Power exhaust by core radiation at COMPASS tokamak

Abstract

Substantial power dissipation in the edge plasma is required for the safe operation of ITER and next-step fusion reactors, otherwise unmitigated heat fluxes at the divertor plasma-facing components (PFCs) would easily exceed their material limits. Traditionally, such heat flux mitigation is linked to the regime of detachment, which is characterised by a significant pressure gradient between upstream and downstream scrape-off layer (SOL). However, the physics phenomena responsible for power dissipation and pressure loss are distinctly different, especially when the power dissipation is achieved by impurity seeding. In principle, it is possible to achieve substantial mitigation of the heat fluxes while maintaining conservation of the pressure along the open field lines in the SOL. This regime can be accessed by injection of medium- or high-Z impurities, which mostly radiate inside the last closed flux surface. The critical question related to such an approach is the effect on confinement and perspective fusion power generation in future thermonuclear reactors. In this work, we report on experiments at COMPASS tokamak, where neon and argon impurities were injected in ohmic or NBI-heated low confinement plasmas. With appropriate seeding waveform, stable scenarios were achieved, avoiding the radiative collapse of plasmas. Significant reduction of heat fluxes at the outer target was observed, with heat flux pattern similar to the one previously achieved by nitrogen seeding. The reduction of downstream pressure was, however, accompanied by an equal reduction of upstream pressure, indicating that the power dissipation occurred inside the separatrix. Indeed, the impurity cooling is causing a significant drop of edge temperature; however, the effect in the plasma centre is much less pronounced.

Overview of the COMPASS results *

“…In addition, a separate paper at the 28 th IAEA FEC was dedicated to more details in COMPASS detachment studies [32].…”

“…Since these elements are heavier than nitrogen, they tend to radiate mostly in the confined region and as such allowed us to explore the scenarios relevant to European DEMO, where a large fraction of power will have to be radiated inside the separatrix [31]. Stable L-mode discharges were achieved [32] and surprisingly, the methods of heat flux footprint characterisation developed for nitrogen in [28] proved to be applicable also in these discharges.…”

Full-radius integrated modelling of ASDEX Upgrade L-modes including impurity transport and radiation

“…An interesting property of this discharge is that, after an initial H-mode phase, it transitions back to L-mode but retains high confinement (H 98 ≈ 0.95) without ELMs. This type of high performing radiative L-modes has been obtained in different machines [95][96][97][98][99] and is currently investigated for potential reactor scenarios [100]. AUG #37041 is an appealing application due to the high impurity content and radiated power, the presence of multiple species (intrinsic B and W, seeded Ar) and the high confinement with no ELM activity.…”

Radiative pulsed L-mode operation in ARC-class reactors

“…Radiative heat exhaust was examined experimentally in a number of devices. Highly radiative L-mode tokamak plasmas have been demonstrated in: ISX-B, ASDEX, TEXTOR94, DIII-D, Alcator C-Mod, TFTR, ASDEX-U, and COMPASS [23,[32][33][34][35][36][37][38][39][40][41][42][43][44][45], and in some cases these plasmas demonstrated performance enhancement above the ITER89P L-mode scaling due to ion temperature gradient (ITG) driven transport suppression from increased effective ion charge, Z eff [46][47][48]. Of particular note, TFTR obtained large f rad in high-heating power discharges using Kr and Xe.…”