Alcator C-Mod

Erid Edlund

Thesis Supervisor: Prof. Porkolab

Updated on July 7, 2006

The inability to produce direct measurements of the hot plasma core poses a great experimental challenge for understanding the dynamics therein. One method for overcoming this hindrance is to use indirect methods such as interferometric techniques. The phase contrast imaging (PCI) diagnostic is one such manifestation of this approach. Utilizing the spatial separation of the Fourier k-space components composing the electron density perturbations, an interferometric technique can be applied using the unscattered ,or zero k component, as the reference beam allowing for enhanced imaging of these perturbations. Because of the line-integrated nature of the PCI system, density perturbations can be measured throughout the plasma.

Figure 1. Cross section of Alcator C-Mod showing the PCI viewing chords in red.

My research is primarily focused on a particular class of core localized modes known as reverse shear Alfvén eigenmodes (RSAE) or Alfvén cascades. The RSAE exist only in plasma configurations with reverse shear q-profiles, or equivalently, hollow current profiles. These conditions are generally found during the ramp-up phase of the plasma when the current is still diffusing, though they can also be generated with external sources such as lower hybrid current drive. What makes these modes especially interesting is that the mode frequency is very sensitive to small changes in minimum of the q-profile, and in this regard can be a direct measurement of this parameter.

Of recent interest has been the recognition that the dynamics of the RSAE can only be accurately described by inclusion of coupling to geodesic acoustic modes. The acoustic influence becomes a dominant factor when the minimum of the q-profile is near an integral value. Under these circumstances, the mode frequency is determined by a combination of the electron and ion temperatures. Modeling with the hybrid acoustic-kinetic code NOVA-K has shown excellent agreement with the experimental measurements.

Figure 2. A color spectrogram of PCI data shows a feature with multiple chirping modes, known as a grand cascade, overlaid in white with the modeled evolution from NOVA-K The non-zero initial frequency of these modes is a signature of strong coupling to the geodesic acoustic modes.

Further studies in ramp-up conditions seek to use the cascades modes as indicators of the current diffusion process. Specifically, the influence of off-axis heating and non-inductive current drive from ion cyclotron current drive or lower-hybrid current drive will be investigated. This class of modes is a valuable diagnostic for understanding the development of the current profiles under these conditions.