High-Speed Videography on HBT-EP
Creators: Angelini, Sarah
Publisher: name: Columbia University
Year: 2014
Subjects: Plasma (Ionized gases)
Physics

description: In this thesis, I present measurements from a high-speed video camera diagnostic on the High Beta Tokamak - Extended Pulse (HBT-EP). This work represents the first use of video data to analyze and understand the behavior of long wavelength kink perturbations in a wall-stabilized tokamak. A Phantom v7.3 camera was installed to capture the plasma's global behavior using visible light emissions and it operates at frame rates from 63 to 125 kfps. A USB2000 spectrometer was used to identify the dominant wavelength of light emitted in HBT-EP. At 656 nm, it is consistent with the D-alpha light expected from interactions between neutral deuterium and plasma electrons. The fast camera in combination with an Acktar vacuum black background produced images which were inverted using Abel techniques to determine the average radial emissivity profiles. These profiles were found to be hollow with a radial scale length of approximately 4 cm at the plasma edge. As a result, the behavior measured and analyzed using visible light videography is limited to the edge region. Using difference subtraction, biorthogonal decomposition and Fourier analysis, the structures of the observed edge fluctuations are computed. By comparing forward modelling results to measurements, the plasma is found to have an m/n = 3/1 helical shape that rotates in the electron drift direction with a lab-frame frequency between 5 and 10 kHz.
      The fast camera was also used to measure the plasma's response to applied helical magnetic perturbations which resonate with the equilibrium magnetic field at the plasma's edge. The static plasma response to non-rotating resonant magnetic perturbations (RMPs) is measured by comparing changes in the recorded image following a fast reversal, or phase flip, of the applied RMP. The programmed toroidal angle of the RMP is directly inferred from the resulting images of the plasma response. The plasma response and the intensityof the RMP are compared under different conditions. I found the resulting amplitude correlations to be consistent with previous measurements of the static response using an array of magnetic sensors.
      My work has shown that high-speed videography can be an extremely useful diagnostic for measuring edge perturbations in a tokamak. Future measurements, such as those using multiple cameras with different views, are expected to improve our understanding of plasma behavior and to detect edge fluctuations with higher temporal and spatial resolution.
      Supplementary Videos:
      Video 1 - This is an example of the video data from Shot 77324, an unforced plasma shot
      taken with the shells inserted.
      Video 2 - The strongest naturally-rotating mode has been extracted from a subset of the
      raw data shown in Video 1 using a biorthogonal decomposition. Long striations can be
      seen which are common in shots that have the shells inserted.
      Video 3 - In this video of the raw data from Shot 77537, the shells are retracted. The
      smooth non-reflective Acktar black background can be seen between the shells.
      Video 4 - The dominant BD mode from Shot 77537 shows pinwheel-like behavior. With
      the shells retracted, the plasma encounters fewer physical structures for neutral recycling
      and this affects the light emissions.
      Video 5 - This video shows the dominant BD modes from Shot 78029 during which a
      phase-flip RMP was used to influence the plasma. The mode seems to slow in its rotation
      as it resonates with the externally-applied field.