Simulating divertor detachment in the TCV and JET tokamaks

Divertor detachment is currently assumed to be a fundamental pre-requisite for the successful operation of future fusion reactors. Only by partially detaching the divertor in the regions of highest power flux density can high performance tokamak operation be made compatible with the technological limits set by the thermo-mechanical properties of surfaces in contact with the plasma. Although the various physics components of the detachment process are thought to be well known, their relative importance and the degree to which each may affect the others, thus determining the final detached state, cannot in general be deduced from any simple analytic approach. Instead, sophisticated two and three dimensional interpretative and predictive code packages have been developed within the fusion community to model the scrape-off layer (SOL) and divertor plasmas of magnetic confinement devices. One of these, the SOLPS code, has for some time been employed as a tool for the design of the divertor in the next step tokamak reactor, ITER. In reality, however, these codes have not, in general, been fully validated against experimental observations from current tokamaks, in particular with regard to divertor detachment. This thesis aims to contribute to such validation by thorough comparisons between numerical simulations, using the SOLPS5 (plasma fluid, Monte Carlo neutral) code pack- age and experimental characterization of the detachment process in two very different tokamaks, TCV and JET. The approach taken has been to test the code against experiment, not only for two machines with a vast difference in size and divertor geometry, but also for plasma operation with either deuterium or helium fuel. Changing the fuel species in a tokamak containing significant graphite first wall components as do TCV and JET, dramatically modifies the impurity production mechanism but also the important atomic physics processes at work, both of which influence the detachment threshold. In TCV, divertor detachment in the simplest of situations is experimentally observed to be anomalous and could not be explained by the first attempts at code modeling prior to this thesis. Evidence is presented for the detachment anomaly being directly linked to enhanced interaction between the graphite main chamber walls at high plasma density due to anomalous convective radial transport. Such interaction is not well modeled by the code and the results presented in this thesis highlight an important area in which the complexities of the real situation are inadequately represented in the numerical model. This work also constitutes the first known application of the SOLPS code to tokamak simulation with consistent modeling of molecular hydrocarbons. Indeed, they are found to be important in producing high degrees of numerical detachment. In JET, experimental data from high density helium plasma operation have been successfully modeled, constituting the first ever simulations of pure He discharges on this machine. Helium detachment is very different to that in deuterium, due in large part to the absence of carbon chemistry. The simulation results demonstrate this together with strong evidence for conclusions to be drawn concerning the principal mechanisms driving the detachment. Similar good agreement is obtained between code and experiment for helium operation on TCV and a comparison of code results between the two devices demonstrates how divertor geometry can have a significant impact on the detachment behavior

Physics research on the TCV tokamak facility: from conventional to alternative scenarios and beyond

The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device’s unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly non-inductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power ‘starvation’ reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in–out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable-configuration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added

Drift effects in SOLPS-ITER simulations for the TCV divertor upgrade

This contribution reports on the progress towards including drift effects in SOLPS-ITER simulations to assess power distribution and detachment onset in the light of the 2019 TCV divertor upgrade [1]. The installation of in-vessel gas baffles (Figure 1a) is predicted to increase the divertor neutral density by a factor ∼ 5 and therefore facilitates the access to detachment [2]. The conditions for the onset of detachment at each strike point depend on the power entering each divertor leg and thus simulations must correctly include the power distribution between the inner and outer divertor that is greatly affected by scrape-off layer drifts. Surprisingly, the sharing of particles and heat between the targets in reversed field conditions is found to be shifted towards the outer target. This counter-intuitive flowpattern is due to an electric potential well below the X-point that occurs in high density reversed field simulations

Hydrosedimentological dynamic on Água Fria Watershed

This study aimed surveying the amount of sediment yielded from the Água Fria watershed (Palmas, Tocantins, Brazil), from February-1998 to January-1999, and investigating the relations between the sediment yield and some environmental and/or antropic factors. The Colby's method was the technique employed for this investigation. The specific sediment yield and sediment delivery ratio were also determined for this period. It was estimated that 138,619 tons of sediment were yielded and the specific sediment yield for the study area was 827 t km-2 y-1, while the sediment delivery ratio was 6.2%. The suspended load was the most dominating fraction in almost all the studied period.<br>Este estudo objetivou estimar a quantidade de sedimento que foi carreada da microbacia do Ribeirão Água Fria (Palmas, TO) entre fevereiro de 1998 e janeiro de 1999. Almejou-se ainda investigar as relações entre a produção de sedimento e alguns fatores antrópicos e ambientais. O método de Colby foi a técnica empregada no estudo. A produção específica de sedimento e o coeficiente de remoção de sedimentos foram parâmetros também investigados neste trabalho. Foi estimada uma quantidade de 138.619 toneladas de sedimento produzido e a produção específica de sedimentos foi estimada como sendo 827 t km-2 ano-1, enquanto que o coeficiente de remoção de sedimentos foi 6,2%. A fração suspensa foi a predominante durante quase todo o período de estudo

Soil loss risk and habitat quality in streams of a meso-scale river basin Risco de perda de solo e qualidade do habitat numa bacia hidrográfica de meso-escala

Soil loss expectation and possible relationships among soil erosion, riparian vegetation and water quality were studied in the São José dos Dourados River basin, State of São Paulo, Brazil. Through Geographic Information System (GIS) resources and technology, Soil Loss Expectation (SLE) data obtained using the Universal Soil Loss Equation (USLE) model were analyzed. For the whole catchment area and for the 30 m buffer strips of the streams of 22 randomly selected catchments, the predominant land use and habitat quality were studied. Owing mainly to the high soil erodibility, the river basin is highly susceptible to erosive processes. Habitat quality analyses revealed that the superficial water from the catchments is not chemically impacted but suffers physical damage. A high chemical purity is observed since there are no urban areas along the catchments. The water is physically poor because of high rates of sediment delivery and the almost nonexistence of riparian vegetation.<br>Expectativa de perda de solo e possíveis relações entre erosão, vegetação ripária e qualidade da água foram estudados na bacia do rio São José dos Dourados (SP). Através de recursos de geoprocessamento e da Equação Universal de Perda de Solos, os dados sobre expectativa de perda de solo foram levantados. Para a área de drenagem total e a faixa tampão dos corpos d'água de 22 sub-bacias aleatoriamente selecionadas, analisou-se a cobertura do solo predominante e qualidade do habitat. Devido principalmente à alta erodibilidade do solo, a área estudada é altamente suscetível ao processo erosivo. As análises de qualidade da água revelaram que as águas superficiais das sub-bacias estão quimicamente não impactadas, mas fisicamente degradadas. A alta pureza química deve-se, possivelmente, à ausência de áreas urbanizadas ao longo das sub-bacias e as alterações nas características físicas são, possivelmente, decorrentes das altas taxas de transferência de sedimento aos corpos d'água e à quase ausência de mata ciliar