H.Q. Wang

Scenario development for high βp low torque plasma with qmin above 2 and large-radius internal transport barrier in DIII-D

A recent experiment on DIII-D, which was conducted by the joint research team from DIII-D and EAST, has extended the previous high βp, high qmin regime, which has been tested in the 2013 DIII-D/EAST joint experiment, to inductive operation at higher plasma current (Ip=0.8 MA) and significantly higher normalized fusion performance (G = H89βN/=q

Advances in understanding of high-Z material erosion and re-deposition in low-Z wall environment in DIII-D

2\atop{95}

Study of the impact of resonant magnetic perturbation fields on gross tungsten erosion using DiMES samples in DIII-D

= 0.16). The experiment aims at exploring high performance scenario with qmin > 2 and reduced torque for long pulse operation, which can be potentially extrapolated to EAST. The effort was largely motivated by the interest in developing a feasible scenario for long-pulse high performance operation with low torque on EAST. Very high confinement, H89 = 3.5 or H98,y2 = 2.1 with βN ~ 3.0, has been achieved transiently in this experiment together with qmin > 2 and reduced NBI torque (3~5 N-m). The excellent confinement is associated with the spontaneous formation of an internal transport barrier (ITB) in plasmas with Ip=0.8 MA at large minor radius (normalized p ~ 0.7) in all channels (ne, Te, Ti, VΦ, especially strong in the Te channel). Fluctuation measurements show a significant reduction in the fluctuation levels, including AE modes and broadband turbulence, at the location where an ITB forms. Linear gyrokinetic simulations also support the decrease of the growth rate of the most unstable mode during strong ITB formation. The simulation implies that strong suppression of turbulence and a positive feedback loop may be active in this process and is responsible for the spontaneous formation of large-radius ITB. Finally, in an unstable ITB phase, an ELM crash is observed to have a positive effect on transient formation of large-radius ITB. The formation of this kind of ITB is found to have a shielding (protecting) effect on the core plasma while isolating the perturbation due to ELM crash.

Effects of low-Z and high-Z impurities on divertor detachment and plasma confinement

A novel type of surface eroding thermocouple (SETC) has been tested and demonstrated in the small angle slot (SAS) divertor of DIII-D for fast local heat flux measurements. The thermojunction of the SETC is formed between two thin (10 μm) ribbons, which are filed over to create microfiber junctions. These thermocouples are able to be exposed directly to the plasma at surface temperatures exceeding 2000 °C and are capable of sub-10 ms time resolution. Before installation in SAS, the SETCs were exposed in the lower DIII-D divertor during L-mode and H-mode discharges, from which results are presented. In preliminary tests, SETCs proved to be a qualified diagnostic to accurately measure both the intra-edge localized mode (ELM) and inter-ELM heat flux during H-mode shots with high frequency ELMs (hundreds of Hz) and to resolve heat flux profiles during strike point sweeps. The heat fluxes measured by using SETCs are consistent with the heat fluxes measured by using IR cameras and Langmuir probes. These new diagnostic capabilities will complement the existing IR camera measurements and will be of particularly significant value to measure surface heat flux in the SAS divertor or other regions where the IR camera lacks line of sight.