Design and application of an efficient probe for precise frequency measurement in high-gradient traveling wave accelerating structures

Purpose The full-energy linac injector project of the Southern advanced photon source (SAPS) will utilize C-band traveling wave accelerating structures to achieve a high acceleration gradient, enabling the generation of higher-energy beams. For compact C-band accelerating structures, microwave measurements must be highly accurate and efficient, with the results being one of the most important indicators for subsequent tuning. However, existing method has difficulty in maintaining the accuracy of measurements after welding, and there is an urgent need to find a reliable solution.
Methods A novel, cost-effective probe design was proposed to measure single-cavity frequencies in welded accelerating structures without pre-tuned couplers, which is featured a thin shielding structure to minimize signal interference. Simulations experiment validated the theoretical framework, while experimental validation utilized the S-band and C-band accelerating structures. Based on the experimental data, thoroughly tuning for the C-band structure was implemented to further prove the performance of the probes. Furthermore, frequency measurements cross-verified against bead-pull method results.
Results Experimental results of the plunger-probe method validated high precision with theoretical data. After tuning, frequencies of C-band structure improved, with quality factors aligning to simulations. Field flatness and local reflection coefficients (optimized to -0.03~0.03) also confirmed probe accuracy and tuning efficacy.
Conclusion The proposed probe design simplifies post-welding cavity tuning by eliminating coupler-matching dependencies, offering a practical solution for high-gradient accelerators measurement.