Whistler Wave Propagation in Inhomogeneous Plasmas [Elektronische Ressource] / Jörg Pfannmöller

Jörg Pfannmöller

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• Physik

Informations

• Publié par: ernst-moritz-arndt-universitat_greifswald
• Publié le: 01 janvier 2011
• Nombre de lectures: 34
• Langue: English
• Poids de l'ouvrage: 21 Mo

Extrait

Whistler Wave Propagation in Inhomogeneous Plasmas
I n a u g u r a l d i s s e r t a t i o n
zur
Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
der
¨Mathematisch-Naturwissenschaftlichen Fakultat
der
Ernst-Moritz-Arndt-Universitat¨ Greifswald
vorgelegt von
J org P. Pfannm oller
geboren am 08.11.1978
in Darmstadt
Greifswald, im M arz 2011Dekan: Prof. Dr. K. Fesser
1. Gutachter: Prof. Dr. O. Grulke
2. Gutachter: Prof. Dr. R. Sydora
Tag der Promotion: 17.06.2011Abstract
In this thesis wave propagation in the whistler wave frequency range! !!ci ce
in the linear magnetized plasma experiment VINETA is investigated. The plasma
is generated by a helicon antenna and has a diameter of about 10 cm. Whistler
waves are launched by a loop antenna with a diameter of 4:5 cm and the uctuat-
_ing magnetic eld is mapped by B-probes. Experiments are carried out for plasmap
parameters 1= 2 under which the only transversal polarized wave according
to plane wave dispersion theory is the whistler wave. Due to the small collision
frequencies 1 cyclotron damping of waves in this parameter regime
is dominant and depends only on the electron plasma-.
The in uence of the inhomogeneous plasma pro le and excitation by a loop an-
tenna is investigated by measurements of the uctuating magnetic eld perpen-
dicular to the ambient magnetic eld in azimuthal and radial axial planes. A
mode characterized by the number of wave lengthsm in the azimuthal direction is
found. The mode structure is modi ed by the speci c shape of the plasma density
pro le. Pro les with a homogeneous density inside the plasma radius are found to
posses a comparably simple mode structure. An agreement in the mode structure
of full-wave simulations in three dimensions, including a Gaussian density pro le
and excitation of the wave by a loop antenna, with the experimental results is
found. Conclusions on the spatial structure of the excited mode are drawn us-
ing the simulations which predict excitation of an m = 2 mode. The wave is
found to be ducted within the plasma radius over a wide parameter range. A
Helmholtz decomposition of the simulations electric eld exhibits the uctuating
space charge as the dominant source for the eld, while the contribution
due to induction is negligible. The magnetic eld is given partially by the electron
and displacement current. Both contributions to the magnetic eld are of the
same order of magnitude.
The frequency dependency of the excited modes spatial damping increment is in-
vestigated using measurements of the magnetic uctuations along the symmetry
axis of the plasma. In order to illustrate the parameter dependency, the electron
4 2plasma- is varied over two orders in magnitude in the range = 410 2:410 .
The experimental result for the spatial damping increment of the mode yields a
strong damping for wave frequencies !=! > 0:5 at maximum plasma-, whichce
shifts to higher frequencies with decreasing . The parameter dependency of the
damping for a xed frequency is studied in an axial ambient magnetic eld gra-
dient. In both cases an excellent agreement between the experimental result and
predictions for cyclotron damping from plane wave dispersion theory is found.
iiiKurzfassung
In dieser Dissertation wird die Ausbreitung von Wellen im Whistlerwellen Fre-
quenzbereich ! !! im linear magnetisierten Plasmaexperiment VINETAci ce
untersucht. Das Plasma wird von einer Helikonantenne erzeugt und hat einen
Durchmesser von 10 cm. Whistlerwellen werden von einer Schleifenantenne mit
_einem Durchmesser von 4:5 cm angeregt und deren Magnetfeld wird mit B-p
Sonden detektiert. Die Experimente werden in einem Parameterbereich 1= 2
durchgefuhrt fur den die Dispersionstheorie fur ebene Wellen als einzige transver-
sal polarisierte Welle die Whistlerwelle vorhersagt. Aufgrund der vernachl assigbar
kleinen Sto frequenz 1 ist die D ampfung der Whistlerwelle in diesem Pa-
rameterbereich durch Zyklotrond ampfung bestimmt welche nur vom Elektronen
Plasma-