Impact of the R=2.14 m FIRE design and AT scenarios on the TF and PF magnet design

The toroidal field coil system of the FIRE tokamak utilizes LN₂ cooled, beryllium copper alloy bitter plate type magnets. They produce a field on axis (R₀=2.14 m) of 10 T with a flat top time of ≥20 s. The central solenoid is a free-standing segmented design that probably will use a Cu-Cr-Zr alloy that combines the strength and conductivity characteristics needed for FIRE. In the course of pre-conceptual design, the machine major radius has increased from 2.0 to 2.14 m and space allocation in the central column has been adjusted, reducing the cross section of the TF inner leg. Addition of a second cooling channel in the TF inner leg has improved cool-down times, but has further degraded TF cross section. Analyses of TF based flat-top times based on refined nuclear heat calculations and refined current diffusion models are presented to qualify the 20 second flat top time for the new build. The TF profile in the structural analysis model has been updated to match the latest radial and vertical builds, and the new layout of the three corner PF coils. The refinements in the TF, and the space reallocation in the central column has reduced the "excess" engineering margin based on TF inner leg stress from 1.3 to 1.18. The primary purpose of the excess margin was to envelope the design changes during preconceptual design. Efforts to recover the excess margin are described. Conversion of PF 1 and 2 to three coils improved their stress and thermal states. Results for both the H-mode, and AT mode scenarios are presented. AT mode utilization of the segments of the CS is different than for the standard mode, and introduces larger relative radial motion. Scenario adjustments to reduce the thermal differential in the coils are described. The addition of advanced tokamak operation will require additional fatigue based analysis and design.