7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Numerical simulation of fuel isotope transport during DEMO start-up phase
K. Miyamae1, H. Yamada1, A. Fukuyama2.
1
Graduate school of Frontier Sciences, The University of Tokyo, 2Kyoto University.
e-mail (speaker): miyamae.kento22@ae.k.u-tokyo.ac.jp

Power [MW]𝑷𝐢𝐧𝑷𝐍𝐁𝐈
𝑷𝐑𝐅

𝑷𝜶Current [MA]Seconds

𝑰𝐏
𝑰𝐍𝐁𝐈

𝑰𝐎𝐇
𝑰𝐑𝐅 𝑰𝐁𝐒

Seconds

Figure 1 Temporal evolution of plasma
current and fusion output in the base casescenario.
Density [1020/m3]

This study has investigated the effect of particle
transport of fuel ions (deuterium (D) and tritium (T)) andhelium ash (He) as well as their heat transport on thenuclear fusion output in a DEMO reactor. The result wasassessed and possible start-up scenarios for DEMO isdiscussed.
While development of a reliable startup scenario for
DEMO that aligns with physical design and engineering
constraints is important, it still remains in research withmany different assumptions over the plasma physics. In
particular, particle control is an open issue because ofimmature physics understanding and insufficienttechnological readiness for that . Establishment of a
fueling and exhaust scheme is inevitable for both stableburning control and efficient self sufficiency of T fuel ina fusion plant.
In this study, the hydrogen isotope effect in D/H/He
mixture plasmas on transport and resultant fusion power
output has been discussed with the integrated tokamakmodeling code TASK/TR [1]. TASK/TR solves onedimensional transport equations for densities,
temperatures, and a poloidal magnetic flux. Thermal
diffusivity and particle diffusivity is calculated as sum of
neoclassical transport and drift wave transport. For drift
wave transport, the CDBM (Current diffusive ballooningmode) model is incorporated. Thermal diffusivity of
CDBM is expressed as below equation.
𝑐 & 𝑣*
𝜒!"#$ = 𝐶𝐹 (𝑠, 𝛼)𝛼 %⁄& &𝜔() 𝑞𝑅𝐷!"#$ = 𝐾 ∗ 𝜒!"#$c is the velocity of light, 𝜔() is the electron plasma
frequency, 𝑣+ is the toroidal Alfvén velocity and q is thesafety factor. Factor F is a function of the magnetic shears and the normalized MHD pressure gradient α. 𝐾 isconstant factor derived from experimental results.
Fusion reactor parameters and reference operationalsequences are based upon the recent tokamak fusion
DEMO design by the Joint Special Design Team [3].
The result of fusion power development and plasma
current for reference case is shown in Fig. 1. Also,density profile at t=1,000 s is shown in Fig. 2 (a). The
result suggests that with consideration of particle
transport, efficient fueling of D and T to the core region
is difficult in particular in the start-up phase without
anomalous inward pinch. Together with the buildup ofHe, fuel ions show hollow density profiles, whichhinders higher fusion output. The fusion output wasdecreased by about 70% compared to reference scenario
with fixed electron and fuel ion density profile.
The density profile of deuterium and tritium withisotope effect incorporated model is shown in Fig. 2 (b).
To incorporate isotope effect as mass dependencies,
𝜒,-./ for each ion is calculated separately, whereAlfvén velocity differs for each ion species due to ion(a)𝒏𝐞𝒏𝐇𝐞𝒏𝐃(c)

𝒏𝐞𝒏𝐓𝒏𝐃,𝐓𝒏𝐇𝐞(b)𝒏𝐞𝒏𝐃,𝐓𝒏𝐇𝐞

Normalized minor radiusFigure 2 Density profile of ions and electron in (a) the basecase scenario, (b) the case with isotope effect of CDBM
integrated , and (c) He enhancement effect incorporated model.
mass. Due to the isotope effect, heavier ions have lessdiffusivity, resulting in the difference of density profiles.
Although He ash built up more and ratio of deuterium
density and tritium density deviated from 1:1, its effectto the fusion power output was relatively small (less than5%).
To discuss the effect of He build up, arbitrary
enhancement of He particle transport is integrated to thediffusion model as bellow equation.
𝐷0) = 𝐴 ∗ (𝐷1! + 𝐷"2 )𝐴 denotes arbitrary constant coefficient expressing theenhancement of particle transport. The density profile
with He particle diffusivity enhancement effectincorporated model is shown in Fig. 2 (c). Whenenhancement factor 𝐴 is set to 1.5, He build up is
mitigated to about 2/3 in the core region. The mitigation
also made possible to increase fueling when considering
electron density limit.
References[1] M. Honda and A. Fukuyama, Nucl. Fusion 46, 580 (2006).
[2] A Fukuyama et al., Plasma Phys. Control. Fusion 37, 611(1995)[3] K. Tobita et al., J. Phys.: Conf. Ser. 1293, 012078 (2019)