A novel high-gradient accelerating structure with low phase velocity, $v/c=0.38$, has been designed, manufactured and high-power tested. The structure was designed and built using the methodology and technology developed for CLIC $100\mathrm{MV}/\mathrm{m}$ high-gradient accelerating structures, which have speed of light phase velocity, but adapts them to a structure for nonrelativistic particles. The parameters of the structure were optimized for the compact proton therapy linac project, and specifically to 76 MeV energy protons, but the type of structure opens more generally the possibility of compact low phase velocity linacs. The structure operates in S-band, is backward traveling wave (BTW) with a phase advance of 150 degrees and has an active length of 19 cm. The main objective for designing and testing this structure was to demonstrate that low velocity particles, in particular protons, can be accelerated with high gradients. In addition, the performance of this structure compared to other type of structures provides insights into the factors that limit high gradient operation. The structure was conditioned successfully to high gradient using the same protocol as for CLIC X-band structures. However, after the high power test, data analysis realized that the structure had been installed backwards, that is, the input power had been fed into what is nominally the output end of the structure. This resulted in higher peak fields at the power feed end and a steeply decreasing field profile along the structure, rather than the intended near constant field and gradient profile. A local accelerating gradient of $81\mathrm{MV}/\mathrm{m}$ near the input end was achieved at a pulse length of $1.2\mu \mathrm{s}$ and with a breakdown rate (BDR) of $7.2\times {10}^{-7}1/\mathrm{pulse}/\mathrm{m}$. The reverse configuration was accidental but the operating with this field condition gave very important insights into high-gradient behaviour and a comprehensive analysis has been carried out. A particular attention was paid to the characterization of the distribution of BD positions along the structure and within a cell.

中文翻译：

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AnS波段，常导低相速度加速结构的高梯度测试

一种新型的低相速高梯度加速结构， $v/C=0.38$，已经过设计，制造和大功率测试。该结构是使用为CLIC开发的方法和技术设计和建造的$100\mathrm{MV}/\mathrm{米}$高梯度加速结构，具有光相速度，但是使它们适合于非相对论粒子的结构。结构的参数针对紧凑的质子治疗直线加速器项目进行了优化，特别是针对76 MeV能量质子，但结构类型更普遍地提出了紧凑的低相速度直线加速器的可能性。该结构在S波段工作，是具有150度相位超前的反向行波（BTW），有效长度为19 cm。设计和测试该结构的主要目的是证明低速粒子，尤其是质子，可以通过高梯度来加速。此外，与其他类型的结构相比，该结构的性能可洞悉限制高梯度运行的因素。使用与CLIC X波段结构相同的协议，将结构成功调整为高梯度。但是，在进行高功率测试之后，数据分析意识到该结构已向后安装，也就是说，输入功率已馈入标称结构的输出端。这导致在馈电端的峰值场更高，并且沿结构的场轮廓急剧减小，而不是预期的接近恒定的场轮廓和梯度轮廓。的局部加速梯度 这导致在馈电端的峰值场更高，并且沿结构的场轮廓急剧减小，而不是预期的接近恒定的场轮廓和梯度轮廓。的局部加速梯度 这导致在馈电端的峰值场更高，并且沿结构的场轮廓急剧减小，而不是预期的接近恒定的场轮廓和梯度轮廓。的局部加速梯度$81\mathrm{MV}/\mathrm{米}$ 在输入端附近达到了脉冲长度为 $1.2\mu \mathrm{s}$ 且故障率（BDR）为 $7.2\times {10}^{-7}\mathrm{1个}/\mathrm{脉冲}/\mathrm{米}$。反向配置是偶然的，但在这种现场条件下进行操作对高坡度行为提供了非常重要的见解，并且已进行了全面分析。BD结构和细胞内BD位置分布的特征得到了特别关注。