The improved H-mode at ASDEX Upgrade: a candidate for an ITER hybrid scenario

Abstract

A stationary regime with improved confinement (H 98(y,2) >1) and, simultaneously, improved stability (N >2.5) compared to standard H-mode has been investigated on ASDEX Upgrade for many years. This socalled "improved H-mode" is characterized by a q-profile with low central magnetic shear and q 0 1 that is obtained by early heating during the current ramp. The existence domain of this scenario is documented for 3.2<q 95 <4.5 and n e /n GW up to 0.85. When compared with standard H-mode, core transport is still governed by drift-wave turbulence but stronger peaked density profiles and, possibly, a higher edge stability may be responsible for the increase in confinement. Impurity control is achieved by central wave heating. The improved stability is due to the q-profile: by q 0 >1 sawteeth as the main trigger of large NTMs are avoided and the low shear significantly reduces the amplitude of (3,2) modes present at high. The stability is eventually limited by the occurrence of a (2,1) mode at typically N~3. As far as the reactor relevance of this regime is concerned, its compatibility with significant central electron heating, with high edge densities and with type-II ELMs is of importance. The improved H-mode is, therefore, seen as candidate for a long pulse ITER "hybrid" operation.

Development of advanced inductive scenarios for ITER

“…At pressures lower than the maximum achieved, there appear to be systematic trends in the type of MHD observed [9,10,13,16]. Small sawteeth are generally seen for In AUG and DIII-D, the fishbones tend to appear at higher density, while at lower density,…”

“…proxies of the ratio of the thermal kinetic to magnetic pressure There is also an apparent correlation of [8,[13][14][15]. Because this limit is a resistive mode rather than an ideal mode, the result is often a dramatic loss of energy confinement rather than an immediate disruption.…”

The performance of improved H-modes at ASDEX Upgrade and projection to ITER

“…Discharges at q 95~5 (0.8MA/2.1T) have similar MHD behaviour, stability and confinement compared to pulses at somewhat lower q 95 [6]. These discharges achieve high values for beta poloidal (~2).…”

“…The maximum values of β N achieved are in most cases near the no wall beta limit (β N =4l i ) over the range of q 95 explored. The beta limit in (improved) H-modes manifests itself as a (2,1) NTM [6].…”

“…At ASDEX Upgrade [4,5,6,7] stationary H-mode discharges have been developed which achieve the required level of confinement and stability for improving ITER's performance at full current (P fus > 400 MW and Q > 10) or for hybrid operation at reduced current. These 'improved H-modes' are characterized by a q-profile with low magnetic shear in the centre and q 0 ~ 1, typically obtained by applying heating during the current rise phase of the discharge.…”