Studies of coaxial multipactor in the presence of a magnetic field

Multipactor discharges consists of electron multiplication between two surfaces by secondary electron emission in resonance with an alternating electric field. They are detrimental to the performance of radio frequency (RF) systems, such as the ICRF (ion cyclotron range of frequencies) antennas for heating of plasmas in the Alcator C-Mod tokamak and other nuclear fusion devices. This work investigates multipactor discharges in the coaxial geometry in the presence of a constant and uniform magnetic field transverse to the direction of electromagnetic wave propagation. Studies on the Coaxial Multipactor Experiment (CMX) show that the magnetic field decreases the degree to which the discharge detunes the RF circuit. However, it enhances the susceptibility of the system to multipactor-induced gas breakdown at low pressures, which appears to cause the observed neutral pressure limits on antenna performance in Alcator C-Mod. Different surface treatment methods involving roughening and in-situ cleaning failed to suppress the multipactor discharges in a consistent and reliable way in experiments on CMX, despite the success of similar techniques in the parallel-plate geometry. Electron trajectories are significantly more complicated in the presence of magnetic fields of different strengths, as shown by a three-dimensional particle-tracking simulation using Monte Carlo sampling techniques. The trends in electron path length, time of flight, impact energy, secondary emission yield and population growth do not account for the experimental observations between the low and high field limits. These appear to be better explained by collective effects not included in the simulations, such as the effect of the magnetic field on charged particle diffusion.

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

2007

Author dc:creator

• Becerra, Gabriel E. (Becerra Toledo)

Advisor dc:contributor.advisor

• Ian H. Hutchinson.

• 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/1721.1/41248

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/41248