Divertor

Contact Persons: Bruce Lipschultz, Ben Dudson and Istvan Cziegler

The simplest method of removing power from the narrow SOL heat flux channel before reaching the divertor targets is through ‘line radiation’ – collisions of the fusion fuel (hydrogen isotopes D and T) and injected impurities (e.g. N) with electrons excite the electrons orbiting the hydrogen or impurity nuclei to a higher level, followed by a photon emitted which thus converts some of the incident electron energy to photons which escape the plasma and absorbed on the surrounding walls. Radiation lowers the plasma temperature and, through pressure balance, allows the density to rise – both enhance the radiation.

Once the temperature of the plasma drops enough then other atomic and molecular processes also start to play a significant role. For example charge exchange reactions between hydrogenic ions and neutrals (an electron switches from the atom to the ion) replaces a hot ion with a cold, slower neutral. If that neutral escapes from the plasma it carries away energy and momentum; A similar loss of energy and momentum can occur in ion-molecule collisions.

This case of where nature and some cleverness leads to the continued dropping of plasma temperature and power flow along the magnetic field occurs close to the divertor target as the density is highest there, higher than in the core plasma, and the temperature becomes very low. Such low temperatures ultimately leads to the plasma near the target being essentially snuffed out – recombination of ions and electrons into atoms occurs! Very little plasma or power remains (our goal). The plasma is "detached" from the target.

There are drawbacks to detachment: The detached region can expand all the way to the separatrix from the divertor target. This puts a cold region and source of radiative losses up against the separatrix and the hot core plasma which, in some cases leads to degradation of the core plasma – to be avoided if possible.

The phenomenon of detachment is a central focus of our research. Or research involves the use of analytic [1] and computer models [2] of how the magnetic field topology could lead to improvements in the radiative losses during detachment and the control of how detachment develops –something that is difficult at the moment since all the processes are so strongly coupled.

We also study the development of detachment using spectroscopy [3] and using camera views of the divertor region which are filtered for Hydrogen Balmer [4,5] and impurity emission lines [4,5] + pictures (next page). This information is used in comparing to 2D models of the divertor as well as direct analysis to study aspects of detachment (e.g. [3]).

Something on divertor turbulence as a next step in expanding our research and comparison to experiment.

Figure x: each of the pictures are tomographic inversions of a camera image, filtered for line emission. The three columns correspond to different times in the progression of detachment from the cold region being at the divertor surface (yellow line) and to reaching the x-point region (red crossed lines of the separatrix). Each row is for a different emission line image - a-c: Balmer alpha line (D_a); d-f: Balmer gamma line (D_g); g-i: Singly-ionized nitrogen (NII); and j-l: The ratio Dg/Da which is indicative of where the recombination of ions with electrons to form neutrals is.

Publications

[1] B. Lipschultz, FI Parra, IH Hutchinson, Sensitivity of Detachment extend to Magnetic Configuration and External Parameters, Nucl. Fusion 56 (2016) 056007

[2] D Moulton, J Harrison, B Lipschultz and D Coster, Using SOLPS to Confirm the Importance of Total Flux Expansion in Super-X Divertors, Plasma Phys. Control. Fusion 59 (2017) 065011

[3] K. Verhaegh, B. Lipschultz , B.P. Duval , et al, Spectroscopic investigations of divertor detachment in TCV, Nuclear Materials and Energy, 12 (2017) 1112

[4] A R Field, I Balboa, P Drewelow, et al, Dynamics and stability of divertor detachment in H-mode plasmas on JET, Plasma Phys. Control. Fusion 59 (2017) 095003

[5] J.R. Harrison, W.A.J. Vijvers , C. Theiler, Detachment evolution on the TCV tokamak, Nuclear Materials and Energy, 12 (2017) 071.