Experimental investigation of plasma-electrode interactions on the ZaP-HD sheared-flow-stabilized Z-pinch device

TL;DR

This study addresses electrode erosion in the ZaP-HD sheared-flow-stabilized Z-pinch, where a solid graphite electrode endures extreme plasma exposure. It combines in-situ S/XB spectroscopy, ex-situ SEM and profilometry, and a mean-free-path analysis to distinguish sublimation- and sputtering-driven erosion and to reveal a carbon recycling mechanism via redeposition. The results show that gross erosion closely follows the sublimation flux and greatly exceeds sputtering, yet sublimed neutrals are ionized within the sheath and redeposited, reducing net erosion, while sputtered carbon governs the net erosion signal. These findings support a recycling/self-healing picture with practical implications for erosion management in high-current, fusion-relevant Z-pinch configurations and inform strategies for electrode longevity in future devices.

Abstract

The ZaP-HD sheared-flow-stabilized (SFS) Z-pinch device is a testbed for experimental investigation of plasma-electrode interactions. The graphite electrode is exposed to a high temperature, high density Z-pinch plasma while supplying large pinch currents. In-situ measurements of the gross carbon erosion flux obtained with S/XB spectroscopy exceed the expected flux from physical sputtering, but have reasonable agreement with the expected sublimation flux. Comparison of the ionization mean free paths of neutrals produced through both erosion processes shows that sublimated carbon is ionized within the sheath while sputtered carbon is ionized beyond the sheath. This suggests a process of electrode recycling and self-healing through redeposition. The sputtered carbon is primarily responsible for net erosion. Ex-situ analysis of electrode material is enabled by the design of a removable coupon. Three different plasma exposure conditions varied the pinch current and number of pulses. Net mass loss measurements support the physical picture of electrode recycling. Erosion rates range from 0.01 to 0.1 mg/C, which are comparable to existing arc discharge devices. Measurements of the microscopic surface morphology and roughness reveal irregular consolidated structures and general smoothing except at high particle fluence. Crack formation suggests the importance of repetitive thermal cycles. Definitive features of sputtering such as pitting and cratering are absent, although further study is needed to attribute the observed changes to other processes. These results indicate some alignment with erosion processes in high-powered arc discharges, which successfully operate solid electrodes in extreme environments. This provides confidence in managing electrode erosion in the SFS Z-pinch configuration.

Figures (13)

• Figure 1: Cross-sectional machine drawing of the ZaP-HD SFS Z-pinch device illustrating the Acceleration and Assembly Regions. Neutral gas is ionized and accelerated down the Acceleration Region. In the Assembly Region, the plasma forms a Z-pinch configuration and is compressed by the axial current. The Z-pinch plasma (shown schematically in magenta) conducts the discharge current and directly contacts the inner electrode at the nose cone. Four rectangular windows in the Assembly Region provide optical access to the Z-pinch plasma and the nose cone for erosion measurements. Two Rogowski coils measure the total plasma current and the compression current. An array of magnetic field probes in the Assembly Region measures the azimuthal magnetic field produced by the Z pinch.
• Figure 2: (a) Cross-sectional machine drawing of the Assembly Region showing the inner electrode positioned 8 cm downstream of $z=0$, providing optical access for spectroscopy measurements through the fused silica windows. The redesigned electrode assembly incorporating the removable coupon is outlined in red. Removal of the windows enables access for coupon replacement through the slots in the outer electrode. (b) Enlarged section-view of the electrode coupon assembly. Fasteners are not visible in the plane of the cross-section. (c) Image of a pristine electrode coupon installed on ZaP-HD.
• Figure 3: Top-down and side views of the graphite electrode coupon with major dimensions labeled. SEM measurements (blue squares) were made in 10 mm increments along perpendicular axes, while profilometer measurements (red square) were limited to the center of the coupon.
• Figure 4: Typical traces of (a) voltages and (b) currents recorded during a ZaP-HD pulse. The initial capacitor bank discharge occurs at 2 $\mu$s, applying a voltage to the Acceleration Region and allowing current to flow through the plasma. After a 20 $\mu$s delay, the second capacitor bank discharges, applying a voltage to the Assembly Region that drives an axial current for pinch compression. The pinch current is calculated using Eq. \ref{['eq:pinchcurrent']} using the azimuthal array of probes at $z=10$ cm.
• Figure 5: Time evolution of the (a) pinch current and (b) flux of eroded carbon for the compression capacitor bank voltages used in the study. In (a), the shaded regions show the standard deviation of the pinch current over all pulses in each campaign. Increasing this voltage setting increases the pinch current, which corresponds to greater measured erosion flux. The red and black dotted lines correspond to theoretical upper limits of the flux from sublimation and physical sputtering, respectively.