midas-xaf — Cross-Axis Faceted HEDM (XAF-HEDM)

Design & simulation toolkit for a one-of-a-kind far-field HEDM measurement performed through a cubic diamond anvil cell with a ~15° conical opening on all six faces.

The technique

• Narrow ω wedges (~16° wide) are collected at ω ≈ 0, ±90, 180° through the four equatorial faces in a given mounting.
• The cell is disassembled and remounted about an orthogonal axis (90° about the beam) so the previously top/bottom faces reach the equator — giving an orthogonal rotation axis in the crystal frame that fills the first mounting's missing cone.
• Two faces are measured in both mountings; embedded high-Z fiducials give an independent rigid-body remount registration.
• Both mountings are merged into a single reciprocal-space reconstruction.

What the toolkit answers

Built on the differentiable midas-diffract forward model, it quantifies — across energy / distance / detector / opening-angle / beam-mode / material knobs — reciprocal-space coverage, spots per grain, Friedel-pair completeness, and above all strain determinability (autograd Jacobian conditioning), including how it degrades with the face opening angle (the 15° vs 20° v2-cell decision).

Physics captured

• Exit-aperture shadowing: the diffracted beam must clear a face opening whose transmitting cone rotates with ω, so different detector sectors go dark at different ω (exit_model="cone", the default).
• Orthogonal-axis merge: single-mounting vs merged strain determinability is compared directly (metrics.cross_axis_gain).

Status: alpha — Phase 1 (forward + metrics + sweep). Phase 2 (registration + reconstruction) in progress. See packages/XAF_HEDM_IMPLEMENTATION_PLAN.md.