Results for "Nathaniel J. Fisch"

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Laser frequency upconversion in plasmas with finite ionization ratesMay 15 2019Laser frequency can be upconverted in a plasma undergoing ionization. For finite ionization rates, the laser pulse energy is partitioned into a pair of counter-propagating waves and static transverse currents. The wave amplitudes are determined by the ... More
Simulations of relativistic-quantum plasmas using real-time lattice scalar QEDFeb 02 2018Real-time lattice quantum electrodynamics (QED) provides a unique tool for simulating plasmas in the strong-field regime, where collective plasma scales are not well-separated from relativistic-quantum scales. As a toy model, we study scalar QED, which ... More
Centrifugal instability in the regime of fast rotationJul 04 2017Centrifugal instability, which stems from a difference between the azimuthal angular drift velocity of ions and electrons, is studied in the limit of fast rotation for which ions can rotate up to twice as fast as electrons. As the angular velocity approaches ... More
Compressing turbulence and sudden viscous dissipation with compression-dependent ionization stateMay 10 2016Jun 14 2016Turbulent plasma flow, amplified by rapid 3D compression, can be suddenly dissipated under continuing compression. This effect relies on the sensitivity of the plasma viscosity to the temperature, $\mu \sim T^{5/2}$. The plasma viscosity is also sensitive ... More
Creating localized plasma wave by ionization of doped semiconductorsMar 21 2019Localized plasma waves can be generated by suddenly ionizing extrinsic semiconductors with spatially periodic dopant densities. The built-in electrostatic potentials at the metallurgical junctions, combined with electron density ripples, offer the exact ... More
Multi-frequency Raman amplifiersFeb 24 2018In its usual implementation, the Raman amplifier features only one pump carrier frequency. However, pulses with well-separated frequencies can also be Raman amplified while compressed in time. Amplification with frequency-separated pumps is shown to hold ... More
Laser Pulse Sharpening with Electromagnetically Induced Transparency in PlasmaMar 21 2017Jul 19 2017We propose a laser-controlled plasma shutter technique to generate sharp laser pulses using a process analogous to electromagnetically-induced transparency in atoms. The shutter is controlled by a laser with moderately strong intensity, which induces ... More
Practicality of magnetic compression for plasma density controlJan 26 2016Plasma densification through magnetic compression has been suggested for time-resolved control of the wave properties in plasma-based accelerators. Using particle in cell simulations with real mass ratio, the practicality of large magnetic compression ... More
Modeling turbulent energy behavior and sudden viscous dissipation in compressing plasma turbulenceOct 15 2017Feb 19 2018We present a simple model for the turbulent kinetic energy behavior of subsonic plasma turbulence undergoing isotropic three-dimensional compression, such as may exist in various inertial confinement fusion experiments or astrophysical settings. The plasma ... More
A lower bound on adiabatic heating of compressed turbulence for simulation and model validationNov 30 2016Apr 09 2017The energy in turbulent flow can be amplified by compression, when the compression occurs on a timescale shorter than the turbulent dissipation time. This mechanism may play a part in sustaining turbulence in various astrophysical systems, including molecular ... More
Reducing parametric backscattering by polarization rotationJun 27 2016Sep 20 2016When a laser passes through underdense plasmas, Raman and Brillouin Backscattering can reflect a substantial portion of the incident laser energy. This is a major loss mechanism, for example, in employing lasers in inertial confinement fusion. However, ... More
Amplification of mid-infrared lasers via magnetized-plasma couplingApr 11 2019Plasmas may be used as gain media for amplifying intense lasers, and external magnetic fields may be applied to improve the performance. For mid-infrared lasers, the requisite magnetic field is on megagauss scale, which can already be provided by current ... More
A lower bound on adiabatic heating of compressed turbulence for simulation and model validationNov 30 2016The energy in turbulent flow can be amplified by compression, when the compression occurs on a timescale shorter than the turbulent dissipation time. This mechanism may play a part in sustaining turbulence in various astrophysical systems, including molecular ... More
