| If
you know exactly what you are going to do, what is the point of doing
it? Pablo
Picasso Every line in your calculations ends with "=0". You are not making much progress! Kate Hammett We don't do the calculation because we don't know the answer, we do it because we have a conscience. Bryan Taylor Let's change the resolution on the Unknown. Gabe Plunk Let me put it differently: suppose we had a reactor... Per Helander Working together takes more than one person. Bill Dorland We can't do ultraviolence to that square bracket. Ian Abel Field lines are forever. Bryan Taylor This is not really an ill-posed problem, this is not a problem at all. Paul Dellar I am just doing mathematics at the moment, but it is mathematics that has some point. Bryan Taylor The equivalent of God in MAST is MHD, which is global and all powerful. Anthony Field I've got a fishbone coming up. Tim Horbury We have crossed all the i's and dotted all the t's. Job's a good'un. Ian Abel Let us not jump in front of the bandwagon! Anonymous It's not the triviality, it's the emptiness of it that bothers me. Bryan Taylor This is a very inefficient way of achieving nothing. François Rincon We Hankel all the way in and then Hankel all the way out. Joseph Parker For the sake of fairness, let me mention that some people have objections to this, none of which are particularly valid. Chris Chen |
It is
better to be roughly right than precisely wrong. Maynard
Keynes Yes, Steve, you have always known what we have only just figured out. Alfred Mallet This is more or less right. Perhaps less. Alfred Mallet You go ahead with your argument. I'll think. Steve Cowley Too simple? At the end of my talk, you will beg for simplicity! Andrey Beresnyak The point is not the point! Anonymous This is why I am presenting this here and nowhere else: nothing is solved! Ian Abel This calculation is not intellectual masturbation: there's no orgasm. Anonymous There are many big Buts here... I like big Buts. Anonymous Previous studies of this problem have been either unsatisfactory or limited in scope.---What is the difference between these?---This means either wrong or irrelevant. Anatoly Spitkovsky, Michael Medvedev and Steve Cowley (in conversation) I don't give a damn about astrophysics, explain to me what is going on. Anatoly Spitkovsky Have you subtracted the baby with the bathwater? Steve Balbus This is not rigour, unless you mean rigor mortis. Ian Abel I wouldn't say these are theoretical arguments... Let's just call them arguments. François Rincon You can never be too happy with the state of your closure. Amitava Bhattacharjee For this plot, 1 is 6.--- Weeell, not even. Steve Cowley and Steve Balbus Это качественное объяснение недостаточно качественное. Eugene Churazov If you mean it seriously, this is actually a very good question. Minhyong Kim |
| Wk | Date |
Time
& place |
Speaker & Topic |
Background reading |
Comings and goings (incl. visitors in town) |
| Thu Jan 4 |
No seminar | ||||
| 0 |
Thu Jan 11 |
11:30 501 DWB |
Plasma Seminar (organised
by ALEX) Adnane Osmane (Aalto, Finland) --- Electron phase-space holes in planetary radiation belts |
ApJ 846, 83 (2017) PRL 111, 235002 (2013) |
|
| Fri Jan 12 |
9:30 Culham Pease Rm |
CCFE Science Meeting (organised
by SARAH NEWTON) Alex Schekochihin (Oxford) --- Two etudes on unexpected behaviour of drift-wave turbulence near stability threshold I will discuss some
recent results — numerical and experimental — on the nature of
drift-wave turbulence in MAST, obtained in the doctoral theses of
Ferdinand van Wyk [1,4], Michael J. Fox [2] and Greg Colyer [3]. At ion
scales, in the presence of flow shear, we find numerically [1,4] a type
of transition to turbulence that is new (as far as we know) in the
tokamaks, but reminiscent of some fluid dynamical phenomena (e.g., pipe
flows or accretion discs in astrophysics): close to threshold, the
nonlinear saturated state and the associated anomalous heat transport
are dominated by long-lived coherent structures, which drift across the
domain, have finite amplitudes, but are not volume filling; as the
system is taken away from the threshold into the more unstable regime,
the number of these structures increases until they overlap and a more
conventional chaotic state emerges. Such a transition has its roots in
the subcritical nature of the turbulence in the presence of flow shear.
