to achieve this rate lower bound in some cases. The rate region of functional compression problem has been an open problem. However, it has been solved for some simple networks under some special conditions. For instance, consider the rate region of a network with two transmitters and a receiver under a condition on source random variables. Here, we derive a rate lower bound for an arbitrary tree network based on the graph entropy. We introduce a new condition on colorings of source random variables’ characteristic graphs called the coloring connectivity condition (C.C.C.). We show that unlike the condition mentioned earlier, this condition is necessary and sufficient for any achievable coding scheme. We also show that unlike the entropy, the graph entropy does not satisfy the chain rule. For one stage trees with correlated sources, and general trees with independent sources, we propose a modularized coding scheme based on graph colorings to perform arbitrarily close to this rate lower bound. We show that in a general tree network case with independent sources, to achieve the rate lower bound, intermediate nodes should perform some computations. However, for a family of functions and RVs called coloring proper sets, it is sufficient to have intermediate nodes act like relays to perform arbitrarily close to the rate lower bound. 11/06/09 Muriel Medard http://ilp-www.mit.edu/display_paper.a4d?paperId=84222 In this paper, we study the multiple unicast network communication problem on undirected graphs. It has been conjectured by Li and Li [CISS 2004] that, for the problem at hand, the use of network coding does not allow any advantage over standard routing. Loosely speaking, we show that under certain (strong) connectivity requirements the advantage of network coding is indeed bounded by 3. 11/06/09 Muriel Medard http://ilp-www.mit.edu/display_paper.a4d?paperId=84221 The impact of Internet content and IP based television on networks is growing. Video is now ubiquitous in the home and on the street. It demands new approaches to video transmission to meet the growing traffic volume. This paper presents a novel strategy for network coding in video transmission. It uses a variety of coding approaches and adds feedback and device discovery to tailor the coding to the receiver ecosystems and can achieve better overall performance than the non coded versions. 11/06/09 Marie-Jose Montpetit http://ilp-www.mit.edu/display_paper.a4d?paperId=84220 Puzzled by the indication of a new critical theory for the spin-1/2 Heisenberg model with a spatially staggered anisotropy on the square lattice, we re-investigate the phase transition of this model induced by dimerization. We focus on studying the finite-size scaling of the observables. We find by performing finite-size scaling using the observable !s2L, which corresponds to the spatial direction with a fixed antiferromagnetic coupling, one would suffer a much less severe correction compared to that of using ps1L. Therefore ps2L is a better quantity than ps1L for finite-size scaling analysis concerning the limitation for the availability of large volumes data in our study. Remarkably, by employing the method of fixing the aspect-ratio of spatial winding numbers squared in the simulations, even from !s1L which receives the most serious correction among the observables considered in this study, we arrive at a value for the critical exponent " which is consistent with the expected O(3) value by using only up to L = 64 data points. 11/06/09 Edward Farhi http://ilp-www.mit.edu/display_paper.a4d?paperId=84219 For cosmological topologically massive gravity at the chiral point we calculate momentum space 2- and 3-point correlators of operators in the postulated dual CFT on the cylinder. These operators are sourced by the bulk and boundary gravitons. Our correlators are fully consistent with the proposal that cosmological topologically massive gravity at the chiral point is dual to a logarithmic CFT. In the process we give a complete classification of normalizable and non-normalizeable left, right and logarithmic solutions to the linearized equations of motion in global AdS3. 11/06/09 Daniel Grumiller http://ilp-www.mit.edu/display_paper.a4d?paperId=84218 Intermediate filaments, in addition to microtubules and microfilaments, are one of the three major components of the cytoskeleton in eukaryotic cells, and play an important role in mechanotransduction as well as in providing mechanical stability to cells at large stretch. The molecular structures, mechanical and dynamical properties of the intermediate filament basic building blocks, the dimer and the tetramer, however, have remained elusive due to persistent experimental challenges owing to the large size and fibrillar geometry of this protein. We have recently reported an atomistic-level model of the human vimentin dimer and tetramer, obtained through a bottom-up approach based on structural optimization via molecular simulation based on an implicit solvent model (Qin et al., PLoS ONE, 2009). Here we present extensive simulations and structural analyses of the model based on equilibration in large-scale atomistic-level simulations in an explicit solvent model, with system sizes exceeding 500,000 atoms and simulations carried out at 20 ns time-scales. We report a detailed comparison of the structural and dynamical behavior of this large biomolecular model with implicit and explicit solvent models. Our simulations confirm the stability of the molecular model and provide insight into the dynamical properties of the dimer and tetramer. Specifically, our simulations reveal a heterogeneous distribution of the bending stiffness along the molecular axis with the formation of rather soft and highly flexible hinge-like regions defined by non-alpha-helical linker. We compute the persistence length of the dimer and tetramer structure of vimentin intermediate filaments for various subdomains of the protein. Our simulations provide detailed insight into the dynamical properties of the vimentin dimer and tetramer intermediate filament building blocks, which may guide the development of novel coarse-grained models of intermediate filaments that could help in understanding assembly mechanisms. 