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We present a technique for handling Dirichlet boundary conditions with theFlux Coordinate Independent (FCI) parallel derivative operator witharbitrary-shaped material geometry in general 3D magnetic fields. The FCImethod constructs a finite difference scheme for $\nabla_\Vert$ by followingfield lines between planes and interpolating within planes. Doing so removesthe need for field-aligned coordinate systems that suffer from singularities inthe metric tensor at null points in the magnetic field (or equivalently, when$q \to \infty$). One cost of this method is that as the field lines are not onthe mesh, they may leave the domain at any point between neighbouring planes,complicating the application of boundary conditions.  The Leg Value Fill (LVF) boundary condition scheme presented here involves anextrapolation/interpolation of the boundary value onto the field line endpoint. The usual finite difference scheme can then be used unmodified. Weimplement the LVF scheme in BOUT++ and use the Method of Manufactured Solutionsto verify the implementation in a rectangular domain, and show that the errorscaling of the finite difference scheme isn't modified. The use of LVF forarbitrary wall geometry is outlined.  We also demonstrate the feasibility of using the FCI approach innon-axisymmetric configurations for a simple diffusion model in a "straightstellarator" magnetic field. A Gaussian blob diffuses along the field lines,tracing out flux surfaces. Dirichlet boundary conditions impose a last closedflux surface (LCFS) that confines the density. Including a poloidal limitermoves the LCFS to a smaller radius.  The expected scaling of the numerical perpendicular diffusion, which is aconsequence of the FCI method, in stellarator-like geometry is recovered. Anovel technique for increasing the parallel resolution during post-processingis also described.

Dirichlet boundary conditions for arbitrary-shaped boundaries in stellarator-like magnetic fields for the Flux-Coordinate Independent method