Effect of impurity radiation and helium particle confinement on tokamak–reactor plasma performance

Abstract

In this paper two important matters for assessments the D–T plasma performance of future tokamak–reactors such as ITER and EU–DEMO projects are discussed. The first issue is the heat removal via the radiation of intrinsic and seeded impurities. The second is the helium particle confinement and ash removal. To study these issues, a simple numerical 0.5D-model is proposed. The impurity radiation in the tokamak high–temperature plasma is described by the coronal model. Coronal radiative cooling used rates are based on the tabulated data published in the recent works. Helium particle confinement and ash exhaust efficiency are determined by the variable parameter ρ* characterizing the ratio between the global helium particle confinement time τ H e ∗ and the energy confinement time τ E . Such approach allows simultaneously obtaining solutions of the particle and energy balance equations in steady–state conditions, taking account of the effect of impurity radiation loss and ash poisoning. Using known energy confinement scaling τ E and design parameters of the ITER and EU–DEMO projects the particle and energy balance equations have been solved. For fixed values of impurities concentration and ratios ρ*, iso–curves of constant fusion power were obtained. These curves are similar to the well–known Plasma OPeration CONtours (POPCON). The POPCON curves are often used to study the operating points of tokamak–reactors. It is shown that for the ITER parameters there is a noticeable margin in the ratio ρ*. For the design parameters of the EU–DEMO project, the margin for ratio ρ* is very small, which casts doubt on the feasibility of the project in its current form (at least with regard to the EU–DEMO steady–state project). Further research in this area is required. The methods and results obtained in this paper can be used for further conceptual analysis of tokamak–reactors operating space.

Toward DEMO Power Plant Concept Selection Under Epistemic Uncertainty

“…On the other hand, the thermal He-4 fraction shows a linear increase in feasibility. These results may indicate that the plasma elongation should be at least 1.83 to fulfill the net electric power output constraint of 400 MW, and greater concentrations of thermal He-4 in the plasma are favorable to achieve the pulselength constraint of 2 h. In fact, it has been shown that greater fusion power is associated with larger elongations [6], and He-4 contributes to increase the energy confinement time when it does not exceed the dilution upper limit [25]. To visualize the combination effects on the feasibility space, the scatterplots of the plasma elongation against the thermal He-4 fraction are displayed on the top right and bottom left figures of Fig.…”

Effect of toroidal rotation on impurity transport in tokamak improved confinement

“…The hybrid scenario, which is an intermediate operation mode, has been investigated in several experimental devices [4][5][6]. On the other hand, significant degradation of core plasma performance has been reported due to radiation losses as a result of accumulation of high-Z impurities such as tungsten (W), which is used for the divertor and vacuum vessel wall material in fusion reactors [7][8][9]. In particular, the impurity accumulation can be accelerated in the ITB region of improved confinement discharges in negative magnetic shear mode and high-β p mode.…”

Analytical model for the combined effects of rotation and collisionality on neoclassical impurity transport

“…In order to withstand the peak heat fluxes during fusion reactor operation, avoid tritium fuel retention and minimize target erosion, next-generation tokamaks like ITER and SPARC are relying on the use of tungsten (W) divertors [1,2]. However, plasma contamination with such a high-Z material can severely restrict the operational space of a reactor [3,4]. Several current devices, like ASDEX Upgrade (AUG) and JET, have transitioned to all-metal walls to study plasma performance in metallic environments in preparation for these future machines [5,6].…”