11/06/09 Markus Buehler http://ilp-www.mit.edu/display_paper.a4d?paperId=84217 Amyloid fibrils are highly ordered protein aggregates associated with several pathological processes such as prion propagation and Alzheimer’s disease. A key issue in amyloid science is the understanding of the mechanical properties of amyloid fibrils and fibers, in order to quantify biomechanical interactions with surrounding tissues and to identify mechanobiological mechanisms associated with changes of material properties as amyloid fibrils grow from nanoscale to microscale structures. Here we report a series of computational studies using atomistic simulation, elastic network modeling and finite element simulation, utilized to elucidate the mechanical properties of Alzheimer’s ABeta](1-40) amyloid fibrils as a function of the length of the protein filament for both 2-fold and 3-fold symmetric amyloid fibrils. We calculate elastic constants associated with the torsional, bending and tensile deformation as a function of the size of the amyloid fibril, covering fibril lengths ranging from nanometers to micrometers. The resulting Young’s moduli are found to be consistent with available experimental measurements obtained from long amyloid fibrils, and predicted to be in the range of 20-30 GPa. Results show that ABeta(1-40) amyloid fibrils feature a remarkable structural stability and mechanical rigidity for fibrils in excess of *hundred nanometers in length. However, local instabilities that emerge at the ends of short fibrils on the order of tens of nanometers reduce their stability and contribute to their disassociation under extreme mechanical or chemical conditions. Moreover, we find that amyloids with lengths shorter than the periodicity of their helical pitch, typically between 90 and 130 nm, feature significant size effects of their bending stiffness due the anisotropy in the fibril’s cross section. Studies reveal the importance of size effects in understanding the mechanical properties of amyloid fibrils, an issue of great importance for the comparison between experimental and simulation results, and for the general understanding of the biological mechanisms during the growth of ectopic amyloid materials. 11/03/09 Markus Buehler http://ilp-www.mit.edu/display_paper.a4d?paperId=84216 The study of multi-scale hierarchical systems as found in natural, biological and synthetic materials require a new approach of analysis to elucidate the inherent structure-property relationships that transcend atomistic, mesoscopic to macroscopic scales. Here we review a systemdependent “finer-trains-coarser” multi-scale paradigm for coarse-grain model development that aims to maintain relevant atomistic mechanics and accurate molecular interactions, focused on interactions at the mesoscale. Parameterization of such a model is achieved through full atomistic results, thereby providing a sound theoretical basis for coarse-grain potentials and a significant reduction of computational expense. The system simplification can be efficiently implemented to reconcile the differences between empirical and simulation results by bridging the time- and length-scale limitations of classical molecular dynamics into physically relevant and experimentally accessible regimes. However, vast the number of potential system variations and combinations of constituent materials requires different model formulations depending on the intended application. Such a system-dependent restriction contradicts the formulation of a universal stepwise coarse-graining procedure. A presented generalized framework elucidates the fundamental principles necessary for coarse-grain model formulation. The framework is illustrated through a thorough discussion of three case studies, with a concentration on the detailed steps of model parameter identification, including: (1) carbon nanotubes and tropocollagen molecules, characterized by a linear bead-spring model and implemented to investigate system properties of carbon nanotube arrays and collagen fibrils at the mesoscale; (2) alphahelical protein domains, using a novel double-well coarse-grain potential to extend atomistic unfolding to inaccessible time- and length-scales, and; (3) polymer-tethered nanoparticles, with a concentration on particle interactions rather than mechanics, as well as elimination of solvent to investigate the aggregation of nanoparticles at the mesoscale. Each case study is motivated by different objectives, illustrating broad categories of structures and materials that can potentially benefit from coarse-grained representations of materials and structures. 11/03/09 Markus Buehler http://ilp-www.mit.edu/display_paper.a4d?paperId=84215 ICEBs1 is an integrative and conjugative element found in the chromosome of Bacillus subtilis. ICEBs1 encodes functions needed for its excision and transfer to recipient cells. We found that the ICEBs1 gene conE (formerly yddE) is required for conjugation and that conjugative transfer of ICEBs1 requires a conserved ATPase domain of ConE. ConE belongs to the HerA/FtsK superfamily of ATPases, which includes the well-characterized proteins FtsK, SpoIIIE, VirB4, and VirD4. We found that a ConE-GFP (green fluorescent protein) fusion associated with the membrane predominantly at the cell poles in ICEBs1 donor cells. At least one ICEBs1 product likely interacts with ConE to target it to the membrane and cell poles, as ConE GFP was dispersed throughout the cytoplasm in a strain lacking ICEBs1. We also visualized the subcellular location of ICEBs1. When integrated in the chromosome, ICEBs1 was located near midcell along the length of the cell, a position characteristic of that chromosomal region. Following excision, ICEBs1 was more frequently found near a cell pole. Excision of ICEBs1 also caused altered positioning of at least one component of the replisome. Taken together, our findings indicate that ConE is a critical component of the ICEBs1 conjugation machinery, that conjugative transfer of ICEBs1 from B. subtilis likely initiates at a donor cell pole, and that ICEBs1 affects the subcellular position of the replisome. 