Technical Note TN-2026-HBN-02: Longitudinal Split-Wafer (Clamshell) Fabrication and Open-Surface ALD Protocol for h-BN Protective Linings in Ultra-High-Gradient PWFA Capillaries

This technical note details the Longitudinal Split-Wafer (Clamshell) Architecture and Open-Surface Atomic Layer Deposition (ALD) protocol for depositing 50.0 nm hexagonal Boron Nitride (h-BN) protective linings inside high-gradient Plasma Wakefield Accelerator (PWFA) capillary channels. To overcome precursor gas diffusion limits (Knudsen transport bottleneck) in ultra-high aspect ratio micro-channels (up to 2,000,000:1), matching semi-circular micro-grooves are precision-machined onto planar synthetic sapphire blocks. This leaves the surface completely open during deposition, guaranteeing zero precursor depletion and uniform 50.0 nm h-BN atomic layer growth across the entire 1.0-meter channel length prior to optical contact fusion. Key Specifications & Results: - Substrate: High-purity synthetic sapphire (0001 c-plane planar slabs) - Capillary Baseline Length: 1000.0 mm (1.0 meter) - Groove Radius / Post-Bonding Inner Bore: r = 250.0 nm, width w = 500.0 nm (ϕ_ad = 500.0 nm) - Surface Roughness: R_a < 0.5 nm (CMP polished optical standard) - Protective Liner Thickness: t_hbn = 50.0 nm ± 1.2 nm - Assembly Method: Direct solid-state thermal fusion at 1100°C in UHV after cleanroom optical contact alignment Includes full fabrication process steps, metrology requirements, and prototype non-recurring engineering (NRE) cost estimates.