Transport barrier and current profile studies on the JET tokamak

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Doctoral thesis (article-based)
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VTT publications, 467
One of the crucial problems in fusion research is the understanding of heat and particle transport in plasmas relevant for energy production. The neo-classical theory of tokamak transport is well-established, but it cannot explain experimental results. Instead, the micro-turbulence driven anomalous transport has been found to be dominant in present tokamak experiments. There are several mechanisms that can locally suppress micro-turbulence and reduce significantly the anomalous transport. These regions of reduced transport are called transport barriers. The presence of Internal Transport Barriers (ITBs) is one of the bases in 'Advanced Tokamak Scenarios'. One of the principal goals in the 'Advanced Tokamak Scenarios' is to improve the fusion power density and confinement with internal transport barriers by controlling the current density profile and maximising the bootstrap current - and ultimately rendering the tokamak compatible with continuous operation. This thesis reports on studies and modelling of internal transport barriers and current density profiles in the Joint European Torus (JET) tokamak with a fluid transport code. Explanations for the following open questions are sought: what are the mechanisms that govern the formation and dynamics of the ITBs in JET and secondly, how can the current density profile be modified and further, how does it affect ITBs and plasma performance? On the basis of the empirical study at the ITB transition, the ωE×B flow shear and magnetic shear appear as strong candidates in determining the onset time, the radial location and the dynamics of the ITBs in JET. This ITB threshold condition, employed in the semi-empirical Bohm/GyroBohm transport model, has been found to be in good agreement with experimental results in predictive transport simulations. On the other hand, the simulation results from the predictive transport modelling with a theory-based quasi-linear fluid transport model strongly emphasise the importance of the density gradient in the ITB formation. According to the current density modelling studies, lower hybrid and electron cyclotron current drive are the most versatile current drive methods in terms of the produced q-profile in the preheating phase in JET. With lower hybrid preheating, a core current hole has been found and a physics-based explanation, confirmed by the transport modelling, is given. The predictive transport simulations indicate that application of lower hybrid current drive during the high performance phase can enhance the fusion performance significantly by increasing the ITB radius.
nuclear fusion, JET tokamak, plasma transport, heat transport, internal transport barriers, current density, modelling, transport models, flow shear, magnetic shear
  • Tala T.J.J., Heikkinen J.A., Parail V.V., Baranov Yu.F. and Karttunen S.J., ITB formation in terms of ω<sub>E×B</sub> flow shear and magnetic shear s on JET, Plasma Phys. Control. Fusion 43 (2001) 507-523. [article1.pdf] © 2001 IOP Publishing. By permission
  • Tala T.J.J., Parail V.V., Becoulet A., Corrigan G., Heading D.J., Mantsinen M.J., Strand P.I. and contributors to the EFDA-JET workprogramme, Comparison of theory-based and semi-empirical transport modelling in JET plasmas with ITBs, Plasma Phys. Control. Fusion 44 (2002) in press. [article2.pdf] © 2002 IOP Publishing. By permission
  • Tala T.J.J., Parail V.V., Becoulet A., Challis C.D., Corrigan G., Hawkes N.C., Heading D.J., Mantsinen M.J., Nowak S. and contributors to the EFDA-JET workprogramme, Impact of different heating and current drive methods on the early q-profile evolution in JET, Plasma Phys. Control. Fusion 44 (2002) in press. [article3.pdf] © 2002 IOP Publishing. By permission
  • Hawkes N.C., Stratton B.C., Tala T.J.J., Challis C.D., Conway G., DeAngelis R., Giroud C., Hobirk J., Joffrin E., Lomas P., Lotte P., Mailloux J., Mazon D., Rachlew E., Reyes-Cortes S., Solano E. and Zastrow K.-D., Observation of Zero Current Density in the Core of JET Discharges with Lower Hybrid Heating and Current Drive, Phys. Rev. Lett. 87 (2001) 115011-1-4. [article4.pdf] © 2001 American Physical Society. By permission
  • Tala T.J.J., Söldner F.X., Parail V.V., Baranov Yu.F., Heikkinen J.A. and Karttunen S.J., Modelling of optimized shear scenarios with LHCD for high performance experiments on JET, Nucl. Fusion 40 (2000) 1635-1649. [article5.pdf] © 2000 IAEA. By permission
  • Heikkinen J.A., Tala T.J.J., Pättikangas T.J.H., Piliya A.D., Saveliev A.N. and Karttunen S.J., Role of fast waves in the central deposition of lower hybrid power, Plasma Phys. Control. Fusion 41 (1999) 1231-1249. [article6.pdf] © 1999 IOP Publishing. By permission
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