Comparison Study of 2D Images of Temperature Fluctuations during Sawtooth Oscillation with Theoretical Models

Abstract

High temporal and spatial resolution two-dimensional (2D) images of electron temperature fluctuations were employed to study the sawtooth oscillation in the Toroidal Experiment for Technically Oriented Research tokamak plasmas. The 2D images are directly compared with the expected 2D patterns of the plasma pressure (or electron temperature) from various theoretical models. The observed experimental 2D images are only partially in agreement with the expected patterns from each model: The image of the initial reconnection process is similar to that of the ballooning mode model. The intermediate and final stages of the reconnection process resemble those of the full reconnection model. The time evolution of the images of the hot spot or island is partially consistent to those from the full reconnection model but is not consistent with those from the quasi-interchange model. The ''sawtooth oscillation'' was discovered in the early days of fusion plasma research [1] and is known as the m=n 1=1 internal kink mode, where m and n are poloidal and toroidal mode numbers, respectively. An excellent review of recent research in the field of sawtooth oscillations is given in Ref. [2]. This Letter presents a direct comparison study between experimentally measured highresolution 2D images of electron temperature fluctuations with the relevant 2D pattern from prominent physical models developed for the sawtooth oscillation physics in high temperature plasmas. The experiment was performed in the Toroidal Experiment for Technically Oriented Research (TEXTOR) tokamak plasma, which has a circular cross section, a major radius of 175 cm, and a minor radius of 46 cm [3]. The toroidal magnetic field in the present work was in the range 1.9-2.4 T, and the corresponding plasma current was <305 kA. Key plasma parameters were as follows: The central electron density and temperature are in the range 1:5-2:5 10 19 m ÿ3 and from 1.2 to 1.6 keV, respectively. The corresponding peak toroidal beta is 1:0%, and the average poloidal beta is between 0.3 and 0.5. The toroidal rotation of the plasma varied from 1 to 8 10 4 m=s. The speed of a thermal electron is 6 10 7 m=s. The Alfvén and ion acoustic speeds are 5 10 6 and 7 10 5 m=s, respectively. Using plasma parameters close to the q 1 surface, the characteristic reconnection time ( c ) is 700 s.High-resolution 2D images of the electron temperature fluctuations in TEXTOR have been measured with a 2D electron cyclotron emission imaging (ECEI) system. The basic principle of the technique is similar to that of conventional 1D ECE radiometers [4,5]. The new feature of the ECEI diagnostic is that measurements are done in a 2D matrix of sample volumes. Since the ECEI diagnostic has recently been published elsewhere [6,7], we only briefly mention it here as an introduction to the comparison with theoretical models. The system has 16 vertical 8 horizontal sampling volumes arranged in a 2D matrix of 16 cm vertical 7 cm radial . The vertical full width at half maximum of the central antenna pattern is 2 cm...

Spontaneous onset of magnetic reconnection in toroidal plasma caused by breaking of 2D symmetry

“…On MST, Choi et al 17 found that small sawtooth events are linearly unstable and hence spontaneous, while larger events require a nonlinear mode coupling for the instability to grow. In sawtooth reconnection, detailed measurements by Park et al 7,18,19 and Munsat et al 20 from the TEXTOR tokamak found toroidally localization of the reconnection events, with reconnection starting at specific toroidal locations with both "good" and "bad" curvature. These observations show that the 2D theories may not capture the essential physics to describe the onset of spontaneous reconnection.…”

Experimental investigation of the trigger problem in magnetic reconnection

“…These measurements, which take advantage of advances in electron cyclotron emission imaging [64,65,66], have shown that sawtooth onset is localized both toroidally and poloidally and that the temperature profile flattening is well-organized as opposed to stochastic [67]. The localization is observed even though the magnetic field geometry is mainly 2D.…”

Final Report: Laboratory Studies of Spontaneous Reconnection and Intermittent Plasma Objects

“…On MST, Choi et al [19] found that small sawtooth events are linearly unstable and hence spontaneous, while larger events require a nonlinear mode coupling for the instability to grow. In sawtooth reconnection, detailed measurements by Park et al [9,20,21] and Munsat et al [22] from the TEXTOR tokamak found toroidally localization of the reconnection events, with reconnection starting at specific toroidal locations with both "good" and "bad" curvature. These observations show that the 2D theories may not capture the essential physics to describe the onset of spontaneous reconnection.…”