IGNITOR DOUBLE-NULL CONFIGURATION: ELECTROMAGNETIC, THERMAL AND STRUCTURAL ANALYSIS WITH INTEGRATED FEM MODELS

A. Capriccioli (*), L. Cornaggia (**), P. Frosi (*), A. Pizzuto (*)(*)ENEA, CR Frascati (**) ALTECO, Genova, Italy

Introduction

Ignitor [1] is a high field compact machine propose and designed to achieve ignition in D-T plasmas. The machine has been conceived as completely integrated system of its major components (toroidal field system, poloidal field system and plasma chamber). The toroidal field magnet is made of 12 modules, each including two toroidal field coils (TFC), conteined by 4 C-Clamp elements. The structural performance of the machine relies on an optimised combination of “wedging”, in the TFC inboard legs and in the outboard of the C-Clamp, and “bucking” between the TFC and the Central Solenoid (CS). The Poloidal Field Coils (PFC) includes 14 coils for the CS and 16 external coils for the equilibrium and shaping system. The CS consists of two concentric sets of coils (7+7), the central steel post, spacers and precompression systems.

IGNITOR Section

IGNITOR Magnet Module

In this analysis a double-null configuration with X-points laying just outside the first wall is taken into account. The separatrix solutions are constrained to have a value q95>3,

resulting in a reduced plasma current of 10 MA.The first step in the analysis sequence is the evaluation of the electromagnetic forces and magnetic flux densities. These results are used as input for the thermal analysis and finally the structural model is performed using the output of both.

     Electromagnetic Model

The aim of the analysis is to calculate and to write on appropriate files nodal magnetic forces and flux densities that take effect on each poloidal coil. This is a 2D model with ANSYS Plane53 elements (8-node and 4-node Magnetic Solid) and Infin110 (2-D Infinite Solid). The model describes all the Poloidal Field System Coils, the Vacuum Vessel and the Plasma Region. The latter is modeled with 78 small square areas, 25 mm thick, that behave as single conductors. In each wire flows a fraction of plasma current. The total current for the given scenario is assigned a spatial parabolic distribution with the maximum density value in the center of the plasma region and the minimum value at the edge. Their ratio is set to 2. For the PFC system, appropriate elements (Circu124) impose to each coil the assigned current and to each element corresponds a turn in the same coil.

Recent studies [2] demonstrate the capability of the Ignitor PFC system to support X-Point configurations scenarios to allow an easier access to H-mode regimes. The plasma dimensions must be reduced from their reference values to allow enough space for the scrape-off layer.

The total FE model includes about 18000 nodes and 14000 elements; the Fig. 1. shows only the solid model with the Coils, the Vacuum Vessel and the Plasma region. This items are surrounded by the “air” and “infinite” regions.

IGNITOR Electromagnetic FE model

B distribution at the set time (4 sec)

ENEA-FUS

The calculation output is saved on files as arrays or tables: these values make up the input data for the next PFC system Thermal Analysis and the whole Load Assembly Structural Analysis.In particular, the magnetic flux density values (B) will be loaded by the Thermal model to calculate the resistivity growth as function of B while the electromagnetic forces will be loaded by the Structural model.

Thermal Model

The model includes, for each coil, all the turns and the cooling holes, the insulation layers between each turn and the ground insulation. This is a 3D model with ANSYS Solid69 elements (3-D Thermal-Electric-Solid) and describes a 2° toroidal sector of each poloidal coil.This is a static analysis and the time evolution of the currents are set as nodal forces to each turn.

To take magneto-resistance effects into account, a specific procedure was developed: values of the magnetic flux density are read, during the calculation, for each turn and with a time step of 0.5 sec, from the files generated by the previous electromagnetic model. These are used to calculate the resistivity growth and to change the material characteristics of the single turn. In this way it is possible to consider the resistivity as function of the magnetic flux density as well.

Temperature distribution in P10 copper turns

The whole calculation cycle involves the sequential analysis of each upper and lower poloidal coil, when the currents scenario is not symmetric. The calculation results are saved on appropriate files for the next step of structural analysis. No manual data handling is necessary in any step of the analysis: the output files are saved with appropriate names in appropriate directories.

Temperature distribution in P10 insulation layer

Temperature distribution in P10: view of a selected detail

Mechanical Model

The model takes into account 1 Toroidal Field Coil, 2 C-Clamps, the Central Post, the OHmic Transformer (OHT) coils with intermediate disks and spacers, the external Coils, the passive and active press rings, and all the precompression systems. This is a 3D model with ANSYS Solid45 (3-D Structural Solid) and Contac52 (3-D Point-to-Point Contact) elements and it describes a 15° angular sector of the whole Load Assembly. The FE model includes about 22000 nodes and 16000 elements.

Between TFC and CC and betwen TFC and OHT there are non linear elements capable of supporting compression in the direction normal to the surfaces and shear (Coulomb friction) in the tangential direction.The same elements are located between all the OHT coils and in all the precompression systems. In such systems the right precompression value is taken by appropriate negative gap values in the initial interference condition. The total number of non linear elements is about 2400.

External coils are constrained to transmit only vertical forces to the C-Clamp structures: free strains are available in radial direction. At this time the bracing ring total force is about 190 MN while the contribution of the active press is 170 MN.This is a non linear analysis and the results are limited to stresses induced by in plane forces. IGNITOR Structural Model

Thermal Mechanical Analysis

The structural analysis shows that the equivalent Von Mises stresses in the TFC coil (312 MPa max value) are very similar to the reference scenario stresses (291 MPa), at the same time (4 sec). The internal stress distribution and the minumum value (35 MPa) also appear to be the same as in the reference case (39 MPa). TFC and PFC coils: Temperature

distribution P1 P10 PFC coils: Temperature distribution

The P1, P2, P3, and P4 poloidal coils show a maximum stress essentially equal or lower (163 MPa) than in the reference case (167 MPa); the same applies for the minimum values (15 MPa DN conf. and 50 MPa Ref.). Stresses in other outer coils are less than those of the reference scenario, with the exception of coils P9 and P10. Nevertheless, the stresses are well below the allowable.

Because of a substantial equivalence in TFC stresses also the C-Clamp stresses appear to be only a little higher than in reference scenario. The differences are localized in small areas between the nose and the flag top. Small adjustments with appropriate fillets could significantly decrease them.In the reference scenario the stress intensity range is 40 - 600 MPa, in the present configuration the range is 39 - 640 MPa.

On the basis of the structural analysis it is possible to state the Ignitor capability to support loads induced by Double Null configuration, at the selected time and with regard to “in plane forces”. Further analyses will be necessary to compute the stress field by “out of plane forces” at the same topical times of the proposed Double Null scenario.

Von Mises Stress in Ignitor TFC

Stresses in Ignitor OHT coils

Stresses in Coils P9 and P10 Stresses in Ignitor C-Clamps

[1]     B. Coppi, A. Airoldi, F. Bombarda, G. Cenacchi, P. Detragiache, C. Ferro, R. Maggiora, L.E. Sugiyama, G. Vecchi “Critical Physics Issues for Ignition Experiments: Ignitor” MIT (RLE) Report PTP 99/06 Cambridge 1999

References

Conclusions

[2] G. Cenacchi and A. Airoldi “Equilibrium configurations for the Ignitor experiment” Consiglio Nazionale delle Ricerche, Italy FP01/1 February 2002