11/03/09 Catherine Lee http://ilp-www.mit.edu/display_paper.a4d?paperId=84214 Integrative and conjugative elements (ICEs), a.k.a. conjugative transposons, are mobile genetic elements involved in many biological processes, including pathogenesis, symbiosis, and the spread of antibiotic resistance. Unlike conjugative plasmids that are extra-chromosomal and replicate autonomously, ICEs are integrated in the chromosome and replicate passively during chromosomal replication. It is generally thought that ICEs do not replicate autonomously. We found that when induced, Bacillus subtilis ICEBs1 undergoes autonomous plasmid-like replication. Replication was unidirectional, initiated from the ICEBs1 origin of transfer, oriT, and required the ICEBs1-encoded relaxase NicK. Replication also required several host proteins needed for chromosomal replication, but did not require the replicative helicase DnaC or the helicase loader protein DnaB. Rather, replication of ICEBs1 required the helicase PcrA that is required for rolling circle replication of many plasmids. Transfer of ICEBs1 from the donor required PcrA, but did not require replication, indicating that PcrA, and not DNA replication, facilitates unwinding of ICEBs1 DNA for horizontal transfer. Although not needed for horizontal transfer, replication of ICEBs1 was needed for stability of the element. We propose that autonomous plasmid-like replication is a common property of ICEs and contributes to the stability and maintenance of these mobile genetic elements in bacterial populations. 11/03/09 Catherine Lee http://ilp-www.mit.edu/display_paper.a4d?paperId=84213 We investigate a stable Casimir force configuration consisting of an object contained inside a spherical or spheroidal cavity filled with a dielectric medium. The spring constant for displacements from the center of the cavity and the dependence of the energy on the relative orientations of the inner object and the cavity walls are computed. We find that the stability of the force equilibrium can be predicted based on the sign of the force, but the torque cannot be. 11/03/09 Robert Jaffe http://ilp-www.mit.edu/display_paper.a4d?paperId=84212 Quantum mechanics predicts an exponentially small probability that a particle with energy greater than the height of a potential barrier will nevertheless reflect from the barrier in violation of classical expectations. This process can be regarded as tunneling in momentum space, leading to a simple derivation of the reflection probability. 11/03/09 Robert Jaffe http://ilp-www.mit.edu/display_paper.a4d?paperId=84211 The Casimir force has been computed exactly for only a few simple geometries, such as infinite plates, cylinders, and spheres. We show that a parabolic cylinder, for which analytic solutions to the Helmholtz equation are available, is another case where such a calculation is possible. We compute the interaction energy of a parabolic cylinder and an infinite plate (both perfect mirrors), as a function of their separation and inclination, H and !, and the cylinder’s parabolic radius R. As H/R ! 0, the proximity force approximation becomes exact. The opposite limit of R/H ! 0 corresponds to the a semi-infinite plate, where the effects of edge and inclination can be probed. 11/03/09 Robert Jaffe http://ilp-www.mit.edu/display_paper.a4d?paperId=84210 The application of lower hybrid waves (LH) in H-mode plasmas on Alcator C-Mod can result in significant reduction of core particle inventory, with no significant degradation of energy confinement. This phenomenon has been observed in steady enhanced D (EDA) H-mode targets, which are sustained by ion cyclotron RF auxiliary heating, in which pedestal density nped is usually tied firmly to plasma current IP and shows a strong resilience to changes in the edge neutral source. Upon application of up to 1MW LH power, nped is reduced by up to 30%, while the temperature profile increases simultaneously such that the pressure pedestal remains constant or is slightly increased. Steady EDA H-mode operation with no edge-localized modes (ELMs) can be maintained while edge collisionality is reduced by factors of reductions of 2—4. Elevation of scrape-off layer (SOL) density and electric currents accompany the application of LH (at levels as low as 400kW) with a fast time response (~10?2 s), while full density pedestal relaxation and core density reduction occur on longer time scales (~10?1 s). A similarly prompt counter-IP change in the edge toroidal velocity is also observed in response to LHRF, followed on longer time scales by a counter-IP change in the central rotation. The range of time scales of the plasma response may indicate that the radial locations of LH interactions (i.e., SOL vs. core), and power deposition mechanisms, are evolving in time. Understanding the responsible physical mechanisms and applying them to a broad range of H-mode discharges could provide a tool for improving density control, affecting edge MHD stability and applying modulation for transport studies. 11/02/09 Amanda Hubbard http://ilp-www.mit.edu/display_paper.a4d?paperId=84209 In the 1970s, large increases in the price of oil were associated with sharp decreases in output and large increases in inflation. In the 2000s, and at least until the end of 2007, even larger increases in the price of oil were associated with much milder movements in output and inflation. Using a structural VAR approach Blanchard and Gali (2007a) argued that this has reflected in large part a change in the causal relation from the price of oil to output and inflation. In order to shed light on the possible factors behind the decrease in the macroeconomic effects of oil price shocks, we develop a new-Keynesian model, with imported oil used both in production and consumption, and we use a minimum distance estimator that minimizes, over the set of structural parameters and for each of the two samples (pre and post 1984), the distance between the empirical SVAR-based impulse response functions and those implied by the model. Our results point to two relevant changes in the structure of the economy, which have modified the transmission mechanism of the oil shock: vanishing wage indexation and an improvement in the credibility of monetary policy. The relative importance of these two structural changes depends however on how we formalize the process of expectations formation by economic agents. 10/30/09 Olivier Blanchard http://ilp-www.mit.edu/display_paper.a4d?paperId=84208 We propose two related equilibrium refinements for voting and agenda-setting games, Sequentially Weakly Undominated Equilibrium (SWUE) and Markov Trembling Hand Perfect Equilibrium (MTHPE), and show how these equilibrium concepts eliminate non-intuitive equilibria that arise naturally in dynamic voting games and games in which random or deterministic sequences of agenda-setters make offers to several players. We establish existence of these equilibria in finite and infinite (for MTHPE) games, provide a characterization of the structure of equilibria, and clarify the relationship between the two concepts. Finally, we show how these concepts can be applied in a dynamic model of endogenous club formation. 