Advances in understanding quiescent H-mode plasmas in DIII-D

Abstract

Recent QH-mode research on DIII-D [J. L. Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research 1996 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] has used the peeling-ballooning modes model of edge magnetohydrodynamic stability as a working hypothesis to organize the data; several predictions of this theory are consistent with the experimental results. Current ramping results indicate that QH modes operate near the edge current limit set by peeling modes. This operating point explains why QH mode is easier to get at lower plasma currents. Power scans have shown a saturation of edge pressure with increasing power input. This allows QH-mode plasmas to remain stable to edge localized modes (ELMs) to the highest powers used in DIII-D. At present, the mechanism for this saturation is unknown; if the edge harmonic oscillation (EHO) is playing a role here, the physics is not a simple amplitude dependence. The increase in edge stability with plasma triangularity predicted by the peeling-ballooning theory is consistent with the substantial improvement in pedestal pressure achieved by changing the plasma shape from a single null divertor to a high triangularity double null. Detailed ELITE calculations for the high triangularity plasmas have demonstrated that the plasma operating point is marginally stable to peeling-ballooning modes. Comparison of ELMing, coinjected and quiescent, counterinjected discharges with the same shape, current, toroidal field, electron density, and electron temperature indicates that the edge radial electric field or the edge toroidal rotation are also playing a role in edge stability. The EHO produces electron, main ion, and impurity particle transport at the plasma edge which is more rapid than that produced by ELMs under similar conditions. The EHO also decreases the edge rotation while producing little change in the edge electron and ion temperatures. Other edge electromagnetic modes also produce particle transport; this includes the incoherent, broadband activity seen at high triangularity. Pedestal values of ν* and βT bracketing, those required for International Experimental Thermonuclear Reactor [Nucl. Fusion 39, 2137 (1999)] have been achieved in DIII-D, demonstrating the QH-mode edge densities are sufficient for future devices.

Edge pedestal control in quiescent H-mode discharges in DIII-D using co-plus counter-neutral beam injection

“…As the mode grows to finite amplitude, its magnetic fields interact with the vacuum vessel wall, slowing the plasma rotation, decreasing the rotational shear and, hence, reducing the drive for the mode, thus allowing the mode to saturate at finite amplitude. Experimentally, we see a decrease in edge rotation and an increase in edge particle transport whenever the EHO starts and grows to finite amplitude [1]. The increased particle transport leads to reduced edge density and edge density gradient, which in turn reduces the edge bootstrap current.…”

“…This theory has recently been extended to provide a semi-quantitative theory of the edge harmonic oscillation (EHO) [13]. The EHO is an electromagnetic mode localized in the edge pedestal region which provides the extra particle transport necessary to keep the edge operating point below the peeling-ballooning boundary in QH-mode [1,12]. Based on the observation that the predicted growth rates for modes with low toroidal mode number € n increase with increasing shear in the edge rotation [13], the theory predicts that the EHO is a low € n peeling mode driven unstable by rotational shear at edge conditions slightly below the ELM stability limit in the absence of rotation.…”

“…In addition, density control and helium ash removal demand sufficient edge particle transport; ELM-induced particle transport could provide this were it not for the impulsive heat load problem. ELM-free QH-mode plasmas have demonstrated that all these requirements can be met simultaneously in discharges which operate with constant density and radiated power [1]. An example of long pulse, ELM-free QH-mode operation is shown in Fig.…”

“…In addition, the QH-mode edge is quite compatible with core transport barriers; indeed, most QH-mode discharges in DIII-D are quiescent double barrier plasmas with both a core and an edge transport barrier. These plasmas exhibit time-averaged edge particle transport more rapid than that produced by ELMs while operating at reactor relevant pedestal beta ( € β ped ~ 1%) and collisionality ( € ν i * = 0.1) [1,8].…”

“…QH-modes in DIII-D have been run for long duration, a bit longer than 4 s, which is about 30 energy confinement times, € τ E , or about 2 current relaxation times € τ R [1]. The maximum duration to date has been limited by neutral beam pulse length.…”