A. Sagara

J. Miyazawa

R. Sakamoto

T. Goto

National Institute for Fusion Science (NIFS-Repository)

        The confinement enhancement factor, γDPE, used in the Direct Profile Extrapolation (DPE) method [1] has been formularized using the peaking factor of the neutral beam (NB) heat deposition profile, (Pdep/Pdep1)avg, which is the line-average of (Pdep/Pdep1) inside ρ ≤ 1, and Pdep (ρ*) = ′Pdep (dV / dρ)0ρ*∫ expdρ .            (1) Pdep1 = Pdep(1) = Pexp is the total NB heating power.      In the DPE method, the gyro-Bohm normalized electron pressure profile is defined by pe,exp(ρ)/(aexp2.4 Rexp0.5 Pexp0.4 Bexp0.8 ne,exp(ρ)0.6). This can be fitted by the zero-order Bessel function of α0J0(2.4ρ/α1), or α0*J0(2.4ρ/α1*), as shown in Fig. 1, where all profile data are used to obtain α0 and α1 (thin solid lines in Fig. 1), while the profile data in the outer region of 0.5 < ρ ≤ 1 are used to obtain α0* and α1* (bold broken lines in Fig. 1). In LHD, the pressure profile occasionally shows flattening in the core region as shown in Fig. 1(a). This is what we call the core confinement degradation. The physics mechanism of this is not understood yet. The latter model with α0* and α1* has been introduced to estimate the core pressure profile from the data in the outer region of 0.5 < ρ ≤ 1 in the case with the core confinement degradation.      In Fig. 2, shown are the dependences of the plasma parameters on the line-averaged electron density, en . The magnetic stored energy, Wp, that shows the gyro-Bohm type density dependence (Wp ∝ 6.0en ) often degrades at high-density. This is called the global confinement degradation, and is clearly observed in the plasmas fuelled by gas puffing (GP). In the plasmas fuelled by hydrogen ice-pellet injection (PI), the gyro-Bohm type density dependence is kept even in the high-density regime reaching 1020 m-3. However, the value is slightly smaller than that expected from low-density GP data (Fig. 2(a)). This global degradation is related to the heating profile that changes from centrally peaked to flat to hollow as the density increases (Fig. 2(d)) and the NB penetration length becomes shallow. Regression analysis using over 800 profile data gives a relation of α0* ∝ (Pdep/Pdep1)avg0.61. If we assume for FFHR-d1 that 1) the core confinement degradation can be avoided, 2) the global confinement is proportional to α0*, and 3) the peaking factor of the alpha heat deposition profile is 0.65, then γDPE is given by  γDPE = (0.65 / (Pdep/Pdep1)avg)0.6.            (2)   1) J. Miyazawa, et al., Fusion Eng. Des. 86 (2011) 2879.  Fig. 1. Gyro-Bohm normalized electron pressure profiles at various line-averaged electron densities of (a) ~1 × 1019 m-3 and (b) ~5 × 1019 m-3. The thin solid curve and the bold broken curve in each figure correspond to the fitting models of α0J0(2.4ρ/α1) and α0*J0(2.4ρ/α1*), respectively. The data in the shaded region of 0.5 < ρ < 1.0 are used to estimate α0* and α1*. 0200400600Wp,exp / Pexp0.4(a)Rexp = 3.65 m, Bexp = 2.71 Tclosed: PI (#53600)open: GP (#53592)global degradationy = 120 x0.6y = 100 x0.6012α0α0α0*(b)core degradation0.1110 ν*2/3,exp(c)00.50 5 10 15(Pdep/Pdep1)avgne,exp (1019 m-3)flatpeakhollow(d)  Fig. 2. Density dependence of (a) the plasma stored energy, Wp, normalized by Pexp0.4, (b) the height of the gyro-Bohm normalized electron pressure profile, α0 and α0*, (c) the collisionality, ν*2/3,exp, at ρ = 2/3, and (d) the peaking factor of the NB heat deposition profile, (Pdep/Pdep1)avg. 238§6. Formularization of the Confinement Enhancement Factor as a Function of the Heating ProfileMiyazawa, J., Goto, T., Sakamoto, R., Sagara, A.

Formularization of the Confinement Enhancement Factor as a Function of the Heating Profile

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