10/30/09 K Acemoglu http://ilp-www.mit.edu/display_paper.a4d?paperId=84207 A notable feature of post-World War II civil wars is their very long average duration. We provide a theory of the persistence of civil wars. The civilian government can successfully defeat rebellious factions only by creating a relatively strong army. In weakly-institutionalized polities this opens the way for excessive ináuence or coups by the military. Civilian governments whose rents are largely unaffected by civil wars then choose small and weak armies that are incapable of ending insurrections. The framework also shows that when civilian governments need to take more decisive action against rebels, they may be forced to build over-sized armies, beyond the size necessary for fighting the insurrection, as a commitment to not reforming the military in the future. 10/30/09 K Acemoglu http://ilp-www.mit.edu/display_paper.a4d?paperId=84205 A variety of mechanisms have been proposed to detect inorganic ions using magnetic resonance imaging (MRI) contrast agents. In the most widely explored approach, analyte binding to a paramagnetic metal chelate withdraws ligands, increasing the number (q) of inner sphere sites available for water coordination at the paramagnetic center. As a consequence, the longitudinal relaxivity (r1) of protons on exchangeable solvent molecules in the complex is similarly increased. Here we demonstrate an extension of this strategy, in which the analyte completely releases the paramagnetic ion from a polydentate chelating ligand to form the aqua complex with q = 6 that serves as an efficient MRI contrast agent. In this facile and flexible new approach, changes in r1 can be tuned by the choice of the metal chelate complex. Measurement of the effect by MRI can then be used to determine the analyte concentration with spatiotemporal resolution. 10/28/09 Stephen Lippard http://ilp-www.mit.edu/display_paper.a4d?paperId=84204 We present a simple solution to the little hierarchy problem in the MSSM: a vector-like fourth generation. With O(1) Yukawa couplings for the new quarks, the Higgs mass can naturally be above 114 GeV. Unlike a chiral fourth generation, a vector-like generation can solve the little hierarchy problem while remaining consistent with precision electroweak and direct production constraints, and maintaining the success of perturbative gauge coupling unification. The new quarks are predicted to lie between ~ 300 -- 600 GeV and will thus be discovered or ruled out at the LHC. This scenario suggests exploration of several novel collider signatures. 10/28/09 Peter Graham http://ilp-www.mit.edu/display_paper.a4d?paperId=84202 Nitric oxide (NO) mediates both physiological and pathological processes. In addition to cardiovascular signaling, NO has been invoked to play a neurochemical role in learning and memory, and it is a powerful necrotic agent wielded by macrophages of the immune system. Whereas considerable effort has been invested to develop metal-based and other probes for detecting nitric oxide, there has been significantly less progress in the synthesis of platforms capable of detecting other reactive nitrogen species (RNS). Of the nitrogen oxides relevant to biology, nitroxyl (HNO), the one electron reduced, protonated analog of nitric oxide, is among the least thoroughly investigated 10/28/09 Stephen Lippard http://ilp-www.mit.edu/display_paper.a4d?paperId=84201 The unique glycolytic metabolism of most solid tumors, known as the Warburg effect, is associated with resistance to apoptosis that enables cancer cells to survive. Dichloroacetate (DCA) is an anticancer agent that can reverse the Warburg effect by inhibiting a key enzyme in cancer cells, pyruvate dehydrogenase kinase (PDK) that is required for the process. DCA is currently not approved for cancer treatment in the USA. Here we present the synthesis, characterization, and anticancer properties of c,t,c-[Pt(NH3)2(O2CHCl2)2Cl2], mitaplatin, in which two DCA units are appended to the axial positions of a six-coordinate Pt(IV) center. The negative intracellular redox potential reduces the platinum to release cisplatin, a Pt(II) compound, and two equivalents of DCA. By a unique mechanism, mitaplatin thereby attacks both nuclear DNA with cisplatin and mitochondria with DCA selectively in cancer cells. The cytotoxicity of mitaplatin in a variety of cancer cell lines exceeds that of all known Pt(IV) compounds and is comparable to that of cisplatin. Mitaplatin alters the mitochondrial membrane potential gradient (delta-psi-m) of cancer cells, promoting apoptosis by releasing cytochrome c and translocating apoptosis inducing factor from mitochondria to the nucleus. Cisplatin formed upon cellular reduction of mitaplatin enters the nucleus and targets DNA to form 1,2- intrastrand d(GpG) cross-links characteristic of its own potency as an anticancer drug. These extraordinary properties of mitaplatin are manifest in its ability to selectively kill cancer cells cocultured with normal fibroblasts and to overcome cisplatin resistance. 10/27/09 Stephen Lippard http://ilp-www.mit.edu/display_paper.a4d?paperId=84200 We show in some lower-dimensional supergravity models that the holographic counterterms which are needed in the AdS/CFT correspondence to make the theory finite, coincide with the counterterms that are needed to make the action supersymmetric without imposing any boundary conditions on the fields. 10/26/09 Daniel Grumiller http://ilp-www.mit.edu/display_paper.a4d?paperId=84199 Oscillatory brain rhythms and evoked responses are widely believed to impact cognition, but relatively little is known about how these measures are affected by healthy aging. The present study used MEG to examine age-related changes in spontaneous oscillations and tactile evoked responses in primary somatosensory cortex (SI) in healthy young (YA) and middle-aged (MA) adults. To make specific predictions about neurophysiological changes that mediate age-related MEG changes, we applied a biophysically realistic model of SI that accurately reproduces SI MEG mu rhythms, containing alpha (7-14Hz) and beta (15-30Hz) components, and evoked responses. Analyses of MEG data revealed a significant increase in prestimulus mu power in SI, driven predominately by greater mu-beta dominance, and a larger and delayed M70 peak in the SI evoked response in MA. Previous analysis with our computational model showed that the SI mu rhythm could be reproduced with a sequence of rhythmic ~10Hz feedforward (FF) input to the granular layers of SI (representative of lemniscal thalamic input) followed by ~10Hz feedback (FB) input to the supragranular layers (representative of input from high order cortical or non-specific thalamic sources) (Jones et al 2009). In the present study, the model further predicted that the rhythmic FF and FB inputs become stronger with age. Further, the FB input is predicted to arrive more synchronously to SI on each cycle of the 10Hz input in MA. The simulated neurophysiological changes are sufficient to account for the age-related differences in both prestimulus mu rhythms and evoked responses. Thus, the model predicts that a single set of neurophysiological changes intimately links these age-related changes in neural dynamics. 