Page 4 - Characterization and calibration of the Thomson scattering diagnostic suite for the C-2W field-reversed configuration experiment

Nucl. Fusion 58 (2018) 126026
B.H. Deng et al
Figure 6. Micro-burst peak frequency (fp, symbols) tracks 2 times the plasma rigid rotor frequency (2frr, solid curve) from 1D equilibrium modelling based on measured density profile data as described in [7].
Figure 7. Time traces of plasma rigid rotor frequency (fn) and plasma moment of inertia per particle (In), both normalized to unity at 2ms.
as shown in figure 7. The plasma moment of inertia is calcu- lated by integration over the plasma column. When divided by the total particle number inventory, the resultant plasma moment of inertia per particle represents the plasma profile shape only. During the steady phase from 1 to 4 ms both In and fn remain constant, indicating that the angular momentum per particle remaining in the system is conserved. In other words the profile change can lead to plasma spin up (or down), while the particles escaping from the system will take the angular momentum away with them.
The micro-burst fluctuation phase at peak time is plotted in figure 8. It spans between −π and π. However, it is interesting to observe that the phase at peak time tends to cluster around ±0.41π. The phase of fluctuations represents the mode spatial variation with respect to the horizontal direction, i.e. the FIR interferom- eter laser beam direction. Therefore, the clustering of the peak phase suggests that there are preferred geometry directions.
2.2. Spatial mode structure of micro-bursts
Using Lissajous’s diagrams, it is found that the bursting fluc- tuation measured by the central chord of FIR interferometer is in phase with that measured by the FIR chord with an impact
Figure 8. Phase at peak of each micro-burst, clustering around ±0.41π (dashed lines).
Figure 9. Bolometer signals from 2 sight lines with azimuthal angles of 60° and 330° are 180° out of phase, which is twice the 90° physical separation, indicating n = 2 mode structure.
parameter of 15 cm, and 180 degrees out of phase with the chord at 30cm, and the amplitude peaks at the 30cm chord. These amplitude-phase characteristics in the interferometer signals are consistent with the n=2 structure measured before [9]. An n = 2 toroidal (azimuthal) mode structure means the fluctuation phase changes by 4π upon one com- plete azimuthal rotation. More straight forwardly the n = 2 mode structure is shown in figure 9, where signals from two bolometer array channels with azimuthal viewing angles of 60° and 330° respectively are 180° out of phase, twice the physical azimuthal angle separation, indicating that the toroidal mode number of the fluctuation is n = 2. When the micro-burst amplitude is sufficiently high, it can be detected by the edge magnetic probes as well, as shown in figure 10. The probe pairs installed with azimuthal angles 180° apart see in phase micro-burst oscillations, again confirming the n = 2 mode structure. In addition, from the ordering of the peaks/ troughs seen by the probes installed around the circumference it is obvious that the mode is rotating clockwise, i.e. in the ion diamagnetic direction.
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