It can be diagnosed in terms of the breaking of the statistical up-down
symmetry of turbulence: this manifests itself in the form of tilted
two-point correlation functions and skewed distributions of the
fluctuating density field, found both in simulations and in
BES-measured density fields in MAST [2]. The governing (order)
parameter in the system is the distance from the threshold, rather than
individual values of equilibrium gradients; the symmetries — and
drift-wave/zonal-flow turbulence of conventional type — are restored
away from the threshold. The experiment appears to lie just at the edge
of this latter transition rather than at the exact stability threshold.
At electron scales in MAST, the conventional streamer-dominated state
of ETG turbulence turns out to be a long-time transient, during which
an initially unimportant zonal component continues to grow slowly,
eventually leading to a new saturated state dominated by zonal modes,
rather similar to ITG turbulence [3]. In this regime, the heat flux
turns out to be proportional to the collision rate, in approximate
agreement with the experimentally observed collisionality scaling of
the energy confinement in MAST. Our explanation of this effect is based
on a model of ETG turbulence dominated by zonal–nonzonal interactions
and on an analytically derived scaling of the zonal-mode damping rate
with the electron–ion collisionality. These developments open some
intriguing possibilities both for enterprising theoreticians tired of
the V&V routine and for ingenious experimentalists interested in
making use of tokamaks to probe transitions to turbulence in nonlinear
plasma systems.
|
[1] JPP 82, 905820609 (2016) [2] PPCF 59, 034002 (2017) [3] PPCF 59, 055002 (2017) [4] PPCF 59, 114003 (2017) |
||
| I |
Thu Jan 18. |
11:30 TBA |
Plasma
Group Meeting (organised
by MICHAEL) Free discussion of current affairs: contributions welcome |
Javier Maurino-Alperovich joins the group to work with Felix Parra as visiting student |
|
| II |
Thu Jan 25. |
11:30 501 DWB |
Plasma
Group Meeting (organised
by MICHAEL) Filomena Nave --- Experimental intrinsic rotation update |
||
| III |
Thu Feb 1. |
11:30 501 DWB |
Plasma
Group Meeting (organised
by FELIX) Javier Maurino-Alperovich (Imperial) --- Dependence of confinement scaling laws on aspect ratio in tokamak plasmas |
||
| IV |
Thu Feb 8. |
11:30 501 DWB |
Plasma
Group Meeting (organised
by MICHAEL) Michele Romanelli (UKAEA) --- Novel aspects of plasma dynamics in JT-60SA |
||
| V |
Thu Feb 15. |
11:30 501 DWB |
Plasma
Group Meeting (organised
by FELIX) Free discussion |
Peter Catto (MIT) |
|
| VI |
Thu Feb 22. |
11:30 501 DWB |
Plasma
Group Meeting Cancelled due to UCU strike |
Peter Catto (MIT) |
|
| VII |
Thu Mar 1 |
11:30 501 DWB |
Plasma Seminar (organised
by ALEX) Peter Catto (MIT) --- Quasilinear theory: gyrokinetics isn't just for low frequencies |
JPP 83, 905830611 (2017) |
Peter Catto (MIT) |
| VIII |
Thu Mar 8 |
11:30 501 DWB |
Plasma Group Meeting Cancelled due to UCU strike |
||
| 9 |
Thu Mar 15 |
11:30 501 DWB |
Plasma Seminar Cancelled due to UCU strike |
Ian Abel (Chalmers) |
|
| Fri Mar 16 |
14:00 Culham |
CCFE Student Seminar Alex Schekochihin (Oxford) --- Phase mixing vs. nonlinear advection in drift-kinetic plasma turbulence |
JPP 82, 905820212 (2016) PoP 23, 070703 (2016) |
||
| Tue Mar 20 |
11:30 501 DWB |
Plasma Seminar (organised
by FELIX) Grisha Kagan (LLNL) --- Thermal and suprathermal particles in ICF implosions Experiments
are indicative of substantial kinetic effects during the course of ICF
implosion. The effects appear as the plasma mean-free-path grows
relative to the background scale making standard rad-hydro single-fluid
description invalid. To understand the consequences it is convenient to
consider the thermal and suprathermal particles separately. For the
former, sharp gradients can drive the inter-ion-species diffusion, so
the fuel composition no longer remains constant unlike what the
single-fluid codes assume. Atomic mix at interfaces is, fundamentally,
due to the same diffusion process. For the latter, the mean-free-path
is much larger than that of their thermal counterparts, so their
distribution function may be far from Maxwellian, even if thermal ions
are nearly equilibrated. It is these suprathermal, or tail, ions that
fuse in subignited implosions. Their distribution is thus the key to
proper interpretation of nuclear diagnostics employed in ICF
experiments in general and to correct fusion yield prediction in
particular. Furthermore, suprathermal electron distribution shows
similar behavior, affecting the X-ray diagnostics. Basic mechanisms
behind and practical consequences of these groups of effects will be
discussed.
