Development of Research on High Performance Scenario
A recent DIII-D/EAST joint experiment on DIII-D by GA and ASIPP researchers extended the previous highβp, high qmin regime, which had been tested in the 2013DIII-D/EAST joint experiment, to inductive operation at higher plasma current (Ip = 0.8 MA) and significantly higher normalized fusion performance (G = H89βN/q952 = 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 scenario developed in this research is a good reference for future long pulse high performance plasma operation on EAST, which is anticipated to be operated at low torque and qmin>2 with RF dominant heating, balanced NBI (nearly perpendicular injection) and strong off-axis current driven by LHW and bootstrap current as well.
Very high confinement, H89 = 3.5 or H98,y2 = 2.1 withβN~3.0 was achieved transiently in this experiment together with qmin>2 and reduced NBI torque (3~5 Nm). 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 (normalizedρ~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. 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.
The research is the result of long-term international and domestic collaboration. Scientists from Zhejiang University, General Atomics, Soochow University, LLNL, University of Wisconsin-Madison and Oak Ridge Associated Universities have participated and supported the research. This work is supported by National Magnetic Confinement Fusion Science Program of China and National Natural Science Foundation of China.
The relevant research result “Scenario development for high βp low torque plasma with qmin above 2 and large-radius internal transport barrier in DIII-D”was published in the leading journal of nuclear fusion community (Nuclear Fusion, 57 (2017) 022016).
(Hongfei DU reports)
The time history of a typical discharge (DIII-D shot number (SN)# 160710)
In this discharge, plasma current ramped up from 0.6 MA to 0.8 MAwith BT = 2.05 T. NBI power varied from 8 to 10 MW, and the injected beam torque between 3~5 Nm. frad = Prad/Pinj was about 40%. βN feedback control technique was used in this discharge, and it was limited to 3.0. Generally, this discharge achieved the target of qmin>2 either in low or high plasma current phase. There were four periods of confinement enhancement in three phases (the first and second periods are in phase I together) as shown in the shaded regions.
The proportional relation between thepedestal-core correlation and thepedestal-ITB correlation
Therefore, it is concluded in this experiment, the formation of large-radius ITB has shielding effect on core plasma while isolating the perturbation due to ELM crash. The pedestal has little effect on core plasma performance with the presence of large-radius ITB.