I-mode pedestal relaxation events in the Alcator C-Mod and ASDEX Upgrade tokamaks

Abstract

In some conditions, I-mode plasmas can feature pedestal relaxation events (PREs) that transiently enhance the energy reaching the divertor target plates. To shed light on their appearance, characteristics and energy reaching the divertor targets, a comparative study between two tokamaks – Alcator C-Mod and ASDEX Upgrade – is carried out. It is found that PREs appear only in a subset of I-mode discharges, mainly when the plasma is close to the H-mode transition. Also, a growing oscillating precursor before the PRE onset is observed in the region close to the separatrix in both devices, and a discussion on a possible triggering mechanism is outlined. The PRE relative energy loss from the confined region is found to increase with decreasing pedestal top collisionality ν* ped. Similarly, also the relative electron temperature drop at the pedestal top, which is related to the conductive energy loss, rises with decreasing ν* ped. Based on these relations, the PRE relative energy loss in future devices such as DEMO and ARC is estimated. Finally, the divertor peak energy fluence due to the PRE is measured on each device. Those values are then compared to the model introduced in [1] for type-I ELMs. The model is shown to provide an upper boundary for PRE energy fluence data, while a lower boundary is found by dividing the model by three. These two boundaries are used to make projections of the PRE divertor energy fluence to DEMO and ARC.

Simulation study of particle transport by weakly coherent mode in the Alcator C-Mod tokamak

“…where R is the major radius and B p is the local poloidal magnetic field, the poloidal wavenumber k θ and its spectrum can be obtained as shown in figure 8. It can be seen that the peak k θ is about 1.8 cm −1 , consistent with that of the WCM (1.5-2 cm −1 ) obtained through the gas-puff imaging [14,38] and PCI observations [8]. Both ω r = 2π f and k θ are positive, implying that the mode propagates in the EDD.…”

“…This makes I-mode naturally stable to the edge-localized modes (ELMs) [9] arising from the coupling of the pressure-driven ballooning and current-driven kink/peeling instabilities [10][11][12]. Under certain circumstances, small (<1% drop in stored energy) and short intermittent ELM-like events occur occasionally in Imode [9,13,14]. Combining both the advantages of L-and H-modes, I-mode is a promising high-performance operation regime for reactor-scale tokamak devices.…”

Characterization of pedestal burst instabilities during I-mode to H-mode transition in the EAST tokamak

“…PBIs can be observed by various diagnostics, such as electron cyclotron emission (ECE), Doppler reflectometer (DR), bolometer, soft x-ray (SXR), Mirnov probes and divertor Langmuir probes (Div-LPs). Similar events were reported in C-Mod [18,31], DIII-D [32] and AUG [33]. These events are called pedestal relaxation events (PREs) in both AUG and C-Mod [31].…”

“…Similar events were reported in C-Mod [18,31], DIII-D [32] and AUG [33]. These events are called pedestal relaxation events (PREs) in both AUG and C-Mod [31]. However, the characteristics of these quasi-periodic instabilities are still not clear.…”

“…The comparison between PBIs and PREs is shown in table 2. In AUG and C-Mod, PREs with the frequency range of 0.1-0.7 kHz mainly occur during the I-H transition, which typically result in relative energy loss of 0.5%-3% [31,52]. Moreover, PREs can be triggered by the core sawtooth instabilities in C-Mod.…”