Alcator C-Mod

Arturo Dominguez

Reflectometry on Alcator C-Mod

In tokamak plasmas, turbulence is responsible for anomalous transport of both energy and particles, often lowering the confinement time by more than an order of magnitude as compared to the values predicted by neoclassical transport theory. My research focuses on characterizing anomalous electron density fluctuations (electron density turbulence) through the use of O-mode reflectometry.

The O-mode reflectometry diagnostic in Alcator C-Mod works as follows: RF waves are injected into the plasma from the low field side midplane such that their wavenumber is perpendicular to the machine's toroidal magnetic field, and the wave electric field is parallel to it. In C-Mod, the plasma conditions are such that one can use the cold plasma approximation to derive the dispersion relation of the injected waves. The waves travel inside the plasma where they propagate until they reach a cutoff, at which point they reflect back towards the direction of the source. It can be shown that the position of the cutoff depends ONLY on the electron density (this from the cold plasma approximation). The density at which the cutoff occurs is n e =(f/89.8) 2 where n e is the electron density in units of 10 20 m -3 and “f” is the frequency of the injected wave in GHz. The waves, then, reflect towards detector horns and the signal is analyzed. Throughout the propagation, the WKB approximation can be used to describe the behavior of the wave since, far from the cutoff, the wavelength is much longer than the characteristic density length (n e /(dn e /dx)). Therefore, the phase information that the wave acquires will be primarily from the cutoff layer. This is the basis behind the reflectometry diagnostic; the information comes primarily from the behavior of the electron density at the cutoff.

Originally, the Alcator C-Mod Reflectometer consisted of 5 different frequency channels of 50, 60, 75, 88 and 110 GHz frequency respectively. It was initially used for the measurement of electron density profiles by locating the cutoff layers for the different frequencies and interpolating intermediate density layers. The RF waves were amplitude modulated by mixing the original GHz frequencies with a 500MHz signal before the injection into the plasma. This created 2 sidebands 1GHz apart that would probe two close cutoff density surfaces. This AM technique was needed in order to approximate phase delay d f /df (change in phase of signal due to change in frequency) to the difference between the measured phase from the two sidebands divided by the modulating frequency, d f /df~( f 2 - f 1 )/f mod

Due to advances of more accurate methods of measurement of density profiles, like Thomson Scattering, Bremsstrahlung emissivity and interferometry, the focus of the reflectometry system has changed towards density fluctuation measurements. The theory behind this is based on simulations using the phase screen model which predict a linear dependence between the fluctuations in phase from the reflected wave and the fluctuations of the electron density at the cutoff layer.

Since the phase delay is not needed to measure fluctuations, the reflectometer was initially modified by Y.Lin, J. Irby et al. such that the 88GHz sidebands were measured independently (baseband measurements) and the phase was not subtracted out, hence increasing the sensitivity of the channel. With the collaboration of PPPL, two higher frequency channels of 132GHz and 140GHz were also installed in C-Mod so as to probe higher density surfaces. These channels are also operated in baseband mode.

I am currently involved in two projects. The lower frequency channels (50, 60, 75 and 110GHz) are currently being upgraded to baseband mode with the purpose of increasing the resolution of the system since phase information will no longer be subtracted out of the received signal. This will increase the sensitivity of the reflectometer for electron densities ranging from 3.1x10 19 m -3 to 1.5x10 20 m -3 .

The second project, in collaboration of the PPPL group, is the installation of a variable frequency source in the 140GHz system. This will enable the system to frequency sweep the injected source (± 5GHz) allowing us to determine the radial correlation lengths of turbulent phenomena at for densities of the order of 2x10 20 m -3 .