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VERSION:2.0
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BEGIN:VEVENT
DTSTAMP:20240328T222105Z
UID:https://www.ipp.mpg.de/events/13672/4054204
DTSTART:20180502T133000Z
DTEND:20180502T143000Z
CLASS:PUBLIC
CREATED:20180412T140529Z
DESCRIPTION:Edge localized modes (ELMs) are magnetohydrodynamic (MHD) insta
bilities that occur in the high confinement regime (H-mode) of magneticall
y confined fusion plasmas. ELMs lead to sudden periodic releases of partic
les and stored energy on a millisecond time scale. These ELM crashes might
cause intolerably high heat fluxes onto the divertor target plates or the
first wall in future fusion devices. According to the broadly accepted li
near peeling-ballooning model these MHD instabilities are driven by edge c
urrent density and steep edge pressure gradient\, which are characteristic
for the H-mode. However\, details of the underlying process responsible f
or ELMs and their nonlinear development during the crashes are not yet ful
ly understood. The focus of this thesis is to determine one of the main ch
aracterizing parameters of MHD instabilities\, which is the periodic magne
tic structure described by the poloidal and toroidal mode numbers m and n.
These structures are investigated for ELMs and associated phenomena on th
e ASDEX Upgrade tokamak. Mode numbers of instabilities are determined by r
ecently upgraded magnetic pick-up coil arrays. It is shown that mode numbe
rs of high frequency oscillations\, f >\; 50 kHz\, can only be re
liably determined if the frequency dependent phase response of the coils i
s taken into account. Furthermore\, a precise ELM synchronization enables
the identification of mode numbers during the fast crash of ELMs\, which w
as never achieved before on ASDEX Upgrade. In addition to that\, mode numb
ers and positions of modes appearing between ELM crashes as well as their
connection to the edge gradient development are determined for the first t
ime\, which is a big step forward in characterizing them and understanding
their role for the ELM itself. Ensembles of modes between ELM crashes are
detected with different rotation velocities and thereby different locatio
ns at the plasma edge. Modes with higher toroidal mode numbers\, n=
7-13\, appear at the position of fastest poloidal plasma rotation\, close
to the maximum pressure gradient and might be interpreted as ideal modes w
ithout additional phase velocity. Modes with lower toroidal mode numbers\,
n=2-7\, exist further outwards close to the separatrix. A similar
low n structure is present during the ELM crash. The detection of this str
ucture and other parameters of the crash such as induced energy losses or
duration enables a quantitative comparison to results from modeling with t
he nonlinear MHD code JOREK for the first time. Here the n=6 compon
ent with smaller structure size is linearly dominant\, but nonlinear coupl
ing in which n=1 is particularly important leads to the dominance o
f larger structure sizes with n=3-5 during the ELM crash which is i
n excellent agreement with experimental observations. Moreover\, the scali
ng of the toroidal and poloidal structure\, intensity and duration of the
ELM crash with plasma parameters is investigated in a database containing
various plasma scenarios. It is found that n increases linearly with the i
nclination of the magnetic field lines\, i.e. decreases with safety factor
q. Furthermore\, no intense ELMs are found at high edge q and no l
ong lasting ELMs are found at low edge q. Other parameters such as
normalized pressure gradient alpha\, bootstrap current density jBS or plasma triangularity delta\, that should have\, according to
linear peeling-ballooning theory\, an impact on n\, do not show clear tre
nds. Introducing a simple geometric model\, the scaling of toroidal struct
ure size with q can be explained by the dominance of one poloidal s
tructure. In order to place the nonlinear phase of ELMs into a wider conte
xt of other nonlinear edge phenomena\, toroidal mode numbers are analyzed
between ELM crashes on the JET tokamak and during ELM crashes mitigated wi
th an external magnetic error field\, ELM crashes of nitrogen seeded disch
arges and ELM-like magnetic bursts of the intermediate confinement regime
(I-phase). The JET edge modes are found with similar properties as the one
s on ASDEX Upgrade. The ELM crash structure is found to adopt to the one o
f the external error field while nitrogen seeding seems not to change it.
I-phase bursts have the same toroidal structure as ELMs.\nVortragende(r):
Felician Mink
LAST-MODIFIED:20180412T140751Z
LOCATION:Garching\, Raum: Seminarraum D3
ORGANIZER:
SUMMARY:Edge Physics Forum: Structure and Nonlinear Development of Edge Loc
alized Magnetohydrodynamic Instabilities on the ASDEX Upgrade Tokamak
URL:https://www.ipp.mpg.de/events/13672/4054204
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