Alcator C-Mod

Laurence Lyons

Advisor: Dr. Brian LaBombard

Udated: 7/24/2006

One of the most fundamental diagnostics for measuring the conditions in a plasma is the Langmuir Probe. This device consists of a small electrode that is inserted into the plasma and, based on the voltage applied to it, draws a current from the free electrons and ions. The relationship between this current and the voltage applied is known as the I-V characteristic and it depends primarily on three plasma parameters: plasma density, electron temperature and plasma potential. Thus, by continuously recording I-V characteristics during a plasma discharge, one can infer the time evolution of these conditions with a high degree of spatial accuracy, being localized by the size of the electrode. Langmuir probes are particularly well suited for plasma measurements in the cooler edge regions of experimental fusion plasmas, such as in Alcator C-Mod, where plasma conditions can exhibit large spatial gradients.

The electronics currently used on Alcator C-Mod to both apply the voltage to the Langmuir probes and measure the resulting current are in need of an upgrade. While the current system is able to apply a voltage waveform and measure current in the low kHz range of frequencies, significant changes to all three of the plasma parameters mentioned above can occur in the 10 kHz to 1 MHz range. Thus, the current Langmuir probe system is unable to follow fast changes, making an investigation of such topics as plasma turbulence nearly impossible. To remedy this situation, a custom-designed package of fast-switching electronics is currently in development to be able to sample the I-V characteristic in the MHz range of frequencies.

The new Langmuir probe bias system is divided into three main components: (1) a master TTL waveform generator, (2) fast-switching MOSFET drive circuit and (3) current-voltage monitor circuit. The MOSFET board will apply a voltage waveform to a Langmuir probe based on the TTL signals received from the master waveform generator. The monitor circuit would then sample the voltage and resulting current at a high bandwidth and allow for a much more accurate measurement of the plasma's I-V characteristic. In the simplest case, output from this monitor circuit would be digitized at a high sample rate, requiring subsequent analysis to extract the information of interest from the I-V characteristic. However, we are also investigating the possibility of reporting these plasma conditions in real-time using a ‘mirror Langmuir Probe' technique, which is currently under development.