Evolution of Graphite Nose‑Cone Surfaces Under Repetitive ZaP‑HD Z‑Pinch Exposure Revealed by Quantitative SEM

Reliable plasma‑facing components are essential for future sheared‑flow‑stabilized (SFS) Z‑pinch reactors [1]. On ZaP‑HD the highest heat and particle fluxes strike a removable graphite cathode coupon located where the Z‑pinch attaches. We investigate how repeated plasma pulses modify this surface by combining qualitative scanning-electron-microscopy (SEM) inspection with image processing and analysis. Different coupons were exposed to varying pulse counts (controlling ion fluence) and capacitor bank voltages (controlling pinch current and ion impact energy). They were then imaged via the ETD secondary-electron detector at multiple magnifications at several radial positions. A Python pipeline extracts quantitative metrics from each SEM micrograph: (i) 2‑D fast‑Fourier transforms give dominant spatial frequencies and an anisotropy index; (ii) A white‑top‑hat filter followed by Otsu thresholding picks out local bright surface protrusions; a separate fixed‑threshold routine captures the dark cracks and valleys; logging total count, average area, average pixel intensity, and average eccentricity over these features; (iii) grayscale histograms capture overall changes in surface roughness. Qualitative examination reveals broad surface pattern changes, and the subsequent surface-analysis quantities objectively test those impressions, eliminating observer bias and providing objective and quantifiable characterization. Comparing these metrics across radius and exposure history indicates ZaP‑HD plasma impact on surface topography, crack morphology, and directional features. The study clarifies microscale plasma-material interaction mechanisms and supplies a quantitative surface-analysis framework to guide the design of more durable electrodes and other plasma-facing components in SFS Z-pinch devices. Overall, ZaP-HD plasma exposure smooths tall surface protrusions, promotes elongated cracking near the pinch attachment, and at larger radii, the machining ridges left from fabrication become more pronounced. These changes depend on both radial location and plasma operating conditions, showing how different exposure conditions can shape the surface in distinct ways.