|
Grisha Kagan (LLNL) Angelo Limone (Siemens) |
||
| Thu Mar 22 |
11:30 501 DWB |
Plasma Seminar (organised
by ALEX) Angelo Limone (Siemens Industry Software Computational Dynamics Ltd Digital Factory, London) --- Simulating the plasma state for industrial applications Plasma physicists traditionally focus their attention on astrophysics and nuclear fusion. However, a composite zoo of plasma applications does exist and---surprisingly---has even a certain economic impact. Thin semiconductor layers, for instance, used in our ubiquitous smartphones and displays are produced also via techniques that use the plasma state as catalyst. In circuit breakers of whichever voltage size the plasma state is something that has to be quenched, and quickly. Some other few applications of industrial plasmas will be described, in particular I will be discussing the tough challenges that the average engineer might have to tackle when trying to model plasma flows via numerical simulations and the problems that a Computational Fluid Dynamics software provider has to face in order to deliver a solver that can help the design engineer with realism and on an "industrially reasonable" timescale. |
Computers & Fluids 88, 872 (2013) J Phys D: Appl Phys 46, 065203 (2013) |
||
| Mon Mar 26 |
11:30 Dobson Room |
Plasma Seminar (organised
by ALEX) Archie Bott (Oxford ALP) --- When is collisional plasma a fluid? (1001 plasma instabilities at high beta) |
preprint from author on request |
Troy Carter (UCLA) |
|
| Wed Mar 28 |
11:30 Dobson Room |
Plasma Seminar (organised
by ALEX) Troy Carter (UCLA) --- Updates from LAPD: turbulence and energetic-ion-driven modes in high-beta plasmas |
RSI 87, 025105 (2016) PPCF 59, 024006 (2017) PRE 91, 013109 (2015) |
||
| Thu Mar 29 |
9:30 Culham HOW Room |
Special JET Seminar (organised
by JON HILLESHEIM) Troy Carter (UCLA) --- Studies of ICRF physics using the Large Plasma Device The Basic
Plasma Science Facility (BaPSF) at UCLA is a US national user facility
for studies of fundamental processes in magnetized plasmas. The
centerpiece of the facility is the Large Plasma Device (LAPD), a 20m
long, magnetized linear plasma device. This LAPD has been utilized to
study a number of fundamental processes, including: dispersion and
damping of kinetic and inertial Alfven waves, flux ropes and magnetic
reconnection, three-wave interactions and parametric instabilities of
Alfven waves, turbulence and transport and interactions of energetic
ions and electrons with plasma waves. A brief overview of research
using the facility will be given, followed by a more detailed
discussion of research relevant to ICRF heating and current drive in
fusion devices. Recent experiments in LAPD have studied high power
(~200kW) fast wave excitation (f ∼ 2 − 10fci). Highlights of this work
include documenting: the structure and scaling of RF sheaths, the
formation of convective cells and associated density modification, and
parasitic coupling to the slow mode in the low density plasma in front
of the antenna [Martin, et al., PRL 119, 205002 (2017)].
|
|||
| Thu Apr 5 |
11:30 501 DWB |
Plasma Group Meeting (organised
by ALEX) TBA |
Troy Carter (UCLA) |
||