29th European Physical Society Conference

Steady-State Operation Scenarios with a Central Current Hole for JT-60SC

H.Tamai, S.Ishida, M.Matsukawa, A.Sakasai, G.Kurita, K.Tsuchiya, S.Sakurai, H.Shirai, Y.Sakamoto, T.Fujita, Y.Miura, and K.Ushigusa
Japan Atomic Energy Research Institute, Naka Fusion Research Establishment Naka, Ibaraki 311-0193, JAPAN

Abstract.
While a central "current hole" in JT-60U reversed shear plasmas has been recently discovered, the potential attractiveness of the current hole has remained open for an operation scenario in a tokamak fusion reactor. This paper presents the feasibility and issues of steady state operation with a central current hole for JT-60SC from the view of reactor applicability of the current hole. The superconducting tokamak, JT-60SC, to be modified from JT-60U is designed towards the establishment of steady-state high performance plasmas in deuterium with high (=3.5-5.5) in a regime of sufficiently low collisionality (ν~0.01) and Larmor radius (^*~0.01), which are relevant to reactor plasmas. Main parameters of JT-60SC are I_p=4 MA, B_t= 3.8 T, R_p=2.8 m, a=0.85 m, ~1.8, ~0.35 and the current flat-top duration of 100 s exceeding a current diffusion time of ~30 s. An impact of the current hole on fusion engineering would stand in no necessity of central current drive leading to a remarkable reduction of neutral beam injection energy. In order to investigate whether such a steady state operation with the current hole is feasible only by off-axis current drive schemes consistent with a high bootstrap current fraction, the 1.5D time-dependent transport code (TOPICS) analysis has been made. Here thermal and particle transport coefficients deduced from observations in JT-60U are used as a function of the magnetic shear, representing the formation of internal transport barriers (ITBs) near q-minimum.
A steady state operation with HH_y2~1.6 and ~4 has been obtained at I_p=1.5 MA, B_t=2 T and q₉₅=4.5 where the non-inductive currents are developed to form a current hole with beam driven currents by tangential off-axis beams of 6.7 MW and bootstrap currents additionally by perpendicular beams of 4.5 MW. The bootstrap fraction increases up to ~75% of the plasma current and the current hole region is enlarged up to ~30% of the minor radius in t=30 s from the discharge initiation. The central current hole is found to be sustained up to t=70 s, when the toroidal electric field becomes sufficiently uniform around zero. The present transport simulation shows that the central current hole can be sustained only by off-axis beam driven currents and bootstrap currents. No current relaxation mechanism due to MHD instabilities is considered on the basis of no observation of MHD instabilities associated with the current hole in JT-60U. The results suggest that a bootstrap current evolution near the current hole region and a relation between the location of ITB and the central current hole region are important to achieve a steady state plasma compatible with the current hole. Evaluation of beta limits for the steady state plasma and extension to higher current plasmas will be presented.