Construction and operation of a Mirror Langmuir Probe diagnostic for the Alcator C-Mod tokamak

Langmuir probe diagnostic systems presently employed on Alcator C-Mod and elsewhere generally suffer from a severe limitation: unless multiple electrode or high-frequency bias techniques are employed, these systems can not resolve the rapid changes in plasma electron temperature, floating potential and ion saturation current that are associated with plasma turbulence. Moreover, no existing system can provide real-time output of these three parameters using a single electrode. To remedy this limitation, an advanced, high-bandwidth Langmuir probe system has been constructed for Alcator C-Mod using state-of-the-art design tools and components. The system produces a fast-switched, three-state probe bias waveform and employs a new method for outputting plasma conditions in real-time, a 'Mirror Langmuir Probe' (MLP), which utilizes high-bandwidth bipolar transistors to electrically simulate a Langmuir probe's response. Detailed information on the design, construction and performance of this new diagnostic is described in this thesis, representing the first proof-of-principle demonstration of the MLP technique. The MLP system was found to meet all the performance goals set forth at the beginning of the project: real-time output of electron temperature, floating potential and ion saturation current, ability to track changes in plasma parameters within a ~1 [mu]s timescale, while utilizing only a single Langmuir electrode. The system was tested using an 'electronic Langmuir probe' and also using an actual Langmuir probe in Alcator C-Mod. In both cases, the system accurately locked onto changing plasma conditions (< ~5% error in outputted parameters), with the exception of some severe transient events found in C-Mod plasmas (ELMs), which challenged the system's accuracy.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2007

Author dc:creator

• Lyons, Laurence Anthony

Advisor dc:contributor.advisor

• Brian LaBombard and Ronald R. Parker.

• Electrical Engineering and Computer Science.

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/1721.1/40321

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/40321