Conditional averaging was utilized in order to find the blobs and the holes in the plasma edge of an L-mode and an H-mode KSTAR plasma. By analyzing the measurement data, it was possible to determine the average poloidal and radial size of the events. The analysis also showed that the structures are propagating both in the radial and the poloidal direction.
The velocities of the propagation were also calculated. Table 6.1 contains these information for both the L-mode and the H-mode shot.
TABLE6.1: Summary of the blob and hole analysis results for the L-mode and the H-mode shots.
Parameter L-mode blobs
L-mode holes
H-mode blobs
H-mode holes Poloidal size 20 mm 20 mm 20 mm 30 mm Radial size 20-40 mm 20 mm 20-40 mm 20 mm Poloidal velocity 225m/s
ion diam.
1000 m/s e−diam.
150m/s ion diam.
166 m/s e−diam Radial velocity 300m/s -400m/s 200m/s -175m/s Generation rate 576s−1 796s−1 318s−1 530s−1
6.4. Discussion of the L-mode and H-mode blob and hole analysis 81 The L-mode and H-mode results cannot be directly compared in detail, since the shots were completely different plasma regimes. However, the same tendency is seen, as it was found on other machines. There is no significant difference in the radial and poloidal sizes of intermittent events between L-mode and H-mode shots. The poloidal velocities of blobs and holes have the same magnitude as on other machines and their direction also coincides with earlier findings. The magnitudes of the radial velocities do not match earlier research, because there, holes tend to have higher absolute radial velocity than blobs in all regimes.
However, that is not the case for the H-mode shot in the DBES measurement. Their direction matches results on other machines: blobs are propagating radially outwards, while holes show inwards movement. The event generation rates show the same tendency as on other machines: the generation rate is generally higher for holes, than blobs. The comparison was based on results from [74,80,109].
83
Chapter 7
Summary and discussion
After an extensive feasibility study including beam modeling and trial measurements, a combined Hydrogen and Lithium beam emission spectroscopy system was successfully put in operation on KSTAR. For the heating beam BES, the first plasma measurements showed the peak noise ratio of the system to be approximately 80, while the signal-to-background ratio was 20. For the diagnostic Lithium BES, the peak SNR was found to be 20, while the highest SBR ratio was around 10. The calculated SBR suggest, that the Lithium filter design was also appropriate, and its lower value is a result of the non-optimal focusing of the Li-beam.
The turbulence detection limit was investigated for the heating Hydrogen beam and the diagnostic Lithium beam BES measurements, as well. An avalanche photo-diode detector (4x16 channels) based camera provided fast (2MHz sampling) fluctuation measurements of the beam line emission with 1cm radial and vertical optical resolution. For the HBES, the highest relative fluctuation amplitude was 4% at the plasma edge (r/a = 1.0), which was two times higher, than the noise level. This shows, that turbulence can be directly detected at the plasma edge with spectral analysis. Towards the plasma core, the value of the relative fluctuation amplitude decreased to 1%, which is the same level as the noise amplitude. This result shows, that turbulence can only be detected towards the core with cross-correlation methods. For the LiBES, the relative noise amplitude exceeds the relative fluctuation am-plitude for all radial positions, but at the plasma edge, in the range ofr/a = [0.95,0.1], the fluctuation amplitude is only 2-3 times lower than the noise amplitude, which allows turbu-lence measurements, but only with cross-correlation methods. Turbuturbu-lence detection is only possible at the plasma edge due to the low penetration depth of the Lithium beam. Poloidal propagation velocity of edge turbulence could also be detected in both LiBES and HBES measurements. Lithium beam measurements can also provide measurement of the absolute radial electron density profile.
Scrape-off layer and edge turbulence measurements were performed on KSTAR with Deuterium beam emission spectroscopy in different plasma scenarios. The light signal fluc-tuations are considered as a proxy to plasma electron density modulations. During initial data analysis spurious oscillations were found in the BES signals originating from the NBI high voltage power source. An oscillation subtraction method was developed in order to remove these disadvantageous effects.
An L-mode and an H-mode shot were chosen for the aim of demonstrating the scrape-off layer turbulence measurement capabilities of the KSTAR Deuterium BES system. The main parameters of the measurements were determined such as the turbulence spectrum, the poloidal time lag profiles (inverse poloidal velocity profiles) and the relative fluctuation and noise amplitudes. Furthermore, the skewness and the kurtosis profiles of the shots were also investigated. It was found that the skewness profile exhibits similar behavior as it was found in other machines. The analysis showed that the birth zone of blobs and holes is 1-2cm inside the separatrix.
In order to analyze the average behavior of blobs and holes, conditional averaging was applied. By utilizing this method, blobs and holes were found in L-mode and H-mode plasmas, as well. While blobs can be detected everywhere in the scrape-off layer, holes can only be seen at the birth location of the blobs and 1-2cm radially inwards. The poloidal and radial propagation direction of both the blobs and holes match the findings on other plasma devices: the average blob is propagating radially outward and poloidally in the ion diamagnetic drift direction, while holes are propagating radially inward and poloidally in the electron diamagnetic drift direction. The two-dimensional BES measurement suggest that the blobs change their shape as they propagate radially outward, but the contribution of beam effects is at present unclear and needs further analysis. The analysis of conditional averaging showed the generation rate of blobs and holes, as well. The hole generation rate was found to be 50% higher than the blob generation rate in both L-mode and H-mode cases Probe measurements were analyzed for the same L-mode shot in order to compare SOL probe measurements to BES measurements. Probe measurements show higher relative fluc-tuation amplitude and higher cutoff frequency than the BES measurement. This is consid-ered to be at effect of the limited effective spatial resolution of the BES diagnostic. It causes the smaller blobs to be averaged out and thus lowering the fluctuation amplitude and the cutoff frequency. This effect can be addressed quantitatively by investigating synthetic diag-nostic signals which will be a subject of a work in the future. Although density fluctuation measurements of probes are superior to BES in respect to spatial resolution and dynamic range, they lack full 2D resolution and they are limited in radial measurement range, dis-charge conditions and operation time. Furthermore, probes perturb the plasma, as well, which makes interpretation of the data difficult.
Based on the results, one can draw the conclusion that the Deuterium beam emission spectroscopy diagnostic on KSTAR can provide two-dimensional scrape-off layer and edge turbulence measurements in L-mode and H-mode plasmas for the entire plasma shot. Blob and hole dynamics can be observed and the results compare well to the same aspects of the dynamics on other machines [74,80,109]. This continuous two-dimensional measurement capability provides an excellent possibility for detailed intermittent event dynamics studies.
However, the downside of the diagnostic is the limited spatial resolution which prevents detection of small blob events (< 1−2cm) and impedes determining accurate blob size scaling, which is an important aspect of blob theory. The effective resolution of the system would improve if the effects of spatial smearing were minimized in the measurement. In order to achieve that a deconvolution method is being developed, which could, to some extent, correct for the effects of the smearing and extend the diagnostic capabilities towards smaller blob events.
7.1 Outlook
This thesis showed various measurement capabilities of the KSTAR BES system. There are numerous possibilities which are not discussed here in detail, for example the possibility of local electron density fluctuation reconstruction in beam emission spectroscopy measure-ments. A method is being developed to subtract the effect of spatial smearing from the BES measurement data. This numerical tool could help characterizing the blobs and holes in more detail and it could remove the spurious spatial smearing from the measurement. Fur-thermore, a higher resolution APDCAM is planned to be installed on KSTAR which could double the poloidal measurement range of the system. This could help the characterization of the larger blobs and it could provide data for an interesting research in the future.
85
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