10/26/09 David Ziegler http://ilp-www.mit.edu/display_paper.a4d?paperId=84198 An experimental study of AC losses was carried out on a 36-strand sub-sized Cable-In-Conduit-Conductor of Nb3Sn superconducting wires used for the International Thermonuclear Experimental Reactor (ITER) Central Solenoid model coils. Ripple field loss tests were performed using the 20 T large bore magnet at the National High Field Magnet Laboratory. Small amplitude, sinusoidal ripple field of 4 Hz, superimposed on a DC magnetic field was produced by modulating the magnet power supply. AC losses were measured with transport currents ranging from 0 to 6 kA at various DC background fields up to 12 T using an isothermal calorimetric method. The inter-strand coupling loss of the 36-strand cable increased gradually and then sharply at approximately 30 kN/m with increasing Lorentz force, and resulted in an 84% increase at the Lorentz force of 50 kN/m. The cable coupling current effective time constant (nsts) at zero current declined from 20 ms to 8 ms after cyclic Lorentz force loading. 10/22/09 Makoto Takayasu http://ilp-www.mit.edu/display_paper.a4d?paperId=84197 The refrigeration power for large current superconductor systems, such as for electrical power distribution, is dominated by current lead losses. The use of multiple cooling stages between room temperature and ~ 70K is investigated as means to decrease the refrigeration power. We show that it is possible to decrease the electrical power requirements for the refrigerator by about 1/3 through the use of two-stages current leads; this computed power saving is based on a conservative estimate of refrigerator performance. Using data from real systems, that is, higher temperature refrigerators operating at higher fractions of their Carnot efficiencies we believe that the refrigerator electrical power requirement can actually be decreased by 1/2. Means have been investigated to optimize current lead performance at lower than maximum current operation. Adjustment of the cooling power of the intermediate temperature refrigerators achieves limited success in power consumption minimization. Other means to optimize the performance will be described. The implications of intermediate stages for stability of the current leads following a short overcurrent period will be described. 10/22/09 C Miles http://ilp-www.mit.edu/display_paper.a4d?paperId=84195 This paper investigates coolant topologies for High Temperature Superconducting (HTS) transmission and distribution cable systems. We explore options that allow for flexibility of operation, low temperature rise in the superconductor and low refrigerator power consumption. Topologies for cooling the cryostat and HTS in long-distance electric power transmission systems are explored. For transmission, the goal is to achieve long spans between cooling stations along the transmission line, and low power consumption. For HTS distribution systems, the issue is cooling the superconductor and the current leads and the goals are to minimize the power consumption and to prevent excessive heating of the superconductor. Means are explored to cool distribution systems where cryogenic loads are dominated by current lead loss. Use of multiple fluids or multiple coolant circuits of the same fluid to decrease the energy ingress in the low temperature environment is described. Potential alternative coolants are proposed. We show that it is possible to reduce electrical consumption by about a factor of 2, while also decreasing the temperature rise of the superconductor. 10/22/09 Leslie Bromberg http://ilp-www.mit.edu/display_paper.a4d?paperId=84194 The use of monolithic High Temperature Superconductors (HTS) for field shaping in stellarators and tokamaks is presented. Design issues relevant to stellarator magnets using single crystal or highly textured YBCO monoliths will be discussed. The excellent properties of YBCO operating at elevated temperatures (> 10 K) will be summarized. High field, cryo-stable, highly complex magnet field topologies can be generated using the techniques discussed in this paper. The diamagnetic properties of the bulk HTS material can be used to provide simple mechanisms for providing field-shaping. Engineering constraints, such as stresses in the superconducting monoliths, support, quench protection, superconducting stability of the monoliths and required external support structure will be described. The limitations imposed by different fusion environments on the performance and lifetime of the HTS monoliths will be reviewed, both for near term experiments as well as long term stellarator fusion reactors. Since the HTS monoliths require no insulation or copper for stability/quench protection, the typical irradiation limits on these elements are eliminated. Nuclear heating, due to the high temperature of operation of the HTS compounds, is also very much relaxed, since at 50 K it is possible to remove more than one order of magnitude higher cryogenic loads than at 4 K, for the same refrigerator power. In addition, irradiation damage limits on HTS, and YBCO in particular, are no lower than for Nb3Sn. 10/22/09 Leslie Bromberg http://ilp-www.mit.edu/display_paper.a4d?paperId=84193 The use of monolithic High Temperature Superconductors (HTS) for field shaping in stellarators and tokamaks is presented. Design issues relevant to stellarator magnets using single crystal or highly textured YBCO monoliths will be discussed. The excellent properties of YBCO operating at elevated temperatures (> 10 K) will be summarized. High field, cryo-stable, highly complex magnet field topologies can be generated using the techniques discussed in this paper. The diamagnetic properties of the bulk HTS material can be used to provide simple mechanisms for providing field-shaping. Engineering constraints, such as stresses in the superconducting monoliths, support, quench protection, superconducting stability of the