GRANITE: High-Resolution Imaging and Electrical Qualification of Large-Area TPC Electrodes

TL;DR

Next-generation dual-phase TPCs require large-area, high-precision electrodes with stringent HV performance. The authors integrate a coaxial HV-scanning head with the GRANITE platform to perform non-contact, high-resolution surface metrology and localized electrical testing over extended electrode lengths, achieving measurements with inception around $-5.2\ \mathrm{kV}$ and imaging over areas of about $2.0\times1.4\ \mathrm{m^2}$. They find that natural discharge hotspots are transient and not reliably linked to static optical features, while controlled abrasive surface damage lowers the discharge inception voltage by about $200\ \mathrm{V}$, establishing a practical QC threshold. The work provides a scalable QA toolkit for large-area electrode production and outlines upgrades toward $3\ \mathrm{m}$-scale electrodes using collaborative robotics for future low-background experiments.

Abstract

Next-generation dual-phase time projection chambers (TPCs) for rare event searches will require large-scale, high-precision electrodes. To meet the stringent requirements for high-voltage performance of such an experiment, we have developed a scanning setup for comprehensive electrode quality assurance. The system is built around the GRANITE (Granular Robotic Assay for Novel Integrated TPC Electrodes) facility: a gantry robot on top of a $2.5\,\text{m}\times1.8\,\text{m}$ granite table, equipped with a suite of non-contact metrology devices. We developed a coaxial wire scanning head to measure and correlate localized high-voltage discharge currents in air with high-resolution surface images. We find that the identified discharge 'hotspots' are transient and show no significant correlation with static visual features. Next, we established a quantitative relationship between artificially induced abrasive surface damage on the wires and a reduction in the discharge inception voltage. This work provides a novel non-invasive tool for qualifying wires dedicated for use in electrodes for future low-background experiments.

Figures (8)

• Figure 1: (\ref{['sec:setup:fig:granite_table:schematic']}) Schematic of the gantry setup on the $2.5 \times 1.8m\squared$ granite table with coordinate system. (\ref{['sec:setup:fig:granite_table:metrology']}) The different metrology components from left to right: Confocal microscope ($i$) and -- slightly higher -- the high resolution (industrial) camera ($ii$), the laser distance sensor ($iii$), and the profile laser scanner ($iv$). (\ref{['sec:setup:fig:opticalSensors:confocal-microscope']}) Illustration of the resolution of the confocal microscope and a $\times50$ lens, and (\ref{['sec:setup:fig:opticalSensors:high-res-cam']}) the high resolution industrial camera. The region marked in blue shows the size of the confocal microscope image. (\ref{['sec:setup:fig:granite_table:photo']}) Photo of the setup: In the foreground an acrylic glass box with a $\sim\!\!1.4m$ diameter electrode can be seen, whilst the arm with the metrology components described in the text is visible on the far, left side of the table.
• Figure 2: Experimental setup for wire scanning. The custom-designed scanning head is mounted on the telecentric lens of the Basler acA4600-7gc camera, which is mounted on the gantry system. The cylindrical ground generates a nearly uniform field on the surface of the wire and includes a slit for the wire. (\ref{['fig:experimental_setup_coaxial_ground']}) Zoomed-in view of the copper ground with the defined coordinate system (along with equalities to relate the axes to the isel-system's coordinate system axes defined in Figure \ref{['sec:setup:fig:granite_table:schematic']}): $y$ axis along the camera viewing direction, $z$ axis along the wire, and $x$ axis perpendicular to both. (\ref{['fig:experimental_setup_scanning_head']}) 3D design and photo of the scanning head, showing the cylindrical ground and its camera attachment. (\ref{['fig:experimental_setup_5wire_frame']}) The 5-wire frame used for HV wire scanning, optical scanning, and cleaning multiple wires. (\ref{['fig:experimental_setup_camera_image']}) Close-up of the wire through the top slit in the coaxial configuration along with a 1mm scale.
• Figure 3: Subplots (\ref{['fig:sim_coax_field_singleXY']}) and (\ref{['fig:sim_coax_field_res_singlexy']}) show FEM simulations of the electric field produced by ground (including the viewing window and slit) and the wire electrode, with a deliberately large lateral offset of 0.5mm in both the $x$ and $y$ directions. A schematic outline of the HV scanning head is overlaid for orientation, with the slit indicated at the bottom. The voltage difference between the cylinder (inner radius 5.0mm) and the wire (radius 0.108mm) is $\Delta U = -5000V$. Two-dimensional cross-sections at the $z$-midplane with (\ref{['fig:sim_coax_field_singleXY']}) the simulated field magnitude, and (\ref{['fig:sim_coax_field_res_singlexy']}) the difference between the simulated realistic field with offset and the field produced by an ideal coaxial electrode system (infinitely long coaxial ground around perfectly centered wire) are shown along with an angular scale in green to be used as reference for (\ref{['fig:sim_fitted_E_zTh']}) and (\ref{['fig:sim_fitted_res_E_zTh']}). The jagged edges near the ground in (\ref{['fig:sim_coax_field_res_singlexy']}) are due to meshing artefacts in the FEM simulation that are not relevant for the analysis, since we focus on the appropriately densely meshed high-field region close to the wire. Subplot (\ref{['fig:sim_fitted_E_zTh']}) shows the electric field magnitude on a cylindrical surface (radius 0.15mm) around the wire, unrolled into $z$ vs. $\theta$, fitted with Eqn. \ref{['sec:highres-scans:subsec:experimental-setup:eq:plateau-function']} to account for wire eccentricity and boundary effects near the edges of the 4mm viewing window ($z=$ 8mm to $z=$ 12mm). Sublot (\ref{['fig:sim_fitted_res_E_zTh']}) shows the difference between simulation and fit. See text for more details.
• Figure 4: Two-dimensional histogram of voltage versus measured discharge current for the wire-ground system from 328 distinct positions for cleaned, undamaged wires. The upper panel shows the measurement density (color scale, capped at 20 counts for clarity) together with mean voltages and standard deviations for 1µA current bins (green points with yellow error bars). The lower panel displays the corresponding voltage distribution for measurements with 0.0µA < $I$ < 4.0µA, with a dashed line at -5350V. The adjacent colour bar represents the mean current per voltage bin, considering only values below 4.0µA and truncated at 1µA for readability.
• Figure 5: Analysis of hotspot location and consistency for five test wires. The histograms show the number of significant current events ($I_\text{e} > \mu_{I_\text{r}} + 3\,\sigma_{I_\text{r}}$ and $I_\text{e} \geq 0.3µA$ for a voltage of $-5350V$) detected at locations 4mm apart along each wire. The colour of the top half of each bar indicates the measurement session, demonstrating that hotspot events appear/disappear after cleaning. The colour of the bottom half represents the hotspot event consistency for repeated scans in a single measurement session (a score of 1.0, yellow, signifies the hotspot event occurred at that location in every scan of a measurement session), demonstrating high short-term repeatability.