Compressing turbulence and sudden viscous dissipation with compression-dependent ionization stateMay 10 2016Oct 27 2016Turbulent plasma flow, amplified by rapid 3D compression, can be suddenly dissipated under continuing compression. This effect relies on the sensitivity of the plasma viscosity to the temperature, $\mu \sim T^{5/2}$. The plasma viscosity is also sensitive ... More
Ignition threshold for non-Maxwellian plasmasSep 08 2015Sep 09 2015An optically thin $p$-$^{11}$B plasma loses more energy to bremsstrahlung than it gains from fusion reactions, unless the ion temperature can be elevated above the electron temperature. In thermal plasmas, the temperature differences required are possible ... More
Plasma Wave Seed for Raman AmplifiersDec 19 2016Apr 10 2017It is proposed to replace the traditional counterpropagating laser seed in backward Raman amplifiers with a plasma wave seed. In the linear regime, namely, for a constant pump amplitude, a plasma wave seed may be found by construction that strictly produces ... More
Anisotropy-driven collisional separation of impurities in magnetized compressing and expanding cylindrical plasmasSep 07 2018When a cylindrically-symmetric magnetized plasma compresses or expands, velocity-space anisotropy is naturally generated as a result of the different adiabatic conservation laws parallel and perpendicular to the magnetic field. When the compression timescale ... More
Alpha Channeling in a Rotating PlasmaAug 28 2008The wave-particle alpha-channeling effect is generalized to include rotating plasma. Specifically, radio frequency waves can resonate with alpha particles in a mirror machine with ExB rotation to diffuse the alpha particles along constrained paths in ... More
Efficiency of Current Drive by Fast WavesJan 12 2005The Rosenbluth form for the collision operator for a weakly relativistic plasma is derived. The formalism adopted by Antonsen and Chu can then be used to calculate the efficiency of current drive by fast waves in a relativistic plasma. Accurate numerical ... More
Cumulative displacement induced by a magnetosonic soliton bouncing in a bounded plasma slabFeb 15 2018The passage of a magnetosonic (MS) soliton in a cold plasma leads to the displacement of charged particles in the direction of a compressive pulse and in the opposite direction of a rarefaction pulse. In the overdense plasma limit, the displacement induced ... More
Effective action approach to wave propagation in scalar QED plasmasMar 12 2016A relativistic quantum field theory with nontrivial background fields is developed and applied to study waves in plasmas. The effective action of the electromagnetic 4-potential is calculated ab initio from the standard action of scalar QED using path ... More
Plasma q-plate for generation and manipulation of intense optical vorticesSep 22 2017Nov 30 2017An optical vortex is a light wave with a twisting wavefront around its propagation axis and null intensity in the beam center. Its unique spatial structure of field lends itself to a broad range of applications, including optical communication, quantum ... More
Radiative transfer dynamo effectSep 21 2017Magnetic fields in rotating and radiating astrophysical plasma can be produced due to a radiative interaction between plasma layers moving relative to each other. The efficiency of current drive, and with it the associated dynamo effect, is considered ... More
Favorable Collisional Demixing of Ash and Fuel in Magnetized Inertial FusionMay 18 2018Sep 18 2018Magnetized inertial fusion experiments are approaching regimes where the radial transport is dominated by collisions between magnetized ions, providing an opportunity to exploit effects usually associated with steady-state magnetic fusion. In particular, ... More
Laser Pulse Compression Using Magnetized PlasmasDec 29 2016Proposals to reach the next generation of laser intensities through Raman or Brillouin backscattering have centered on optical frequencies. Higher frequencies are beyond the range of such methods mainly due to the wave damping that accompanies the higher ... More
On the role of magnetosonic solitons in perpendicular collisionless shock reformationFeb 28 2017The nature of the magnetic structure arising from ion specular reflection in shock compression studies is examined by means of 1d particle in cell simulations. Propagation speed, field profiles and supporting currents for this magnetic structure are shown ... More