monoliths and required external support structure will be described. The limitations imposed by different fusion environments on the performance and lifetime of the HTS monoliths will be reviewed, both for near term experiments as well as long term stellarator fusion reactors. Since the HTS monoliths require no insulation or copper for stability/quench protection, the typical irradiation limits on these elements are eliminated. Nuclear heating, due to the high temperature of operation of the HTS compounds, is also very much relaxed, since at 50 K it is possible to remove more than one order of magnitude higher cryogenic loads than at 4 K, for the same refrigerator power. In addition, irradiation damage limits on HTS, and YBCO in particular, are no lower than for Nb3Sn. 10/22/09 Leslie Bromberg http://ilp-www.mit.edu/display_paper.a4d?paperId=84192 Lower hybrid (LH) waves have the attractive property of damping strongly via electron Landau resonance on relatively fast tail electrons and consequently are well-suited to driving current. Established modeling techniques use WKB expansions with self-consistent non-Maxwellian distributions. Higher order WKB expansions have shown some effects on the parallel wave-number evolution and consequently on the damping due to diffraction. A massively parallel version of the TORIC full wave electromagnetic field solver valid in the LH range of frequencies has been developed and coupled to an electron Fokker-Planck solver CQL3D in order to self-consistently evolve non-thermal electron distributions characteristic of LH current drive experiments in devices such as Alcator C-Mod and ITER. These simulations represent the first ever self-consistent simulations of LHCD utilizing both a full wave and Fokker{Planck calculation in toroidal geometry. 10/22/09 John Wright http://ilp-www.mit.edu/display_paper.a4d?paperId=84191 Lower hybrid (LH) waves have the attractive property of damping strongly via electron Landau resonance on relatively fast tail electrons at (2:5 -- --3)vte, where vte  (2Te=me)1=2 is the electron thermal speed. Consequently these waves are well-suited to driving current in the plasma periphery where the electron temperature is lower, making LH current drive (LHCD) a promising technique for off-axis (r=a  0:60) current profile control in reactor grade plasmas. Established techniques for computing wave propagation and absorption use WKB expansions with non-Maxwellian self-consistent distributions. Higher order WKB expansions have shown some effects from diffraction on the parallel wavenumber evolution and consequently on the damping location. In typical plasma conditions with electron densities of several 10(19) m(3) and toroidal magnetic fields strengths of 4 Telsa, the perpendicular wavelength is of the order of 1mm and the parallel wavelength is of the order of 1cm. Even in a relatively small device such as Alcator C-Mod with a minor radius of 22 cm, the number of wavelengths that must be resolved requires large amounts of computational resources for the full wave treatment. These requirements are met with a massively parallel version of the TORIC full wave code that has been adapted specifically for the simulation of LH waves. This model accurately represents the effects of focusing and diffraction that occur in LH propagation. It is also coupled with a Fokker-Planck solver, CQL3D, to provide self-consistent distribution functions for the plasma dielectric as well as a synthetic HXR diagnostic for direct comparisons with experimental measurements of LH waves. The wave solutions from the TORIC zero FLR model will be compared to the results from ray tracing from the GENRAY/CQL3D code via the synthetic HXR diagnostic and power deposition. 10/22/09 John Wright http://ilp-www.mit.edu/display_paper.a4d?paperId=84190 The three-dimensional interaction of a magnetized, collisionless flowing plasma with a non-emitting conducting sphere is solved in the entire range of physically allowed parameters, in the ion-collecting regime. This can be considered as the “spherical Mach probe” problem, establishing how the ion flux to the surface varies with orientation and external velocity; the study is however of broader interest, as the sphere can also be seen as a dust particle or any ionospheric body. The core tool developed for this study is the fully parallelized (particle + field solver) Particle-In-Cell code SCEPTIC3D, three-dimensional evolution of SCEPTIC, accounting for the full ion distribution function and Boltzmann electrons. Investigations are first carried out in the quasineutral limit. Results include a report of ion current dependence on the external plasma parameters, as well as a theoretical calibration for transverse Mach probes with four electrodes oriented at 45o to the magnetic field in a plane of flow and magnetic field, valid for arbitrary temperature and ion magnetization. The analysis is preceded by an independent semi-analytic treatment of strongly magnetized ion collection by oblique surfaces, successfully validating SCEPTIC3D’s behaviour. The finite shielding length regime is more complex, and an important transition in plasma structure occurs when the Debye length goes over the average ion Larmor radius. Studies of ion collection show that the ion current can exceed the (unmagnetized) OML limit at weak magnetization, and the Mach probe calibration method proposed in the context of quasineutral plasmas holds up to Debye lengths equal to about 10% of the probe radius. A further analysis consists in calculating the force exerted by the flow on spherical dust. In short Debye length plasmas a strong drag component antiparallel to the convective electric field forms, causing the dust to spin faster than what predicted by its Larmor frequency. At intermediate and large Debye length the ion-drag in the direction of transverse flow is found to reverse in subsonic conditions, but the internal Laplace force appears to be positive, and larger in magnitude than the negative iondrag. 