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Fusion and rising worldwide demand for energy
2012-01-12
av
Christophe Eléhn
The world's energy consumption is increasing as the driving forces of globalization, industrialization and markets wealth are growing.
The world's energy consumption is increasing as the driving forces of globalization, industrialization and markets’ wealth are growing. A rising worldwide demand for efficient, energetic and long-term sources of energy is becoming vital. In the absence of long term options of energy sources, Sara Moradi and her colleagues at Chalmers University of Technology take action in own hands. “One of few long term options of energy sources is fusion energy – energy harnessed from controlled fusion of magnetically confined light nuclei. Key features of fusion make it the most attractive option as a future energy source. A fusion reaction is about four million times more energetic than a chemical reaction, for instance, burning of fossil fuels as coal, oil or gas”, says Sara.
Research in fusion energy has increased key fusion plasma performance parameters dramatically - by a factor of 10,000 - over the last 50 years. Research is now close – less than a factor of 10 away – to produce the core of a fusion power plant. In order to establish societal use of fusion energy, there remain challenges that have to be overcome. “A fundamental challenge is for scientists to understand the mechanisms of transport of matter, electric charge, energy and momentum. These are the most important goals of research in the field of plasma physics”, says Sara.
Practically all applications of plasmas are limited in one way or another to transport phenomena. Although well-developed theories, such as Collisional transport theory (Neoclassical Theory in toroidal geometry), explain the process of transport, experimental observations have proven the transport to deviate from the Collisional transport theory.
One of the reasons for this theoretical discrepancy to experimental observations is the fact that Collisional theory assumes that the plasma in equilibrium and axisymmetric. But in reality, tokamak (a doughnut-shaped magnetic confinement device) plasmas always show the presence of a broad spectrum of fluctuations, e.g. in plasma density, temperature and electromagnetic fields, thus real tokamak plasmas are turbulent. Sara and her colleagues stress the importance of incorporating the effect of the turbulent fluctuations that give rise to transport across the equilibrium magnetic surface in a comprehensive transport theory.
“Most of the instabilities that seem to be responsible for the observed plasma turbulence have a very small component of wave number vector parallel to the magnetic field, compared to the perpendicular component. In other words, most of the turbulent eddies are quasi-perpendicular to the toroidal magnetic field. Therefore, we can expect that turbulence dominate perpendicular transport; in this case the transport is called anomalous. The influence of the plasma turbulence on the parallel transport is rather small, and this is confirmed by experimental observations”, Sara says.
By the definite discrepancy between theory and experiment, Sara and her colleagues in the fusion theory group (http://ft.nephy.chalmers.se) at Chalmers are focusing their research on developing a new physical model and perform numerical simulations in order to expand our understanding, primarily on experimentally observed phenomena in existing magnetic confinement devices, and to make predictions on the implications for the design of future reactors.
Sara and her colleagues mainly point out two micro-instabilities on the impurity turbulent driven transport in Tokamak plasmas:
Micro-instabilities: In fusion devices, a variety of waves can develop and propagate in plasma as a response to a perturbation of a stationary state. In an inhomogeneous plasma, the density and temperature gradients give rise to electron and ion diamagnetic drifts. These drifts will give rise to collective oscillations, which are called drift waves. In many tokamak experiments around the world, the universal appearance of a broadband of drift wave fluctuations with frequencies: 50-500 kHz at perpendicular wave numbers: 1-15 cm-1 have been observed.
Drift wave instabilities are a particularly important class of plasma micro-instabilities (with wavelengths of the order of ion Larmor radius), which have been invoked as the source of fine-scale plasma turbulence i.e. anomalous transport, in tokamak plasmas. Two of the most important micro-instabilities responsible for the anomalous heat and particle transport in the core of the tokamak plasmas, are the electrostatic drift modes: the Trapped Electron mode (TE mode), and the Ion Temperature Gradient mode (ITG mode).
“To study the anomalous particle and heat transports due to ITG and TE instabilities we use a gyro-phase averaged kinetic approach (the so-called gyrokinetic approach). Due to its complexity, usually the gyrokinetic equation is solved numerically.” Sara says.
Figure 1 shows a snapshot from a numerical simulation of plasma turbulence in the ASDEX Upgrade tokamak with the nonlinear gyrokinetic code GENE (Gyrokinetic Electromagnetic Numerical Experiment) (http://www.ipp.mpg.de/~fsj/gene/). Impurities: are always present in fusion plasmas. In a fusion reactor, the helium produced as a result of the fusion process is an internal source of impurity. Moreover, impurities are released from the material surfaces surrounding the plasma by a variety of processes: by radiation from the plasma, or as a result of sputtering, arcing and evaporation. Impurities in tokamak plasmas introduce a range of problems. The most immediate effect is the radiated power loss (radiative cooling). Another effect is dilution: impurity ions produce many electrons and in view of the operating limits on density and pressure, this has the effect of replacing fuel ions. For example, at a given electron density, ne, each fully ionized carbon ion (used in the wall materials in the form of graphite) replaces six fuel ions, so that a 7% concentration of fully ionized carbon in the plasma core, would reduce the fusion power to one half of the value in a pure plasma. Therefore, for all tokamaks it is an immediate and continuing task to reduce impurities to acceptably low concentrations. On the other hand, the presence of impurities, with control, can be beneficial for the plasma performance and also it can lead to reduction of strong plasma heat loads on the plasma facing walls. The radiative cooling effect which was mentioned above can be used at the edge of the plasma in order to distribute the plasma heat more evenly on the whole surface of the vessel walls and therefore, reduce significantly plasma heat bursts on the small regions of the divertor or limiter tiles. Up to 90% of the heating power can be radiated from a rather thin belt at the periphery of the confined plasma. In certain experiments, impurities are introduced deliberately in order to reduce the temperature at the plasma edge (few centimeters from the last closed magnetic surface). This can mitigate the large amplitude Edge Localized Modes (ELMs), which are the results of plasma Magneto Hydro-Dynamic (MHD) instabilities due to the high pressure gradients in the edge region of High confinement (H)-mode regimes (ELMy H-mode is the foreseen regime of operation for ITER tokamak). This way the lifetime of the plasma facing wall components is prolonged. Thus, the impurities' positive impact on the plasma performance offers a possibility to better harness the fusion power. However, it is vital for a fusion reactor to have feedback controls in order to keep impurities at the plasma edge and limit their accumulation in the plasma core where the fusion reactions are happening. In order to have control over the impurity transport we first need to understand different mechanisms responsible for its transport.
Uppdaterad:
12 januari 2012
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