Enhancement of the stabilization efficiency of a neoclassical magnetic island by modulated electron cyclotron current drive in the ASDEX upgrade tokamak

Recent progress in the interaction between energetic particles and tearing modes

The dynamics of energetic particles and tearing modes and the interactions between them are of great significance for magnetically confined fusion plasmas. In this review, we focus on these issues in the context of tokamak plasmas. The interaction between energetic particles and tearing modes is considered from two perspectives: (i) the influence of energetic particles on tearing modes and (ii) the transport of energetic particles by tearing modes. The influence of energetic particles on tearing modes is described on the basis of a general dispersion relation for tearing modes. The effects of energetic particles are considered separately in the outer region and the island region of a tearing mode. The physics mainly results from the modification of the perturbed parallel current by energetic particles without wave-particle resonance. In addition, the resonance between energetic particles and tearing modes is also reviewed. For the transport of energetic particles, transport of both circulating and trapped energetic particles by tearing mode is reviewed. Our descriptions of physical phenomena here are based on an analytical approach, while the experiments and simulations are used to illustrate and confirm our results. Finally, a number of open issues are discussed.

A spectroscopic electric field vector imaging diagnostic for electron cyclotron heating systems

An experimental measurement of the wave electric field vector provides important data that can be used to directly compare against 3D full-wave simulations. This direct comparison yields the fastest approach toward identifying missing physics in computational models and providing a high fidelity validation platform. In this paper, we present a diagnostic that is capable of imaging the Electron Cyclotron (EC) wave electric field vector by acquiring filtered images of polarized D _β spectral satellites. The diagnostic is designed to have a spatial and temporal resolution on the order of 100 μm and 100 μs, respectively. The diagnostic purpose is to provide experimental data for the direct validation of full-wave codes used to predict EC beam propagation and absorption and to provide real-time monitoring of EC waves.

Control system of neoclassical tearing modes in real time on HL-2A tokamak

The stability and performance of tokamak plasmas are routinely limited by various magneto-hydrodynamic instabilities, such as neoclassical tearing modes (NTMs). This paper presents a rather simple method to control the NTMs in real time (RT) on a tokamak, including the control principle of a feedback approach for RT suppression and stabilization for the NTMs. The control system combines Mirnov, electron cyclotron emission, and soft X-ray diagnostics used for determining the NTM positions. A methodology for fast detection of 2/1 or 3/2 NTM positions with 129 × 129 grid reconstruction is elucidated. The forty poloidal angles for steering the electron cyclotron resonance heating (ECRH)/electron cyclotron current drive launcher are used to establish the alignment of antenna mirrors with the center of the NTM and to ensure launcher emission intersecting with the rational surface of a magnetic island. Pilot experiments demonstrate the RT control capability to trace the conventional tearing modes (CTMs) in the HL-2A tokamak. The 2/1 CTMs have been suppressed or stabilized by the ECRH power deposition on site or with the steerable launcher.

Avoiding Tokamak Disruptions by Applying Static Magnetic Fields That Align Locked Modes with Stabilizing Wave-Driven Currents

Nonrotating ("locked") magnetic islands often lead to complete losses of confinement in tokamak plasmas, called major disruptions. Here locked islands were suppressed for the first time, by a combination of applied three-dimensional magnetic fields and injected millimeter waves. The applied fields were used to control the phase of locking and so align the island O point with the region where the injected waves generated noninductive currents. This resulted in stabilization of the locked island, disruption avoidance, recovery of high confinement, and high pressure, in accordance with the expected dependencies upon wave power and relative phase between the O point and driven current.

Oblique electron-cyclotron-emission radial and phase detector of rotating magnetic islands applied to alignment and modulation of electron-cyclotron-current-drive for neoclassical tearing mode stabilization

A two channel oblique electron cyclotron emission (ECE) radiometer was installed on the DIII-D tokamak and interfaced to four gyrotrons. Oblique ECE was used to toroidally and radially localize rotating magnetic islands and so assist their electron cyclotron current drive (ECCD) stabilization. In particular, after manipulations operated by the interfacing analogue circuit, the oblique ECE signals directly modulated the current drive in synch with the island rotation and in phase with the island O-point, for a more efficient stabilization. Apart from the different toroidal location, the diagnostic view is identical to the ECCD launch direction, which greatly simplified the real-time use of the signals. In fact, a simple toroidal extrapolation was sufficient to lock the modulation to the O-point phase. This was accomplished by a specially designed phase shifter of nearly flat response over the 1-7 kHz range. Moreover, correlation analysis of two channels slightly above and below the ECCD frequency allowed checking the radial alignment to the island, based on the fact that for satisfactory alignment the two signals are out of phase.

Sawtooth pacing by real-time auxiliary power control in a tokamak plasma

In the standard scenario of tokamak plasma operation, sawtooth crashes are the main perturbations that can trigger performance-degrading, and potentially disruption-generating, neoclassical tearing modes. This Letter demonstrates sawtooth pacing by real-time control of the auxiliary power. It is shown that the sawtooth crash takes place in a reproducible manner shortly after the removal of that power, and this can be used to precisely prescribe, i.e., pace, the individual sawteeth. In combination with preemptive stabilization of the neoclassical tearing modes, sawtooth pacing provides a new sawtooth control paradigm for improved performance in burning plasmas.

A line-of-sight electron cyclotron emission receiver for electron cyclotron resonance heating feedback control of tearing modes

An electron cyclotron emission (ECE) receiver inside the electron cyclotron resonance heating (ECRH) transmission line has been brought into operation. The ECE is extracted by placing a quartz plate acting as a Fabry-Perot interferometer under an angle inside the electron cyclotron wave (ECW) beam. ECE measurements are obtained during high power ECRH operation. This demonstrates the successful operation of the diagnostic and, in particular, a sufficient suppression of the gyrotron component preventing it from interfering with ECE measurements. When integrated into a feedback system for the control of plasma instabilities this line-of-sight ECE diagnostic removes the need to localize the instabilities in absolute coordinates.