GRANITE: Mechanical Characterization and Optical Inspection of Large-Area TPC Electrodes

TL;DR

The paper presents GRANITE, a robotic, non-contact QA system for large-area TPC electrodes, integrating laser distance sensing, optical imaging, and a confocal microscope on a granite-table gantry. It demonstrates high-precision mechanical measurements: relative electrostatic sagging at $20 μm$ and absolute gravitational sagging down to $200 μm$, improving to $≈50 μm$ with model corrections, meeting sub-millimetre requirements for future detectors. It also documents an optical survey of the XENON1T cathode grid, using an autoencoder to classify wire segments by anomaly, revealing widespread features but no clear link to SE emission without further tests. The work outlines a scalable QA pathway for XLZD-scale electrodes and motivates dedicated studies of defect-induced field emission to ensure safe, performant deployment in next-generation dual-phase TPCs.

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 mechanical stability and high-voltage performance of such an experiment, we have developed a scanning setup for electrode quality assurance called GRANITE: Granular Robotic Assay for Novel Integrated TPC Electrodes. GRANITE is built around 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 demonstrate the setup's capabilities in two key areas: first, using laser scanners, we characterize wire tension, and in an independent measurement wire deflection due to gravity and electrostatic forces is determined. The setup achieves a precision of $20\,μ\text{m}$ for the relative measurement of only electrostatic displacement. Furthermore, GRANITE can measure gravitational sag down to $200\,μ\text{m}$ in an absolute measurement; this precision improves to $50\,μ\text{m}$ when applying model-based corrections for systematic effects. The performance achieved exceeds the needs for the characterisation of the electrode sagging in future experiments, which typically aims to ensure a maximal sag on the order of $500\,μ\text{m}$. Second, we use GRANITE's high resolution camera to image all wires of XENON1T's cathode grid. Subsets of these images are then hand sorted and used to train an autoencoder, to reliably classify wire images as either pristine wires or images containing severe anomalous features. These anomalies appear e.g. as staining and may be potential defects. The interpretation of the classification results is complicated by the fact that most wire segments are not spotless, but show a varying amount of anomalous features. Follow-up studies are needed to identify the exact nature of such features on wires.

Figures (12)

• 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: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. (\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 ($iv$).
• Figure 2: Illustration of the resolution of the (\ref{['sec:setup:fig:opticalSensors:confocal-microscope']}) 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:subsec:calibration:pointlaser:fig:fulltable:withcuts']}) Data obtained with the optoNCDT 2300-20 laser distance sensor when measuring the distance of the sensor to the empty table. The sagging of the "gantry bridge" can be seen as the valley at $x_\text{isel}\!\!\sim\!\!600mm$. Oscillations in $z_\text{meas}$ as function of $y_\text{isel}$ are visible as well.
• Figure 3: (\ref{['sec:setup:subsec:gantry:features:fig:explanation:sketch']}) A graphic from the data-sheet muepsilon:scanCONTROL3000 of the scanCONTROL 3000 profile laser scanner overlaid with the measurement coordinate system used in this paper. (\ref{['sec:setup:subsec:gantry:features:fig:linelaserexplanation:datanoncorr']}) Six height profiles above the flat table surface measured by the laser scanner ($y_{\text{isel}}=0mm$). Colours are used to distinguish the measurements at different $x_{\text{isel}}$ positions. The profile laser scanner's tilt is visible from the left to right in every profile. (\ref{['sec:setup:subsec:gantry:features:fig:linelaserexplanation:datacorr']}) The same data as in (\ref{['sec:setup:subsec:gantry:features:fig:linelaserexplanation:datanoncorr']}), but with the tilt correction applied. The height differences between the profiles are due to the sagging of the gantry bridge, which changes the distance to the granite table for different $x_{\text{isel}}$ positions.
• Figure 4: (\ref{['sec:sagging:test-wires:fig:stretched_wires:grid']}) The frame on which wires for the sagging measurement are stretched and fixed; the distance over which the wires hang free is 880mm. (\ref{['sec:sagging:test-wires:fig:stretched_wires:fixation']}) Detailed image of how wires are fixed with screws, before being passivated with Kapton tape. (The images do not show specific wires used during the measurement, as wires were changed during the course of the work presented here.)
• Figure 5: Illustration of the wire tension measurement. From left to right: Distance measurement by the optoNCDT 2300-20 laser distance sensor, zoom into the first panel highlighting the oscillation of the wire, FFT of the data in the first panel where the harmonic frequencies found by our algorithm are marked, zoom into the second panel focusing at the fundamental frequency.