a, Three-dimensional presentation of a diamond anvil featuring a toroidal profile milled by FIB on the surface of a conventional Boehler–Almax-type single-bevelled diamond anvil with a 40-μm culet. A hemisphere of transparent nanocrystalline diamond (NCD)17, which was FIB-milled from a single ball with a diameter of 12 to 14 μm, was placed over the tip to realize a double-stage DAC design. Two anvils of this kind were forced together as shown in the scheme in Extended Data Fig. 1. b, Pressure profile along the cross-section through the centre of dsDAC #1 after pulsed laser-heating. Diffraction patterns were collected at each point with a step of 0.5 μm at the ID11 beamline at ESRF, and pressure was determined according to the equation of state from ref. 27 using the lattice parameters of Re found from powder diffraction data.