## Wolfgang Pauli Institute (WPI) Vienna

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## Talks of the past month

 Alexander Lorz (KAUST and Université Pierre et Marie Curie) HS 13, 2nd floor of Fak.Mathematik Uni Wien Sat, 29. Jul 17, 16:20 Mathematics meets oncology: from Adaptive evolution to Zebrafish Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 James Greene (Rutgers University) HS 13, 2nd floor of Fak.Mathematik Uni Wien Sat, 29. Jul 17, 15:40 The role of induced drug resistance in cancer chemotherapy Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Lisa Gabler (Medical University, Vienna) HS 13, 2nd floor of Fak.Mathematik Uni Wien Sat, 29. Jul 17, 15:10 Co-expression network-based identification of molecular subtypes in cancer Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 John King (University of Nottingham) HS 13, 2nd floor of Fak.Mathematik Uni Wien Sat, 29. Jul 17, 14:10 Mathematical modeling of biological tissue growth Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Dominik Wodarz (University of California, Irvine) HS 13, 2nd floor of Fak.Mathematik Uni Wien Sat, 29. Jul 17, 13:30 Oncolytic virus therapy: Dynamics of virus spread at low infection multiplicities Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Min Tang (Shanghai Jiao Tong University) HS 13, 2nd floor of Fak.Mathematik Uni Wien Sat, 29. Jul 17, 11:10 The role of intracellular pathways on the E.coli population dynamics Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Maria Lukácová-Medvidová (University of Mainz) HS 13, 2nd floor of Fak.Mathematik Uni Wien Sat, 29. Jul 17, 10:30 Mathematical and numerical modelling of cancer invasion Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Dana-Adriana Botesteanu (University of Maryland) HS 13, 2nd floor of Fak.Mathematik Uni Wien Sat, 29. Jul 17, 9:40 Modeling cancer cell growth dynamics in vitro in response to antimitotics Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Christoph Bock (Center for Molecular Medicine, Vienna) HS 13, 2nd floor of Fak.Mathematik Uni Wien Sat, 29. Jul 17, 9:00 Bioinformatics for personalized medicine: Looking beyond the genome Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Bernhard Englinger (Medical University, Vienna) HS 13, 2nd floor of Fak.Mathematik Uni Wien Fri, 28. Jul 17, 17:00 Mathematical models to predict intracellular drug distribution – Do they work? Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Michael Breitenbach (University of Salzburg) HS 13, 2nd floor of Fak.Mathematik Uni Wien Fri, 28. Jul 17, 16:10 The human NADPH oxidase, Nox4, its S. cerevisiae ortholog, Yno1, and its role in regulating the actin cytoskeleton Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Natalia Komarova (University of California, Irvine) HS 13, 2nd floor of Fak.Mathematik Uni Wien Fri, 28. Jul 17, 15:30 Stochastic Calculus of Stem Cells Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Thomas Mohr (Medical University, Vienna) HS 13, 2nd floor of Fak.Mathematik Uni Wien Fri, 28. Jul 17, 14:40 Deciphering gene co-expression networks in tumor endothelium Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Michael Speicher (Medical University, Graz) HS 13, 2nd floor of Fak.Mathematik Uni Wien Fri, 28. Jul 17, 14:00 Inferring expressed genes by whole-genome sequencing of plasma DNA Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Heyrim Cho (University of Maryland) HS 13, 2nd floor of Fak.Mathematik Uni Wien Fri, 28. Jul 17, 11:30 Modeling the chemotherapy-induced selection of drug-resistant traits during tumor growth Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Anna Marciniak-Czochra (University of Heidelberg) HS 13, 2nd floor of Fak.Mathematik Uni Wien Fri, 28. Jul 17, 10:50 Mathematical Modeling of Clonal Dynamics in Acute Leukemias Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Michael Medvedev (Kansas) WPI, OMP 1, Seminar Room 08.135 Fri, 28. Jul 17, 10:00 Quasi-nonlinear theory of the Weibel instability Astrophysical and high-energy-density laboratory plasmas often have large-amplitude, sub-Larmor-scale electromagnetic fluctuations excited by various kinetic-streaming or anisotropy-driven instabilities. The Weibel (or the filamentation) instability is particularly important because it can rapidly generate strong magnetic fields, even in the absence of seed fields. Particles propagating in collisionless plasmas with such small-scale magnetic fields undergo stochastic deflections similar to Coulomb collisions, with the magnetic pitch-angle diffusion coefficient representing the effective "collision" frequency. We show that this effect of the plasma "quasi-collisionality" can strongly affect the growth rate and evolution of the Weibel instability in the deeply nonlinear regime. This result is especially important for understanding cosmic-ray-driven turbulence in an upstream region of a collisionless shock of a gamma-ray burst or a supernova. We demonstrate that the quasi-collisions caused by the fields generated in the upstream suppress the instability slightly but can never shut it down completely. This confirms the assumptions made in the self-similar model of the collisionless foreshock. Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Michael Bergmann (Medical University, Vienna) HS 13, 2nd floor of Fak.Mathematik Uni Wien Fri, 28. Jul 17, 9:50 The understanding of the DNA damage response in solid tumors and the development of oncolytic influenza viruses Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Benoit Perthame (Université Pierre et Marie Curie) HS 13, 2nd floor of Fak.Mathematik Uni Wien Fri, 28. Jul 17, 9:10 Modeling of living tissues and free boundary asymptotics Thematic program: Mathematical models in Biology and Medicine (2017/2018) Event: Workshop on "Mathematical Methods in Biology and Medicine" (2017)

 Denis St-Onge (Princeton) WPI, OMP 1, Seminar Room 08.135 Thu, 27. Jul 17, 16:00 Plasma dynamo Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Dmitri Uzdensky (UC Boulder) WPI, OMP 1, Seminar Room 08.135 Thu, 27. Jul 17, 10:30 Nonthermal particle acceleration in relativistic collisionless magnetic reconnection As a fundamental process converting magnetic to plasma energy in high-energy astrophysical plasmas, relativistic magnetic reconnection is a leading explanation for the acceleration of particles to the ultrarelativistic energies necessary to power nonthermal emission (especially X-rays and gamma-rays) in pulsar magnetospheres and pulsar wind nebulae, coronae and jets of accreting black holes, and gamma-ray bursts. An important objective of plasma astrophysics is therefore the characterization of nonthermal particle acceleration (NTPA) effected by reconnection. Reconnection-powered NTPA has been demonstrated over a wide range of physical conditions using large two-dimensional (2D) kinetic simulations. However, its robustness in realistic 3D reconnection -- in particular, whether the 3D relativistic drift-kink instability (RDKI) disrupts NTPA -- has not been systematically investigated, although pioneering 3D simulations have observed NTPA in isolated cases. Here we present the first comprehensive study of NTPA in 3D relativistic reconnection in collisionless electron-positron plasmas, characterizing NTPA as the strength of 3D effects is varied systematically via the length in the third dimension and the strength of the guide magnetic field. We find that, while the RDKI prominently perturbs 3D reconnecting current sheets, it does not suppress particle acceleration, even for zero guide field; fully 3D reconnection robustly and efficiently produces nonthermal power-law particle spectra closely resembling those obtained in 2D. This finding provides strong support for reconnection as the key mechanism powering high-energy flares in various astrophysical systems. We also show that strong guide fields significantly inhibit NTPA, slowing reconnection and limiting the energy available for plasma energization, yielding steeper and shorter power-law spectra. Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Vladimir Zhdankin (UC Boulder) WPI, OMP 1, Seminar Room 08.135 Thu, 27. Jul 17, 10:00 Particle acceleration in relativistic kinetic turbulence We present results from particle-in-cell simulations of driven turbulence in magnetized, collisionless, and relativistic pair plasmas. We find that the fluctuations are consistent with the classical k −5/3 ¡Ñ magnetic energy spectrum at fluid scales and a steeper k −4 ¡Ñ spectrum at sub-Larmor scales, where k¡Ñ is the wave vector perpendicular to the mean field. We demonstrate the development of a nonthermal, power-law particle energy distribution f(E)¡­E−¥á, with an index ¥á that decreases with increasing magnetization and increases with an increasing system size (relative to the characteristic Larmor radius). Our simulations indicate that turbulence can be a viable source of energetic particles in high-energy astrophysical systems, such as pulsar wind nebulae, if scalings asymptotically become insensitive to the system size. Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Jonathan Squire (Caltech) WPI, OMP 1, Seminar Room 08.135 Wed, 26. Jul 17, 16:00 Resonant instabilities: dust-gas coupling and others? It is shown that grains streaming through a fluid are generically unstable if their velocity, projected along some direction, matches the phase velocity of a fluid wave. This can occur whenever grains stream faster than a fluid wave. The wave itself can be quite general--sound waves, magnetosonic waves, epicyclic oscillations, and Brunt-V\"ais\"al\"a oscillations each generate instabilities, for example. A simple expression for this "resonant drag instability" (RDI) growth rate is derived. This expression (i) illustrates why such instabilities are so virulent and generic, and (ii) allows for simple analytic computation of RDI growth rates and properties for different fluid systems. As examples, we introduce several new instabilities, which could see application across a variety of astrophysical systems from protoplanetary disks to galactic outflows. Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Archie Bott (Oxford) WPI, OMP 1, Seminar Room 08.135 Wed, 26. Jul 17, 10:00 When are plasmas collisional? Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Nuno Loureiro (MIT) WPI, OMP 1, Seminar Room 08.135 Tue, 25. Jul 17, 16:00 Fully-kinetic versus reduced-kinetic modelling of collisionless plasma turbulence Pulsed-power driven magnetic reconnection experiments We report the results of a direct comparison between different kinetic models of collisionless plasma turbulence in two spatial dimensions. The models considered include a first principles fully-kinetic (FK) description, two widely used reduced models [gyrokinetic (GK) and hybrid-kinetic (HK) with fluid electrons], and a novel reduced gyrokinetic approach (KREHM). Two different ion beta (â i ) regimes are considered: 0.1 and 0.5. For â i =0.5 , good agreement between the GK and FK models is found at scales ranging from the ion to the electron gyroradius, thus providing firm evidence for a kinetic Alfv'en cascade scenario. In the same range, the HK model produces shallower spectral slopes, presumably due to the lack of electron Landau damping. For â i =0.1 , a detailed analysis of spectral ratios reveals a slight disagreement between the GK and FK descriptions at kinetic scales, even though kinetic Alfv'en fluctuations likely still play a significant role. The discrepancy can be traced back to scales above the ion gyroradius, where the FK and HK results seem to suggest the presence of fast magnetosonic and ion Bernstein modes in both plasma beta regimes, but with a more notable deviation from GK in the low-beta case. The identified practical limits and strengths of reduced-kinetic approximations, compared here against the fully-kinetic model on a case-by-case basis, may provide valuable insight into the main kinetic effects at play in turbulent collisionless plasmas, such as the solar wind. Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Francois Rincon (Toulouse) WPI, OMP 1, Seminar Room 08.135 Tue, 25. Jul 17, 10:00 Some thoughts on theoretical problems and appoaches in dynamo theory Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Nuno Loureiro (MIT) WPI, OMP 1, Seminar Room 08.135 Mon, 24. Jul 17, 16:45 MHD turbulence + magnetic reconnection The current understanding of magnetohydrodynamic (MHD) turbulence envisions turbulent eddies which are anisotropic in all three directions. In the plane perpendicular to the local mean magnetic field, this implies that such eddies become current-sheetlike structures at small scales. We analyze the role of magnetic reconnection in these structures and conclude that reconnection becomes important at a scale ¥ë¡­LS −4/7L, where SL is the outer-scale (L) Lundquist number and ¥ë is the smallest of the field-perpendicular eddy dimensions. This scale is larger than the scale set by the resistive diffusion of eddies, therefore implying a fundamentally different route to energy dissipation than that predicted by the Kolmogorov-like phenomenology. In particular, our analysis predicts the existence of the subinertial, reconnection interval of MHD turbulence, with the estimated scaling of the Fourier energy spectrum E(k¡Ñ)¡ðk−5/2¡Ñ, where k¡Ñ is the wave number perpendicular to the local mean magnetic field. The same calculation is also performed for high (perpendicular) magnetic Prandtl number plasmas (Pm), where the reconnection scale is found to be ¥ë/L¡­S−4/7LPm−2/7. Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Alex Schekochihin (Oxford) WPI, OMP 1, Seminar Room 08.135 Mon, 24. Jul 17, 16:00 MHD turbulence in 2017: end of the road? ++kinetic extensions Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Yohei Kawazura (Oxford) WPI, OMP 1, Seminar Room 08.135 Mon, 24. Jul 17, 10:30 Hybrid GK-isothermal electrons code + ion heating calculations Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Lev Arzamasskiy (Princeton) WPI, OMP 1, Seminar Room 08.135 Mon, 24. Jul 17, 10:00 Hybrid-kinetic simulations of solar wind turbulence Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 David Hatch (UT Austin) WPI, OMP 1, Seminar Room 08.135 Thu, 20. Jul 17, 16:00 Flow Shear Suppression of Pedestal Turbulence--A First Principles Theoretical Framework A combined analytic and computational gyrokinetic approach is developed to address the question of the scaling of pedestal turbulent transport with arbitrary levels of E×B shear. Due to strong gradients and shaping in the pedestal, the instabilities of interest are not curvature-driven like the core instabilities. By extensive numerical (gyrokinetic) simulations, it is demonstrated that pedestal modes respond to shear suppression very much like the predictions of a basic analytic decorrelation theory. The quantitative agreement between the two provides us with a new dependable, first principles (physics based) theoretical framework to predict the efficacy of shear suppression in burning plasmas that lie in a low-shear regime not accessed by present experiments. Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Denis St-Onge (Princeton) WPI, OMP 1, Seminar Room 08.135 Wed, 19. Jul 17, 16:30 The Dimits Shift in a One-Field Fluid Model The two-dimensional Terry-Horton equation is shown to exhibit the Dimits shift when suitably modified to capture both the nonlinear enhancement of zonal/drift-wave interactions and the existence of residual Rosenbluth-Hinton states. This phenomena persists through numerous simplifications of the equation, including a quasilinear approximation as well as a four-mode truncation. Analytic progress on the truncated system is reported, focused on determining the growth rates of zonal flows and calculating the upper bound of the Dimits shift. The results for the truncated system are then used to estimate the Dimits shift of the fully nonlinear system. A new understanding is thus developed on the fundamental nature of the Dimits shift, both on its operation and its eventual termination. Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Justin Ball (EPFLausanne) WPI, OMP 1, Seminar Room 08.135 Wed, 19. Jul 17, 10:00 Optimized up-down asymmetry to drive fast intrinsic rotation in tokamaks Breaking the up-down symmetry of the tokamak poloidal cross-section can significantly increase the spontaneous rotation due to turbulent momentum transport. In this work, we optimize the shape of flux surfaces with both tilted elongation and tilted triangularity in order to maximize this drive of intrinsic rotation. Nonlinear gyrokinetic simulations demonstrate that adding optimally-tilted triangularity can double the momentum transport of a tilted elliptical shape. This work indicates that tilting the elongation and triangularity in an ITER-like device can reduce the energy transport and drive intrinsic rotation with an Alfv\'{e}n Mach number on the order of 1% . This rotation is four times larger than the rotation expected in ITER and is sufficient to stabilize MHD instabilities. It is shown that this optimal shape can be created using the shaping coils of several experiments. Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Alessandro Geraldini (Oxford) WPI, OMP 1, Seminar Room 08.135 Tue, 18. Jul 17, 16:00 Gyrokinetic treatment of a grazing angle magnetic presheath We develop a gyrokinetic treatment for ions in the magnetic presheath, close to the plasma-wall boundary. We focus on magnetic presheaths with a small magnetic field to wall angle, $\alpha \ll 1$ (in radians). Characteristic lengths perpendicular to the wall in such a magnetic presheath scale with the typical ion Larmor orbit size, ${\rho }_{{\rm{i}}}$. The smallest scale length associated with variations parallel to the wall is taken to be across the magnetic field, and ordered $l={\rho }_{{\rm{i}}}/\delta$, where $\delta \ll 1$ is assumed. The scale lengths along the magnetic field line are assumed so long that variations associated with this direction are neglected. These orderings are consistent with what we expect close to the divertor target of a tokamak. We allow for a strong component of the electric field ${\bf{E}}$ in the direction normal to the electron repelling wall, with strong variation in the same direction. The large change of the electric field over an ion Larmor radius distorts the orbit so that it is not circular. We solve for the lowest order orbits by identifying coordinates, which consist of constants of integration, an adiabatic invariant and a gyrophase, associated with periodic ion motion in the system with $\alpha =\delta =0$. By using these new coordinates as variables in the limit $\alpha \sim \delta \ll 1$, we obtain a generalised ion gyrokinetic equation. We find another quantity that is conserved to first order and use this to simplify the gyrokinetic equation, solving it in the case of a collisionless magnetic presheath. Assuming a Boltzmann response for the electrons, a form of the quasineutrality equation that exploits the change of variables is derived. The gyrokinetic and quasineutrality equations give the ion distribution function and electrostatic potential in the magnetic presheath if the entrance boundary condition is specified. Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)

 Silvia Espinosa (MIT) WPI, OMP 1, Seminar Room 08.135 Tue, 18. Jul 17, 10:00 Pedestal radial flux measuring method to prevent impurity accumulation The use of high-z wall materials attempts to shift the fusion challenge from heat handling to impurity removal. We demonstrate that not only the impurity density in-out asymmetry but also the poloidal flow has a major impact on the radial impurity flux direction. This realization provides the first method of measuring the flux from available diagnostics, without the need of a computationally demanding kinetic calculation of the full bulk ion response. Moreover, it affords insight into optimal tokamak operation to avoid impurity accumulation while allowing free fueling. Thematic program: Models in Plasmas, Earth and Space Science (2017/2018) Event: 10th Plasma Kinetics Working Group Meeting (2017)