Sawteeth and energy confinement in the Madison Symmetric Torus reversed-field pinch

Abstract

Most Madison Symmetric Torus ͑MST͒ ͓Fusion Technol. 19, 131 ͑1991͔͒ reversed-field pinch discharges exhibit sawtooth oscillations with a period of 2-5 ms, corresponding to magnetohydrodynamic ͑MHD͒ instability and increased transport. However, in discharges where the plasma-facing wall has been boronized, the plasma resistivity is reduced, and sawteeth are often suppressed for periods up to 20 ms. The energy confinement time during these sawtooth-free periods is triple the normal value, corresponding to a higher plasma temperature and lower Ohmic input power. In addition, the steady growth of the dominant magnetic fluctuations normally observed between sawtooth crashes is absent. © 1996 American Institute of Physics. ͓S1070-664X͑96͒02703-5͔Sawtooth oscillations have been observed in several reversed-field pinch ͑RFP͒ experiments. 1-3 Similar to the case in tokamaks, the RFP sawtooth crash coincides with a sudden transfer of energy from the core plasma to the edge and is interpreted as magnetohydrodynamic ͑MHD͒ relaxation. During the slow sawtooth rise phase in the RFP, the profile, where (r)ϭ 0 J-B/B 2 , peaks as a result of resistive diffusion and the peaked parallel equilibrium inductive electric field. The amplitudes of the dominant poloidal mode number mϭ1 magnetic fluctuations ͑or modes͒, resonant in the plasma core, gradually increase during this phase. The faster sawtooth crash phase results in the redistribution of parallel current, flattening (r). The crash is a discrete dynamo event, during which edge current is driven, and toroidal flux is generated. Sawteeth degrade confinement in two ways. First, the growth of magnetic fluctuations increases transport. Second, plasma-wall interaction, most prominent in the crash phase, injects radiative impurities into the plasma. Hence, control of sawtooth oscillations and understanding of their origin is essential.In the Madison Symmetric Torus ͑MST͒ RFP, 4 sawteeth with periods of several ms have, until recently, accompanied nearly all discharges. 5 In this paper we report the production of discharges, following boronization 6 of the plasma-facing wall, in which sawteeth are absent for extended periods ͑up to 20 ms͒. We believe that these sawtooth-free periods ͑SFP͒ are due primarily to decreased current diffusion ͓and slowed peaking of (r)͔ arising from a reduced plasma resistivity. During SFP we observe that ͑1͒ the energy confinement time triples relative to normal sawtoothing discharges, and ͑2͒ the steady growth of the dominant mϭ1 modes normally observed between sawtooth crashes is absent. Also observed in SFP ͑and in some boronized discharges without SFP͒ are discrete bursts of mϭ0, toroidal mode number nϭ1 mode activity, each corresponding to a small increase in the toroidal flux.Shown in Fig. 1 are data from a typical MST discharge in which sawteeth occur throughout. Each sawtooth crash causes a rapid increase in the overall toroidal magnetic flux ͓Fig. 1͑a͔͒, while enhancing the reversal of the toroidal magnetic field at the edge ͓Fig. 1͑b͔͒. Increased ...

Active-Feedback Control of the Magnetic Boundary for Magnetohydrodynamic Stabilization of a Fusion Plasma

“…The measured increase of the temperature in the island region can be attributed to a local improved confinement, but the overall reduction of the transport due to the suppression of the higher n modes and reduction of the core field line stochasticity [24] can also play a key role. The dynamics also has similarities to that detected in other RFPs operating with a thick ideal wall [27,28]. Stationary singlehelical states are observed in numerical simulations [29][30][31].…”

Scaling of magnetic turbulence with Lundquist number in relaxed state devices

“…This implies that a discrete dynamo may be more desirable than a continuous dynamo. This contrasts with an observation by Chapman et al 14 from Madison Symmetric Torus 32 ͑MST͒, indicating that a continuous dynamo has better confinement than a discrete dynamo, at least in the neighborhood of Sϳ10 6 . Together, these two results imply that as S increases, the discrete dynamo may eventually have superior energy confinement.…”

E×B flow shear and enhanced confinement in the Madison Symmetric Torus reversed-field pinch

“…In contrast to sawtooth crashes, the dynamo events occurring during EC discharges originate with the mϭ0 modes, resonant in the edge. 17,18,27 The momentary increase in b rms caused by these events is dominantly mϭ0. Note that these events occur during PPCD discharges as well ͑as will be shown below͒.…”

“…Such current generation characterizes the RFP dynamo. Another common form of RFP dynamo event is the sawtooth crash, [25][26][27] which arises from the rapid destabilization of the core-resonant mϭ1 modes. In contrast to sawtooth crashes, the dynamo events occurring during EC discharges originate with the mϭ0 modes, resonant in the edge.…”