Session VI03: Invited: MFE VI - Advances in Tokamak Operating Scenarios

Simulation and experiment of EC steering of LH deposition on EAST

Localization of lower hybrid (LH) deposition has been predicted in time-slice simulations using the ray-tracing code GENRAY¹ and the Fokker-Planck solver CQL3D², and is confirmed experimentally for the first time. This work demonstrates the application of Electron Cyclotron (EC) power to create a local population of fast electrons in phase space, on which the LH power will preferentially damp, thus concentrating the LH power damping at the EC resonance layer. Scans changing the deposition location of 2.2 MW of EC from a radius of r = 0 to 0.25 across discharges reduced the internal inductance from l_i = 1.31 to 0.94, indicating a broader current profile. As the EC waves were aimed for co-Ip current drive (CD), a portion of this change is the direct result of moving the ECCD location radially outward. However, a simultaneous broadening is observed in the hard x-ray (HXR) profile, which measures bremsstrahlung radiation from the fast electron population created by the LH waves. This confirms that the LH deposition profile moved radially outward along with the EC. Shifting the toroidal injection angle of a subset of the ECCD power from co-I_p to counter-I_p to heating only at fixed EC and LH power levels showed that the synergistic effect between EC and LH depends on the fast electron population created by co-I_p ECCD, not just on the locally increased T_e. The combination of the steerable ECCD and efficient LHCD provides a new tool for broadening and tailoring steady-state current profiles, which opens new avenues in scenario development for advanced tokamak concepts that require efficient off-axis current drive.

[1] Smirnov A.P., et al. Bull. Amer. Phys. Soc. (1995).

[2] Harvey, R.W., et al. Proceedings of the IAEA Technical Committee Meeting on Simulation and Modeling of Thermonuclear Plasmas (1992).   

Progress of joint ITER/EAST experiments with tungsten plasma-facing materials in support of the ITER new baseline

Recent joint ITER/EAST experiments were performed to address specific ITER questions on the reliability and operability of EAST with tungsten (W) as a plasma-facing material. Plasma start-up and stationary limiter plasmas were achieved on the EAST outboard, fully actively cooled W limiter, when on-axis ECH heating is applied. A constant radiated fraction is found in the stationary phases of limiter plasmas over a significant density range compatible with a self-regulating sputtering mechanism for W which is seen in ITER modeling: higher plasma densities can significantly reduce the influx of medium and high-Z impurities leading to similar radiative fractions as low densities with higher impurity influxes. Boronizations assisted by ICWC and GDC have been compared to investigate the uniformity and quality of the boron coating as well as the retained fuel and its removal. A more uniform plasma was obtained with both toroidal and vertical magnetic field with ICWC leading to more uniform coatings. The neutral particle spectra in ICWC were measured for the first time by the low-energy neutral particle analyzer in ITER relevant conditions, which suggests the importance of uniform gas fueling and minimizing the neutral pressure thereby increasing the mean free path of energetic ions for achieving a uniform particle flux. High confinement H-mode operation has been successfully achieved under both boronized and unboronized wall conditions with type II ELMs at q95 ~ 6 and with ITER-like low input torque/no-torque with total input powers from 3 MW to 5 MW by ECH+NBI/LHW. On the contrary type-I ELMy H-modes in similar conditions show poorer energy confinement associated with high W impurity sources, this is alleviated with ELM suppressed by n = 2 RMPs but without a significant improvement of energy confinement. In addition, long pulse type II H-modes (~ 60 s) with dominant electron heating have been successfully achieved with the W wall without boron coatings.   

Intrinsically Grassy ELM Edge Near Peeling Boundary in High Beta Poloidal Regime on DIII-D

Recent experiments on the DIII-D tokamak have discovered an intrinsically grassy ELM regime operating at reactor-relevant low pedestal collisionalities near the peeling stability boundary, distinct from the typical high-collisionality grassy ELMs near the ballooning boundary. This regime supports a high-confinement, high-β_N core and offers substantial benefits for ITER and future FPPs, addressing critical issues like divertor heat load management and efficient impurity exhaust. These plasmas feature a high-performance Hybrid core (β_N ~ 3.5, β_P ~ 2.0, H₉₈ ~ 1.6, n/n_GW ~ 0.6) coupled with a grassy ELM edge (ELM frequency ~ 400Hz and ELM size △W_ELM/W_ped < 1%) at low edge collisionality (ν^∗_e ~ 0.1). Notably, the divertor heat flux widths in both inter- and intra-grassy ELM phases are about 50% wider than those observed during Type-I inter-ELM phases, displaying a small variation throughout the grassy ELM phases. Accessing this low-collisionality grassy ELM regime involves synthesizing factors such as low pedestal density gradients, high β_P, and widening of density and temperature pedestals. The high β_P induces a strong Grad-Shafranov shift, which stabilizes the ballooning component of the peeling-ballooning modes. Numerical modeling suggests a weak global peeling-ballooning mode, producing much less power flux into SOL compared to large ELM, fosters these grassy ELMs. Unique edge turbulences observed in this regime enhances edge transport, facilitating wide pedestals with gradients below the EPED-KBM scaling predictions. Detachment is achieved through divertor nitrogen injection, however, the increased collisionality correlates with a loss of unique edge turbulences and an increase in ELM size. Since ITER and FPPs can maintain low collisionality pedestals even at high densities, this intrinsically grassy ELM regime offers a promising core-edge integration solution.   

Multi-scale Interaction for Edge-Localized-Mode Suppression in Turbulent Pedestal in DIII-D plasmas

The multi-scale pedestal turbulence interaction shows a viable means for suppression of Edge Localized Modes (ELMs). Specifically, how interactions between large-scale MHD and small-scale drift wave turbulence modulate particle flux is studied in DIII-D wide pedestal quiescent H-mode (WPQH). The large-scale, low-frequency MHD (10-60 kHz) rotates in the ion-diamagnetic direction and is identified as weakly excited Peeling-Ballooning (PB) mode; the small-scale, high-frequency turbulence (60kHz-2MHz) rotates in the electron-diamagnetic direction and is comprised of electron drift waves. Alternating evolution of PB mode fluctuations, electron drift waves, and background density/temperature gradients are observed in WPQH mode pedestals. BES velocimetry analysis reveals that strong bicoherence, negative inward turbulent particle flux, and scatter of the cross phase between density and radial velocity perturbation of MHD during an electron drift wave burst. Such results demonstrate that the interplay between scale-separated modes plays a crucial role in determining ELM dynamics. Synergistic numerical modeling demonstrates that small-scale electron drift waves scatter the cross phase of the pressure and radial velocity perturbation of PB mode, resulting in decoherence of the PB-driven flux. This scattering interaction prevents PB growth and suppresses the ELM. A theoretical model to quantify the impact of electron drift wave scattering on PB modes has also been developed. This work yields a novel nonlinear prediction of the shift of the ELM onset boundary induced by the ambient electron drift waves, thereby indicating when a turbulent pedestal can be maintained in a quiescent state in this scenario. In addition, this work showcases a new type of multi-scale interaction physics, which can play a role in a wide range of physical systems.