Next seminars

Séminaires des laboratoires de la DR4

CPHT's Seminars:

Séminaire de Physique des Particules
CPHT-Ecole Polytechnique et Groupe Théorie IPN Orsay

Jeudi 28 juin 2018 à 11h
CPHT, Ecole Polytechnique, Salle de Conférence Louis Michel (Bât. 6)

Matthias Burkardt
(New Mexico State University)

Nucleon tomography

Generalized parton distributions embody information on both the longitudinal momentum of the quarks as well as their transverse position and thus allow determining a 3-dimensional image of the nucleon. For transversely polarized quarks and or nucleons, the resulting transverse deformation of these images provides new insight on nucleon spin structure and spin-orbit correlations. Twist-3 GPDs provide additional information on the dependence of transverse forces on the impact parameter.


Bi-séminaire CORDES

Jeudi 7 juin 2018
CPHT, Ecole Polytechnique, Salle de Conférence Louis Michel (Bât. 6)

10:30 Amir Kashani-Poor (LPTENS, Paris)

Topological strings and 6d SCFTs

The topological string is a simplified version of physical string theory. It is of interest because it computes the BPS spectrum of relevant string theory compactifications, but also because it shares structural properties of physical string theory. Dualities and symmetries which often must be argued for arduously in the physical string can often be verified by computation in the topological setting. The central observable of the theory is the topological string partition function Z_top. This quantity has an eerie habit of making surprise appearances in many areas of mathematical physics. Numerous techniques exist for its computation in various expansions in parameters of the theory, yet to date, no satisfactory closed form for this quantity is known. In this talk, after reviewing notions of topological string theory with an emphasis on the interplay between worldsheet and target space physics (one of the structural similarities between the physical and the topological string alluded to above), I will report on progress in computing Z_top in settings where it is related to enigmatic 6d theories.


11:45 Rob Leigh (University of Illinois, Urbanna) 

String Zero Modes, T-duality, Vertex States and Entanglement

In this talk, I will touch on a variety of topics related to the notion of edge modes. I’ll discuss their relevance in simple string theory settings, leading to an interpretation in terms of non-commutative geometry. This will lead into a discussion of topology change in Lorentzian quantum gravity, and relations to quantum entanglement.


Séminaire de Physique des Particules
CPhT-Ecole Polytechnique et Groupe Théorie IPN Orsay
Mardi 20 février 2018 à 11h00
CPhT, Ecole Polytechnique, Salle de Conférence Louis Michel (Bât. 6)
Kasper Larsen
(Southampton U.)
Algebraic geometry applied to multi-loop scattering amplitudes/span>
The computation of two-loop amplitudes forms a current bottleneck to computing precisionlevel cross sections for LHC phenomenology. In this talk I will discuss several new methods for analytic evaluation of two-loop amplitudes, drawing inspiration from algebraic geometry and modern unitarity. More specifically, the methods involve efficient determination of a basis of integrals in terms of which the amplitude is decomposed; derivation of the integral identities needed to perform the decomposition; and evaluation of the basis integrals via differential equations. The first step of determining a basis of integrals has been implemented in the publicly available code Azurite which I will discuss. I will also discuss future implementations of the methods and their application to computing the two-loop five-gluon QCD amplitud


Journée des thèses du CPHT

Mardi 6 février 2018 à partir de 10h30

CPHT, Ecole Polytechnique, Salle de Conférence Louis Michel



Séminaire de Physique des Particules
CPHT-Ecole Polytechnique et Groupe Théorie IPN Orsay
Vendredi 15 décembre 2017 à 14h00
CPHT, Ecole Polytechnique, Salle de Conférence Louis Michel
Cédric Mezrag
(INFN. Roma)
Exploring the nucleon structure
Understanding the emergence of the nucleon structures in terms of QCD degrees of freedom, i.e. quarks and gluons, is one of the major challenges of hadron physics. Two main types of non-perturbative techniques have been developed to tackle this challenge, discretised ones using large computer simulations (LQCD), and continuum ones relying on Dyson-Schwinger equations (DSEs). The latter suggest for instance that dynamical diquarks are generated inside the nucleon, creating eective degrees of freedom. Since both LQCD and DSEs techniques can be used in order to compute local matrix elements related to the structures of the Nucleon through for instance Distributions Amplitudes (DAs), we will explore in this talk the possibility of understanding the nucleon structure in terms of dynamical degrees of freedom.


