Laser Induced Desorption as Hydrogen Retention Diagnostic Method

Laser Induced Desorption Spectroscopy (LIDS) is a diagnostic method to measure the hydrogen content in the surface of a material exposed to a hydrogen isotope (H, D, T) plasma. It is developed mainly to monitor hydrogen retention in the walls of magnetic fusion devices that have to limit the amount of their fuel tritium mainly due to safety reasons. The development of fusion increasingly focusses on plasma-wall interactions for which in situ diagnostics like LIDS are required that work during plasma operation and without tile removal. The method has first been developed for thin amorphous hydrocarbon (a-C:H 95%) desorption already within 1.5 ms pulse duration. The retained hydrogen atoms are desorbed locally, recombine to molecules and migrate promptly to the surface via internal channels like pores and grain boundaries. Whereas, in W the retained hydrogen atoms have to diffuse through the bulk material, which is a relatively slow process also directed into the depth. The desorbed hydrogen fraction can thus be strongly reduced to 18-91% as observed here. This fraction is measured by melting the central part of a previously heated spot ca. 40 μm deep with a Ø2 mm, 3 ms laser pulse, releasing the remaining hydrogen. W samples exposed to different plasmas in TEXTOR, Pilot-PSI, PSI-2, PADOS and PlaQ show that the desorption fraction of LID mainly decreases due to higher sample temperature during plasma exposure. The heat causes deeper hydrogen diffusion and/or stronger hydrogen trapping due to creation of traps with higher binding energy. Such effects can lead to the observed desorption fractions as simulations (TMAP7 code) of heat and H diffusion during the laser pulse show. These experiments are performed in a vacuum chamber outside the tokamak, where the desorbed gases are quantified by a quadrupole mass spectrometer, thus representing the ex situ method LID-QMS.In the tokamak TEXTOR the in situ diagnostic method LIDS is used utilizing the same physics for heating, desorption and surface modifications. Understanding the latter becomes important to mitigate material release into the plasma. Here, the quantification of the desorbed hydrogen is done by passive spectroscopy of the Balmer Hα and Dα light (656 nm) observed coaxially to the laser beam as a double line by a spectrometer and from the side by a camera with gated image intensifier using a narrow-band H&D filter. A simplified data evaluation has been developed which determines the plasma radius of the light intensity maximum of the LIDS light, takes the electron density and temperature at this radius measured by edge plasma diagnostics and looks up the corresponding quotient of ionisation to excitation rate S/XB(ne,Te) in a database (ADAS). A second factor takes into account the dominant plasma processes which yield only one atom from one hydrogen molecule for pure hydrogen release and even less for desorbed hydrocarbons. The combined light-to-particle conversion factor is ca. 30 H atoms/Hα photons which agrees with simulations of the LIDS light (ERO code). While the simulated spatial light distribution is very sensitive to the details of the plasma edge profiles, the total photon amount stays very constant, thus justifying the simplified data evaluation. The experimental FWHM of the light in toroidal/poloidal direction is 30-40 mm and has an e-folding decay length of 15-20 mm in radial direction. Its intensity maximum is typically at ne ≈ 4∙10^18 e−/m³ and kB Te ≈ 60 eV close to the last closed flux surface.A measurement series shows good reproducibility of LIDS with a standard deviation of ±13%, while the estimated uncertainty of a single LIDS measurement is −47% to +43%. LIDS measurements are also in agreement with results from LID-QMS, slow thermal desorption (TDS) or nuclear reaction analysis (NRA). The lower detection limit of LIDS is determined by the Hα background fluctuations in TEXTOR to 8∙10^20 H/m² for ohmic and 5∙10^21 H/m² for neutral beam heated plasmas for a Ø2.6 mm laser spot. The upper measurement limit due to local plasma cooling by the cooler desorbed gas lies at ca. 6∙10^22 H/m² for TEXTOR conditions

Impact of synergistic heat and particle loads on plasma-facing materials - new capabilities to include the impact of neutron damage on lifetime and fuel retention

Impact of synergistic heat and particle loads on plasma-facing materials - new capabilities to include the impact of neutron damage on lifetime and fuel retention Bernhard Unterberg, Arkadi Kreter, Christian Linsmeier, Dirk Nicolai, Gerald Pintsuk, Lothar Scheibl, Marius Wirtz and Miroslaw ZlobinskiInstitut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Trilateral Euregio Cluster, D- 52425 Jülich, GermanyMaterial damage due to transient heat loads limits the lifetime of plasma facing components. Both the combined plasma impact and associated hydrogen embrittlement and as well as neutron damage further deteriorate the thermo-mechanical properties of plasma facing materials. In this contribution, we summarize the current understanding of the impact of synergistic heat and plasma loads on damage thresholds and fuel retention of plasma facing materials and discuss the additional impact of neutron damage.We introduce new experimental facilities to study the impact neutron irradiation prior to the heat and plasma exposure, which are currently set-up in the High Temperature Material Laboratory at Forschungszentrum Jülich. These experiments comprise the new heat load facility JUDITH 3 (60 kW electron beam gun with an acceleration voltage of 120 -150 kV) and the linear plasma device JULE-PSI (with steady state ion particle flux densities up to 1023 m-2s-1 and heat flux up to 2 MWm-2 including target biasing) which is equipped with a pulsed laser to simulate transient plasma loads to wall materials. JULE-PSIwill also include a flexible target analysis chamber, where in-vacuo surface diagnostics based on laser techniques are installed (laser induced desorption analysis and laser induced break down spectroscopy).JUDITH-3 and JULE-PSI will be located in two new hot cells, the design of which aims at shielding of radiation corresponding to the maximum allowed amount of radioactive substances in this particular section of HML, for which the handling limits with respect to contained tritium are up to 2.5 1013 Bq and with respect to tungsten (contained but in form of dust) up to 1015 Bq (W-181, W-185) and 1014 Bq (W-187

Hydrogen Retention in Tungsten Materials Studied by Laser Induced Desorption

Development of methods to characterise the first wall in ITER and future fusion devices without removal of wall tiles is important to support safety assessments for tritium retention and dust production and to understand plasma wall processes in general. Laser based techniques are presently under investigation to provide these requirements, among which Laser Induced Desorption Spectroscopy (LIDS) is proposed to measure the deuterium and tritium load of the plasma facing surfaces by thermal desorption and spectroscopic detection of the desorbed fuel in the edge of the fusion plasma. The method relies on its capability to desorb the hydrogen isotopes in a laser heated spot. The application of LID on bulk tungsten targets exposed to a wide range of deuterium fluxes, fluences and impact energies under different surface temperatures is investigated in this paper. The results are compared with Thermal Desorption Spectrometry (TDS), Nuclear Reaction Analysis (NRA) and a diffusion model

Investigation of the Impact on Tungsten of Transient Heat Loads induced by Laser Irradiation, Electron Beams and Plasma Guns

This contribution gives an overview of different simulation methods for transient events and damages induced in tungsten. The investigations were focussed on the resulting crack networks and special attention was paid to crack distance, width and depth. The results indicate that the different techniques show, in general, similar damage behaviours and the same damage thresholds