Development of a prototype superconducting radio-frequency cavity for conduction-cooled accelerators

The higher efficiency of superconducting radio-frequency (SRF) cavities compared to normal-conducting ones enables the development of high-energy continuous-wave linear accelerators (linacs). Recent progress in the development of high-quality ${\mathrm{Nb}}_3\mathrm{Sn}$ film coatings along with the availability of cryocoolers with high cooling capacity at 4 K makes it feasible to operate SRF cavities cooled by thermal conduction at relevant accelerating gradients for use in accelerators. A possible use of conduction-cooled SRF linacs is for environmental applications, requiring electron beams with energy of 1–10 MeV and 1 MW of power. We have designed a 915 MHz SRF linac for such an application and developed a prototype single-cell cavity to prove the proposed design by operating it with cryocoolers at the accelerating gradient required for 1 MeV energy gain. The cavity has a $\sim 3\mathrm{\mu }\mathrm{m}$ thick ${\mathrm{Nb}}_3\mathrm{Sn}$ film on the inner surface, deposited on a $\sim 4\mathrm{mm}$ thick bulk Nb substrate and a bulk $\sim 7\mathrm{mm}$ thick Cu outer shell with three Cu attachment tabs. The cavity was tested up to a peak surface magnetic field of 53 mT in liquid He at 4.3 K. A horizontal test cryostat was designed and built to test the cavity cooled with three Gifford-McMahon cryocoolers. The rf tests of the conduction-cooled cavity, performed at General Atomics, achieved a peak surface magnetic field of 50 mT and stable operation was possible with up to 18.5 W of rf heat load. The peak frequency shift due to microphonics was 23 Hz. These results represent the highest peak surface magnetic field achieved in a conduction-cooled SRF cavity to date and meet the requirements for a 1 MeV energy gain.

Figure 1

Average beam energy (a), transverse rms beam size and longitudinal rms bunch length (b) along the beam axis for a 915 MHz, 1 MeV, 1 A linac obtained from a simulation with gpt. The schematic layout on top is not to scale.

Figure 2

3D model used for the thermal analysis. Each color represents a different material: cyan indicates Nb, blue indicates stainless steel, orange represents electroplated Cu, and fuchsia represents Cu with $\mathrm{R}\mathrm{R}\mathrm{R}=100$. The nomenclature “Stage 1-3” means stage 1 of cryocooler number 3.

Figure 3

Stage 2 temperature as a function of the stage 2 heat load for two different stage 1 heat load values for a GM cryocooler model RDE418-D4 measured at Jefferson Lab.

Figure 4

Empirical field dependence of the surface resistance used in the thermal analysis calculated at three different temperatures.

Figure 5

Temperature distribution for the cavity with $\sim 1\mathrm{MW}$ of rf input power and operating at $B_p=47\mathrm{mT}$.

Figure 6

Results from the rf test in superfluid He at 2.0 K of the 952.6 MHz prototype single-cell Nb cavity. A picture of the cavity is shown in the inset.

Figure 7

Cross-section layout of the 952.6 MHz prototype cavity for conduction-cooling demonstration.

Figure 8

(a) Maximum and average equivalent (von Mises) stress as a function of temperature and (b) equivalent strain at 4 K in the ${\mathrm{Nb}}_3\mathrm{Sn}$ layer for the prototype cavity with the Cu structure shown in Fig. 7 on the outer surface.

Figure 9

(a) Low ($\times 350$) and (b) High magnification ($\times 3.5\mathrm{k}$) SEM images of the ${\mathrm{Nb}}_3\mathrm{Sn}$ film on a Nb witness sample after the second coating.

Figure 10

Results from the rf tests in LHe at 4.3 K of the 952.6 MHz prototype single-cell cavity after coating the inner surface with ${\mathrm{Nb}}_3\mathrm{Sn}$. The inset shows a picture of the inner surface after forming the ${\mathrm{Nb}}_3\mathrm{Sn}$ film.

Figure 11

Pictures taken during the fabrication of the Cu structure on the outer cavity surface: during cold-spray (a), during fitting of the equator ring on the Cu layer after the first electroplating and machining (b) and during the final machining (c).

Figure 12

SEM image of the cross section of a Nb/Cu sample with the interface Cu layer between Nb and electroplated Cu being deposited by cold spray.

Figure 13

Results from the rf test in LHe at 4.3 K of the 952.6 MHz prototype single-cell cavity after coating the outer surface with Cu. The inset shows a picture of the fully assembled cavity.

Figure 14

$R_s(T)/R_s(15\mathrm{K})$ between 14.5–19.5 K measured before and after growing the Cu structure on the cavity outer surface.