PhD Defense at CPHT: 

Alain Marx
(Magnetized Plasma)

"Two major steps for XTOR code: enhanced parallelization and free boundary geometry"

Thursday, November  23th at 2:00pm, Room conference "Curie", Ecole Polytechnique

Abstract: The XTOR-2F code simulates the 3D dynamics of full bi-fluid MHD instabilities in tokamak plasmas. The first part of the thesis was dedicated to the parallelisation of XTOR-2F code. The code has been parallelised significantly despite the numerical profile of the problem solved, i.e. a discretisation with pseudo-spectral representations in all angular directions, the stiffness of the two-fluid stability problem in tokamaks, and the use of a direct LU decomposition to invert the physical pre-conditioner. The execution time of the parallelised version is an order of magnitude smaller than the sequential one for low-resolution cases, with an increasing speedup when the discretisation mesh is refined. Moreover, it allows to perform simulations with higher resolutions, previously forbidden because of memory limitations. The second part of the thesis was dedicated to the development of free boundary condition. The original fixed boundary computational domain of the code was generalised to a free-boundary one, thus approaching closely the geometry of today’s and future large experimental devices. The initial conditions are given by the CHEASE equilibrium code inside the plasma. Outside the plasma, fitting the magnetic potential at the CHEASE computation domain boundary with a set of external poloidal magnetic coils extends the solution. The boundary conditions use Green functions to construct a response matrix matching the normal and tangential components of the outside magnetic field with the inside solution. A thin resistive wall can be added to the computational domain. This new numerical setup generalises the investigation field from internal MHD instabilities towards external instabilities. The code linear behaviour is validated with two families of instabilities, n = 0 axisymmetric modes and n = 1 / m = 2 external kinks. In order to validate the nonlinear behaviour, nonlinear resistive MHD simulations of tearing modes at zero beta evolving to a stationary state have been performed.


PhD Defense at CPHT: 

Alexander Efremov
(Mathematical Physics)

"Renormalization of SU(2) Yang-Mills theory with flow equations"

Wednesday, September  27th at 3:00pm, Room conference, CPHT, Building 6

Abstract: The problem of perturbative renormalization of SU(2) Yang-Mills theory is studied in four dimensional Euclidean space with Lorenz gauge fixing. The analysis is based on the renormalization group flow equations. This is a unified approach which permits to study a large class of field theories without recourse to Feynman diagrams. It is remarkable that the flow equations allow us to construct all vertex functions of the theory using only the renormalization conditions. An important part of the work consists in establishing upper bounds in momentum space on all vertex functions at all loop orders. These bounds have a very natural graphical interpretation in form of trees. In order to properly define the vertex functions one must introduce ultraviolet and infrared cut-offs. But this regularization breaks BRST invariance. Thus it is essential to prove at all loop orders that the construction can be accomplished in such a way that BRST invariance is restored when the UV cut-off goes to infinity. Furthermore, substantial effort is made to provide a physical renormalization scheme which is explicitly independent of the flow parameter.

Séminaire de Physique des Particules
CPHT-Ecole Polytechnique et Groupe Théorie IPN Orsay
Jeudi 28 Septembre 2017
CPHT, Ecole Polytechnique, Salle de Conférence du bât. 6
Rainer Stiele
(Lyon, IPN et Frnakfurt U.)
Phase structure and thermodynamics of strongly-interacting matter
Currently, large effort is undertaken, experimentally in heavy ion collisions and theoretically doing simulations on supercomputers, to explore the state of matter under the extreme conditions of the largest temperatures and densities in the universe. Effective models, such as Polyakov-loop-extended chiral models, which capture the main properties of the strong interaction, i.e. the creation of con- stituent quark masses and connement, play the role of an important guidance that allow access at all temperature and density domains. Results of such a model, the Polyakov-loop-extended Quark-Meson truncation of QCD, for the phase diagram of strongly-interacting matter are presented and it will be discussed how constraints from the high-energy domain (lattice calculations) as well as low energy domain (astrophysics measurements on compact stars) can contribute to pin it down more precisely. The importance of including the quark backreaction on the gauge eld dynamics is demonstrated and its impact on the structure of the phase diagram and on the surface tension for nucleation in a rst order transition region discussed. Finally, the necessity to improve the description of the gauge part of the strong interaction with phenomenological Polyakov-loop potentials will be motivated and current ways to improve will be outlined.

