Measurement and Modelling of Tearing Mode Stability for Steady-State Plasmas in DIII-D
High-beta, quasi-steady state scenarios represent a fundamental step towards the performance required for future fusion reactors. In DIII-D steady-state scenario discharges, the normalized beta {beta}{sub N} {triple_bond} {beta}(%) {center_dot} a(m) {center_dot} B{sub T}(T)/I{sub p}(MA) (where {beta} is the ratio of the plasma pressure to the magnetic field pressure, {alpha} the plasma minor radius, B{sub T} the toroidal magnetic field and I{sub p} the plasma current) exceeds the no-wall ideal kink beta limit. The performance of this scenario is limited by the onset of an n = 1 tearing mode, which appears on the resistive evolution time-scale (1-2 s) at constant pressure and causes both a loss of confinement and a radial redistribution of the current density from which the available current drive sources cannot recover. It is routinely observed that the injection of electron cyclotron current drive (ECCD), with a broad deposition localized around {rho} {approx} 0.35, can prevent the mode from appearing. It must be noted that this is not a case of a direct stabilization due to the interaction with the mode's rational surface. These variations of the scenario are illustrated in Fig. 1, where the total injected power [neutral beam injection (NBI) and ECCD], {beta}{sub N} and the n = 1 magnetic perturbation at the outer wall are shown. In case (a), the onset of the n = 1 mode is observed when the EC power is not present or if it is stopped before the end of the high {beta} phase, whereas in case (b) the difference is pointed out between broad and narrow current deposition (with the narrow deposition case becoming unstable). The current density profile evolution and the MHD modes of several sets of significant discharges with and without ECCD (at different locations) have been analyzed, using motional Stark effect (MSE) spectroscopy measurements for the former and edge magnetic probes measurements, toroidal rotation profiles and fast electron cyclotron emission (ECE) data for the latter. One equilibrium based on EFIT reconstruction [1] with kinetic data has been perturbed by adding local current density at a specific radius, mimicking the application of EC waves, and the changes in the stability for a sequence of equilibria with the current perturbed at various radii, have been evaluated by means of the DCON [2], GATO [3] and PEST3 [4] codes.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 963531
- Report Number(s):
- LLNL-CONF-414273; TRN: US0903371
- Resource Relation:
- Conference: Presented at: 36th EPS Conference on Plasma Physics, Sofia, Bulgaria, Jun 29 - Jul 03, 2009
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
BEAM INJECTION
CONFINEMENT
CURRENT DENSITY
CYCLOTRONS
DEPOSITION
DOUBLET-3 DEVICE
ELECTRIC CURRENTS
ELECTRONS
KINETICS
MAGNETIC FIELDS
MAGNETIC PROBES
MODE RATIONAL SURFACES
PLASMA PRESSURE
ROTATION
SPECTROSCOPY
STABILITY
STABILIZATION
STARK EFFECT
THERMONUCLEAR REACTORS