10/22/09 Leonardo Patacchini http://ilp-www.mit.edu/display_paper.a4d?paperId=84189 The Fusion Technology and Engineering Division (FTED) of the MIT Plasma Science and Fusion Center (PSFC) completes the second year of a five year cooperative agreement at the end of FY09. Our mission is to carry out a research program aimed at advancing the state of the art and development of magnet technology for fusion applications. This work is being carried out under funding from the Enabling Magnet Technology Program of the Virtual Laboratory for Technology (VLT). The Project is lead by Dr. Joseph V. Minervini (PI) and Dr. Makoto Takayasu (Co-PI). The work is being out in the research laboratories of the MIT Plasma Science and Fusion Center. The objectives of the program are to perform basic research in superconductor and magnet technology which 1) provides understanding of present state-of-the-art superconductors and magnet systems, and 2) to investigate new directions in applied superconductivity and use these, as appropriate, to advance the state-of-the-art in fusion magnet technology towards future application to DEMO and other advanced magnetic confinement devices. Although this MIT group has previously been involved in the design, analysis, magnet fabrication, and component R&D for ITER magnets since the inception of the ITER program in 1987, including CDA, EDA, and preparation for ITER (2003-2006) phases, our involvement in ITER Magnets has been completely eliminated since February 2008. The focus of the D&T-01 tasks was thus shifted to basic studies of magnet technology issues. Although these results can sometimes be directly applied to the present design of the ITER magnets and conductors, they are primarily oriented towards development of magnet technologies anticipated to be used in fusion applications in addition to, or beyond ITER. The studies performed here are intrinsically fundamental in nature, or in a very early technology development stage. We have selected technical topics that are well suited for study by an academic institution, but the solutions are always approached with a practical experience background of implementing new technology in working devices, and with an eye on eventual technology transfer to industry. 10/22/09 Joel Schultz http://ilp-www.mit.edu/display_paper.a4d?paperId=84188 The Fusion Technology and Engineering Division (FTED) of the MIT Plasma Science and Fusion Center (PSFC) is nearing the end of the first year of a five year cooperative agreement to carry out a research program aimed at advancing the state of the art and development of magnet technology for fusion applications. This work is being carried out under funding from the Enabling Magnet Technology Program of the Virtual Laboratory for Technology (VLT). The Project is lead by Dr. Joseph V. Minervini (PI) and Dr. Makoto Takayasu (Co- PI). The work is being out in the research laboratories of the MIT Plasma Science and Fusion Center. The objective of the program is to perform basic research in superconductor and magnet technology which provides understanding of present state-of-the-art superconductors and magnet systems, with the ultimate goal of advancing these technologies for use in future magnetic confinement fusion devices. In previous years this work was oriented towards R&D associated with ITER Magnets, but after the US ITER program was formalized into an official national project under the auspices of the US ITER Project Office (USIPO) the focus of the D&T-01 tasks was shifted to basic studies of magnet technology issues. Although these results can sometimes be directly applied to ITER magnet and conductor applications they are primarily oriented towards magnet technologies anticipated to be used in fusion applications in addition to, or beyond ITER. The studies performed here are intrinsically fundamental in nature, or in a very early technology development stage. We have selected technical topics which are well suited for study by an academic institution, but the solutions are always approached with a practical experience background of implementing new technology in working devices, and with an eye on eventual technology transfer to industry. The work reported here is primarily structured around the thesis topic work of our 3 present graduate students. These include Luisa Chiesa (cable transverse stress studies), Matteo Salvetti (fundamental analysis of strain effects in filaments), and Scott Mahar (development of a fiber-optic temperature and strain diagnostic). Chiesa and Mahar will finish their studies by early in the Fall 2008-2009 term and Matteo is expected to finish by Spring Term in 2009. 10/22/09 Joseph Minervini http://ilp-www.mit.edu/display_paper.a4d?paperId=84187 An electromagnetic bougienage method to lengthen esophageal segments of infants born with esophageal-atresia was developed thirty years ago by Hendren and Hale. In this treatment magnetic material bougie bullets were placed in the two ends of the esophagus. A magnetic field applied across the infant’s chest was used to attract the bougies to stretch the esophageal tissue. When the two esophageal segments were sufficiently close after one to two months of treatment, they could be surgically joined together. This approach has been performed successfully and confirmed to be feasible for infants whose esophageal separations are too wide for direct surgical repair. However, the technique was not further developed at that time. Recently Magplane Technology, Inc. has developed a next generation device for electromagnetic bougienage that meets modern hospital standards for safety and ease of use. In this report we present characteristics of a prototype bougienage magnet system that has been fabricated and tested. Two types of bullets, solid rounded rod bullet and hollow cylinder bullet for an electromagnetic bougienage have been developed. Measurements of magnetic forces acting on bougie bullets in the magnet field have been discussed along with calculation results of analytical methods developed to obtain the magnetic forces on the bougie. The calculation results were confirmed to agree with the experimental results. The analytical equations obtained can be used for an active bougienage force control system while in use during hospital operation. 10/22/09 Makoto Takayasu http://ilp-www.mit.edu/display_paper.a4d?paperId=84186 Cooperative binding of transcription factors (TFs) to promoter and other regulatory regions is essential for precise gene expression. The classical model of cooperativity requires direct interactions between TFs, thus constraining the arrangement of TFs sites in regulatory regions. Recent genomic and functional studies, however, demonstrate a great deal of flexibility in such arrangements with variable distances, numbers of sites, and identities of the involved TFs. Such flexibility is inconsistent with cooperativity by direct interactions between TFs. Here we demonstrate that strong cooperativity among noninteracting TFs can be achieved by their competition with nucleosomes. We find that the mechanism of nucleosome-mediated cooperativity is analogous to cooperativity in another multimolecular complex of hemoglobin. This surprising analogy provides deep insights, with parallels between heterotropic regulation of hemoglobin (e.g. Bohr effect) and roles of nucleosome-positioning sequences and chromatin modifications in gene expression. Nucleosome-mediated cooperativity is consistent with several experimental studies, allows substantial evolutionary flexibility in and modularity of regulatory regions, and provides a rationale for a broad range of genomic and evolutionary observations. Striking parallel between cooperativity in hemoglobin and in transcription regulation points to a general mechanism that may be used in various biological systems. 