Meeting Location: 7.2-001
Zoom Raum 2 (ID: 948-1054-7070 / PW: 021741)
Dear colleagues,
Thank you for your continued interest in divertor design!
The next colloquium is scheduled for Tuesday 19.03. from 16:30 to 17:30 in Room 7.2-001 and Zoom Raum 2 (ID: 948-1054-7070 / PW: 021741)
https://event.ipp-hgw.mpg.de/event/1324/
On the Agenda is the multi reservoir model of Core, Edge, Plasma SOL, Divertor SOL, and PFR for tokamaks and stellarators and the resulting exhaust and screening efficiencies.
Slides and Minutes from the previous colloquia can be found under:
https://event.ipp-hgw.mpg.de/category/63/
Below are the Minutes from last week:
We talked about the mandatory function to survive sputtering and heat. This resulted in the additional reverse function of impurity release. Further we discussed mechanisms to lower T_i and T_e, and in general the plasma energy.
Sputtering:
Sputtering erodes the target material and is dependent on the energy of the incoming particles. In Maxwellian distributions the temperature can be seen as an energy equivalent. However, in the tail of the distribution or for transients i.e. runaway electrons, kinetic energies play a role.
Heat:
The targets can receive heat through conduction of plasma particles and through radiation. Heat conduction through hardware is possible, if a neighboring part has a higher temperature.
The neutralization of plasma brings a necessary heat load to the divertor target. During surface recombination the target receives the full energy of the particle, consisting of ionization, thermal, and kinetic energy. To spread these particle related heat fluxes out a wider strikeline is beneficial. This can be achieved by an increase in perpendicular transport or through magnetic flux expansion. A technological solution which requires active control would be strike line sweeping.
If these energies are detached from the target, we can speak of particle, thermal, and momentum detachment. During volume recombination the ionization energy is radiated isotropic around the point of neutralization, the thermal and kinetic energy are passed onto the neutral gas in the volume.
Additional radiation sources are:
Neutron radiation (Energy and material defects)
Electromagnetic plasma radiation (Main ion and Impurity) – here the radiation location and stability are relevant
Nuclear radiation of activated materials
Auxilary heating systems
Further a divertor needs to survive:
Neutron, He, and fuel implantation and accumulation in the material
Transient events i.e. Runaway electrons and disruptions if not avoidable.
A high wall temperature can help with the brittleness of materials, as well as the carnot efficiency when using the cooling water for electricity production.
A low wall temperature can help with thermal stresses.
It is desirable to achieve a steady wall temperature, without strong transient changes.
The second topic were mechanisms to lower the plasma temperature or in general the energy.
The identified processes are ionistaion of neutrals, dissociation, molecular assisted dissociation, molecular assisted recombination, electron impact recombination and charge exchange.
Additionally impurity seeding and resulting impurity radiation can be used.
B flux expansion can be used to spread the plasma and therefore decrease density and temperature.
The ratio of perpendicular to parallel transport can be increased, to increase the spreading per meter connection length.
At a fixed ratio, a longer connection length, will lead to a wider strike line.
The strike line symmetry between inner and outer leg are dependent on X-loop location, target location and are further drift dependent.
References:
European DEMO divertor target: Operational requirements and material-design interface
https://www.sciencedirect.com/science/article/pii/S2352179115300788
The diffusion limit of ballistic transport in the scrape-off layer
https://doi.org/10.1063/1.5133839
Effect of the magnetic flux geometry of a poloidal divertor on the profiles and parameters at the target
https://iopscience.iop.org/article/10.1088/0029-5515/32/4/I13
Effects of divertor geometry on tokamak plasmas
https://iopscience.iop.org/article/10.1088/0741-3335/43/6/201/meta
Radial density profiles in a poloidal divertor modelled by diffusion across a region with variable connection length
https://onlinelibrary.wiley.com/doi/abs/10.1002/ctpp.2150320344
Thank you for your continued interest, input and the lively discussion!
Best,
Thierry