Evolution of E × B shear and coherent fluctuations prior to H-L transitions in DIII-D and control strategies for H-L transitions

Abstract

While operating a magnetic fusion device in H-mode has many advantages, care must be taken to understand and control the release of energy during the H-L back transition, as the extra energy stored within the H-mode transport barrier will have the potential to cause damage to material components of a large future tokamak such as ITER. Examining a scenario where the H-L back transition sequence begins before the E × B shearing layer decays on its own, we identify a long-lived precursor mode that is tied to the events of the H-L sequence and we develop a robust control strategy for ensuring gradual release of energy during the transition sequence. Back transitions in this scenario commonly begin with a rapid relaxation of the pedestal, which was previously shown to be inconsistent with ideal peeling-ballooning instability as the trigger [Eldon et al., Phys. Plasmas 22, 052109 (2015)], despite being otherwise similar to a large type-I Edge Localized Mode (ELM). This so-called transient occurs when the E × B shearing rate ωE×B is significantly larger than the turbulence decorrelation rate ωT, indicating that this is not the result of runaway turbulence recovery. The transient is always synchronous with amplitude and propagation velocity modulations of the precursor mode, which has been dubbed the Modulating Pedestal Mode (MPM). The MPM is a coherent density fluctuation, which, in our scenario at least, reliably appears in the steep gradient region with f≈70 kHz, kθ≈0.3 cm−1, and it exists for ≳100 ms before the onset of back transitions. The transient may be reliably eliminated by reducing toroidal rotation in the co-current direction by the application of torque from counter-injecting neutral beams. The transient in these “soft” H-L transitions is then replaced by a small type-III ELM, which is also always synchronous with the MPM, and MPM shows the same behavior in both hard and soft cases.

The role of turbulence–flow interactions in L- to H-mode transition dynamics: recent progress

“…Preliminary data on the back-trans ition sequence have been obtained [50,105], and hysteresis has been observed between the L-H transition threshold power and the power level where a back-transition is initiated. In regimes where LCOs are present, the back-transition is typically initiated by transients resembling type I or type III ELMs, although well below peeling-ballooning stability limits [106,107]. The energy release and the evolution of sheared flow during the backtransition have been investigated for a specific scenario [106,107].…”

“…In regimes where LCOs are present, the back-transition is typically initiated by transients resembling type I or type III ELMs, although well below peeling-ballooning stability limits [106,107]. The energy release and the evolution of sheared flow during the backtransition have been investigated for a specific scenario [106,107]. Once the pedestal pressure gradient has been substantially reduced, LCOs with increasing frequency are often observed before an LCO-L-mode transition occurs with increasing turbulence.…”

Comparison of dynamical features between the fast H-L and the H-I-L transition for EAST RF-heated plasmas

“…Unfortunately, these scaling laws carry large uncertainties which reflects the impact of other 'hidden parameters', such as magnetic geometry [20], main ions [42], wall materials [30], etc. On the other hand, the fundamental dynamical process of the H-L transition may be quite different from that of the L-H transition [43], as the initial plasma conditions preceding the transition sequence, such as plasma profiles and turbulence characteristics [44,45], are significantly different. Unfortunately, there are very few, if any [44][45][46][47][48], dedicated studies of the H-L transition dynamics and databases of the power hysteresis [7,8].…”

“…On the other hand, the fundamental dynamical process of the H-L transition may be quite different from that of the L-H transition [43], as the initial plasma conditions preceding the transition sequence, such as plasma profiles and turbulence characteristics [44,45], are significantly different. Unfortunately, there are very few, if any [44][45][46][47][48], dedicated studies of the H-L transition dynamics and databases of the power hysteresis [7,8]. In this paper we thus present detailed investigations of the dynamical features of the H-L transitions in the EAST tokamak, comprising distinct analysis of the fast H-L transition as well as the H-I-L-transition involving the I-phase with limit cycle oscillations.…”

The power dependence of the maximum achievable H-mode and (disruptive) L-mode separatrix density in ASDEX Upgrade

“…Therefore, the H-L back transition line will be the decisive operation limitation here. It is worth noting that the sudden release of stored energy during an H-L back transition (akin to a large ELM) risks damaging plasma facing components [49]. Therefore, even a gap between the H-L back transition and the disruptive limit exists, one may still want to avoid H-L back transitions in a reactor.…”