% pde_helmholtz.m — A variable-coefficient surface Helmholtz problem.
%
% surfaceop handles general elliptic operators on surfaces of any genus.
% Here we solve (Delta_Gamma + c(x)) u = f on a genus-1 stellarator, with
% a spatially varying coefficient c.

mip load --install flatironinstitute/flatironinstitute/surfacefun

p = 16;
nu = 8;
nv = 24;
dom = surfacemesh.stellarator(p + 1, nu, nv);

% Operator: Laplacian plus a zeroth-order term c(x,y,z).
pdo = [];
pdo.lap = 1;
pdo.c = @(x, y, z) 300*(1 - z);

f = -1;
L = surfaceop(dom, pdo, f);
u = L.solve();

figure(1);
plot(u), colorbar
title('Variable-coefficient Helmholtz on a stellarator');