<p>Theoretically, copper–niobium (Cu-Nb) composite superconducting cavities have excellent potential for high thermal and mechanical stability. They can appropriately exploit the high-gradient surface processing recipes developed for the bulk niobium (Nb) cavity and the thick copper (Cu) layer’s high thermal conductivity and rigidity, thereby enhancing the operational stability of the bulk Nb cavities. This study conducted a global review of the technical approaches employed for fabricating Cu-Nb composite superconducting cavities. We explored Cu-Nb composite superconducting cavities based on two technologies at the Institute of Modern Physics, Chinese Academy of Sciences (IMP, CAS), including their manufacturing processes, radio-frequency (RF) characteristics, and mechanical performance. These cavities exhibit robust mechanical stability. First, the investigation of several 1.3 GHz single-cell elliptical cavities using the Cu-Nb composite sheets indicated that the wavy structure at the Cu-Nb interface influenced the reliable welding of the Cu-Nb composite parts. We observed the generation and trapping of magnetic flux density during the <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="41365_2025_1641_Article_IEq1.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="17" /> </InlineMediaObject> <EquationSource Format="TEX">${T_\text {c}}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>T</mi> <mtext>c</mtext> </msub> </math></EquationSource> </InlineEquation> crossing of Nb in cooldown process. The cooling rates during the <InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="41365_2025_1641_Article_IEq2.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="17" /> </InlineMediaObject> <EquationSource Format="TEX">${T_\text {c}}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>T</mi> <mtext>c</mtext> </msub> </math></EquationSource> </InlineEquation> crossing of Nb exerted a substantial impact on the performance of the cavities. Furthermore, we measured and analyzed the surface resistance <InlineEquation ID="IEq3"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="41365_2025_1641_Article_IEq3.gif" Format="GIF" Height="16" Rendition="HTML" Resolution="72" Type="Linedraw" Width="19" /> </InlineMediaObject> <EquationSource Format="TEX">${R_\text {s}}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>R</mi> <mtext>s</mtext> </msub> </math></EquationSource> </InlineEquation> attributed to the trapped magnetic flux induced by the Seebeck effect after quenching events. Second, for the first time, a low-beta bulk Nb cavity was plated with Cu on its outer surface using electroplating technology. We achieved a high peak electric field <InlineEquation ID="IEq4"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="41365_2025_1641_Article_IEq4.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="27" /> </InlineMediaObject> <EquationSource Format="TEX">${E_\text {pk}}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>E</mi> <mtext>pk</mtext> </msub> </math></EquationSource> </InlineEquation> of <InlineEquation ID="IEq5"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="41365_2025_1641_Article_IEq5.gif" Format="GIF" Height="6" Rendition="HTML" Resolution="72" Type="Linedraw" Width="17" /> </InlineMediaObject> <EquationSource Format="TEX">$\sim$</EquationSource> <EquationSource Format="MATHML"><math> <mo>∼</mo> </math></EquationSource> </InlineEquation> 88.8 MV/m at 2 K and the unloaded quality factor <InlineEquation ID="IEq6"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="41365_2025_1641_Article_IEq6.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="22" /> </InlineMediaObject> <EquationSource Format="TEX">$ {Q_0}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>Q</mi> <mn>0</mn> </msub> </math></EquationSource> </InlineEquation> at the <InlineEquation ID="IEq7"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="41365_2025_1641_Article_IEq7.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="27" /> </InlineMediaObject> <EquationSource Format="TEX">${E_\text {pk}}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>E</mi> <mtext>pk</mtext> </msub> </math></EquationSource> </InlineEquation> of 88.8 MV/m exceeded <InlineEquation ID="IEq8"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="41365_2025_1641_Article_IEq8.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="58" /> </InlineMediaObject> <EquationSource Format="TEX">$1\times 10^{10}$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>1</mn> <mo>×</mo> <msup> <mn>10</mn> <mn>10</mn> </msup> </mrow> </math></EquationSource> </InlineEquation>. This demonstrated that the electroplating Cu on the bulk Nb cavity is a practical method of developing the Cu-Nb composite superconducting cavity with superior thermal stability. The results presented here provide valuable insights for applying Cu-Nb composite superconducting cavities in superconducting accelerators with stringent operational stability requirements.</p>

Exploration of the copper–niobium composite superconducting cavities for pursuing extremely high operational stability at IMP

期刊论文
发表日期：

2025年3月20日

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Shi-Chun Huang,
Yuan He,
Long Peng,
Chun-Long Li,
Sheng-Xue Zhang,
Meng-Xin Xu,
Zi-Qin Yang,
Hao Guo,
Lu-Bei Liu,
Ping-Ran Xiong,
An-Dong Wu,
Qing-Wei Chu,
Xiao-Fei Niu,
Teng Tan,
Zhi-Jun Wang,
Jun-Hui Zhang,
Sheng-Hu Zhang,
Hong-Wei Zhao,
Wen-Long Zhan

摘要

Theoretically, copper–niobium (Cu-Nb) composite superconducting cavities have excellent potential for high thermal and mechanical stability. They can appropriately exploit the high-gradient surface processing recipes developed for the bulk niobium (Nb) cavity and the thick copper (Cu) layer’s high thermal conductivity and rigidity, thereby enhancing the operational stability of the bulk Nb cavities. This study conducted a global review of the technical approaches employed for fabricating Cu-Nb composite superconducting cavities. We explored Cu-Nb composite superconducting cavities based on two technologies at the Institute of Modern Physics, Chinese Academy of Sciences (IMP, CAS), including their manufacturing processes, radio-frequency (RF) characteristics, and mechanical performance. These cavities exhibit robust mechanical stability. First, the investigation of several 1.3 GHz single-cell elliptical cavities using the Cu-Nb composite sheets indicated that the wavy structure at the Cu-Nb interface influenced the reliable welding of the Cu-Nb composite parts. We observed the generation and trapping of magnetic flux density during the ${T_\text {c}}$ $T_{c}$ crossing of Nb in cooldown process. The cooling rates during the ${T_\text {c}}$ $T_{c}$ crossing of Nb exerted a substantial impact on the performance of the cavities. Furthermore, we measured and analyzed the surface resistance ${R_\text {s}}$ $R_{s}$ attributed to the trapped magnetic flux induced by the Seebeck effect after quenching events. Second, for the first time, a low-beta bulk Nb cavity was plated with Cu on its outer surface using electroplating technology. We achieved a high peak electric field ${E_\text {pk}}$ $E_{pk}$ of $\sim$ $\sim$ 88.8 MV/m at 2 K and the unloaded quality factor $ {Q_0}$ $Q_{0}$ at the ${E_\text {pk}}$ $E_{pk}$ of 88.8 MV/m exceeded $1\times 10^{10}$ $1 \times 10^{10}$ . This demonstrated that the electroplating Cu on the bulk Nb cavity is a practical method of developing the Cu-Nb composite superconducting cavity with superior thermal stability. The results presented here provide valuable insights for applying Cu-Nb composite superconducting cavities in superconducting accelerators with stringent operational stability requirements.