PhD Defense at CPHT: 

Pascal Delange
(Condensed Matter)

"Many-electron effects in transition metal and rare earth compounds: Electronic structure, magnetic properties and point defects from first principles"

Friday, september 29th 11:00 AM, Amphi Becquerel, Ecole Polytechnique

Abstrac : The structural, spectral, magnetic, and many other properties of materials are mostly determined by the behavior of the electrons that glue the atoms together.

Some of the most important technological advances of the 20th century were made in the field of semi-conductors, where electrons behave like a dilute gas of weakly interacting particles. In contrast, many of the most promising materials for technological applications today are oxides. In transition metal or rare earth oxides, but also in some pure metals or alloys, the Coulomb repulsion between the electrons can give rise to exotic properties and phase transitions.

In this thesis, we developed and applied first-principles (i.e. based on fundamental laws of physics) methods to evaluate the behavior of electrons interacting with each other and the physical properties stemming from it. We use these tools to study several materials of fundamental and technological interest, in particular iron, vanadium dioxide and hard magnets.


Soutenance de thèse au CPHT :

Pierre Chopin 
(Equipe Plasmas magnétisés)

"Modélisation à grande échelle pour les phénomènes éruptifs"

Vendredi 29 septembre, 14h, Amphi Bécquerel, Ecole Polytechnique

Résumé de la thèse : Cette thèse a pour objet la modélisation du champ magnétique de la couronne solaire à l’aide du code de reconstruction non-linéaire XTRAPOLS, avec une attention particulière pour les phénomènes éruptifs. Le caractère novateur des études menées porte sur l’aspect sphérique global de la méthode.
Trois études principales de cas sont présentées dans cette thèse. La première concerne les évènements éruptifs de février 2011, géoeffectif, faisant figurer une région active étendue. Nous mettons en évidence plusieurs structures de tubes de flux torsadés, et caractérisons leur lien avec les structures à grande échelle. La deuxième concerne les évènements du 3 et 4 août 2011. Plusieurs régions actives sont présentes sur le disque solaire, et deux d’entre elles présentent une activité éruptive importante. Là encore, nous mettons en évidence des tubes de flux torsadés dans chacune de ces deux régions active, et mettons en lumière les liens topologiques qui existent entre elles. La Troisième concerne une étude faite dans le cadre d’un groupe NLFFF, pour l’étude de la modélisation non- linéaire globale de la couronne. La date correspondant à la reconstruction est celle de l’éclipse totale de soleil du 20 mars 2015. Nous discutons ici de l’impact de différents types de données et de modèles utilisés, et soulignons l’importance de la cohérence temporelle et de l’inclusion du courant dans les régions actives.
Les travaux présentés dans cette thèse ont donc permis de caractériser l’en- vironnement global des régions actives éruptives et d’étudier les liens entre les éléments à différentes échelles. Nous présentons en guise d’ouverture différente méthode pour étendre la modélisation au-delà de la surface source.

Colloquium CPHT - Ecole polytechnique
Mardi 12 septembre à 14h
CPHT, Ecole Polytechnique, Salle de Conférence du bât. 6

Eric Akkermans, Universite Technion, Israel
(bureau au CPhT, aile zero 10.14)

Observing a Scale Anomaly in Graphene : a Universal Quantum PhaseTransition

Abstract :
Scale invariance is a common property of our everyday environment. Its breaking gives rise to less common but beautiful structures like fractals. At the quantum level, breaking of continuous scale invariance is a remarkable exemple of quantum phase transition also known as scale anomaly. The general features of this transition will be presented at an elementary quantum mechanics level. Then, we will show recent experimental evidence of this transition in graphene.

PhD Defense at CPHT: 
The defence will be held in English  Friday, September 8th at 02:00pm at EcolePolytechnique in amphi Becquerel.

