14C-CO2 Measurements with Accelerator Mass Spectrometry

Accelerator mass spectrometry (AMS) is a technique developed from mass spectrometry and it is able to measure single very rare isotopes from samples with detection capability down to one atom in 10^16. It uses an accelerator system to accelerate the atoms and molecules to break molecular bonds for precise single isotope detection. This thesis describes the optimization of University of Helsinki's AMS system to detect the rare radioactive isotope 14C from CO2 gas samples. Using AMS to detect radiocarbon is a precise and fast way to conduct radiocarbon dating with minimal sample sizes. Solid graphite samples have been in use before but as the ion source has been adopted to use also gaseous CO2 samples, optimizations must be made to maximize the carbon current and ionization efficiency for efficient 14C detection. Parameters optimized include cesium oven temperature, CO2 flow, carrier gas helium flow and their dependencies with each other. Both carbon current and ionization efficiency is looked at in the optimizations. The results are analyzed and discussed for further optimizations or actual measurements with gas. Ionization occurring in the ion source can be understood better with the results. Standard samples of CO2 were measured to determine the background and precision of the AMS system in gas use by comparing the results with literature. The current system was found to have tolerable background of 1.5% of the standard and the Fraction modern value of actual sample was 2.4% higher than values from literature. Ideas to improve background were discussed. A new theory of negative-ion formation in a cesium sputtering ion source by John S. Vogel is reviewed and taken into account in the discussion of optimization. Utilizing the theory, possible future upgrades to improve the ionization efficiency are presented such as cathode material choices to reduce competitive ionization and cesium excitation by laser.Kiihdytinmassaspektrometria (AMS) on massaspektrometriasta jatkokehitetty tekniikka, joka pystyy mittaamaan yksittäisiä harvinaisia isotooppeja näytteistä havaintotarkkuudella yksi atomi 10^16 atomista. AMS käyttää kiihdytinsysteemiä kiihdyttääkseen atomit ja molekyylit. Törmäyttämällä molekyylit jalokaasuun molekyylien sidokset voidaan hajottaa tarkkaa yhden tietyn isotoopin mittaamista varten. Tämä tutkielma kuvaa Helsingin yliopiston AMS-systeemin optimointia CO2-kaasunäytteillä tehtäviä harvinaisen radioaktiivisen 14C isotoopin mittauksia varten. AMS:n käyttäminen radiohiilen mittaamiseen on tarkka ja nopea tapa tehdä radiohiiliajoituksia hyvin pienistäkin näytteistä. Kiinteitä grafiittinäytteitä on käytetty mittauksissa aiemmin, mutta koska ionilähde on muunnettu käyttämään myös CO2-kaasunäytteitä, täytyy laitteistoa optimoida mahdollisimman suuren hiilivirran sekä ionisaatiotehokkuuden saavuttamiseksi tehokasta 14C-havaitsemista varten. Optimoituja parametrejä ovat esimerkiksi kesium-uunin lämpötila, CO2-virtausnopeus, kuljetuskaasu heliumin virtausnopeus ja näiden riippuvuudet keskenään. Sekä hiilivirta että ionisaatiotehokkuudet on huomioitu optimoinnissa. Tulokset on analysoitu ja pohdittu jatko-optimointeja tai oikeita kaasumittauksia varten. Ionisaatiota ionilähteessä pystyy ymmärtämään paremmin tulosten kanssa. Standardinäytteet mitattiin CO2-kaasulla taustan ja AMS-systeemin tarkkuuden määrittämiseksi kaasukäytössä vertaamalla tuloksia kirjallisuusarvoihin. Tämänhetkisellä systeemillä havaittiin 1,5 % tausta standardiin verrattuna ja oikean näytteen Fraction modern –tulos oli 2,4 % korkeampi kuin kirjallisuudessa. Ideoita taustan parantamiseen pohdittiin. Uusi John S. Vogelin ionisaatioteoria cesiumionilähteessä käytiin läpi, ja teoriaa hyödynnettiin optimoinnin tuloksien pohdinnassa. Tulevia mahdollisia ionilähteen päivityksiä on esitetty teoriaan perustuen. Näihin päivityksiin kuuluvat esimerkiksi katodin materiaalivalinnat kilpaillun ionisaation vähentämiseksi ja cesiumin virittäminen laserilla

Deposition of 13C tracer and impurity elements on the divertor of Wendelstein 7-X