Radiation in equilibrium with plasma and plasma effects on cosmic microwave backgroundMar 13 2019The spectrum of the radiation of a body in equilibrium is given by Planck's law. In plasma, however, waves below the plasma frequency cannot propagate, consequently, the equilibrium radiation inside plasma is necessary different from the Planck spectrum. ... More
Current in Wave Driven PlasmasJan 12 2005A theory for the generation of current in a toroidal plasma by radio-frequency waves is presented. The effect of an opposing electric field is included, allowing the case of time varying currents to be studied. The key quantities that characterize this ... More
Inverse Bremsstrahlung current driveSep 21 2017The generation of the plasma current resulting from Bremsstrahlung absorption is considered. It is shown that the electric current is higher than the naive estimates assuming that electrons absorb only the photon momentum and using the Spitzer conductivity ... More
Current limiting effects on laser compression by resonant backward Raman scatteringJan 05 2011Through resonant backward Raman scattering, the plasma wave mediates the energy transfer between long pump and short seed laser pulses. These mediation can result in pulse compression at extraordinarily high powers. However, both the overall efficiency ... More
Laser-plasma interaction in magnetized environmentDec 01 2017Propagation and scattering of lasers present new phenomena and applications when the plasma medium becomes strongly magnetized. With mega-Gauss magnetic fields, scattering of optical lasers already becomes manifestly anisotropic. Special angles exist ... More
Three-wave scattering in magnetized plasmas: from cold fluid to quantized LagrangianMay 27 2017Large amplitude waves in magnetized plasmas, generated either by external pumps or internal instabilities, can scatter via three-waves interactions. While three-wave scatterings in either forward or backward geometry are well-known, what happens when ... More
Resonant Interactions Along the Critical Line of the Riemann Zeta FunctionSep 18 2013Nov 11 2013We have studied some properties of the special Gram points of the Riemann zeta function which lie on contour lines ${\bf Im}(\zeta ( s )) = 0$ which do not contain zeroes of $\zeta ( s )$. We find that certain functions of these points, which all lie ... More
Bulk hydrodynamic stability and turbulent saturation in compressing hot spotsFeb 19 2018For hot spots compressed at constant velocity, we give a hydrodynamic stability criterion that describes the expected energy behavior of non-radial hydrodynamic motion for different classes of trajectories (in $\rho R$ --- $T$ space). For a given compression ... More
Alpha Channeling with High-field Launch of Lower Hybrid WavesAug 10 2015Oct 02 2015Although lower hybrid waves are effective at driving currents in present-day tokamaks, they are expected to interact strongly with high-energy particles in extrapolating to reactors. In the presence of a radial alpha particle birth gradient, this interaction ... More
Do pulsars rotate clockwise or counterclockwise?Mar 04 2019Pulsars are rotating neutron stars which emit lighthouse-like beams. Owing to their unique properties, pulsars are a unique astrophysical tool to test general relativity, inform on matter at extreme densities, and probe galactic magnetic fields. Understanding ... More
Maximal energy extraction under discrete diffusive exchangeAug 14 2015Waves propagating through a bounded plasma can rearrange the densities of states in the six-dimensional velocity-configuration phase space. Depending on the rearrangement, the wave energy can either increase or decrease, with the difference taken up by ... More
Coupling of alpha-channeling to parallel wavenumber upshift in lower hybrid current driveAug 29 2014Feb 08 2016Although lower hybrid waves have been shown to be effective in driving plasma current in present-day tokamaks, they are predicted to strongly interact with the energetic alpha particles born from fusion reactions in eventual tokamak reactors. However, ... More
Ladder Climbing and Autoresonant Acceleration of Plasma WavesApr 01 2015Jul 21 2015When the background density in a bounded plasma is modulated in time, discrete modes become coupled. Interestingly, for appropriately chosen modulations, the average plasmon energy might be made to grow in a ladder-like manner, achieving up-conversion ... More
Green's function for rf-driven current in a toroidal plasmaFeb 01 2004The Green's function for rf-driven currents in a toroidal plasma is calculated in the long mean-free-path limit.