Loïc Herviou

"Topological phases and Majorana fermions"

Abstract of the thesis:

In this thesis, we study theoretically different aspects of topological systems. These models present resilient properties due to a non-trivial topology of their band structures, and in particular exotic edge excitations such as Majorana fermions. Entanglement markers have been fundamental to the study of these systems and of gapless systems in general, but are challenging to measure. Bipartite charge fluctuations were proposed as a weak measurement of entanglement entropy. We extend results on standard Luttinger Liquids to generic families of one- and two-dimensional non-interacting topological systems. A volume law arises, and is linked to the Quantum Fisher information, with  non-analyticities at the phase transitions. Critical points are characterized by universal coefficients that reveal the topological aspect of the transitions.
In a second time, we include interactions and show that some of these signatures are preserved in interacting topological superconductors. Through analytical and numerical methods, we study  two Coulomb-coupled topological superconducting wires. The interplay between unconventional superconductivity and strong interactions leads to exotic phases. We show the appearance of orbital currents spontaneously breaking the time-reversal symmetry, and of an unusual gaplessphase that is the extension of two critical Majorana modes.
Finally, we focus on electronic transport mediated by Majorana fermions. We study a floating superconducting island carrying several such impurities, a potential building block for a quantum computer. The Majorana fermions affect the statistics of the charge carriers, which leads to very resilient fractionalized transport. We extend previous studies to the charge degenerate case and map it to the Multi-Channel Kondo model at large interaction, reinterpreted in terms of a particle moving in a high-dimensional, dissipative lattice.

This work was done both at Centre de Physique Théorique and at LaboratoirePierre Aigrain (ENS) under the joint supervision of Karyn Le Hur and Christophe Mora.

Séminaire de Physique des Particules

CPHT-Ecole Polytechnique et Groupe Théorie IPN Orsay

Jeudi 6 juillet 2017
CPHT, Ecole Polytechnique, Salle de Conférence du bât. 6

Bo-Wen Xiao
(CCNU, Wuhan and Hua-Zhong Normal U.)


Inclusive dijet productions in pp and AA collisions

In this talk, I will rst discuss the systematic calculation of dijet angular correlation and dijet asymmetries in pp collisions. Then, using the above pp results as a baseline, we can further put in the quark-gluon plasma medium effect and extract the corresponding medium transport coefficient at RHIC and the LHC. The dijet processes can serve as direct probes of QGP medium at NLO accuracy and play important roles at RHIC and the LHC. In the end, I will also discuss the further development of jet physics in heavy ion physics.

Séance de présentation des stages de master

Mercredi 21 juin 2017

Ecole Polytechnique, CPHT, Aile zéro, Salle de Conférence

Le laboratoire organise une séance de présentation des stages de master le mercredi 21 juin, à 14h, dans la salle de conférence de l’aile 0. Chaque étudiant disposera de dix minutes pour présenter son sujet de stage de manière non technique et compréhensible par des non spécialistes. Cette séance sera suivie d’une collation vers 16h30.

Programme :

Lorentz invariance and n-independence

- B. DAGALLIER (J.-R. Chazottes)
Quasicrystals: properties and theoretical application

- J. DOUÇOT (M. Petropoulos)
The Hawking effect and the black hole information problem

- C. FLEMING et T. COUDARCHET (H. Partouche)
Quantum stability of flat spacetime: a superstring point of view

- G. GIACALONE (C. Marquet)
Phenomenology of strong interaction at high energy

- M. HUMBERT (M. Petropoulos)
Holographic reconstruction in higher dimensions

- T. LESAFFRE (S. Billard)
Influence du régime de reproduction sur l'évolution de l'investissement reproducteur chez les Angiospermes

- A. LI (C. Lorcé)
Angular momentum in the scalar diquark model

- S. POEGEL (G. Bossard)
Hidden Symmetries in Supergravity

- C. SHI (G. Bossard)
Gauge and gravitational anomalies in Standard Model

- F. YANG (Karyn Le Hur)
Topological Phases and Majorana Fermions

The Tan contact for 1D Bose gases

Séminaire de Physique des Particules

CPHT-Ecole Polytechnique et Groupe Théorie IPN Orsay

Jeudi 22 juin2017

IPN, Orsay, Bât. 100, Salle A201

Shayan Ghosh
(Indian Institute of Science, Bangalore)


Analytic solutions to massive two-loop sunset diagrams,
and applications to Chiral Perturbation Theory

In this talk, we begin by showing how it is possible to obtain full analytic results for the two-loop three distinct mass scale sunset diagrams that appear in the expressions for the masses and decay constants of the pion, kaon and eta particles in two-loop SU(3) chiral perturbation theory. We then talk about two applications of these results: a) how it allows one to obtain small quark mass ratios to arbitrarily high powers, and b) how it allows one to make ts to lattice data to extract greater information about low energy constants. We then change track and discuss in more detail the Mellin- Barnes method used in our calculations, and demonstrate how this method can be extended using analytic continuation to obtain results for Feynman diagrams whose mass value parameters are such that it is impossible to use a direct calculation. We show how such methods of analytic continuation can also be helpful in contexts where a direct calculation does yield results, and thus set the stage for more widespread application of this technique.


