To operate fusion reactors efficiently, a high confinement is needed. Above a certain threshold in input heating power, a tokamak plasma spontaneously makes a transition from a low confinement (L-mode) regime, to a high confinement (H-mode) regime, almost doubling the confinement. This H-mode regime is presently the main scenario to operate tokamaks, and necessary to obtain a positive energy balance in future reactors. The gain in confinement in H-mode is caused by the development of a region of steep temperature and density gradients at the plasma edge, the so called H-mode pedestal. Unfortunately, this steep gradient, along with the high current density, is also the cause of an instability that causes the pedestal to periodically collapse; the edge localized mode (ELM). During these collapses, a significant part of the confined energy and particles is expelled from the plasma. The high peak heat loads during ELMs could prove difficult to handle, possibly unacceptably shortening the life time of plasma facing components in future reactors.
Improving our understanding of ELMs is therefore crucial for the success of future fusion reactors. To experimentally verify the current theories of ELMs, accurate measurements of the edge current density and the temperature and density gradients with high enough spatial and time resolution are needed.
A novel tangential Thomson scattering diagnostic is being completed in collaboration with the ASDEX Upgrade team. It can provide temperature and density profiles in the pedestal with 1-2 mm spatial resolution and 10 kHz sampling rate. This is sufficient to spatially resolve the pedestal shape and temporally resolve the ELM dynamics. Understanding these dynamics during H-mode transitions (both L-H and H-L transitions) will also enable accurate modelling of this evolution. An exciting new feature of the tangential Thomson scattering diagnostic is that it can also measure the toroidal electron velocity distribution. This constitutes the electron contribution to the current density and provides information on the current density profile at the edge. These measurements will be used as input to Fokker-Planck modelling to constrain the analysis of the edge current density, and the bootstrap current fraction. The Fokker-Planck code calculates the velocity distributions of the electrons, and hence the electron contribution to the (bootstrap) currents in the pedestal region. Experimental validation of these code calculations is needed, as due to the very large temperature and density gradients in the pedestal, nonlinear effects could arise. Tangential Thomson scattering will hence provide a validation of the Fokker-Planck code, and provide valuable information on the otherwise hard to diagnose edge current density.
What are the dynamics of the pedestal temperature and density profiles during H-mode transitions?
How large is the bootstrap current in the pedestal?
- Finalize TTS diagnostic;
- Measure detailed temperature and density profiles during L-H and H-L transitions;
- Prepare scientific papers and conference communications;
- Participation in acquiring additional funds for this research line;
- Supervision of bachelor and master students;
- Contribute to the scientific and collaborative research environment at DIFFER.
This position is an one year postdoc position. The candidate will be stationed at the Max-Planck-Institut für Plasmaphysik in Garching, Germany for the duration of the contract.
The candidate must hold a PhD degree in Physics or Engineering, and should have experience with Thomson Scattering diagnostics.
The successful candidate should have good programming skills. Good verbal and written communication skills (in English) are mandatory.
NWO-I prefers candidates who have qualifying experience (e.g. as PhD student or postdoctoral researcher) in a scientific research institute abroad.
Conditions of employment
You will be employed by NWO-I, the Institutes Organisation of NWO, for a fixed period of one year. Your salary will be up to a maximum of 4,089 euro gross per month, depending on your level of experience. The salary is supplemented with a holiday allowance of 8 percent and an end-of-year bonus of 8.33 percent. The conditions of employment of NWO-I are laid down in the Collective Labour Agreement for Research Centres (Cao-Onderzoekinstellingen), more exclusive information is available at this website under Personeelsinformatie (in Dutch) or under Personnel (in English). General information about working at NWO-I can be found in the English part of this website under Personnel. The 'Job interview code' applies to this position.
Further information on the vacancy is available through the leader of the fusion energy department, Prof.dr. M.R. de Baar, +31 40 333 49 52.
In case you are interested you can apply by submitting an application letter, a full resume (including grade list). Please refer to vacancy 17.001.