Spectral measurements of runaway electrons in the TEXTOR tokamak [Elektronische Ressource] / vorgelegt von Timur Kudyakov

Riks

Sujets

• Physik

Informations

• Publié par: heinrich-heine-universitat_dusseldorf
• Publié le: 01 janvier 2009
• Nombre de lectures: 33
• Langue: Deutsch
• Poids de l'ouvrage: 4 Mo

Extrait

Spectral measurements of runaway
electrons in the TEXTOR tokamak
Inaugural-Dissertation
zur
Erlangung des Doktorgrades der
Mathematisch-Naturwissenschaftlichen Fakult¨at
der Heinrich-Heine-Universit¨at Dusseldo¨ rf
vorgelegt von
Timur Kudyakov
aus Bischkek (Kg)
Mai 2009Aus dem Institut fur¨ Laser- und Plasmaphysik
der Heinrich-Heine-Universit¨ at Dusseldorf¨
Gedruckt mit Genehmigung der
Mathematisch-Naturwissenschaftlichen Fakult¨ at der
Heinrich-Heine-Universitt Dusseldorf.¨
Referent: Prof. Dr. O. Willi
Koreferent: Prof. Dr. U. Samm
Tag der mundlic¨ hen Prufung:¨ 22. 07. 2009Zusammenfassung
W¨ ahrend der Plasmadisruption innerhalb eines Tokamaks werden hochener-
getische Elektronen im Bereich von mehreren MeV, sog. ”Runaways”, emit-
tiert. Fur¨ den Betrieb des zukunftigen¨ Tokamaks ITER spielen Runaway-
Elektronen eine entscheidende Rolle, da sie die Betriebsdauer wesentlich re-
duzieren k¨ onnen.
In dieser Arbeit wurden die zu Grunde liegenden physikalischen Effekte der
Entstehung und dem Transport der Runaway-Elektronen am Tokamak TEX-
TOR des Forschungszentrums Julic¨ h untersucht und eine Absch¨ atzung der
Folgen auf den Betrieb von ITER gemacht. Eine hierfur¨ speziell entwickelte
Sonde erlaubt, die vom Plasma emittierte Anzahl an Runaway-Elektronen,
ihre Energieverteilung und die hervorgerufene Energiedeposition im Material
zu messen. Ihre Funktionsweise basiert auf dem Einsatz von YSO Kristallen,
welche durch die Elektronen im sichtbaren Spektralbereich szintillierenden.
Das Licht wird mit Hilfe von Lichtwellenleitern auf Photovervielfacher geleitet
und zeitaufgel¨ ost registriert. Eine Energieaufl¨ osung wird dadurch erreicht, dass
einzelne Kristalle in Schichten aus Edelstahl unterschiedlicher Dicke eingebet-
tet sind. Mit Hilfe von neun Kristallen konnte ein Energieintervall zwischen
4 MeV und 30 MeV aufgel¨ ost werden. Die Sonde wurde am Elektronen-
Linearbeschleuniger ELBE am Forschungszentrum Dresden-Rossendorf abso-
lut kalibriert; die Ergebnisse stimmen gut mit numerischen Monte-Carlo Sim-
ulationen mit dem Code Geant4 ub¨ erein.
Im TEXTOR-Experiment wurde der Transport der Runaway-Elektronen
in Anwesenheit externer oder intrinsischer Magnetfelder studiert. Es wurde
gefunden, dass der Anstieg der Runaway-Verluste bei abnehmendem Magnet-
feld mit einer Erh¨ ohung der magnetischen Fluktuationen verbunden ist. Diese
Messung ist neuartig und konnte bisher nicht mit anderen Meßmethoden ge-
funden werden.
Beim Anlegen eines externen magnetischen St¨ orfeldes durch den Dynamis-
chen Ergodische Divertor (DED) an TEXTOR wurde auch hier die unter-
schiedliche Abh¨ angigkeit des Transportes als Funktion der Runaway - Energie
gefunden. Auch hier ist der Transport der niederenergetischen Runaways h¨ oher
als derjenige der hochenergetischen. Der verst¨ arkte Verlust der niederener-
getischen Elektronen macht sich bei den hochenergetischen mit einer zeitlichen
Verz¨ ogerung bemerkbar.
Es wurden erstmals Messungen mit der Sonde bei Disruptionen durchgefuhrt,¨
um den Runaway-Fluss, die Energieverteilung der Runaways als Funktion der
Zeit und die W¨ armelast zu bestimmen. Die Sonde zeigte einen zeitlich struk-
turierten Runaway-Verlust. Zuerst - im sog. thermal quench - gehen Runaways
verloren, die in der Startphase der Entladung entstanden sind. Danach - in
der sog. current quench Phase - beobachtet man Runaway-Elektronen, die in
der Phase beschleunigt werden, in denen die magnetische Energie des Toka-
maks dissipiert wird. Die gewonnenen Daten erlauben eine Absch¨ atzung der
thermischen Effekte im Rahmen des ITER-Konzepts.
iAbstract
The generation of multi-MeV runaway electrons is a well known effect re-
lated to the plasma disruptions in tokamaks. The runaway electrons can sub-
stantially reduce the lifetime of the future tokamak ITER.
In this thesis physical properties of runaway electrons and their possible
negative effects on ITER have been studied in the TEXTOR tokamak. A new
diagnostic, a scanning probe, has been developed to provide direct measure-
ments of the absolute number of runaway electrons coming from the plasma,
its energy distribution and the related energy load in the material during low
density (runaway) discharges and during disruptions. The basic elements of
the probe are YSO crystals which transform the energy of runaway electrons
into visible light which is guided via optical fibres to photomultipliers. In