Séminaire de Physique des Particules

CPHT-Ecole Polytechnique et Groupe Théorie IPN Orsay

Vendredi 28 avril 2017

(Attention : Jour inhabituel!)

Ecole Polytechnique, CPHT, bât. 6, Salle de Conférence

Zsolt Szép
(Eötvös University, Budapest)

Thermodynamics of the vector meson extended quark-meson model

We investigate the effects of (axial)vector mesons on the chiral phase transition of the model containing 2+1 constituent quarks and Polyakov-loop degrees of freedom. A x2 minimization procedure is used to parameterize the model based on tree-level decay widths and vacuum scalar and pseudoscalar curvature masses which includes the vacuum and thermal contribution of the constituent quarks. The pressure and the derived thermodynamical observables determined from it at nite temperature T and chemical potential using a simple approximation for the grand potential are compared to lattice results. The best parametrization of the model allows for the existence of the critical end point (CEP) of the T phase diagram.


Colloquium du CPHT, Ecole Polytechnique
Mercredi 15 mars 2017 à 15h00

Salle de séminaire du CPHT, bâtiment 6

Sylvain Carrozza
(Perimeter Institute, Waterloo Canada)


Quantum fields with tensorial locality

Abstract: In recent years, generalizations of matrix models known as Tensor Models and Group Field Theories have been developed into a consistent formalism. The common feature of these field theories is an abstract notion of locality, known as tensorial locality, which encodes the combinatorial structure of the elementary field interactions. It has initially been introduced in the context of quantum gravity, where indeed the absence of a non-dynamical background space-time renders the standard notion of locality inoperative. I will provide an overview of this approach, focusing on general features of the phase diagrams of tensorial theories, and of their possible applications to quantum gravity and statistical physics. I will also discuss a new range of applications at the crossroad between condensed matter and the AdS/CFT correspondence: following a recent insight from Witten, tensorial techniques are being taken advantage of to investigate the properties of so-called Sachdev-Ye-Kitaev models of randomly interacting fermions.


Colloquium du CPHT, Ecole Polytechnique
Vendredi 10 mars 2017 à 14h

Salle de séminaire du CPHT, bâtiment 6

Leonardo Mazza
(Ecole Normale Supérieure, Paris)


Towards the quantum-Hall-effect with ultra-cold atoms and synthetic dimensions

Abstract: Synthetic ladders pierced by a magnetic field realized with one-dimensional alkaline-earth(-like) gases represent a promising environment for the investigation of many-body quantum physics with ultracold atoms. A fundamental question is whether these setups can give access to pristine two-dimensional phenomena, such as the fractional quantum Hall effect, and how. I discuss the existence of a hierarchy of fractional insulating and conducting states by means of both analytical techniques (bosonization) and numerical methods (density-matrix renormalization group algorithm). I show that such states can be exploited for constructing a topological Thouless pump where the charge transported after one cycle is quantized to fractional values and demonstrate this behavior with a full many-body time-dependent calculation. I conclude by presenting numerical signatures of bosonic and fermionic Laughlin-like states that can be observed in these setups.

1 - S. Barbarino, L. Taddia, D. Rossini, L. Mazza, R. Fazio, Nat. Commun. 6, 8134 (2015)
2 - S. Barbarino, L. Taddia, D. Rossini, L. Mazza, R. Fazio, New J. Phys. 18, 035010 (2016)
3 - L. Taddia, E. Cornfeld, D. Rossini, L. Mazza, E. Sela, R. Fazio, arXiv:1607.07842 (2016)
4 - M. Calvanese Strinati, E. Cornfeld, D. Rossini, S. Barbarino, M. Dalmonte, R. Fazio, E. Sela, L. Mazza, arXiv:1612.06682 (2016)


Colloquium du CPHT, Ecole Polytechnique
Vendredi 3 mars 2017 à 11h

Salle de séminaire du CPHT, bâtiment 6

Charles Grenier
(Ecole Normale Supérieure de Lyon)

Mesoscopic transport with ultracold atoms

Abstract: The last years have seen the emergence of a new trend in cold atoms experiments towards the simulation of quantum transport.
In this talk, I will show how one can use cold gases to simulate mesoscopic devices, and how adding a cold atom flavour to a transport
setup can help pushing further our understanding of low dimensional structures. In particular here, I will focus on the evolution of particle
and spin conductances as functions of the interaction strength.

