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Fusion
Published in William J. Nuttall, Nuclear Renaissance, 2022
Once the helium nuclei lose their energy, they become a problem for the plasma. They inhibit future D–T fusion because they dilute the fuel and, with time, they cause a cooling of the plasma. Consistent with its status as the waste generated by the fusion process, the alpha particle by-product has become known as ‘helium ash’. It is something that must be removed from the plasma, although it is important to stress that it is not a radioactive waste. The task of removing the helium ash is performed by the divertor at the bottom of the tokamak (see Section IV.9.2). The divertor also removes other charged impurities, such as material ablated, vaporised and ionised from the walls of the tokamak. Plasma research has shown that any sharp corners of, for example, graphite tiles sticking out from the smooth wall of the vessel will glow white hot and release carbon nuclei into the plasma—a process known as carbon bloom. As with helium ash, these ions inhibit the process of nuclear fusion and must be removed. Graphite is mentioned as a possible plasma-facing first-wall material merely as an example. First-wall materials (usually in the form of replaceable tiles) are a major area of fusion R&D and a very wide range of materials have been considered and remain under investigation. The impact of high-energy fusion neutrons on fusion device materials has been reviewed by Marek Rubel in 2019 [14].
Investigation of the effect of thermal cycle on SS/CRZ brazed joint sample
Published in B. Raneesh, Nandakumar Kalarikkal, Jemy James, Anju K. Nair, Plasma and Fusion Science, 2018
K. P. Singh, Alpesh Patel, Kedar Bhope, S. Belsare, Nikunj Patel, Prakash Mokaria, S. S. Khirwadkar
Development of the joining technique for the plasma facing material with structural material is one of challenging area in fusion research [1]. In ITER like tokamak first wall and divertor plasma facing component (PFC) module, SS/CuCrZr (CRZ) joining has the mandatory requirement of good in thermal transfer and sound in structural joints [1].
Experimental Studies of Alfvén Eigenmodes
Published in Sergei Sharapov, Energetic Particles in Tokamak Plasmas, 2021
The history of FS instabilities driven by energetic ions started from an oscillatory “fishbone” instability, with the dominant mode numbers n = 1 and m = 1, first observed in experiments with perpendicular NBI on the Poloidal Divertor Experiment (PDX) tokamak [7.17]. The instability occurs in repetitive bursts, with the mode frequency decreasing by about a factor of 2 during each burst. Large fishbone bursts cause losses of NBI-produced energetic ions, thus reducing the efficiency of plasma heating. Experimentally, the fishbone mode structure was found to be of the “top hat” type similar to the internal kink mode [7.18] associated with the surface q = 1 in tokamak plasma. The frequency of the fishbone oscillations on PDX was found to be close to the magnetic precession frequency of the trapped beam ions, as well as to the diamagnetic frequency of thermal ions. Two different regimes have been identified for the linear phase of fishbone instability. The first regime of the so-called “precessional” fishbones [7.19] refers to the case when the mode frequency is much greater than the thermal ion diamagnetic frequency. In this case, trapped energetic ions via the resonance with their precessional motion destabilise the n = 1, m = 1 mode emerging from the Alfvén continuum. The mode frequency is essentially determined by the energetic particle population in this case, and the fishbone represents an energetic particle mode. In this case, the mode structure has singularities at the radii of local Alfvén resonances (5.23) and significant continuum damping. Such fishbones are excited at relatively high values of the energetic ion pressure which overcomes the threshold value as determined by the continuum damping. In the non-linear phase, such fishbones could sweep in frequency due to the re-distribution of energetic ions and the MHD nonlinearity of the continuum damping [7.20]. The second regime [7.21,7.22] corresponds to the case of comparable precessional frequency of the beam ions and the diamagnetic frequency of thermal ions. The mode frequency could then reside in a low-frequency “gap” in the continuum associated with the diamagnetic frequency, thus avoiding continuum damping.
The Impacts of Liquid Metal Plasma-Facing Components on Fusion Reactor Safety and Tritium Management
Published in Fusion Science and Technology, 2019
Paul W. Humrickhouse, Brad J. Merrill, Su-Jong Yoon, Lee C. Cadwallader
One of the outstanding challenges on the path to magnetic confinement fusion is engineering of the plasma-facing components (PFCs), which must be able to withstand the high heat and particle fluxes incident upon them. A solid divertor, for which tungsten alloys are presently the only viable candidate materials, must do so with a minimum of erosion, melting, and cracking, in the face of plasma-surface interactions, high-radiation damage, temperature gradients, and concomitant thermal stresses. Such difficulties are exacerbated by any attempt to design a more compact reactor. One potential solution to these issues is to employ a liquid metal (LM) as a plasma-facing surface. Such a continually replenished PFC has the potential to mitigate some of the aforementioned structural concerns and remove heat via convection or evaporation.
Identification of the Postulated Initiating Events of Accidents of a CPS-Based Liquid Metal Divertor for the EU DEMO Fusion Reactor
Published in Fusion Science and Technology, 2022
A. C. Uggenti, G. F. Nallo, A. Carpignano, N. Pedroni, R. Zanino
During the normal operation of the reactor (and in particular during the flat-top phase of the plasma pulse, lasting ~2 h in the EU DEMO reference scenario15), the major function of the divertor is to exhaust the nonradiated fraction of the power associated with the alpha particle source and conducted and advected by the plasma along magnetic field lines in the scrape-off layer. In the LM concept considered in the present study, this is achieved by actively cooling the PFCs by means of pressurized water.14 The correct water flow and cooling parameters are guaranteed by the presence of a water circuit.
Elemental Characterization of Neutron-Irradiated Tungsten Using the GD-OES Technique
Published in Fusion Science and Technology, 2019
Nathan C. Reid, Lauren M. Garrison, Chase N. Taylor, Jean Paul Allain
In addition to the performance of PFCs, the performance of the plasma itself is strained by plasma-material interactions within the W divertor. To mitigate the effects of fuel retention, damage to the divertor by high heat flux, and high-Z contamination of the core plasma, it is imperative to optimize the divertor region by protecting it from plasma instabilities and disruptions and to obtain a net erosion condition that minimizes W entering the plasma and prevent fuel from accumulating in the W.