1st IAEA TM, First Generation of Fusion Power Plants Design and Technology -, Vienna, July 5-7, 25 Computational Study of Non-Inductive Current Buildup in Compact DEMO Plant with Slim Center Solenoid Y. Nakamura, K. Tobita, H. Tsutsui 1), Y. Takase 2), S. Nishio, M. Sato and N. Takei, Naka Fusion Research Establishment, JAERI, Mukoh-yama, Naka-city, Ibaraki, 311-193, Japan 1) Research Lab. for Nuclear Reactors, Tokyo Institute of Technology, Tokyo 152-855, Japan 2) Department of Complexity Science and Engineering, The University of Tokyo, Chiba, 277-8561, Japan - 1 -
Outline Consistent simulations of non-inductive current buildup were carried out from the following control aspects of low aspect ratio, CS-less tokamak : stable ITB-formation to obtain high BS current, integrated scenario based on reasonable confinement/mhd physics, new challenging technique of external ITB-control. Fully non-inductive current buildup was demonstrated, meeting the control and physics requirements set by : plasma shaping, available NB-heating power, avoidance of CH, reasonable HH factor and allowable Greenwald density limit. A new scenario was shown that the downsized Slim CS operation can afford to control the ITB profile over a wide range from positive to negative magnetic shear, and vice versa. - 2 -
Primary Issues of Non-Inductive Buildup CS-less tokamak : VECTOR I p = 14 MA, β N = 6, P F = 2.5 GW 8 6 4 D1 D2 Very slow buildup feasible? τ ~ 1 sec by ECCD, LHCD, NBCD to reduce "Return Current" τ > τ = cf. ~ 1 sec by inductive at present Collaborative with plasma shaping? 2 a µ η( ) Z (m) 2-2 -4 Recharging of (D1, D2) currents to keep q a constant leads to I p ramp down. Safe takeoff from limiter to divertor? In future study, safe landing control as well. -6-8 2 4 6 8 1 R (m) - 3 - Stable ITB-formation feasible? High BS fraction, e.g. f bs > 5 %, needed to save driving power.
Consistent Simulation Modelling "Slim CS" Long timescale buildup, plasma shaping & ITB-control by external CD, ITB-generated BS current + "Slim CS", avoiding CH-formation under over driving condition Operational requirements from non-inductive techniques j p (1) power limit CD (2) external CD limit I cd = < 4 MA Operational requirements from confinement, MHD Physics Ip (MA) (1) density limit n < ngw = 2 π a τe (2) energy confinement HH = 13. (?) τ, (3) equilibrium limit P CD, P NB < 1 MW p η P Ey CD nr e β R / ~. p a 15 2-4 -
1 8. 2. 4. 6. 8 1 1 8 6 4 2 Numerical Model on TSC Momentum Eq. Turbulent & Neoclassical Transports m + F v ( m ) = j B p t with Faraday's law & Ohm's law B = E ; t E + v B = η j oh ITB joh = jtotal jbs δ ETB Neoclassical Transport in Prescribed ETB Region (ρ >.95) NB Heating & Current Drive with Fixed Deposition Profile 1 χ = χ + NC χcdbm with D =.1 χ 1. p(ρ) (x 1 3 N/m 3 ) 6 4 2 T e p n e ETB q T e (ρ) (kev) n e (ρ) (x 1 19 /m 3 ) P NB (ρ) (arb.).5 PNB j ext.5 j ext (ρ) (arb.) ρ - 5 -.2.4.6.8 1 ρ
NB Heating and Current Drive for 1 MA/1 sec Buildup Very slow scenario : Target I p = 1 MA, Buildup time = 1 sec The plasma density n e is adjusted by feedback control to the lineaveraged density n e prescribed. NB heating up to 75 MW and external CD enhanced up to 3.5 MA 15 5 5 1 Ip (MA) 1 5 target I p n e 4 3 2 1 ne (x 119 /m3) ICD (MA) 4 3 2 1 (1) (2) I CD (3) P NB 8 6 4 2 PNB (MW) 5 1 15 time (sec) - 6 - -2 5 1 15 time (sec)
Fully Non-inductive Buildup and Steady State Control Stable non-inductive buildup by 75 MW NB heating + 3.5 MA external drive 11% over drive 1% non-inductive control with ~45% BS drive ITB oscillation due to interplay of BS current and magnetic shear stable buildup with NS profile w/o CH low β p low β p < 1.5 safety PS to NS transition higher NB triggers large amplitude oscillations. takeoff to divertor - 7 -
Stable ITB-formation & Avoidance of CH - 8 -
Mechanism of Spatio-temporal Oscillation at High NB Heating increase of heatflux to edge high NB heating, α-heating (?) inward drifting & flattening of ITB formation of H- mode-like ETB propagation of positive E-field into core region (~ 8 sec) positive "Return Current" & E-field in edge decrease of I bs at edge Tiny E-field plays an important role in full non-inductive CD plasmas. increase of I bs at edge negative "Return Current" & E-field in edge propagation of negative E-field into core region (~ 8 sec) loss of H-modelike ETB reduction of heatflux to edge - 9 - outward drifting & steepening of ITB
A New Scenario of External ITB-control via "Slim CS" External ITB-control technique has been required for advanced tokamak reactors. Compact "Slim CS" is capable of supplying 1 Vsec, less than 1% of total plasma flux. The q-profile undergoes a drastic change by supplying tiny E-field via "Slim CS" for longer time than E-field diffusion time, τ ~ 1 sec. - 1 -
q-profile Control via Tiny E-field The q-profile undergoes a drastic change by supplying tiny E-field for longer time than E-diffusion time. Positive E-field, +.5, drags the ρ s inwards. Eventually, the magnetic shear changes from negative to positive profiles. Negative E-field, -.3, drags the ρ s outwards. Finally, the magnetic shear changes from negative to positive profiles. - 11 -
Time-evolution of j-profile during External ITB Control - 12 -
External Control of ITB Profile via "Slim CS"..5. ρ 1.....5. ρ 1..5. ρ 1..5. ρ 1. - 13 -
Loss of ITB and Getting Back Control An external control of the ITB profile was demonstrated via "Slim CS". Clear ITB at t = 17 sec Loss of ITB at t = 19 sec Getting back of ITB at t = 21 sec Enhanced ITB at t = 23 sec - 14 -
Summary A fully non-inductive drive scenario on CS-less tokamak was studied via consistent numerical simulation using TSC. Stable ITB formation and associated increase of the BS current were confirmed to enhance the current buildup efficiency, meeting non-inductive technique and confinement, MHD physics requirements. In high power NB-heating, a self-organized, spatio-temporal oscillation of the plasma pressure and current was predicted to occur. It was shown that tiny E-field plays a key role in ITB-formation of full non-inductive CD plasmas. A new operation scenario using "Slim CS" was proposed for the external ITB control. Future Studies : Validation through JT-6U CS-less buildup experiment Day-long control of α-heated, burning plasmas - 15 -