Carbon impurity transport and deposition were investigated in the Wendelstein 7-X stellarator by injecting isotopically labelled methane ((CH4)-C-13) into the edge plasma during the last plasma operations of its Operational Phase (OP) 1.2B experimental campaign. C-13 deposition was measured by secondary ion mass spectrometry (SIMS) on three upper divertor tiles located on the opposite side of the vessel to the(13)CH(4) inlet. The highest C-13 inventories were found as stripe-like patterns on both sides of the different strike lines. These high deposition areas were also analysed for their impurity contents and the depth profiles of the main elements in the layers. Layered deposition of different impurity elements such as Cr, Ni, Mo and B was found to reflect various events such as high metallic impurities during the OP1.2A and three boronizations carried out during OP1.2B.Peer reviewe

Hydrogen isotope exchange experiments in high entropy alloy WMoTaNbV

Plasma–facing components in future fusion reactors must endure high temperatures as well as high fluxes and fluences of high energy particles. Currently tungsten has been chosen as the primary plasma-facing material due to its good thermal conductivity, low erosion rate and low fuel retention. Materials with even better properties are still being investigated to be used in reactor regions with demanding plasma conditions. High entropy alloys (HEA) are a new class of metallic alloys and their exploitation in fusion applications has not been widely studied. In this work, the hydrogen isotope exchange effect in an equiatomic HEA containing W, Mo, Ta, Nb, and V was studied. Deuterium was implanted into HEA samples with 30 keV/D energy and the HEA and reference samples were annealed in H2 atmosphere and in vacuum at various temperatures up to 400 °C, respectively. The near-surface D concentration profiles were measured with ERDA and the isotope exchange was observed to remove over 90 % of the trapped deuterium from the implantation region at temperatures above 200 °C. TDS was used to measure retention deeper in the bulk in which the reduction of trapped deuterium was significantly lower. High total retention of H was found in the bulk after H2 atmosphere annealing which indicates permeation and deep trapping of H in the material.Plasma-facing components in future fusion reactors must endure high temperatures as well as high fluxes and fluences of high energy particles. Currently tungsten has been chosen as the primary plasma-facing material due to its good thermal conductivity, low erosion rate and low fuel retention. Materials with even better properties are still being investigated to be used in reactor regions with demanding plasma conditions. High entropy alloys (HEA) are a new class of metallic alloys and their exploitation in fusion applications has not been widely studied. In this work, the hydrogen isotope exchange effect in an equiatomic HEA containing W, Mo, Ta, Nb, and V was studied. Deuterium was implanted into HEA samples with 30 keV/D energy and the HEA and reference samples were annealed in H2 atmosphere and in vacuum at various temperatures up to 400 °C, respectively. The near-surface D concentration profiles were measured with ERDA and the isotope exchange was observed to remove over 90 % of the trapped deuterium from the implantation region at temperatures above 200 °C. TDS was used to measure retention deeper in the bulk in which the reduction of trapped deuterium was significantly lower. High total retention of H was found in the bulk after H2 atmosphere annealing which indicates permeation and deep trapping of H in the material.Peer reviewe

Ex Situ LIBS Analysis of WEST Divertor Wall Tiles after C3 Campaign

Fuel retention monitoring in tokamak walls requires the development of remote composition analysis methods such as laser-induced breakdown spectroscopy (LIBS). The present study investigates the feasibility of the LIBS method to analyse the composition and fuel retention in three samples from WEST divertor erosion marker tiles after the experimental campaign C3. The investigated samples originated from tile regions outside of strong erosion and deposition regions, where the variation of thin deposit layers is relatively small and facilitates cross-comparison between different analysis methods. The depth profiles of main constituents W, Mo and C were consistent with depth profiles determined by other composition analysis methods, such as glow-discharge optical emission spectroscopy (GDOES) and secondary ion mass spectrometry (SIMS). The average LIBS depth resolution determined from depth profiles was 100 nm/shot. The averaging of the spectra collected from multiple spots of a same sample allowed us to improve the signal-to-noise ratio, investigate the presence of fuel D and trace impurities such as O and B. In the investigated tile regions with negligible erosion and deposition, these impurities were clearly detectable during the first laser shot, while the signal decreased to noise level after a few subsequent laser shots at the same spot. LIBS investigation of samples originating from the deposition regions of tiles may further clarify LIBS’ ability to investigate trace impurities

