Characterization of low-frequency electric potential oscillations near the edge of a plasma confined by a levitated magnetic dipole

A vertically adjustable electrostatic probe array was made to observe the previously seen low-frequency angular oscillations in LDX and identify if they are related to computationally expected convective cells. The array rests one meter from the centerline and measures edge fluctuations at field lines near the separatrix. It spans ninety degrees and has 24 probes mounted on it for total probe tip separation of 6.8cm. Bispectral analysis of the fluctuations show that that an inverse cascade of energy is present at times in LDX. The cascade transfers energy from small spatial scale structures to large scale structures. The wavenumber spectrum is xc k-1.4 to cx k-25 at high wavenumbers, which encompasses the inverse energy cascade regime of c k-5/3. The plasma also has a linear dispersion relation which gives a phase velocity of 2-16 k. This phase velocity is inversely correlated with neutral gas pressure in the vessel. The velocity also has a local maximum at 5 pTorr which is the pressure that produces maximum plasma density. The radial E x B drift velocities are observed to have a mean near zero, which indicates a closed structure like a convective cell. The instantaneous radial drift velocities are on the order of the ion sound speed, which is 35 km/s.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Nuclear Science and Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2009

Author dc:creator

• Bergmann, Ryan M

Advisor dc:contributor.advisor

• Jay Kesner and Darren T. Garnier.

• Nuclear Science and Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/53240

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/53240