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Resistive TF Coil

Resistive coil geometry

A much simpler inboard mid-plane geometry is used for resistive TF coils, as shown in Figure 6. The most important difference is the absence of the lateral steel casing structure. Three main sections can be distinguished:

  • The bucking cylinder: radial thickness dr_tf_nose_case (iteration variable 57), is present to support the centering forces. Its presence is however not mandatory and can be can be removed setting TODO.
  • The conductor area: radial thickness dr_tf_wp_with_insulation (iteration variable 140). Ground insulation, corresponding to the dark grey area in Figure 6 is included in this section by convention.
  • The outer cylinder: radial thickness dr_tf_plasma_case. This cylinder plays no role in the structural models in PROCESS.
Res_geom

Figure 6: Resistive TF coil inboard leg structure at mid-plane. The disjoint steel case (no lateral case) are shown in light grey. The ground insulation wrapping the turns is shown in grey and the conductor region containing the turns in light blue. The dotted blue illustrate the location of the 6 coils turn.


The conductor layer is made of N_\mathrm{turn} turns per coil, set by the n_tf_coil_turns user input. The entire coil is wrapped with ground insulation of thickness dx_tf_wp_insulation. Inside the ground insulation, each conductor ("turn") is wrapped in a turn insulation, thickness dx_tf_turn_insulation. The coolant occupies using a fraction of the mid-plane cross-section given by fcoolcp (iteration variable 23).

Res_geom

Figure 7: Schematic view of a single TF coil cross-section with four resistive turns. The positions of the cooling channels are not specified in PROCESS - only the area occupied by coolant and the channel diameters.


Resistive centre-post

In a spherical tokamak the inboard legs of a resistive TF coil are known collectively as the "centre-post" (CP).

As the resistive heating depends on the magnet cross-section (in the plan perpendicular to the current direction), heating can be substantially reduced by flaring the central section of the TF coils. The corresponding shape is illustrated in the right hand side of Figure 8. The radius of the inboard leg then increases away from the midlane as shown. This design is used by default for spherical tokamaks (itart = 1).

tok_tfcoil

Figure 8: Mid-plane toroidal (left) and vertical (right) cross-section of a magnet using the itart == 1 geometry option. The toroidal cross-section (left) shows the presence of vaulted turn geometry with a bucking cylinder (that is not present by default for copper magnets) with insulation and cooling. The vertical cross-section (right) shows the presence of 4 sliding joints for remote maintenance purposes.


The radius of the top and bottom of the inboard leg r_cp_top (iteration variable 174) can be set in three different ways:

  • Calculated (i_r_cp_top = 0, default): , the top CP radius is calculated from the positions of the X-points. This option generally leads to a relatively large degree of flare.

  • User input (i_r_cp_top = 1): the user sets the value of r_cp_top, or selects it as an iteration variable (174). If r_cp_top is less than 1.01R_\mathrm{TF}^\mathrm{mid} (TF inboard mid-plane outer radius), the TF top radius is set to 1.01*r_tf_inboard_out with an error warning. If r_cp_top is too large, causing the centre-post to interfere with the X-point, an error message is generated.

  • Mid/top TF radius ratio (i_r_cp_top = 2): r_cp_top is set as a ratio of the inboard mid-plane radius to the input parameter f_r_cp, defined as \frac{ R_\mathrm{TF}^\mathrm{top} }{R_\mathrm{TF}^\mathrm{out}} . If r_cp_top is too large, causing the centre-post to interfere with the X-point, an error message is generated. This option allows the shape of the centre-post to remain constant when varying the machine size.

The resistive heating, cooling requirement and material masses are calculated taking the flaring into account, parametrized with an arc. The cross-sectional area of the coolant channels is uniform throughout the centre-post, making the coolant fraction smaller at the top where less resistive heating is expected due to the larger conductor section.

Resistive TF coil class | ResistiveTFCoil(TFCoil)