2024-07-14T16:34:54Z
https://ir.soken.ac.jp/oai
oai:ir.soken.ac.jp:00000522
2023-06-20T16:01:37Z
2:427:12
Transient Electron Heat Transport in LHD
Transient Electron Heat Transport in LHD
YAKOVLEV, MYKHAYLO
ヤコブレフ, ミカイロ
YAKOVLEV, Mykhaylo
総合研究大学院大学
博士（学術）
A decrease in global energy confinement with increase in the heating power is observed in the Large Helical Device (LHD) even in the inward shifted (R<SUB>ax</SUB>, = 3.5m) magnetic configuration where the heat transport predicted by the neoclassical theory is small. Moreover, the flattening of temperature profile inside the low order rational surface and saturation of core T<SUB>e</SUB> are observed with respect to increase of the heating power. These observations suggest that the turbulence and rational surfaces play an important role on heat transport in the LHD. The turbulence is usually driven by plasma free energy and thus it restricts the gradients of the local thermodynamic variables in a plasma. When the turbulence-driven-transport is dominant, the heat flux is not proportional to electron temperature gradient ∇T<SUB>e</SUB> but is a non-linear function of electron temperature T<SUB>e</SUB> and ∇T<SUB>e</SUB>. The effect of magnetic configuration on transport is also important because it influences the turbulence in many aspects. In addition, the magnetic field structure (e.g. magnetic island) directly affects the over all plasma confinement. In this thesis, the non-linearity in the heat transport and effect of the rational surfaces are studied by using the transient transport analysis in the LHD. <br /> The perturbative analysis is recognized as a powerful tool to study an effect of turbulence on transport and its unique feature makes possible to study the transport in the almost flat electron temperature region, where the usual analysis based on the calculated power deposition and measured gradients is not applicable. To study the transport features of the confined plasmas, the heat pulse propagation experiments are performed by on-axis electron cyclotron heating (ECH) power modulation on the LHD plasmas with neutral beam injection (NBI). The heat pulse propagation can not be explained by transport features obtained from the steady state analysis. Requirement of larger (3-5 times) heat diffusivity to reproduce the heat pulse propagation in Co-and balanced NBI plasmas indicates the non-linearity of the heat transport. The analysis method for the heat pulse propagation based on the non-linear dependence of heat diffusivity X<SUB>e</SUB> on T<SUB>e</SUB> and ∇T<SUB>e</SUB> is established. One of the first principle turbulence transport model, the critical temperature gradient scale length model, is tested for quantitative understanding of non-linearity of heat transport in the LHD. The critical temperature gradient scale length model with optimized parameters, which is obtained from the heat pulse propagation, can also explain the results of the cold pulse propagation experiment. <br /> The effect of the rational surface on the heat transport is found to be more important in the counter dominant NBI heated plasmas. A unique feature of heat pulse propagation is observed near the m/n = 2/1 rational surface (m, n are the poloidal and toroidal mode numbers, respectively). A simultaneous response of the temperature perturbation on radially separated flux surfaces is observed. This non-monotonic heat pulse propagation can not be explained even if the heat transport is strongly non-linear. The change in the magnetic field topology due to enlargement of a magnetic island structure is used to explain this non-monotonic heat pulse propagation phenomenon. The estimated O-point position of the island is located near the m/n = 2/1 rational surface. The m/n = 2/l island healing with decrease in electron collisionality is also observed as was predicted by theories. The magnetic island enlargement is considered to be related to a direction and profile of plasma current mainly driven by NBI. The simple equation of heat pulse propagation in slab geometry with time-dependent boundary conditions is used to evaluate the heat diffusivity inside the magnetic island. The estimated electron heat diffusivity inside an m/n = 2/l magnetic island has same order as X<SUB>e</SUB> obtained from the power balance analysis in the Co- NBI plasmas. <br /> The core T<SUB>e</SUB> flattening region in the presence of the m/n = 2/1 island is found to be a stiff structure. No increase in ∇T<SUB>e</SUB> is observed with respect to the change in the power of on-axis ECH. High power ECH above a critical value injection can break this stiff structure, and steep T<SUB>e</SUB> profile is formed just inside the rational surface. The role of the m/n -2/1 rational surface and presence of an island in the formation of internal transport barrier (ITB) is discussed in Ctr-NBI plasmas, where the enlargement of the m/n =2/1 island is indicated.
総研大甲第897号
thesis
2005-09-30
application/pdf
https://ir.soken.ac.jp/record/522/files/甲897_要旨.pdf
eng