EFFECT OF BUILDING STRUCTURES ON TOKAMAK PLASMA INITIATION

ABSTRACT

The stray field produced by a tokamak is high enough to magnetize steel reinforcement in concrete walls of the tokamak building. As a result, the reinforced walls may produce a significant magnetic field that would affect plasma initiation. The paper is devoted to assessment and correction of the stray field with the toroidal mode n = 0 to secure plasma initiation.

АННОТАЦИЯ

Магнитное поле, создаваемое токамаком, способно вызвать намагничивание стальной арматуры здания реактора. В свою очередь, намагниченные армированные конструкции создают поля рассеяния, негативно влияющие на процесс образования плазы. В работе исследованы возмущения магнитного поля с тороидальной модой n = 0, вызываемые намагниченной арматурой здания токамака и определена величина корректирующих токов для компенсации их влияния. 

Keywords: tokamak, plasma, stray field, steel reinforcement, magnetization, simulation.

Ключевые слова: токамак, плазма, поле рассеяния, стальная арматура, магнетизация, численное моделирование.

Introduction

The ITER tokamak complex which is now under construction consists of several buildings located in a common pit. The tokamak itself is located in the bioshield pit of the main nuclear building [1]. Typically, concrete walls of the buildings are reinforced with steel rebar. The stray field produced by the tokamak during operation may magnetize the rebar. The magnetized rebar in its turn produces an additional stray field in the plasma region. The axisymmetric component of this field with the toroidal mode n = 0 may affect plasma initiation. Therefore, correction of plasma initiation scenarios may be needed to provide a wide “magnetic null” under the prefill gas breakdown (BD). This makes the knowledge of the n=0 stray field an important issue in the plasma initiation scenarios. A numerical study has been performed to evaluate the anticipated stray field near the tokamak and elaborate correction strategy.

Field computation

The effect of steel rebar should be taken into account when a scenario of currents in the tokamak poloidal field coils is designed. From the design criteria, the poloidal field in the BD region shall not exceed 20 G at the plasma initiation.

The field from the magnetized steel rebar is assessed with a simplified global magnetic model of the tokamak complex (MMTC-2.2) [2]. The model reflects the volumetric steel fractions in the tokamak building structures. The model is illustrated in Fig. 1.

Figure 1. 3D computational model of tokamak building

1 – seismic basemat, 2 – walls (taken from [2]). Coordinate system is related to tokamak center

Building structures are modelled with a set of 3D elements. Each element is assumed homogeneously magnetized in the field produced by the tokamak coils and plasma. Simulations were performed for two basic scenarios of the plasma discharge, the first plasma and 15 MA DT. The field from the steel rebar is averaged along the circles passing through reference points (R, Z) on the plane Y = 0.

Fig.2 shows the simulated field for two scenarios at BD.

Table 1 lists the reference points and parameters of the tokamak coils used in the computations. Calculated correction currents required to neutralize the effect of the anticipated stray field are presented in the ΔI columns. The maximal change in currents of about 30% occurred for PF3 and PF4 coils.

Table 1.

CS and PF coils position, size, current capacity and required correction currents for basic plasma scenarios

Fig. 3 shows the stray field maps obtained with the introduced correction currents. The maps demonstrate that after correction the stray field in the plasma breakdown region does not exceed the design limit of 20 G.

Conclusions

At plasma BD the reinforced wall structures of the tokamak building are capable to produce the stray field of about 100-120 G in the plasma region. This field is high enough to be dangerous for the plasma initiation. A proposed remedy is adjustment of the coil currents. The effect of the building structures can be compensated by relatively low currents of less than 0.1 MAt. This is sufficient to provide the stray field below the 20 G design limit in the plasma region.

References:

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