Layer 1: Erosion and thermal protection. The outermost surface survives direct contact with the Debye sheath. Tungsten-rhenium alloy tiles at thermal hotspots — the equatorial region where field intensity peaks, the plasma injection ports where local density is highest. The remaining surface is triazite-alloy — a hafnium-niobium-carbon refractory developed by Metallic Sciences for the XR-1 program. Triazite was selected over nickel superalloys because nickel produces unacceptable eddy current losses at THz operating frequencies. The radiation load at the hull surface is 2–5 kW/m² from bremsstrahlung and line emission — significant, but well within the thermal budget of refractory metals.^[12]

Layer 2: Electromagnetic ground plane. A 3 mm copper-clad mu-metal composite — six alternating layers of electrodeposited copper and mu-metal ($\mu_r > 50{,}000$). Three simultaneous functions: defines the electromagnetic boundary condition for the active layers below, provides structural load transfer, and attenuates the DC field from the superconducting coils to protect the magnetostrictive layer from saturation.

Layer 3: CCTO dielectric. Calcium copper titanate — CaCu&sub3Ti&sub4O&sub1&sub2 — with relative permittivity exceeding 100,000. The extreme permittivity enables near-perfect electromagnetic coupling between the hull surface and the Debye sheath: coupling coefficient $\kappa \approx 1 - 2/\varepsilon_r \approx 0.99998$. The hull and the sheath are electromagnetically fused — perturbations in hull surface charge produce immediate, lossless perturbations in sheath charge. Functionally graded from $\varepsilon_r = 10{,}000$ at the ground plane to $120{,}000$ at the metamaterial interface to eliminate THz reflection at the boundary.^[13]

Layer 4: Bi-Mg metamaterial. One hundred micrometers of bismuth nanowires in a magnesium matrix — the THz waveguide that carries control signals to the sheath. Bismuth is a semimetal with anomalously low carrier density ($3\times10^{23}$ m$^{-3}$, five orders below copper) and low effective mass ($m^* \approx 0.001\,m_e$), placing its plasma frequency at approximately 500 GHz — squarely in the THz band. Surface plasmon polariton modes propagate along the Bi-Mg layer at THz frequencies, carrying sheath control signals from the 512 hull-embedded gyrotrons to sub-millimeter spatial resolution at the plasma boundary. Conventional metals support SPPs only at optical frequencies. Bismuth is the THz plasmonic medium. Graphene — gate-tunable, atomically thin, with 3× the carrier mobility — is under development by Plasma Press for the Block 2 hull.^[14]

Layer 5: Terfenol-D magnetostrictive layer. The innermost active layer. Tb&sub0·&sub3Dy&sub0·&sub7Fe&sub2 — the highest-performance magnetostrictive material known, developed at the Naval Ordnance Laboratory in the 1970s. Magnetostriction coefficient: 1,200 ppm pre-stressed, producing 10–20 μm mechanical displacement per actuation cycle. The 500 kHz control system modulates current in dedicated coils, driving the Terfenol-D layer into mechanical oscillation that couples through the CCTO and ground plane to modulate the soliton field geometry at the MHD breathing frequency. This is the bulk actuator — the low-frequency, high-authority correction layer.^[15]