High resolution soft-x-ray tomography of sawtooth oscillations on the Tokamak de Varennes

Abstract

Up to five arrays of soft-x-ray detectors are used with the Fourier-Bessel harmonic reconstruction method to generate 2D images of sawtooth collapses during Ohmic plasma discharges on the Tokamak de Varennes. For low-harmonic reconstructions (M< 2), a quasi-interchange behavior (Wesson model) is observed during the sawtooth crash, but a reconnectionlike behavior (Kadomtsev model) is recovered using higher harmonics {M>L 3) on the same data. This indicates that the Kadomtsev model is a better description for the sawtooth collapse, although the collapse time remains shorter than predicted.PACS numbers: 52.55. Fa, 52.30.Jb, 52.35.Py, 52.70.La Sawtooth oscillations' are internal disruptive instabilities observed on most tokamaks. This periodic phenomenon is observed on many plasma diagnostics and each cycle consists of a slow^ increase in the central temperature and density followed by a sudden collapse. As an explanation, Kadomtsev proposed the total reconnection model, ^ where an m = l mode disappears nonlinearly through resistive reconnection at the q = 1 surface, resulting in a flattening of the temperature, density, and current profile, and a release of energy. Detailed investigations using nonlinear two-dimensional codes ^"^ have shown that this model is consistent with MHD theory, and is able to explain several experimental observations on small tokamaks. ^"^ Recently though, this model has been called into question in the light of new experimental results obtained on large tokamaks.^'^ The total reconnection model would fail to explain the fast sawtooth crash, and to a lesser extent, the lack of large precursors and the presence of large slowly decaying successor oscillations. A number of recent works have attempted to explain these new observations, *^"^^ but most of the attention has been focused on the quasi-interchange (Wesson) model'^ since soft-x-ray-emission reconstructions on the Joint European Torus (JET) seem to agree with topological predictions of the sawtooth collapse phase. ^''^ In this model, a flattened q profile leads to an unstable m = l mode, which drives a cold bubble toward the center of the plasma, expelling the initial hot core through an interchange mechanism. Qualitative diff*erences between the total reconnection model and the quasi-interchange model should make clear experimental identification possible through tomographic imaging of soft-x-ray emission during the sawtooth collapse. In the total reconnection model, the hot core of the q < 1 region moves almost rigidly to one side, whereas in the quasiinterchange model, the initial hot core becomes a crescent when the cold bubble penetrates the center region due to convective flow, ^ ^ Despite such contrasting topological diff*erences, experimental identification is not straightforward since spatial inversion techniques must be used to unfold line-integrated data obtained with discrete detectors. On tokamaks, several limitations on viewing angles and the number of detectors favor the Fourier-harmonic-reconstruction technique, whic...

Recent experiments in the EAST and HT-7 superconducting tokamaks

“…It shows that, a large heat flow transfer across the x point of (both the m=1 and m=2) islands (C-E) and then the transferred energy gradually spreads poloidally over the peripheral region near the q=1 surface (E-F), while the reconstructed sawtooth crash process from total signals, as shown in the middle frames in Fig. 18, seems to be consistent with the Kadomtsev's model, which was also concluded in Ref [11]. The asymmetric heat flow can occur both in low field side and high field side, which is different from the observations in high β plasmas on TFTR [12].…”

“…Tomography of high-resolution soft-x-ray arrays can give the sawtooth crash structure. The Fourier-Bessel inversion method is used in the reconstruction by choosing M=2-3 and L=6-8 [11] . In order to present a clear picture of the crash, the sawtooth activity in Lower Hybrid Current Driven (LHCD) plasma is analyzed due to the large fluctuating amplitude (20-30% of the total emission in the central sightline) for distinguishing the crash and precursor stage.…”

Tomography of full sawtooth crashes on the Tokamak Fusion Test Reactor

“…So, the heat conduction and the particle convection along the field line, which are much greater than that across the flux surface, causes the very fast transfer of the heat and particles across the qϭ1 surface. This model is consistent with soft x-ray tomography, 2 but it is inconsistent with q profile measurements. 3 It has been suggested that a three-dimensional ͑3-D͒ reconnection 4 plays an important role in the sawtooth crash.…”

Sawtooth Oscillation in Current-Carrying Plasma in the Large Helical Device

“…Here, m and n are the poloidal and the toroidal mode numbers, respectively. This model is consistent with the soft x-ray tomography measurement [4], but is not consistent with the observed crash time [5] or with the q profile measurement [6]. Considered three dimensionally, the full reconnection model implies that the set of reconnection points forms an m 1 helical line around the torus [7].…”