Investigation of magnetic reconnection during a sawtooth crash in a high-temperature tokamak plasma

Abstract

In this paper a laboratory investigation is made on magnetic reconnection in high-temperature Tokamak Fusion Test Reactor (TFTR) plasmas [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 51]. The motional Stark effect (MSE) diagnostic is employed to measure the pitch angle profile of magnetic field lines, and hence the q profile. An analytical expression that relates pitch angle to q profile is presented for a toroidal plasma with circular cross section. During the crash phase of sawtooth oscillations in plasma discharges, the ECE (electron cyclotron emission) diagnostic measures a fast flattening of the two-dimensional (2-D) electron temperature profile in a poloidal plane, an observation consistent with the Kadomtsev reconnection theory. On the other hand, the MSE measurements indicate that central q values do not relax to unity after the crash, but increase only by 5%–15%, typically from 0.7 to 0.8. The latter result is in contradiction with the 2-D models of Kadomtsev and/or Wesson. In the present study this puzzle is addressed by a simultaneous analysis of electron temperature and q profile evolutions. Based on a heuristic model for magnetic reconnection during the sawtooth crash, the small change of q, i.e., partial reconnection, is attributed to the precipitous drop of pressure gradients that drive the instability and the reconnection process, as well as flux conserving plasma dynamics.

Magnetic Reconnection

“…Simultaneously, a rapid reflux of thermal energy occurs through the reconnection region along newly connected field lines which connect the inside and the outside of q = 1 surface (Lichtenberg, 1984). The precipitous drop of the pressure gradient, which occurs within a short period of 100-200 µsec τ Sweet−P arker removes the free energy to drive the kink instability, inhibiting the full reconnection process proposed by Kadomtsev. Similar changes of central q values were measured in the sawtoothing plasmas of circular cross section tokamaks by two groups (Soltwisch, 1988) and (Levinton et al, 1993;Yamada et al, 1994;Nagayama et al, 1996). Although the final values of the central q after the crash are different, all reported ∆q < 0.1 during sawteeth.…”

“…This was evidenced by the small changes in the q profile which were documented by (Levinton et al, 1993;Yamada et al, 1994;Nagayama et al, 1996). This change of q value represents magnetic reconnection.…”

“…The observations raise an important question as to why the magnetic field lines inside the q = 1 region do not form a flat q ∼ 1 inner region after the crash as suggested by Kadomtsev (1975), while the temperature gradient diminishes to zero as predicted by his full reconnection theory. Simultaneous measurements of T e (r, θ) and q(R) profile evolutions (Levinton et al, 1993;Yamada et al, 1994) were made in TFTR tokamak. Based on these results, a heuristic model was proposed for the sawtooth crash.…”

Experimental Study of the Hall Effect and Electron Diffusion Region During Magnetic Reconnection in a Laboratory Plasma

“…In the situations where magnetic energy is abundant, explosive phenomena such as magnetospheric substorms and solar flares are thought to be driven by this dynamical process [1,2]. Magnetic reconnection also plays an important role in determining the relaxation, stability and transport properties of laboratory fusion plasmas [3][4][5][6]. This process involves the breaking and reconnecting of magnetic field lines in a narrow "diffusion region" where the ideal plasma "frozen-in" condition is violated.…”

Magnetic Reconnection Triggering Magnetohydrodynamic Instabilities during a Sawtooth Crash in a Tokamak Plasma

“…Whilst the sawtooth instability was first observed in 1974 [5], the process by which this periodic collapse of the core plasma temperature occurs is still only partially understood. Detailed diagnosis of the sawtooth crash has shown that the temperature profile is initially essentially axisymmetric, but is then deformed by a helical instability before a very rapid temperature collapse re-establishes an axisymmetric profile with a lower value at the magnetic axis [6,7].Tokamak plasmas are susceptible to sawtooth oscillations when the safety factor, q = rB φ /RB θ is less than unity [8], where r, R are the minor and major radii and B φ , B θ are the toroidal and poloidal magnetic fields. The helical perturbation which arises during the crash has an m = n = 1 structure, where m, n are the poloidal and toroidal periodicity of the wave.…”