H-mode Pedestal Region Research Articles

Local destabilization of the ideal or infinite-n ballooning mode in the H-mode pedestal region with high triangularity

Unlike the usual belief that the access to the 2nd stability regime of the H-mode pedestal is facilitated by strong plasma shaping, high triangularity is shown to induce a local breakdown of the access by destabilizing the ideal or infinite-n ballooning mode in the outer-well region, where n is the toroidal mode number. This destabilization in high triangularity regime is conjectured mainly due to the substantial increase of the magnetic shear, which can then make the stabilizing magnetic well effect sub-dominant. A large similarity observed in the mode excitation condition and location suggests that these locally-destabilized modes may be a possible source of the broadband fluctuations which are believed to trigger the wide pedestal quiescent-H (QH) mode in low toroidal rotation limit.

Effective current drive in the pedestal region of high-confinement tokamak plasma using electron cyclotron waves

Large-amplitude edge-localized modes (ELMs) are a major concern in the development of tokamak fusion energy. According to the peeling–ballooning theoretical model, effective current drive in the pedestal region of high-confinement (H-mode) tokamak plasmas can alter the current profile in this region and hence affect ELM instabilities. Using ray-tracing and Fokker–Planck quasilinear codes, effective and localized current drive by electron cyclotron (EC) waves in the H-mode pedestal region is studied in this paper. Numerical investigations are performed under present-day medium-sized tokamaks with an aspect ratio (A = R/a) covering a typical range of [2.7, 4.0]. Localized non-inductive current can be generated effectively by the Ohkawa mechanism of EC waves in the low-electron collisionality H-mode pedestal region. The Ohkawa mechanism-dominated current drive (OKCD) is much more effective than traditional electron cyclotron current drive (ECCD), which is the Fisch–Boozer mechanism-dominated one. Under 1–2 MW EC power, the current density of both co-OKCD and counter-OKCD has the same magnitude as edge bootstrap current density, which is calculated using the Sauter model. However, edge ECCD is much smaller than the bootstrap current. Consequently, the use of edge ECCD to control ELM failed in experiments, and only the EC heating can affect ELM behavior. Simulations on OKCD are performed in the H-mode pedestal region of realistic DIII-D tokamak plasma, and the results also support the above conclusions. The dependence of effective edge OKCD on the ratio of magnetic field and EC frequency is also found in tokamaks with a typical aspect ratio range. This work opens up a new possible field for active control of ELMs (triggering or suppressing an ELM) by edge current drive in present-day medium-sized tokamaks.

A one-dimensional tearing mode equation for pedestal stability studies in tokamaks

Starting from expressions in Connor et al. (Phys. Fluids, vol. 31, 1988, p. 577), we derive a one-dimensional tearing equation similar to the approximate equation obtained by Hegna & Callen (Phys. Plasmas, vol. 1, 1994, p. 2308) and Nishimura et al. (Phys. Plasmas, vol. 5, 1998, p. 4292), but for more realistic toroidal equilibria. The intention is to use this approximation to explore the role of steep profiles, bootstrap currents and strong shaping in the vicinity of a separatrix, on the stability of tearing modes which are resonant in the H-mode pedestal region of finite aspect ratio, shaped cross-section tokamaks, e.g. the Joint European Torus (JET). We discuss how this one-dimensional model for tearing modes, which assumes a single poloidal harmonic for the perturbed poloidal flux, compares with a model that includes poloidal coupling Fitzpatrick et al. (Nucl. Fusion, vol. 33, 1993, p. 1533).

Numerical simulation of Li pellet ablation in the H-mode pedestal region

The use of non-fuel pellets is desirable for ITER due to its decoupling ELM pacing from fueling. In this paper, the Neutral Gas Shielding Model (Parks et al., Nucl, Fusion 34(1994)417) has been applied to the ablation of Li pellets in the H-mode pedestal region. The results show that the injection velocity of pellets is a more convenient parameter for one to control the deposition profile and location of the injected species in the pedestal. The information obtained from this work can further be used as input for fluid transport codes to track the trajectories of the impurity species or as input for advanced MHD codes for better understanding the non-linear destabilization of ELMs.

Anomalous transport in the H-mode pedestal of Alcator C-Mod discharges

Anomalous transport in the H-mode pedestal region of five Alcator C-Mod discharges, representing a collisionality scan is analyzed. The understanding of anomalous transport in the pedestal region is important for the development of a comprehensive model for the H-mode pedestal slope. In this research, a possible role of the drift resistive inertial ballooning modes (Rafiq et al 2010 Phys. Plasmas 17 082511) in the edge of Alcator C-Mod discharges is analyzed. The stability analysis, carried out using the TRANSP code, indicates that the DRIBM modes are strongly unstable in Alcator C-Mod discharges with large electron collisionality. An improved interpretive analysis of H-mode pedestal experimental data is carried out utilizing the additive flux minimization technique (Pankin et al 2013 Phys. Plasmas 20 102501) together with the guiding-center neoclassical kinetic XGC0 code. The neoclassical and neutral physics are simulated in the XGC0 code and the anomalous fluxes are computed using the additive flux minimization technique. The anomalous fluxes are reconstructed and compared with each other for the collisionality scan Alcator C-Mod discharges. It is found that the electron thermal anomalous diffusivities at the pedestal top increase with the electron collisionality. This dependence can also point to the drift resistive inertial ballooning modes as the modes that drive the anomalous transport in the plasma edge of highly collisional discharges.

