ELM temperature in JET and COMPASS tokamak divertors

Abstract

Analysis of the divertor ELM electron temperature at a uniquely high temporal resolution (1e-5 s) was reported at the JET tokamak [C. Guillemaut et al. Nucl. Fusion 58 (2018) 066006]. By collecting divertor probe data obtained during many dozens of ELMs, the conditional-average technique (CAV) yields surprisingly low peak electron temperatures, much below the pedestal ones (a reduction by 70% up to 99%!) which we, however, question. This result was interpreted through the collisional free-streaming kinetic model of ELMs, by a transfer of most of the electron energy to ions, implying a high tungsten sputtering for unmitigated ELMs in future fusion devices like ITER. Recently, direct microsecond temperature measurements on the COMPASS tokamak, however, showed that the electron temperature peak of ELM filaments measured in the divertor is reduced by less than ⅓ with respect to the pedestal one. This was further confirmed by a dedicated 1D PIC simulation and tends to prove that the pedestal electrons can transfer only their parallel energy to ions (due to low collisionality), thus less than ⅓ as is predicted by the collision-less free-streaming model. This finding is in strong contradiction with the JET observations. We have therefore compared the CAV to the direct (microsecond) ball-pen and Langmuir probes measurements in COMPASS and found very good agreement between them. Revisiting the mentioned JET CAV analysis yields indeed that the electron temperatures are much higher than previously reported, nearly as predicted by the PIC simulation, and thus the ion energy seems not significantly increasing in the SOL.

Investigation of helium exhaust dynamics at the ASDEX Upgrade tokamak with full-tungsten wall

“…Therefore, a precise estimate of the ion impact energies at the ELM peak does not have a great impact to our modeling. To this regard, according to a recent publication, the free-streaming estimate of the intra-ELM ion impact energies, which we employ, may be overestimated by up to a factor of 3 [92]. Even considering this, the impact energies at the ELM peak would be still at the order of one to few thousands of electron volts, i.e.…”

Investigation of ELM-related Larmor ion flux into toroidal gaps of divertor target plates

“…The latter is necessary because ions expelled by edge localised modes (ELMs) from the hot plasma pedestal region into the scrape-off layer (SOL) are predicted, according to the free streaming model (FSM) of ELM propagation [8,9], to not fully thermalise on their way to the divertor, thus arriving at the target plates with a significant fraction of their pedestal thermal energy. This has in fact been directly linked to the observed intra-ELM W sputtering flux on the JET [10][11][12] and DIII-D [13,14] tokamaks. For ITER, typical intra-ELM ion temperatures of up to 5 keV [5] are expected in baseline burning plasma discharges at Q DT = 10.…”

“…As discussed in section 3.1, the Pt marker on the samples in the reversed I p , B t experiment had an area density of n Pt a = 126.5 × 10 15 at cm −2 . For incident deuterium ions during an ELM with T i = 1 keV and assuming intra-ELM T e ≈ 250 eV [12], the sputter yield of Pt [36], averaged over the ion temperature distribution function, is Y D→Pt ≈ 0.024. The additional contribution of the typically <1% fraction [37] of low-Z residual impurity ions is neglected here, because it is significantly smaller (e.g.…”

Mitigation of divertor edge localised mode power loading by impurity seeding

“…We assume that impurity cooling is the only loss channel of the ELM and we ignore the transport effects which may impact the cooling. All other interactions with the SOL plasma are also ignored (including the effect of the target sheath [25]) as well as energy transfer between ions and electrons, which was estimated by the free streaming model [26] to account for up to 1 3 of the electron energy (this magnitude of energy transfer was later confirmed experimentally at COMPASS [27] and measurements consistent with these predictions were also reported at JET [28]). In this greatly simplified picture the energy of the ELM changes as:…”