Figure 15

Normalized shift in resonance frequency of the $\mathrm{Cu}/\mathrm{Nb}/{\mathrm{Nb}}_3\mathrm{Sn}$ cavity along with the normalized thermal expansion of Cu as a function of temperature.

Figure 16

$R_s(T)$ in the normal state measured for the $\mathrm{Cu}/\mathrm{Nb}/{\mathrm{Nb}}_3\mathrm{Sn}$ cavity. The solid line is a least-square fit with the real part of Eq. (1).

Figure 17

Picture of the TLA used to connect the cavity to stage 2 of the CCR.

Figure 18

Thermal conductance of the Cu TLA shown in Fig. 17 as a function of temperature. The inset shows a picture of the TLA mounted to stage 2 of the CCR at Jefferson Lab.

Figure 19

Cross-section view of the 3D layout of the HTC on a support cart.

Figure 20

Approximate locations of heaters (black rectangles), temperature sensors (red circles) and FGMs (blue cylinders) installed on the prototype cavity inside the HTC.

Figure 21

Picture of the prototype cavity being assembled in the HTC at General Atomics.

Figure 22

Average temperature and ambient magnetic field at the cavity equator as a function of time, starting at the beginning of the cooldown, during the first cooldown of the cavity in the HTC. The dashed line is at $T_c\sim 17.7\mathrm{K}$.

Figure 23

$Q_0$ as a function of the rf field for the conduction-cooled prototype cavity measured at General Atomics for different cool-down conditions summarized in Table 3. All the data points were measured in steady-state conditions and the symbols’ colors correspond to the average equator ring temperature. The cavity performance was limited by thermal instability beyond the lowest measured $Q_0$-value. The inset shows a photo of the HTC.

Figure 24

$T_{\mathrm{avg}}$ and $B_a$ along three orthogonal directions at the cavity equator during the second cooldown for which the CCRs were power cycled on and off. The dashed line is at $T_c\sim 17.7\mathrm{K}$.

Figure 25

$Q_0$ as a function of the rf field for the conduction-cooled prototype cavity measured at General Atomics after the second cooldown for different values of dc heat applied by HTR 6. All the data points were measured in steady-state conditions and the symbols’ colors correspond to the average equator ring temperature. The cavity performance was limited by thermal instability beyond the lowest measured $Q_0$-value, except for $P_{\mathrm{d}\mathrm{c}}=2$ W for which the cavity test was stopped before reaching thermal instability.

Figure 26

$T_{\mathrm{avg}}$ as a function of the total heat load, $P_{\mathrm{tot}}=P_{\mathrm{r}\mathrm{f}}+P_{\mathrm{d}\mathrm{c}}$, for different values of $P_{\mathrm{d}\mathrm{c}}$. For each value of $P_{\mathrm{d}\mathrm{c}}$, $P_{\mathrm{r}\mathrm{f}}$ was increased up to the thermal stability limit, except for $P_{\mathrm{d}\mathrm{c}}=2\mathrm{W}$.

Figure 27

$T_{\mathrm{avg}}$ and $B_a$ along three orthogonal directions at the cavity equator during the third cooldown for which the cavity temperature was controlled by adjusting the power to heaters 5–7. The heaters location is shown in Fig. 20. The dashed line is at $T_c\sim 17.7\mathrm{K}$.

Figure 28

$B_a$ at different locations of the cavity equator ring, along three orthogonal directions, and temperature difference $T_{\mathrm{B}\mathrm{T}\mathrm{1}}-T_{\mathrm{avg}}$ as a function of $P_{\mathrm{r}\mathrm{f}}$, measured during the high-power rf test after the first cooldown.

Figure 29

Total thermal conductance between the cavity and CCRs 2 and 3 as a function of temperature obtained from the data measured during the high-power rf test after the second cooldown.

Figure 30

Snapshot of the shift in the resonant frequency of the prototype cavity inside the HTC, due to microphonics.

Figure 31

Magnitude of the FFT of $\delta f(t)$ shown in Fig. 30. The inset shows an expanded view of the peaks below 50 Hz.

Figure 32

Magnitude of the FFT of the vertical acceleration measured on the HTC support cart (a) and on top of CCR1 (b). The accelerometer is circled in red in the inset photos.

Figure 33

$R_s(B_p)$ data for the Nb prototype cavity with ${\mathrm{Nb}}_3\mathrm{Sn}$ film on the inner surface cooled by LHe and with Cu outer layer tested both in LHe and with cryocoolers. Solid lines are least-squares fit to each dataset.

Figure 34

Temperature distribution for the 952.6 MHz prototype cavity cooled by 3 CCRs, calculated with Ansys for $B_p=47\mathrm{mT}$ at the maximum $P_{\mathrm{r}\mathrm{f}}=16.9\mathrm{W}$ which resulted in a stable solution. The temperature color map scale is in units of Kelvin.