Microturbulence in DIII-D tokamak pedestal. II. Electromagnetic instabilities

Gyrokinetic simulations have been used to identify electromagnetic microinstabilities in the H-mode pedestal region of DIII-D shot 131 997 using global gyrokinetic code GTC. It was found that dominant instability at the top of the pedestal is the ion temperature gradient mode (ITG). In the maximum gradient location the most unstable mode is the kinetic ballooning mode (KBM) for the dominant poloidal wavenumber cm−1. For shorter wavelengths the dominant instability is the trapped-electron mode (TEM). We have demonstrated the ITG–KBM transition at the pedestal top, and TEM–KBM transition in the steep pressure gradient region as plasma pressure increases while gradients remain unchanged.

Influence of poloidal equilibrium rotation in MHD simulations of edge-localized modes

In nonlinear MHD simulations of edge localized modes (ELMs) with the code JOREK, a strong m = n = 0 poloidal flow localized in the H-mode pedestal region shears density filaments off the separatrix. This flow is nonlinearly induced by the ballooning instability (Huysmans and Czarny 2007 Nucl. Fusion 47 659, Huysmans et al 2009 Plasma Phys. Control. Fusion 51 124012). An approach of the reduced resistive MHD equations used in JOREK leads to an analytical interpretation of the equilibrium flow in both circular and X-point plasmas. Transitions between two different flow structures have been found. Similar transitions between equilibrium flow states had been predicted by Strauss (1995 Phys. Plasmas 2 1229). Finally, results concerning the influence of the equilibrium flow on the linear and nonlinear stabilities of the ELM are presented. ELM-like multiple crashes were simulated.

Edge stability of stationary ELM-suppressed regimes on DIII-D

We discuss the MHD stability of the H-mode pedestal region in two edge localized mode (ELM)-suppressed H-mode regimes on DIII-D, the quiescent (Q)H-mode and resonant magnetic perturbation (RMP) H-mode. The QH-mode is obtained at low density with most of the neutral beam power injected counter to the plasma current, and is characterized by a continuous, low toroidal mode number (n), edge harmonic oscillation (EHO). QH-mode is observed to transition back to ELMing H-mode as the rotational shear is decreased by increasing the co-current neutral beam power fraction or by increasing the plasma to conducting wall distance, consistent with a model for the EHO as a peeling instability with added drive from rotational shear, which saturates through self transport of momentum or damping of rotation through drag of the mode on the conducting wall. In RMP H-mode, an n = 3 coil provides the non-axisymmetric perturbation. Two regimes of RMP ELM suppression are observed. At low collisionality with large resonant field perturbation, the pedestal pressure gradient and current density are reduced below the peeling-ballooning mode stability limit; while at higher collisionality with large non-resonant perturbation there is little change in the pedestal parameters and type II ELMs are enhanced.

A theory-based transport model with comprehensive physics

A new theory-based transport model with comprehensive physics (trapping, general toroidal geometry, fully electromagnetic, electron-ion collisions, impurity ions) has been developed. The core of the model is the new trapped-gyro-Landau-fluid (TGLF) equations, which provide a fast and accurate approximation to the linear eigenmodes for gyrokinetic drift-wave instabilities (trapped ion and electron modes, ion and electron temperature gradient modes, and kinetic ballooning modes). The new TGLF transport model is more accurate, and has an extended range of validity, compared to its predecessor GLF23. The TGLF model unifies trapped and passing particles in a single set of gyro-Landau-fluid equations. A model for the averaging of the Landau resonance by the trapped particles makes the equations work seamlessly over the whole drift-wave wave-number range from trapped ion modes to electron temperature gradient modes. A fast eigenmode solution method enables unrestricted magnetic geometry. The transport model uses the TGLF eigenmodes to compute quasilinear fluxes of energy and particles. A model for the saturated intensity of the turbulence completes the flux calculation. The intensity model is constructed to fit a large set of nonlinear gyrokinetic turbulence simulations with kinetic electrons. The TGLF model is valid in new physical regimes that GLF23 was not. These include the low aspect ratio spherical torus, which has both a high trapped fraction and strong shaping of magnetic flux surfaces. The TGLF model is also valid close to the magnetic separatrix so the transport physics of the H-mode pedestal region can be explored.

Measurements of large poloidal variations of impurity density in the Alcator C-Mod H-mode barrier region

Simultaneous high resolution measurements of the soft x-ray emission at the top and outboard edge of the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. Impurity density profiles are derived from these measurements. The impurity density varies considerably on a flux surface in the high confinement mode (H-mode) pedestal region. The soft x-ray and impurity density pedestals are not on the same flux surface at the top and outboard edge. The pedestal widths are consistently larger at the outboard edge in the Enhanced D-alpha (EDA) H-mode, suggesting a ballooning-like character of the EDA quasi-coherent mode. The usual separation of time scales between radial and parallel impurity transport does not hold in the H-mode pedestal region of Alcator C-Mod. Thus, one should not necessarily expect that the impurity density be constant on a flux surface in this region.

Edge localized mode particle losses from the DIII-D tokamak

Particle losses associated with edge localized mode (ELM) activity on the DIII-D tokamak [J. Luxon et al., Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1986, Vol. I, p. 159] are evaluated quantitatively using density profile data obtained from a Thomson scattering system. It is shown that up to 10% of the total core particle content is lost with each ELM. The particle loss varies inversely with ELM frequency. The temporally averaged ELM particle loss is shown to be about 25% of the total particle loss from the confined region under a wide variety of plasma conditions. Although this ELM loss is a small fraction of the total ion flux, it is large compared to the particle input from neutral beam heating. Hence ELM particle losses are sufficient to control the density rise associated with H-mode plasma operation with neutral beam heating. In addition to controlling the average density by enhancing the total ion flow, albeit only by 25%, it is posited that the ELMs play a role in determining the density profile in the H-mode pedestal region.