X-Ray Amplification by Stimulated Brillouin ScatteringMay 24 2017Jul 23 2017Plasma-based parametric amplification using stimulated Brillouin scattering offers a route to coherent x-ray pulses orders-of-magnitude more intense than those of the brightest available sources. Brillouin amplification permits amplification of shorter ... More
Opportunities for plasma separation techniques in rare earth elements recyclingOct 12 2017Rare earth elements recycling has been proposed to alleviate supply risks and market volatility. In this context, the potential of a new recycling pathway, namely plasma mass separation, is uncovered through the example of nedodymium - iron - boron magnets ... More
Plasma filtering techniques for nuclear waste remediationApr 27 2015Nuclear waste cleanup is challenged by the handling of feed stocks that are both unknown and complex. Plasma filtering, operating on dissociated elements, offers advantages over chemical methods in processing such wastes. The costs incurred by plasma ... More
Theory of electromagnetic wave frequency upconversion in dynamic mediaApr 19 2018Aug 07 2018Frequency upconversion of an electromagnetic wave can occur in ionized plasma with decreasing electric permittivity and in split-ring resonator-structure metamaterials with decreasing magnetic permeability. We develop a general theory to describe the ... More
Beyond nonlinear saturation of backward Raman amplifiersJan 21 2016Backward Raman amplification is limited by relativistic nonlinear dephasing resulting in saturation of the leading spike of the amplified pulse. Pump detuning is employed to mitigate the relativistic phase mismatch and to overcome the associated saturation. ... More
Strongly Enhanced Stimulated Brillouin Backscattering in an Electron-Positron PlasmaDec 02 2015Jan 07 2016Stimulated Brillouin backscattering of light is shown to be drastically enhanced in electron-positron plasmas, in contrast to the suppression of stimulated Raman scattering. A generalized theory of three-wave coupling between electromagnetic and plasma ... More
Thermal to Visible Synthesis of Face Images using Multiple RegionsMar 20 2018Synthesis of visible spectrum faces from thermal facial imagery is a promising approach for heterogeneous face recognition; enabling existing face recognition software trained on visible imagery to be leveraged, and allowing human analysts to verify cross-spectrum ... More
Harnessing mass differential confinement effects in magnetized rotating plasmas to address new separation needsJul 03 2017The ability to separate large volumes of mixed species based on atomic mass appears desirable for a variety of emerging applications with high societal impact. One possibility to meet this objective consists in leveraging mass differential effects in ... More
What is the fate of runaway positrons in tokamaks?Apr 17 2015Massive runaway positrons are generated by runaway electrons in tokamaks. The fate of these positrons encodes valuable information about the runaway dynamics. The phase space dynamics of a runaway position is investigated using a Lagrangian that incorporates ... More
Statistical Mechanics of an Optical Phase Space CompressorFeb 10 2005We describe the statistical mechanics of a new method to produce very cold atoms or molecules. The method results from trapping a gas in a potential well, and sweeping through the well a semi-permeable barrier, one that allows particles to leave but not ... More
Short-Pulse Amplification by Strongly-Coupled Stimulated Brillouin ScatteringJul 01 2016Sep 07 2016We examine the feasibility of strongly-coupled stimulated Brillouin scattering as a mechanism for the plasma-based amplification of sub-picosecond pulses. In particular, we use fluid theory and particle-in-cell simulations to compare the relative advantages ... More
Laser Amplification in Strongly-Magnetized PlasmaDec 22 2018We consider backscattering of laser pulses in strongly-magnetized plasma mediated by kinetic magnetohydrodynamic waves. Magnetized low-frequency scattering, which can occur when the external magnetic field is neither perpendicular nor parallel to the ... More
Pseudoholomorphic quilts with figure eight singularityOct 14 2014We show that the novel figure eight singularity in a pseudoholomorphic quilt can be continuously removed when composition of Lagrangian correspondences is cleanly immersed. The proof of this result requires a collection of width-independent elliptic estimates ... More
Supercharacter theories of type $A$ unipotent radicals and unipotent polytopesDec 10 2015Even with the introduction of supercharacter theories, the representation theory of many unipotent groups remains mysterious. This paper constructs a family of supercharacter theories for normal pattern groups in a way that exhibit many of the combinatorial ... More
Analysis and Probability on Infinite-Dimensional SpacesJul 13 2016Sep 07 2016These lecture notes contain an introduction to some of the fundamental ideas and results in analysis and probability on infinite-dimensional spaces, mainly Gaussian measures on Banach spaces. They originated as the notes for a topics course at Cornell ... More