10/22/09 Leonid Mirny http://ilp-www.mit.edu/display_paper.a4d?paperId=84185 In this paper we propose a general framework to characterize and solve the optimization problems underlying a large class of sparsity based regularization algorithms. More precisely, we study the minimization of learning functionals that are sums of a differentiable data term and a convex non differentiable penalty. These latter penalties have recently become popular in machine learning since they allow to enforce various kinds of sparsity properties in the solution. Leveraging on the theory of Fenchel duality and subdifferential calculus, we derive explicit optimality conditions for the regularized solution and propose a general iterative projection algorithm whose convergence to the optimal solution can be proved. The generality of the framework is illustrated, considering several examples of regularization schemes, including `1 regularization (and several variants), multiple kernel learning and multi-task learning. Finally, some features of the proposed framework are empirically studied. 10/21/09 Lorenzo Rosasco http://ilp-www.mit.edu/display_paper.a4d?paperId=84184 To find interesting, personally relevant web content, we often rely on friends and colleagues to pass links along as they encounter them. In this paper, we study and augment link-sharing via e-mail, the most popular means of sharing web content today. Armed with survey data indicating that active sharers of novel web content are often those that actively seek it out, we present FeedMe, a plug-in for Google Reader that makes directed sharing of content a more salient part of the user experience. Our survey research indicates that sharing is moderated by concern about relevancy to the recipient, a desire to send only novel content to the recipient, and the effort required to share. FeedMe allays these concerns by recommending friends who may be interested in seeing the content, providing information on what the recipient has seen and how many emails they have received recently, and giving recipients the opportunity to provide lightweight feedback when they appreciate shared content. FeedMe introduces a novel design space for mixed-initiative social recommenders: friends who know the user voluntarily vet the material on the user’s behalf. We present a two week field experiment (N=60) demonstrating that FeedMe’s recommendations and social awareness features made it easier and more enjoyable to share content that recipients appreciated and would not have found otherwise. 10/21/09 David Karger http://ilp-www.mit.edu/display_paper.a4d?paperId=84183 Decentralized systems, such as distributed hash tables, are subject to the Sybil attack, in which an adversary creates many false identities to increase its influence. This paper proposes a routing protocol for a distributed hash table that is strongly resistant to the Sybil attack. This is the first solution to this problem with sublinear run time and space usage. The protocol uses the social connections between users to build routing tables that enable Sybil-resistant distributed hash table lookups.With a social network of n well-connected honest nodes, the protocol can tolerate up to O(n/ log n) “attack edges” (social links fromhonest users to phoney identities). This means that an adversary has to fool a large fraction of the honest users before any lookups will fail. 10/21/09 M Kaashoek http://ilp-www.mit.edu/display_paper.a4d?paperId=84182 Online, forward-search techniques have demonstrated promising results for solving problems in partially observable environments. These techniques depend on the ability to efficiently search and evaluate the set of beliefs reachable from the current belief. However, enumerating or sampling action-observation sequences to compute the reachable beliefs is computationally demanding; coupled with the need to satisfy real-time constraints, existing online solvers can only search to a limited depth. In this paper, we propose that policies can be generated directly from the distribution of the agent’s posterior belief. When the underlying state distribution is Gaussian, and the observation function is an exponential family distribution, we can calculate this distribution of beliefs without enumerating the possible observations. This property not only enables us to plan in problems with large observation spaces, but also allows us to search deeper by considering policies composed of multi-step action sequences. We present the Posterior Belief Distribution (PBD) algorithm, an efficient forward-search POMDP planner for continuous domains, demonstrating that better policies are generated when we can perform deeper forward search. 10/21/09 Nicholas Roy http://ilp-www.mit.edu/display_paper.a4d?paperId=84181 Most US consumers are charged a near-constant retail price for electricity, despite substan- tial hourly variation in the wholesale market price. This paper evaluates the Örst program to expose residential consumers to hourly real time pricing (RTP). I Önd that enrolled households are statistically signiÖcantly price elastic and that consumers responded by conserving energy during peak hours, but remarkably did not increase average consumption during o°Ë-peak times. Welfare analysis suggests that program households were not su¢ ciently price elastic to generate e¢ ciency gains that substantially outweigh the estimated costs of the advanced electricity me- ters required to observe hourly consumption. Although in electricity pricing, congestion pricing, and many other settings, economistsíintuition is that prices should be aligned with marginal costs, residential RTP may provide an important real-world example of a situation where this is not currently welfare-enhancing given contracting or information costs. 10/21/09 John Parsons