S. Krinner et al., Proceedings of the National Academy of Sciences 113 (29), 8144-8149


Séminaire de Physique des Particules

CPHT-Ecole Polytechnique et Groupe Théorie IPN Orsay

Mardi 28 février 2017 à 11h00
(Attention : Jour inhabituel!)

Ecole Polytechnique, CPhT, bât. 6, Salle de Conférence

Daniel Kroff
(IFT-Unesp, Sao Paulo)

The Linde problem on R2 x S1 x S1

Thermal eld theory provides the natural framework to describe the thermodynamic properties and to study phase transitions of systems describe by quantum eld theories, in particular, the quark-gluon plasma. However, its perturbative realization faces important technical difficulties whenever massless bosons are considered, due to divergences in the IR sector.

After briefly reviewing these IR divergences in thermal eld theory, and especially the Linde problem, we analyze the IR behavior of Yang-Mills theory in a Torus -- R2 x S1 x S1 -- and show that, in this geometry, any perturbative calculation is expected to break-down already at O(g2).


Colloquium du CPHT, Ecole Polytechnique
Jeudi 23 février 2017 à 11h

Salle de séminaire du CPHT, bâtiment 6

Jamal Jalilian-Marian
"Visiting Professor"  au Centre de Physique Théorique,
programme "Jean d'Alembert" de l'Université Paris-Saclay

An introduction to high energy scattering in QCD


After a brief introduction to Quantum ChromoDynamics (QCD), the fundamental theory of strong interactions, we consider scattering of hadrons (protons and neutrons) and nuclei at very high energies. It is shown that scattering cross sections at high energy are dominated by components of the hadron/nucleus wave function that contain a large number of soft partons (gluons and sea quarks carrying a small fraction x of the hadron energy). An effective action, known as Color Glass Condensate, is proposed that describes the dynamics of this many body system of gluons at small x. Applications to particle production at the Large Hadron Collider (LHC), Relativistic Heavy Ion Collider (RHIC) and the proposed Electron Ion Collider (EIC) will be discussed in detail and connections to other branches of physics, cosmic rays, high energy neutrinos, Quark Gluon Plasma and statistical physics will be elucidated.


Colloquium du CPHT, Ecole Polytechnique
Jeudi 2 février 2017 à 11h

Salle de séminaire du CPHT, bâtiment 6

Jeffrey Brown
Université de Louisiane, Baton Rouge et
Centre de Physique Théorique, Ecole polytechnique  

Modeling ultrashort laser pulses in gases


The interaction of short, intense (10^14 W/cm^2) laser pulses with atomic and molecular gases produces a wide range of interesting nonlinear optical phenomena.  Accurately modeling how microscopic quantum processes give rise to macroscopic optical properties in a computationally feasible manner is challenging. For determining which "ingredients" are necessary in a model to capture the most dominant quantum processes, we investigated the utility of exactly solvable,
quantum-based laser-matter interaction models. From this new understanding, we have developed a novel laser-matter interaction model based on Stark resonant states, which provides an accurate and computationally efficient method of representing the nonlinear response of a gaseous medium.

Colloquium du CPHT, Ecole Polytechnique
Jeudi 12 janvier 2017 à 11h

Salle de séminaire du CPHT, bâtiment 6

Timothée Nicolas SPC-EPFL, Lausanne, Suisse

  MHD instabilities : from tokamaks to stellarators


We will explore different aspects of magnetohydrodynamic (MHD) instabilities in tokamaks and stellarators. Both devices require a helical magnetic field to efficiently confine a hot fusion plasma in a torus. External coils generate a main field of the order of 2-4 T. Tokamaks generate the second component of the field using a large electric current in the plasma itself. Stellarators however generate all the field using only external coils. This gives rise to different kinds of instabilities in both devices, mainly current driven in tokamaks, mainly pressure driven (Rayleigh-Taylor like) in stellarators. We will review some theoretical issues related to the MHD stability in both tokamaks and stellarators. We will see why they are crucial in practice, in view of future experiments such as ITER. We will also try to give a taste of the numerical challenge faced by whoever tries to simulate the complex nonlinear behavior of such plasmas.