Accurate closed-form trajectories of light around a Kerr black hole using asymptotic approximantsJun 29 2018Oct 02 2018Highly accurate closed-form expressions that describe the full trajectory of photons propagating in the equatorial plane of a Kerr black hole are obtained using asymptotic approximants. This work extends a prior study of the overall bending angle for ... More
Polarimetric Thermal to Visible Face Verification via Self-Attention Guided SynthesisApr 15 2019Polarimetric thermal to visible face verification entails matching two images that contain significant domain differences. Several recent approaches have attempted to synthesize visible faces from thermal images for cross-modal matching. In this paper, ... More
Threshold-Range Scaling of Excitable Cellular AutomataApr 02 1993Each cell of a two-dimensional lattice is painted one of k colors, arranged in a "color wheel." The colors advance (0 to k-1 mod k) either automatically or by contact with at least a threshold number of successor colors in a prescribed local neighborhood. ... More
Enhanced Conductivity Along Lateral Homojunction Interfaces of Atomically Thin SemiconductorsDec 12 2016Energy band realignment at the interfaces between materials in heterostructures can give rise to unique electronic characteristics and non-trivial low-dimensional charge states. In a homojunction of monolayer and multilayer MoS$_2$, the thickness-dependent ... More
Metastability in the Greenberg-Hastings ModelMar 23 1993The Greenberg-Hastings Model (GHM) is a family of multitype cellular automata that emulate excitable media, exhibiting the nucleation and spiral formation characteristic of such complex systems. In this paper we study the asymptotic frequency of nucleation ... More
Synthesis of High-Quality Visible Faces from Polarimetric Thermal Faces using Generative Adversarial NetworksDec 12 2018The large domain discrepancy between faces captured in polarimetric (or conventional) thermal and visible domain makes cross-domain face verification a highly challenging problem for human examiners as well as computer vision algorithms. Previous approaches ... More
The combinatorics of $\mathrm{GL}_n$ generalized Gelfand--Graev charactersMay 04 2015Introduced by Kawanaka in order to find the unipotent representations of finite groups of Lie type, generalized Gelfand--Graev characters have remained somewhat mysterious. Even in the case of the finite general linear groups, the combinatorics of their ... More
Nonzero coefficients in restrictions and tensor products of supercharacters of $U_n(q)$Dec 09 2009The standard supercharacter theory of the finite unipotent upper-triangular matrices $U_n(q)$ gives rise to a beautiful combinatorics based on set partitions. As with the representation theory of the symmetric group, embeddings of $U_m(q)\subseteq U_n(q)$ ... More
On Yoccoz Return FunctionsAug 23 2009We study the dynamics of complex polynomials. We obtain results on Poincare return maps defined on certain neighborhoods of a point with bounded orbit under a polynomial. We introduce a generalization of the Yoccoz tau-function, the Yoccoz return function, ... More
Reading Wikipedia to Answer Open-Domain QuestionsMar 31 2017Apr 28 2017This paper proposes to tackle open- domain question answering using Wikipedia as the unique knowledge source: the answer to any factoid question is a text span in a Wikipedia article. This task of machine reading at scale combines the challenges of document ... More
Sudden viscous dissipation of compressing turbulenceOct 28 2015Feb 23 2016Compression of turbulent plasma can amplify the turbulent kinetic energy, if the compression is fast compared to the viscous dissipation time of the turbulent eddies. A sudden viscous dissipation mechanism is demonstrated, whereby this amplified turbulent ... More
Excitation of Large-ktheta Ion-Berstein Waves in TokamaksFeb 28 1995The mode-converted ion-Bernstein wave excited in tokamaks is shown to exhibit certain very interesting behavior, including the attainment of very small poloidal phase velocities, the reversal of poloidal direction, and up-down asymmetries in propagation ... More
Cough Detection Using Hidden Markov ModelsApr 28 2019Respiratory infections and chronic respiratory diseases impose a heavy health burden worldwide. Coughing is one of the most common symptoms of many such infections, and can be indicative of flare-ups of chronic respiratory diseases. Whether at a clinical ... More
Axiomatic geometrical optics, Abraham-Minkowski controversy, and photon properties derived classicallyMar 27 2012Nov 21 2012By restating geometrical optics within the field-theoretical approach, the classical concept of a photon (and, more generally, any elementary excitation) in arbitrary dispersive medium is introduced, and photon properties are calculated unambiguously. ... More
