Observations of sawtooth postcursor oscillations in JET and their bearing on the nature of the sawtooth collapse

Abstract

An analysis of sawtooth postcursor oscillations in JET is presented. From electron cyclotron emission measurements, the radial Te profile on the equatorial plane is obtained with high time resolution. The plasma rotation is used to reconstruct the constant temperature contours in the poloidal plane from the Te profile. The postcursor oscillation observed after a partial collapse shows a cold region with a structure very much like that of an m = 1 magnetic island, while the hot plasma core is displaced but still has concentric Te contours. After a full collapse, a cold region of similar structure is seen, while the hot core is displaced over a much larger distance and is squeezed into a crescent shape. Also a large amount of energy is lost from the hot core. It is concluded that partial and full collapses are likely to be due to the same instability and that reconnection is involved in the non-linear evolution of the instability. It is suggested that the amplitude at which the instability saturates determines the type of sawtooth collapse and the amplitude of the postcursor oscillation.

Simultaneous microwave imaging system for density and temperature fluctuation measurements on TEXTOR (invited)

“…Electron cyclotron emission measurements have been routinely used to measure profiles 13 and fluctuations of electron temperature of the plasmas such as sawteeth oscillations, 14,15 in magnetized toroidal plasmas since 1974. In a conventional ECE radiometer, a horn antenna receives the ECE radiation at the outboard side, which is separated into different frequency bands, each corresponding to a different horizontal location in the plasma as illustrated in Fig.…”

Gradients of electron temperature and density across m = 2 magnetic islands in RTP

“…With sufficiently detailed measurements, it could be possible to determine the temperature and pressure profiles inside the island. Detailed experimental observations of large magnetic islands mainly concern the (m,n)=(1,1) pre-and post cursors of the sawtooth instability [1], the m/n=1 region of high density observed in JET (the so-called snake) [2], and the (m,n)=(2,1) mode that occurs as a precursor of a major disruption [3][4][5][6]. In the case of the (m,n)=(1,1) islands, measurements of T e confirmed the theoretical expectation [1], with T e -contours mapping out the island structure.…”

Chapter 3: Microwave Diagnostics

“…In addition, they have been used to measure large amplitude fluctuations such as sawtooth oscillations. 43,44 However, information regarding small amplitude, turbulent temperature fluctuations was not available until 1993, when the intensity interferometric technique was first applied. 45,46 This is due to the coarse temperature resolution limit determined by the intrinsic radiation noise, which is given by DT rms ϭ~T e ϩ T n !…”

“…91 Another way to obtain 2-D T e profiles relies on the assumption that the plasma rotates poloidally as a rigid body; therefore, the temporal variations measured by a 1-D ECE system near the plasma midplane can be interpreted as poloidal variation. 43,44 Important experimental results regarding plasma equilibria 91 and MHD fluctuations 43,44 have been obtained using this approach. In the ECEI approach, the single antenna of a conventional heterodyne ECE radiometer is instead replaced by an array of antennas, using large-diameter optics to image the plasma onto the array, which transforms the 1-D radiometer into a true 2-D imaging diagnostic, thereby permitting time-resolved 2-D fluctuation measurements.…”