Fundamental research of the hydrogen retention in tungsten under fusion relevant conditions (PhD)

Plasma wall interaction is a very broad and complex area in fusion that will largely determine the success of future fusion reactors. The plasma needs to reach temperatures around several millions Kelvin and should be contained in a tokamak by magnetic fields. The plasma facing components that are facing the plasma should have an acceptable lifetime from the safety and economical points of view.

Plasma facing materials for future fusion reactors (ITER/DEMO) have to meet many requirements. They need to be resistant against chemical, physical sputtering and erosion, structural integrity needs to be kept even under severe thermal loads, thermally-induced mechanical stresses and cyclic loading conditions. Excellent properties such as thermal conductivity, high temperature strength, low ductile to brittle transition temperature; high recrystallization temperature, low tritium capturing… needs to be maintained even after neutron irradiation damage. Tungsten-based materials are the most promising because of their superior thermo-mechanical properties, excellent physical properties and low tritium inventory.

The fuel used for the fusion reaction consists of deuterium (D) and tritium (T). After the fusion reaction, helium and neutrons are produced. There is a limit on the amount of the radioactive element T due to safety reasons and licensing. Retention of T in the material is therefore not desirable. A database should be set-up to define the T recycling/retention, resilience to steady state fluences, surface erosion and neutron irradiation degradation as a contribution to the understanding of the interaction with tungsten materials.

The plasmatron VISION I will simulate the steady state plasma. Build-in plasma diagnostics as well as several post-mortem tools will be used to evaluate the H isotopes retention in tungsten. Interpretation of the results obtained by existing and/or new diagnostics should be based on physical models that will be set-up during the PhD.

The synergistic effects of the plasma (H/D/He composition) and material related properties (grain sizes, He bubbles, surface state) on the retention characteristics will be studied. Fundamental modeling will be performed for a better understanding of the experimental results as well as for the extrapolation to ITER/DEMO conditions.

The objective of the project is to understand the behavior of first wall materials under plasma wall interaction phenomena. More specifically, the focus will be put on tungsten materials. The influence of the various hydrogen isotopes (H, D, T) on the retention in tungsten should be understood as well as the effect of Helium on the retention. The additional effect of the decay of tritium to helium in steady-state operation will create helium bubbles in the bulk and neutron induced traps will create additional sinks for tritium retention.

The following issues should be investigated:

-          Determination of the fundamental understanding of the variables on the materials behaviour (desorption, solubility, diffusivity, recycling, implantation, reflection coefficient, sputtering yield, retention…) and investigation of the materials in-depth with metallography (grain size, defects …). Well known reference un-damaged tungsten samples will be used.

-          Effect of the plasma parameters (density, ion temperature, flux, …) and sample surface on the H retention of the same reference un-damaged tungsten sample.

-          Effects of various concentrations of H/D and the effect of the addition of small concentrations of He to the H/D ratios on surface morphology (blisters, bubbles, fuzz, …) and retention of the reference specimens.

-          Additional effect of pre-damaged surfaces (roughening, cracks, erosion, …) on the H retention.

-          Well-defined and specific tests will be conducted with T plasma depending on the results of previous tests campaigns with H/D.

A low temperature, high flux plasma (VISION I) will be used to expose the samples to a H/D/He plasma. The plasma parameters will be monitored by Langmuir probes, ion energy analyzers, current, voltage, pressure, … It will be very important to know the exact plasma parameters in order to make definite conclusions on the final retention. Continuation of the set-up improvement will be necessary as well as additional models for the understanding of the data by the several diagnostics.

The retention/recycling itself will be examined in-situ during plasma exposure with the help of a plasma monitor (with very accurate QMS). After plasma impact it will be analyzed with a TDS (thermal desorption spectroscopy) method. The in-situ and ex-situ methods may give estimations on the transient and steady state retention in the material. Additional ex-situ methods for the in-depth study of the effect of H/D/He by SEM (surface morphology studies) and TEM (helium/hydrogen clusters, created defects, …). For the interpretation of the obtained data some fundamental modeling will be necessary and consists out of:

-          Fundamental modeling of the H retention by determination of the trapping sites and defects (ion implantation, He clusters, …) from the in-situ TDS spectra in comparison with TEM observations.

-          Comparison of the data with literature to establish empirical relationships related to flux density, ion temperature, sample surface temperature etcetera to understand the differences of test methods and conditions (plasma simulators, tokamaks, ion beams, …) in order to be able to extrapolate the H inventory for specific conditions to ITER/DEMO.