ELM resolved energy distribution studies in the JET MKII Gas-Box divertor using infra-red thermography

Abstract

Using infra-red (IR) thermography, power loads onto the MKII Gas-Box divertor targets have been investigated in Type-I ELMy H-Mode plasmas at JET in medium current discharges (I p = 2.6 MA and B T = 2.7 T). Heat fluxes are calculated from the measured divertor target tile surface temperatures taking into account the influence of co-deposited surface layers on tile surfaces. This is particularly important when estimating the energy deposition during transient events such as ELMs. Detailed energy balance analysis is used, both from IR and tile embedded thermocouples, to demonstrate an approximately constant ELM-averaged in/out divertor target asymmetry of ≈0.55 and to show that the ELM in/out energy deposition ratio ranges from 1 : 1 to 2 : 1. The inter-ELM in/out ratio is close to the ELM-averaged value at low pedestal collisionalities and decreases down to values close to zero when the inner target plasma detaches at the highest pedestal collisionalities. The fraction of ELM transported energy is observed to behave differently for the inner and the outer divertor. At higher pedestal collisionalities nearly the full inner target load is due to the ELMs whereas for the outer target the ELM transported energy never exceeds values of ≈0.3 of the total energy deposited there. The fraction of ELM energy arriving at the divertor compared with the pedestal loss energy in JET is found to be in the range of 0.75 for small ELMs down to 0.4 for large ELMs * This work has been conducted under the European Fusion Development Agreement.

ELM induced divertor heat loads on TCV

“…The latter are subsequently converted into radial profiles of surface temperature assuming an emissivity of 0.85. Heat flux profiles are then determined using the 2D finite difference code THEODOR [7], including temperature dependent thermal properties of the polycrystalline graphite target tiles, together with a simple model for the thin layers present on the tile surfaces [4]. Whilst these thin deposited layers complicate the derivation of heat fluxes from surface temperatures, they are also quite useful since the temperature rise on a layered surface can exceed that of clean graphite surfaces by a factor of 3 for the same incident heat flux.…”

“…3) can be accounted for in a simple way by assuming a layer with zero heat capacity and using a heat transmission coefficient a [4]. In this case, the bulk temperature is related to the measured IR surface temperature by T bulk = T IR À q/a, with q the incident heat flux density.…”

“…rise times to peak heat flux) on the divertor targets [1], the extent to which deposition profiles are broadened during the ELM compared with the inter-ELM periods and the presence of filaments in the profile (and if they are present, the energy fraction that they carry) [2]. In addition, an important diagnostic issue is the influence of surface layers, present in any real tokamak experiment, on bulk target material and the consequences of these layers for correct interpretation of the heat fluxes deduced from the IR surface temperature measurements [3,4]. A new fast IR diagnostic, viewing the outer divertor target, has been installed on the TCV tokamak and has recently provided first data for transient interactions in ELMing ohmic H-mode, of which Type-IIIs are shown in this article.…”

Main chamber sources and edge transport of tungsten in H-mode plasmas at ASDEX Upgrade

“…A rough estimate of the ELM frequency can be gained from the assumption that 1/3 of the power transported across the separatrix P sep is exhausted within ELMs [36]. Using P heat =120 MW and P rad,core =0.25P heat gives P sep =90 MW and f ELM =30 Hz.…”

Comment on “Magnetic geometry and physics of advanced divertors: The X-divertor and the snowflake” [Phys. Plasmas 20, 102507 (2013)]

“…Among these are as follows: À Processes in the pedestal region just inside the main separatrix (affected by magnetic shear, resonant magnetic perturbations, velocity shear, and prompt ion losses) that influence both the SOL structure between intermittent edge localized modes (ELMs 8 ) and "germination" of ELMs. À ELMs and their effect on the heat load (in particular, significant asymmetries in the energy deposition between outer and inner targets, with more energy dumped into the inner target 9 ). À Energy and particle flux redistribution between multiple divertor legs and associated separatrix strike points as in the snowflake divertor.…”