order to obtain the energy distribution of runaways, the crystals are covered
with layers of stainless steel (or tungsten in two earlier test versions) of dif-
ferent thicknesses. The final probe design has 9 crystals and can temporally
and spectrally resolve electrons with energies between 4 MeV and 30 MeV.
The probe is tested and absolutely calibrated at the linear electron accelerator
ELBE in Rossendorf. The measurements are in good agreement with Monte
Carlo simulations using the Geant4 code.
The runaway transport in the presence of the internal and externally ap-
plied magnetic perturbations has been studied. The diffusion coefficient and
the value of the magnetic fluctuation for runaways were derived as a function
of B . It was found that an increase of runaway losses from the plasma witht
the decreasing toroidal magnetic field is accompanied with a growth of the
magnetic fluctuation in the plasma. The magnetic shielding picture could be
confirmed which predicts that the runaway loss occurs predominantly for low
energy runaways (few MeV) and considerably less for the high energy ones.
In the case of the externally applied magnetic perturbations by means of
the dynamic ergodic divertor (DED) runaway electrons with different energies
demonstrate a different sensitivity to the DED. Again, highly relativistic elec-
trons are less sensitive to the stochastic magnetic field than the low energy
ones.
Measurements of runaway electrons during the plasma disruptions have
been carried out by the new probe. The probe has shown two distinct losses of
runaways during the thermal quench (runaways were produced at the start up
of the discharge) and during the current quench (runaways were produced due
to the dissipation of the magnetic field). Important parameters, such as the
runaway flux, the energy distribution, the temporal evolution and the thermal
load in materials have been studied. The obtained results allow to estimate
the thermal load due to runaway electrons in the ITER tokamak.
iiiAcknowledgments
First of all, I would like to thank my advisor, Prof. Oswald Willi for giving
me an opportunity to work on challenging and extremely interesting projects
over the past four years. I get very valuable support and advise in all questions
during the whole time of my work with him.
With a great pleasure I would like to thank my co-advisor, Dr. Karl Heinz
Finken. I am very lucky that Dr. Finken has been my co-adviser. It is very
hard to transfer in words that enormous amount of knowledge and experience,
which he has transferred to me. I have appreciated his support any time dur-
ing these four years.
I can only say that it has been a pleasure to work with and learn from such
an extraordinary individuals as Prof. Willi and Dr. Finken.
A lot of thanks to Dr. Marcin Jakubowski, Dr. Yuhong Xu and Herr Albert
Hiller. During the past four years they have done everything to make the work
extremely interesting and very successful.
These work would not have been possible without the TEXTOR team and
personal support of Prof. Ulrich Samm. I would also like to thank Prof. Samm
that he agreed to referee my Ph.D thesis.
My colleagues at the institute have enriched my graduate life in many ways
and deserve a special thanks, especially Mirela Cerchez, Toma Toncian, Ralph
Jung, Jens Osterholz, Munib Amin, Ariane Pipahl, Thomas K¨ onigstein and
Bernhard Hidding. I would like to thank Renate ter Horst, Claudia Dingle,
Christiane Braun and Katherina Vogt without their efforts the organization of
my work would have been impossible. My ”scientific baby” would never have
been born without the help of Manfred Rosemann, Werner Sigosch, Andreas
Karmann, Christoph Bolten and Helmut Borrmann.
I would also like to acknowledge TEXTOR colleagues Dr. Sergey Bozhenkov,
Dr. Michael Lehnen, Dr. Wolfgang Biel, Dr. Bernd Schweer who have collab-
orated with me on the interesting experiments and help me a lot.
A great thanks to colleagues in Forschungszentrum Dresden− Rossendorf
Axel Jochmann, Karl Zeil, Dr. Stephan Kraft, Dr. Ulrich Schramm during a
few days and nights we can breathe new opportunities in my probe.
A lot of ideas are based on discussions with colleagues from the theoretical
groups, in particular Dr. Tunde¨ Ful¨¨ op and Dr. Sadrilla Abdullaev.
I owe my deepest thanks to my family - my wife, my parents, my sister
and all my best friends, for their support, encouragement and understanding.
A lot of thanks to my small daughter, she withstands courageously that her
daddy was busy all the time and did not play enough with her.:)
I would like to acknowledge financial support by the DFG programme
GRK1203 and F and E contract from FZ Julic¨ h.
It is impossible to remember all,