17 / 2021-04-08 17:06:51

Edge density effects of the helium beam at W7-X

Edge plasma, Divertor, edge transport

The second operational campaign of (OP 1.2a) W7-X in 2018 operated with an edge island divertor allowing for considerably longer discharges of typically several 10’s of seconds.

Even at stationary plasma conditions a toroidal plasma current can develop, which affects the magnetic geometry of the magnetic island chain.

This evolving current leads to an increase of the confined 5/5 edge island and a general increase of the connection length in the edge region. This change of topology can also be observed in the edge profiles measured with reciprocating probes mounted on the Multi-purpose Manipulator (MPM).



Additionally, a local increase of the density profiles measured with the Multi-purpose manipulator, at the position of the local temperature minimum, is observed. It is connected to the puffing of helium from the helium beam diagnostic. This observation is paired with measurements from the overview spectrometer, the HEXOS impurity measurements and ERO2.0 modelling.



This density increase is suspected to cause an increase of the measured radiation in the edge region and a reduction of the peak heat flux on the connected divertor modules. The edge plasma along with the density increase also experiences a localized radiation increase and decrease of the divertor heat fluxes.

This observed the reduction of the divertor heat flux is superimposed on the known phenomenon of the evolving toroidal current causing a redistribution of the heat flux and a broadening of the heat flux profile.



It is necessary to quantify and differentiate the two effects on the heat flux, the evolving toroidal current is compared to the expected redistribution with HINT equilibria and field line diffusion and also to the total reduction.



Acknowledgements: This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

The second operational campaign of (OP 1.2a) W7-X in 2018 operated with an edge island divertor allowing for considerably longer discharges of typically several 10’s of seconds.

Even at stationary plasma conditions a toroidal plasma current can develop, which affects the magnetic geometry of the magnetic island chain.

This evolving current leads to an increase of the confined 5/5 edge island and a general increase of the connection length in the edge region. This change of topology can also be observed in the edge profiles measured with reciprocating probes mounted on the Multi-purpose Manipulator (MPM).



Additionally, a local increase of the density profiles measured with the Multi-purpose manipulator, at the position of the local temperature minimum, is observed. It is connected to the puffing of helium from the helium beam diagnostic. This observation is paired with measurements from the overview spectrometer, the HEXOS impurity measurements and ERO2.0 modelling.



This density increase is suspected to cause an increase of the measured radiation in the edge region and a reduction of the peak heat flux on the connected divertor modules. The edge plasma along with the density increase also experiences a localized radiation increase and decrease of the divertor heat fluxes.

This observed the reduction of the divertor heat flux is superimposed on the known phenomenon of the evolving toroidal current causing a redistribution of the heat flux and a broadening of the heat flux profile.



It is necessary to quantify and differentiate the two effects on the heat flux, the evolving toroidal current is compared to the expected redistribution with HINT equilibria and field line diffusion and also to the total reduction.



Acknowledgements: This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.