Rescuing Massive Photons from the SwamplandOct 12 2018Stringent Swampland conjectures aimed at effective theories containing massive abelian vectors have recently been proposed (arXiv:1808.09966), with striking phenomenological implications. In this article, we show how effective theories that parametrically ... More
Adiabatic nonlinear waves with trapped particles: II. Wave dispersionJul 15 2011Nov 12 2011A general nonlinear dispersion relation is derived in a nondifferential form for an adiabatic sinusoidal Langmuir wave in collisionless plasma, allowing for an arbitrary distribution of trapped electrons. The linear dielectric function is generalized, ... More
Vlasov equation and collisionless hydrodynamics adapted to curved spacetimeJun 18 2010Sep 16 2010The modification of the Vlasov equation, in its standard form describing a charged particle distribution in the six-dimensional phase space, is derived explicitly within a formal Hamiltonian approach for arbitrarily curved spacetime. The equation accounts ... More
On the nature of kinetic electrostatic electron nonlinear (KEEN) wavesFeb 25 2014An analytical theory is proposed for the kinetic electrostatic electron nonlinear (KEEN) waves originally found in simulations by Afeyan et al [arXiv:1210.8105]. We suggest that KEEN waves represent saturated states of the negative mass instability (NMI) ... More
Non-equilibrium piezo-thermal effect in spinning gasApr 01 2016A spinning gas, heated adiabatically through axial compression, is known to exhibit a rotation-dependent heat capacity. However, as equilibrium is approached, a new effect is identified here wherein the temperature does not grow homogeneously in the radial ... More
Are perytons signatures of ball lightning?May 15 2014Aug 18 2014The enigmatic downchirped signals, called "perytons", that are detected by radio telescopes in the GHz frequency range may be produced by an atmospheric phenomenon known as ball lightning (BL). If BLs act as nonstationary radiofrequency cavities, their ... More
Ponderomotive forces on waves in modulated mediaFeb 24 2014Nonlinear interactions of waves via instantaneous cross-phase modulation can be cast in the same way as ponderomotive wave-particle interactions in high-frequency electromagnetic field. The ponderomotive effect arises when rays of a probe wave scatter ... More
Positive and negative effective mass of classical particles in oscillatory and static fieldsNov 28 2007A classical particle oscillating in an arbitrary high-frequency or static field effectively exhibits a modified rest mass m_eff derived from the particle averaged Lagrangian. Relativistic ponderomotive and diamagnetic forces, as well as magnetic drifts, ... More
Diagnostic Setup for Characterization of Near-Anode Processes in Hall ThrustersOct 16 2003A diagnostic setup for characterization of near-anode processes in Hall-current plasma thrusters consisting of biased and emissive electrostatic probes, high-precision positioning system and low-noise electronic circuitry was developed and tested. Experimental ... More
Adiabatic nonlinear waves with trapped particles: I. General formalismJul 15 2011Nov 12 2011A Lagrangian formalism is developed for a general nondissipative quasiperiodic nonlinear wave with trapped particles in collisionless plasma. The adiabatic time-averaged Lagrangian density $\mcc{L}$ is expressed in terms of the single-particle oscillation-center ... More
Adiabatic nonlinear waves with trapped particles: III. Wave dynamicsJul 15 2011Nov 12 2011The evolution of adiabatic waves with autoresonant trapped particles is described within the Lagrangian model developed in Paper I, under the assumption that the action distribution of these particles is conserved, and, in particular, that their number ... More
Dressed-particle approach in the nonrelativistic classical limitNov 12 2008For a nonrelativistic classical particle undergoing arbitrary oscillations, the generalized effective potential Y is derived from nonlinear eigenfrequencies of the particle-field system. Specifically, the ponderomotive potential is extended to a nonlinear ... More
Surfatron acceleration along magnetic field by oblique electrostatic wavesNov 20 2011Charged particles can be accelerated via surfatron mechanism along dc magnetic field by obliquely propagating electrostatic waves. In plasma, this mechanism can, in principle, produce an average parallel current, even when the wave frequency is much larger ... More
Nonlinear dispersion of stationary waves in collisionless plasmasMay 01 2011A nonlinear dispersion of a general stationary wave in collisionless plasma is obtained in a non-differential form from a single-particle oscillation-center Hamiltonian. For electrostatic oscillations in nonmagnetized plasma, considered as a paradigmatic ... More
Extended propagation of powerful laser pulses in focusing Kerr mediaJun 01 2016Powerful incoherent laser pulses can propagate in focusing Kerr media much longer distances than can coherent pulses, due to the fast phase mixing that prevents transverse filamentation. This distance is limited by 4-wave scattering, which accumulates ... More
