Chapter 9
Blob generation at TJ-K
Experimental observations of blob generation in many different fusion experiments indicate that usually two features are involved in the generation process:
1. A radial acceleration of structures of increased density as soon as they reach a certain region (specified below).
2. A shear in the poloidal velocity close to the region of increased radial propa-gation that leads to a detachment of a part of the initial density perturbation.
The detached part forms an isolated structure and is observed as blob.
This seems to be a robust observation, independent from the instability that gives rise to the coherent density structures. The characteristic change of the two velocities is often observed around the separatrix in fusion experiments (Refs. [51, 52]), but can also be observed in experiments that have no confinement region, but only open field lines (Ref. [47]). The radial outward motion appears to be a prerequisite for blob generation. As described in Sec. 3.2, in magnetically confined toroidal plasmas curvature induced interchange-like instabilities can explain such a propagation of the blob. These instabilities depend on the normal curvature κn (Sec. 2.2) and it is shown in this chapter that in TJ-K blobs are generated in the SOL region where the mean normal curvature hκni is negative.
Furthermore, in some of the discharges analyzed in the course of this work, a zonal flow (explained below) seems to be involved in the blob generation process.
A possible explanation for this is given and a future experiment is proposed, which could shed light on this connection.
9.1 Blob generation region
There are two distinct regions for blobs in the SOL of TJ-K: In Sec. 7.1.3 it was shown that there is a blob region, in which blobs exist. Inside the blob region there is the smaller generation region, where the majority of blobs originates from. This generation region is evident in 2D histograms of blob events detected in the SOL of
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TJ-K (e. g. Fig. 7.9). The measurements with the poloidal probe arrays, presented in Sec.7.1.1, revealed that the blob filaments are elongated along the magnetic field lines and extend between the two limiter disks. Hence, the blob generation region follows the magnetic field lines and the observed location of the generation region in a poloidal cross section depends on the toroidal position.
In previous blob studies at TJ-K [48, 73], this generation region was not ob-served. The blobs seemed to origin mainly from the mid-plane, where the local normal curvature κn is high. At that time, fast imaging was not available at TJ-K.
Therefore, it cannot be ruled out that a similar feature indeed existed in those ex-periments, but was not obvious in the performed probe measurements. However, since the limiter setup was different in these experiments (only one limiter disk at port O1), both the parallel length of the blobs lk and the poloidal profile of hκni were different. In the following two question will be answered to shed light on what determines the location of the blob generation region:
1. What have the different generation regions in both limiter setups in common?
2. Is there a reason why the generation region discovered in Sec. 7.1.5 has not been observed in the prior experiments?
In Fig. 9.1 the poloidal profile of hκni is depicted for the setup with two limiter disks (closed line) used in the experiments presented in this work, evaluated at the toroidal position of the probe measurement (port O6). The generation region observed in this setup with the fast camera is marked by “GR2”, located inside the region where hκni is negative and blobs can be generated. Also shown in Fig. 9.1 is the hκni profile for the setup with just one limiter used in Ref. [48] (dashed line).
“GR1” indicates the generation region reported in the reference. Assuming that lk is also determined by the SOL connection length in this setup with only one limiter disk, the blob generation region should be located again in the region where hκni is negative, which is indeed the case. So in both cases, the generation region is located in the bad curvature region. This immediately answers the second question, the generation region of the setup with two limiter is located in the good curvature region in the setup with just one limiter disk and, hence, no blob drive exists and blobs should not be generated. It is, however, also apparent from Fig. 9.1 that the generation region is not located at the minimum of hκni, where the effect of the curvature on the blob drive is strongest. The close connection between blob generation and drift waves in the edge of TJ-K implies, however, that the stability of drift waves in the edge plasma plays a major role in the blob generation process.
Therefore, the reason for the strongly increased blob detection in the generation region may lie in the confined plasma rather than in the SOL.
Detailed three dimensional measurements of the turbulent transport in TJ-K [110, 129] revealed that, beside the great importance of local quantities like the normal curvature κn and the geodesic curvature κg, a noticeable toroidal asymmetry in the turbulent transport is observed. The turbulent transport is increased in a small re-gion around a specific magnetic field line. This field line crosses the location where
9.1. Blob generation region 129
−1.0 −0.5 0.0 0.5 1.0
θ(π)
−2
−1 0 1 2 3
hκni(m−1 )
v
θGR
1GR
22 limiters 1 limiter
Figure 9.1: hκni as a function of the poloidal angle evaluated at the toroidal position of port O6 for the setup used in this work (closed line) and the one used in Ref. [48] (dashed line). GR1 and GR2 mark the observed generation regions in the setup with 1 and 2 limiter disks, respectively. The small arrow indicates the poloidal propagation direction.
the microwave heating mainly deposits its power at the upper hybrid resonance [61]
and, therefore, instabilities may be driven due to an interaction of the microwave and the plasma. A possible explanation was given in Ref. [110] namely that trapped particles may exist, which destabilize drift waves and increase the radial transport.
In the limiter setup, this particular field line intersects the limiter plates, there-fore, it is unclear if it plays a role for the stability in the confined region. It is noteworthy, however, that the poloidal angle where the generation region is ob-served at port O6 agrees remarkably well with the poloidal angle where this field line crosses the poloidal plane of O6. If in analogy to the standard setup without limiters instabilities along a particular field line exist in the confinement region, these may indeed be responsible for the generation region.
For the moment this remains as an open question. A possible experiment to assess the role of the localized microwave heating on the generation region would be to heat the plasma in a different toroidal segment and observe if the generation point changes its location in the poloidal cross section.