4634 add fuel ion equilibrium constraint

[grmtrkngtn]

This makes fuel-ion equilibrium an explicit equality constraint in PROCESS.

Previously, the required plasma fuelling rate was calculated and set internally from the fusion rate and burnup fraction. This hides one of the coupled relationships required for PROCESS to arrive at a self-consistent solution.

The change separates:

molflow_plasma_fuelling, representing the plasma fuelling rate;

molflow_plasma_fuelling_equilibrium, calculated from the plasma state.

Constraint 93 then explicitly enforces fuel-ion equilibrium by requiring:

molflow_plasma_fuelling = molflow_plasma_fuelling_equilibrium

The equilibrium requirement is calculated in the physics model, while the constraint simply compares the two quantities.

Motivation

This is part of the work required to introduce a PROCESS solution mode for uncertainty-quantification evaluations.

For UQ, we want to evaluate PROCESS at sampled inputs without performing a full design optimisation. PROCESS must therefore be able to solve the coupled equations required for a physically self-consistent plant state independently of the optimisation problem.

Making fuel-ion equilibrium an explicit closure equation is one step towards separating:

• solving the PROCESS model for a self-consistent state, from

• optimising the design.

This will allow a future solution mode to solve the required internal equilibrium equations for each UQ sample while keeping the sampled/design quantities fixed.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.92%. Comparing base (2029202) to head (d7e9340).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/physics/physics.py	0.00%	3 Missing ⚠️
process/core/io/plot/summary.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4639   +/-   ##
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  Coverage   49.92%   49.92%           
=======================================
  Files         151      151           
  Lines       30225    30242   +17     
=======================================
+ Hits        15089    15099   +10     
- Misses      15136    15143    +7     

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[timothy-nunn]

@grmtrkngtn could you possibly explain (and justify) why the regression tests have changed results here? Its not immediately obvious why this has happened