This change modifies the analytical equilibrium in order to simplify the
computation of the poloidal flux normalization and the derivatives.
In the power law parametrisation of the safety factor, ρ_t is replaced
with ρ_p and, similarly, the normalised poloidal radius is now
identified with ρ_p, instead of ρ_t.
With the same parameters (q₀,q₁,α...), this choice slightly changes the
plasma current distribution, but enables us to obtain a closed form for
ψ_a = ψ(r=a) and the relation ρ_t(ρ_p). In fact, both expressions are
now obtained by integrating the q(ρ_p), instead of 1/q(ρ_t), which has
no elementary antiderivative.
As the normalisation is now computed exactly, the values of the
normalised flux ψ_n = ψ/ψ_a and the gradient ∇ψ (entering the raytracing
equations in X and ∇X, respectively) are computed to the same precision.
Previously, ψ_n was computed to a lower precision due to the use of a
simple trapezoid integration of 1/q(ρ_p) for ψ_a, while ∇ψ was computed
up to machine precision using an exact formula.
This error effectively caused a very slight decoupling between X=ω_p²/ω²
and ∇X that introduced a systematic error in the numerical solution of
the raytracing equations.
The error manifests itself as a bias with a weak dependency on X in the
values taken by the dispersion function Λ(r̅, n̅) on the phase-space
points generated by the integrator. More specifically,
lim h→0 Λ(r̅_i, n̅_i) = -kX(r̅_i)
where h is the integrator step size;
r̅_i is the position at the i-th step;
k ≈ -3.258⋅10⁻⁵ and depends only on the number of points used to
perform the trapedoid integral for ψ_a (as ~ 1/n²).
After this change Λ behaves consistently with being a conserved quantity
(zero) up to the cumulative integration error of the 4° order
Runge-Kutta method. In fact we now have that:
Λ(r̅_i, n̅_i) ∝ - h⁴ ‖∂⁴X(r̅_i)/∂r̅⁴‖
It must be said that within this model the relation ρ_p(ρ_t) can't be
computed analytically (inverting ρ_t(ρ_p) produces a trascendental
equation of the form b = x + c x^α). However, this relation is not
necessary for raytracing and is easily solved, up to machine
precision, using minpack.
In addition, this change also makes the model consistetly use the
cocos=3 and fully implements the ability to force the signs of I_p, B_φ
(via equilibrium.sgni,sgnb) and rescaling the field (via
equilibrium.factb).