On Convergence to Essential SingularitiesJan 05 2018An iterative optimization method applied to a function $f$ on $\mathbb{R}^n$ will produce a sequence of arguments $\{\mathbf{x}_k\}_{k \in \mathbb{N}}$; this sequence is often constrained such that $\{f(\mathbf{x}_k)\}_{k \in \mathbb{N}}$ is monotonic. ... More
Hypoelliptic heat kernels on infinite-dimensional Heisenberg groupsOct 30 2013Dec 12 2013We study the law of a hypoelliptic Brownian motion on an infinite-dimensional Heisenberg group based on an abstract Wiener space. We show that the endpoint distribution, which can be seen as a heat kernel measure, is absolutely continuous with respect ... More
Short-term heterologous immunity after severe influenza A outbreaksJul 18 2010Conventional wisdom holds that influenza A and B are such genetically dissimilar viruses that infection with one cannot confer cross-immunity to the other. However, our examination of the records of the past 25 influenza seasons in the U.S. reveals that ... More
Hopf monoids from class functions on unitriangular matricesMar 07 2012Oct 16 2012We build, from the collection of all groups of unitriangular matrices, Hopf monoids in Joyal's category of species. Such structure is carried by the collection of class function spaces on those groups, and also by the collection of superclass function ... More
Key-Value Memory Networks for Directly Reading DocumentsJun 09 2016Directly reading documents and being able to answer questions from them is a key problem. To avoid its inherent difficulty, question answering (QA) has been directed towards using Knowledge Bases (KBs) instead, which has proven effective. Unfortunately ... More
Gravitational Waves from Supersymmetry BreakingFeb 11 2009In theories of supersymmetry breaking, it is often the case that there is more than one metastable vacuum. First-order phase transitions among such metastable vacua may generate a stochastic background of gravitational waves, the observation of which ... More
A Language for Planning with StatisticsMar 20 2013When a planner must decide whether it has enough evidence to make a decision based on probability, it faces the sample size problem. Current planners using probabilities need not deal with this problem because they do not generate their probabilities ... More
Outer space for untwisted automorphisms of right-angled Artin groupsDec 19 2012Mar 31 2016For a right-angled Artin group $A_\Gamma$, the untwisted outer automorphism group $U(A_\Gamma)$ is the subgroup of $Out(A_\Gamma)$ generated by all of the Laurence-Servatius generators except twists (where a {\em twist} is an automorphisms of the form ... More
The Weak Scale from Weak GravityApr 17 2019We explore the prospects for bounding the weak scale using the weak gravity conjecture (WGC), addressing the hierarchy problem by violating the expectations of effective field theory. Building on earlier work by Cheung and Remmen, we construct models ... More
Transchromatic generalized character mapsOct 14 2011Sep 07 2012In "Generalized Group Characters and Complex Oriented Cohomology Theories", Hopkins, Kuhn, and Ravenel develop a way to study cohomology rings of the form E^*(BG) in terms of a character map. The character map can be interpreted as a map of cohomology ... More
ISS-flationJan 14 2008Inflation may occur while rolling into the metastable supersymmetry-breaking vacuum of massive supersymmetric QCD. We explore the range of parameters in which slow-roll inflation and long-lived metastable supersymmetry breaking may be simultaneously realized. ... More
Trapped-Electron Runaway EffectApr 01 2015In a tokamak, trapped electrons subject to a strong electric field cannot run away immediately, because their parallel velocity does not increase over a bounce period. However, they do pinch towards the tokamak center. As they pinch towards the center, ... More
A Search for Laser Emission with Megawatt Thresholds from 5600 FGKM StarsApr 08 2017We searched high resolution spectra of 5600 nearby stars for emission lines that are both inconsistent with a natural origin and unresolved spatially, as would be expected from extraterrestrial optical lasers. The spectra were obtained with the Keck 10-meter ... More
Strategies for Advantageous Differential Transport of Ions in Magnetic Fusion DevicesJan 19 2018In a variety of magnetized plasma geometries, it has long been known that highly charged impurities tend to accumulate in regions of higher density. Here, we examine how this "collisional pinch" changes in the presence of additional forces, such as might ... More
Disassembling the Clockwork MechanismApr 25 2017Aug 07 2017The clockwork mechanism is a means of naturally generating exponential hierarchies in theories without significant hierarchies among fundamental parameters. We emphasize the role of interactions in the clockwork mechanism, demonstrating that clockwork ... More
Laser-Plasma Interactions Enabled by Emerging TechnologiesApr 30 2019An overview from the past and an outlook for the future of fundamental laser-plasma interactions research enabled by emerging laser systems.