The DIII-D plasma control system (PCS), initially deployed in the early 1990s, now controls nearly all aspects of the tokamak and plasma environment. Versions of this PCS, supported by General Atomics, are presently used to control several tokamaks around the world, including the superconducting tokamaks Experimental Advanced Superconducting Tokamak and Korean Superconducting Tokamak Advanced Research. The experimental challenges posed by the advanced tokamak mission of DIII-D and the variety of devices supported by the PCS have driven the development of a rich array of control algorithms, along with a powerful set of tools for algorithm design and testing. Broadly speaking, the PCS mission is to utilize all available sensors, measurements, and actuators to safely produce a plasma state trajectory leading to and then maintaining the desired experimental conditions. Often new physics understanding leads to new or modified control requirements that use existing actuators in new ways. We describe several important DIII-D PCS design and test tools that support implementation and optimization of algorithms. We describe selected algorithms and the ways they fit within the PCS architecture, which in turn allows great flexibility in designing, constructing, and using the algorithms to reliably produce a desired complex experimental environment. Control algorithms, PCS interfaces, and design and testing tools are described from the perspective of the physics operator (PO), who must operate the PCS to achieve experimental goals and maximize physics productivity of the tokamak. For example, from a POs (and experimental team leader's) standpoint, a PCS algorithm interface that offers maximum actuator, algorithmic, and measurement configuration flexibility is most likely to produce a successful experimental outcome. However, proper constraints that limit flexibility in use of the PCS can also help to maximize effectiveness. For example, device limits and safety must be built into the PCS, sometimes at the algorithm level. We show how the DIII-D PCS toolset enables rapid offline testing of a new or modified algorithm in a simulated tokamak environment. Finally, we illustrate usage of PCS-based checklists and procedures that enhance experimental productivity, and we describe an asynchronous condition detector system within the PCS that enhances device safety and enables complex experiment design.
High performance discharges under steady-state condition in the Hefei Tokamak-7 (HT-7) have been investigated. Lower hybrid current drive (LHCD) was used both for sustaining plasma current and current density profile control. The experiments demonstrated that features of ion Bernstein wave heating in controlling the electron pressure profile can be integrated into LHCD plasmas and the assist localization of LHCD. This local synergy effect was used to tailor the current density profile in a negative shear configuration and to control the internal transport barrier (ITB) under steady-state conditions. The high performance with a stationary ITB at the footprint of the minimum q and βNH89>2 was sustained for >220 τE or >20 τCR. The fraction of noninductive plasma current was larger than 80% in such discharges with considerable bootstrap current. The duration at H89>1.2 and βN∼1 has been extended to nearly 8 s, longer than 400 τE. More than 90% of the plasma current was sustained by LHCD and bootstrap current. High-plasma performance and sustaining time was limited by the magnetohydrodynamic instabilities and the recycling, which caused an uncontrollable rise of the electron density.
Chinese Fusion Engineering Test Reactor (CFETR) based on the tokamak approach with superconducting magnet technology is envisioned to provide 200-MW fusion power and operate with a goal of an annual duty factor of 0.3-0.5. This report based on a zero-dimensional system study using extrapolations of current physics by considering engineering constraints, is focused on qualitative determination of the engineering parameters of the device. Conservative assumptions of plasma performance based on present day existing experiments were made to assure achievable goals, since CFETR could be a near-term project to bridge the gaps between ITER and DEMO. The baseline of 200-MW fusion power in standard H-mode for a duration longer than 1000 s and in a modest improved H-mode (or hybrid mode) with H98 ≤ 1.3 for steady-state operation derive a device of R=5.7 m, a=1.6 m in size with Bt=5 T, and total heating and current drive source power of 80 MW. More ambitious operating modes with higher fusion power reaching the alpha-particle dominated self-heating regime for burning plasma study is possible with the same device hardware, if the more advanced physics is incorporated. Since large vacuum chamber design, possible upgrades both on physics and technologies enable operation of the device with larger plasma configuration and provide potentials to demonstrate key physics issues relevant to DEMO.
In the 2014 year's campaign of Experimental Advanced Superconducting Tokamak (EAST), a series of Magnetohydrodynamics (MHD) instabilities were observed as the launching of Neutral Beam Injection (NBI), the most interesting one of which is the neoclassical tearing mode (NTM).Evidence clearly shows that a kink mode present after a strong sawtooth-like (ST-like) crash leaves a perturbation near the location of the magnetic island, providing the initial seed.The interaction of energetic ions makes the magnetic island oscillate both in island width and in rotation frequency.Analysis indicates that the bulk plasma still 2 dominates the dynamics of NTM, and the orbit excursion of energetic ions induces a polarization current and modifies the width and rotation frequency of the neoclassical magnetic island.
Two distinct regimes of turbulence are identified with Langmuir probe arrays in the edge plasmas of the HuanLiuqi (HL)-2A tokamak for the first time. The spatial and temporal coherent characteristics of the low frequency fluctuations (LFFs) of 20–100 kHz are found in significant contrast to the high frequency ambient turbulence (HFAT) of 100 kHz or higher. In the LFF regime, the deviations from the regular linear dispersion relations of the HFAT are observed. The poloidal and toroidal correlation lengths of the former are measured one order of magnitude longer than that of the latter. The ratio of the temporal scales of the fluctuations in the LFF and HFAT regimes is estimated to be of the same order as that for the spatial scales. The LFF may coexist with and differentiate from the geodesic acoustic modes. The bispectrum analysis of the data indicates that nonlinear three wave coupling between the LFF and HFAT is a possible creation mechanism for the former. The possible correlation of the results with the theory and simulation predictions on quasimodes is discussed.
The interchange-like transport is observed between two resonant surfaces (q = 1 and q = 4/3, where q is the safety factor) in a finite small positive magnetic shear regime with mild core oscillations in the experimental advanced superconducting tokamak strong on-axis electron heating H-mode plasmas. It is synchronized with the increasing gradient of the soft X-ray profile and the intensifying electron density fluctuations in the core. The analysis of two-fluid simulations combined with experimental measurements indicates the destabilization of collective resistive interchange modes with several toroidal mode numbers. The overall effect of modes leads to strong perturbations at the two resonant surfaces in contrast to that between them where the anomalous electron flux is low. Their radial displacement is beyond the resistive layer width which satisfies the condition for the nonlinear destabilization of tearing modes [L. Comisso et al., Phys. Plasmas 23, 100702 (2016)]. Evidence and analysis shown in this paper tend to understand the mechanism of mild oscillations in the core.
Evidence of a nonlinear transition from mitigation to suppression of the edge localized mode (ELM) by using resonant magnetic perturbations (RMPs) in the EAST tokamak is presented. This is the first demonstration of ELM suppression with RMPs in slowly rotating plasmas with dominant radio-frequency wave heating. Changes of edge magnetic topology after the transition are indicated by a gradual phase shift in the plasma response field from a linear magneto hydro dynamics modeling result to a vacuum one and a sudden increase of three-dimensional particle flux to the divertor. The transition threshold depends on the spectrum of RMPs and plasma rotation as well as perturbation amplitude. This means that edge topological changes resulting from nonlinear plasma response plays a key role in the suppression of ELM with RMPs.
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