Irradiation Damage Independent Deuterium Retention in WMoTaNbV

High entropy alloys are a promising new class of metal alloys with outstanding radiation resistance and thermal stability. The interaction with hydrogen might, however, have desired (H storage) or undesired effects, such as hydrogen-induced embrittlement or tritium retention in the fusion reactor wall. High entropy alloy WMoTaNbV and bulk W samples were used to study the quantity of irradiation-induced trapping sites and properties of D retention by employing thermal desorption spectrometry, secondary ion mass spectrometry, and elastic recoil detection analysis. The D implantation was not found to create additional hydrogen traps in WMoTaNbV as it does in W, while 90 at% of implanted D is retained in WMoTaNbV, in contrast to 35 at% in W. Implantation created damage predicted by SRIM is 0.24 dpa in WMoTaNbV, calculated with a density of 6.044×1022 atoms/cm3. The depth of the maximum damage was 90 nm. An effective trapping energy for D in WMoTaNbV was found to be about 1.7 eV, and the D emission temperature was close to 700 °C

Hydrogen isotope exchange mechanism in tungsten studied by ERDA

Future fusion reactors use a D–T plasma mixture as fuel. A fraction of hydrogen species can escape the plasma confinement and hit the first wall. Hydrogen isotope exchange, a process in which trapped T atoms are replaced with lighter hydrogen isotopes D or H, is a potential method to minimize radioactive T retention in the wall materials. The present work extends our systematic research on isotope exchange by reversing the process, i.e. by implanting H ions into tungsten followed by subsequent annealing at different constant temperatures in D2 atmosphere. Elastic Recoil Detection Analysis was used to determine the H and D concentrations. The results show that the isotope exchange process takes place regardless of the mass of the active hydrogen isotope. This indicates that the isotope exchange is a statistical phenomenon in which the abundance of the neighboring hydrogen near the trapped hydrogen isotope defines the efficiency of the process.Peer reviewe

Deuterium retention in tungsten studied by sequential implantations at ELM-relevant energies

Plasma edge-localized modes (ELMs) can cause considerable fuel retention in fusion reactor vessel walls by implanting plasma particles with high energies and fluxes. The effect of deuterium ions implanted into tungsten with ELM-relevant energies was studied in laboratory conditions using ion beams. Deuterium implantations were done at room temperature with low fluxes with energies and fluences corresponding to JET and ITER estimates in a single ELM event for both inter ELM and intra ELM conditions at the divertor during high power operation. Deuterium implantations with 100 eV/D correspond to an inter-ELM phase, whereas implantations with 5 keV/D or 20 keV/D were used to mimic intra-ELM phases at JET and ITER, respectively. Resulted deuterium retention from these single energy implantations was compared with sequential implantations of low energy – high energy sequences as well as high energy – low energy sequences. Retention was measured by ERDA to obtain the amount of deuterium at the irradiated depth as well as the depth profiles for each implantation. High energy – low energy sequential implantations showed increased retention profile around the implantation-induced damage maximum. In low energy – high energy sequential implantations the observed retention was increased throughout the analyzed depth.Peer reviewe

Ex Situ LIBS Analysis of WEST Divertor Wall Tiles after C3 Campaign

Fuel retention monitoring in tokamak walls requires the development of remote composition analysis methods such as laser-induced breakdown spectroscopy (LIBS). The present study investigates the feasibility of the LIBS method to analyse the composition and fuel retention in three samples from WEST divertor erosion marker tiles after the experimental campaign C3. The investigated samples originated from tile regions outside of strong erosion and deposition regions, where the variation of thin deposit layers is relatively small and facilitates cross-comparison between different analysis methods. The depth profiles of main constituents W, Mo and C were consistent with depth profiles determined by other composition analysis methods, such as glow-discharge optical emission spectroscopy (GDOES) and secondary ion mass spectrometry (SIMS). The average LIBS depth resolution determined from depth profiles was 100 nm/shot. The averaging of the spectra collected from multiple spots of a same sample allowed us to improve the signal-to-noise ratio, investigate the presence of fuel D and trace impurities such as O and B. In the investigated tile regions with negligible erosion and deposition, these impurities were clearly detectable during the first laser shot, while the signal decreased to noise level after a few subsequent laser shots at the same spot. LIBS investigation of samples originating from the deposition regions of tiles may further clarify LIBS’ ability to investigate trace impurities