A." Sagara

H. Yamada

J. Miyazawa

R. "Sakamoto

National Institute for Fusion Science (NIFS-Repository)

Pellet refueling conditions to sustain self-burningplasma has been investigated by extrapolating conﬁne-ment property of the LHD plasma which appear to begoverned by gyro-Bohm type conﬁnement property. Apower balance of burning plasma is calculated takinginto account the proﬁle change with the pellet depositionand subsequent density relaxation. Self-burning plasmais achieved within the scope of conventional pellet injec-tion technology. However, very small burn-up rate as0.18 % is predicted.A power-balance analysis has been performed ac-cording to the following conditions. (1) Only pellet in-jection is considered for the fueling and a particle depo-sition proﬁle is estimated using a conventional neutralgas shielding (NGS) model. (2) Density proﬁle relax-ation process after the pellet fueling is calculated by as-suming particle transport coeﬃcients which are extrapo-lated from the LHD experiment. (3) Temperature proﬁlechange is calculated by using the direct proﬁle extrapo-lation (DPE) method1) in which the normalized plasmapressure proﬁle obtained from the LHD experiment is di-rectly extrapolated into a burning plasma by assuminggyro-Bohm type parameter dependence.To characterize the particle transport property ofthe pellet fueled high density plasmas in LHD, densityproﬁle changes were ﬁtted into one-dimensional diﬀusionequation with cylindrical approximation,∂n∂t= D∂2n∂r2+(Dr+∂D∂r−V)∂n∂r−(Vr+∂V∂r)n+S, (1)where n, r and S denote the plasma density, minor ra-dius and particle source proﬁle, respectively. It is as-sumed that the particle ﬂux is consist of diﬀusion term;D and convection term; V . In regard to a thermal trans-port property, pressure proﬁle which is typically observedin LHD experiment is directly extrapolated to burningplasma condition assuming the gyro-Bohm type ther-mal transport property, namely, following relationship is0.1 1 10Time   [s]0.1 1 10Time   [s]0.1 1 10Time   [s]43210100050000.1 1 10Time   [s]6420ne [1020 m-3]Te [10 keV]Pheat [MW]Pnf [GW]PαPLPbPAUXPnfTeneTint= 50 ms (20.0 Hz) Tint= 75 ms (13.3 Hz) Tint= 80 ms (12.5 Hz) Tint= 100 ms (10.0 Hz)formed between LHD plasma (subﬁx exp) and burningplasma (subﬁx bp).pnor(ρ) =pexp(ρ)P 0.4expnexp(ρ)0.6B0.8exp=pbp(ρ)P 0.4bp nbp(ρ)0.6B0.8bp(2)where pnor is normalized pressure proﬁle. p, P and Bdenote pressure proﬁle, heating power and ﬁeld strength.Temperature proﬁle change can be expressed as followsin consideration of an energy conﬁnement time τE,∂T∂t=1τE((Pbpn)0.4B0.8bp pnor − T). (3)Long duration sustainability of pellet fueled burningplasmas have been investigated taking into account theabove mentioned plasma conﬁnement properties. Figureshows waveforms of typical plasma parameters and fu-sion output in the case of 7.1 mm pellet injection at avelocity of 1.2 km/s. Though there is a ﬂuctuation ofthe fusion output due to a drastic proﬁle change by pel-let fueling in the initial phase, quasi steady-state burn-ing plasma is sustained after termination of auxiliaryheating if the pellet injection interval is shorter than 75ms. The minimum fusion output to sustain self-burningquasi steady-state plasma is about 3.1 GW and the fu-sion output at the quasi steady-state phase increases asthe injection interval shortens. The fusion output ﬂuc-tuation due to pellet refueling is ±34 MW and this valueis suﬃciently small as compared with the averaged fu-sion output of 3.1 GW. On the other hand, self-burningplasma sustainment cannot be achieved with the longerpellet injection interval above 80 ms. The fusion outputgradually declines after termination of auxiliary heat-ing and the plasma ﬁnally collapse due to losing a heatbalance. In order to sustain a self-burning plasma, ad-equately short pellet injection interval is required andminimum required DT particles are 1.2× 1024 DT parti-cles per second in the case of 7.1 mm pellet injection at1.2 km/s. These parameters can be accomplished withinthe scope of present-day technology. Since a net DT fuelconsumption rate at 3.1 GW fusion output is 2.2× 1021/s, a burn-up ratio which is deﬁned by fusion reactedparticle to injected fuel particles is only 0.18 %.1) Miyazawa J. et al 2011 Fusion Eng. Design 86 2879241§9. Pellet Fueling Requirements to Allow Self-burning on Helical Type Fusion ReactorSakamoto, R., Miyazawa, J., Yamada, H., Sagara, A.

Pellet Fueling Requirements to Allow Self-burning on Helical Type Fusion Reactor

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