Medium-velocity superconducting cavity for high accelerating gradient continuous-wave hadron linear accelerators

We present the first rf studies of the medium-$\beta$ superconducting radio frequency (SRF) elliptical cavities designed for Michigan State University’s Facility for Rare Isotope Beams (FRIB) energy upgrade linac. The proposed energy upgrade for this continuous-wave (CW) superconducting linac will double the final beam energy from 200 to $400\mathrm{MeV}/\mathrm{u}$ for the heaviest uranium ions within the 80 m of space available in the FRIB tunnel. Two prototype ${\beta }_{\mathrm{opt}}=0.65644\mathrm{MHz}$ five-cell elliptical SRF cavities were fabricated and tested to validate the novel cavity design with three conventional rf recipes: (1) Electropolishing (EP-only), (2) $\mathrm{EP}+48\mathrm{h}$ $120°\mathrm{C}$ bake ($\mathrm{EP}+\text{baking}$), and (3) Buffered chemical $\text{Polishing}+48\mathrm{h}$ $120°\mathrm{C}$ bake ($\mathrm{BCP}+\text{baking}$). The EP-only recipe achieved a 2 K quality factor ($Q_0$) of $2.3\times {10}^{10}$ at the FRIB energy upgrade design accelerating gradient ($E_{\text{acc}}$) of $17.5\mathrm{MV}/\mathrm{m}$, and $Q_0$ of $1.2\times {10}^{10}$ at a maximum gradient of $26\mathrm{MV}/\mathrm{m}$, where the gradient was ultimately limited by the available rf amplifier power available for this test. These results validate the potential of the novel 644 MHz medium-$\beta$ cavity design and motivate its use in future high-${\mathrm{Q}}_0$ development work. The multipacting band, which exists at $E_{\mathrm{acc}}\approx 10\mathrm{MV}/\mathrm{m}$, was completely conditioned in the continuous-wave mode. We also observed that combining the 48 h $120°\mathrm{C}$ baking treatment with EP did not improve the EP-only cavity performance at 2 K due to increased residual resistance ($R_0$) and increased medium-field Q-slope. $\mathrm{BCP}+\text{baking}$ was also found to produce lower ${\mathrm{Q}}_0$ in this cavity due to increased medium-field Q-slope. The mechanical modes of this cavity were measured at room temperature, verifying that the quality factor of the dominant “accordion” mechanical mode these medium-$\beta$ cavities are particularly vulnerable to is close to that of the 1.3 GHz TESLA cavities. Thus, this mode arising from the novel geometry of these cavities is shown to not be of excessive concern for resonance control.

Figure 1

S65-001 surface removal with bulk EP. (a) EP cathode integrated with the cavity. This also shows the doughnuts on the cathode at the equator locations and Teflon shielding at the iris locations. Red arrows indicate the locations for ultrasonic thickness measurements. (b) “Doughnut” to increase the cathode radius at the equator locations. Five doughnuts are attached to a cylindrical aluminum tube with 34 mm diameter; the remaining area of the aluminum tube is masked by Teflon tape. (c) Surface removal at the locations of the beampipes and equators. Since the outer surfaces of the irises were not easy to access due to stiffening rings, we measured the beampipes as a representation of the surfaces at a small radius. PU and FPC denote the pickup and fundamental power coupler, respectively.

Figure 2

Results of the high-power rf testing of 644 MHz ${\beta }_{\mathrm{opt}}=0.65$ cavities at $\mathrm{MSU}/\mathrm{FRIB}$. (a) 2 K Q curves comparing EP only vs $\mathrm{EP}+120°\mathrm{C}$ baking recipes in S65-001. The blue circles are the EP-only case and the red circles are the $\mathrm{EP}+120°\mathrm{C}$ baking case. The open circles are field-emission X rays, as read on the right-hand vertical scale. In all trials FE remained well below $100\mathrm{mR}/\mathrm{h}$, where noticeable degradation in Q due to FE ocurrs at $100\mathrm{mR}/\mathrm{h}$ and above [25]. The color code is the same as for ${\mathrm{Q}}_0$. (b) The same results as in (a) but scaled to show the rf surface resistance $R_{\mathrm{s}}$. The dashed lines are second-degree polynomial fits to each data set. These fits were done for the data, for which $B_{\mathrm{pk}}$ is in the range of 20 to 100 mT, but they are extrapolated into the range of 1 to 120 mT for readability.

Figure 3

$R_{\mathrm{s}}$ as a function of $T_{\mathrm{c}}/T$, where $T_{\mathrm{c}}=9.2\mathrm{K}$, the critical temperature of niobium. $T$ is taken as the liquid helium bath temperature. These data were recorded at constant $E_{\mathrm{acc}}$ of $2\mathrm{MV}/\mathrm{m}$ for the EP-only case, and at approximately $4\mathrm{MV}/\mathrm{m}$ for the $\mathrm{EP}+120°\mathrm{C}$ baking case. This difference in field level is not regarded as significant because the total surface resistance, $R_{\mathrm{s}}$, is relatively field independent at these low fields. Both datasets are fitted to Eq. (2), $R_{\mathrm{s}}=\frac{A{\omega }^2}{T}{e}^{-\frac{\mathrm{\Delta }}{K_{B}T}}+R_0$, where the fitted parameters are (1) EP-only: $A=2.6\times {10}^{-24}$, $\frac{\mathrm{\Delta }}{k_B{T}_c}=1.9$, $R_0=2.2\mathrm{n}\mathrm{\Omega }$, and (2) $\mathrm{EP}+\text{baking}$: $A=2.1\times {10}^{-24}$, $\frac{\mathrm{\Delta }}{k_B{T}_c}=2.0$, $R_0=4.8\mathrm{n}\mathrm{\Omega }$.

Figure 4

Background magnetic field in the vertical test dewar. The fields were measured by lowering fluxgate magnetostatic sensors in increments to create a vertical scan at three different horizontal positions. The sensor locations were offset from the center axis by the same distance as the cavity equator, and they are spaced by 120° in the azimuthal direction from each other. These were measured with the cavity installed in the dewar together with the dewar lid; however, this measurement could only be conducted at room temperature, not at 2K.

Figure 5

Results of the high-power rf testing of 644 MHz ${\beta }_{\mathrm{opt}}=0.65$ cavities at $\mathrm{MSU}/\mathrm{FRIB}$. 2 K Q curves of the $\mathrm{BCP}+120°\mathrm{C}$ baking treatment (green) compared to the previous $\mathrm{EP}+120°\mathrm{C}$ baking treatment (red). The maximum gradient was limited in the $\mathrm{BCP}+\text{baking}$ test by field emission. The open circles depict field-emission x rays, read on the right-hand vertical scale.

Figure 6

RF conditioning of the cavity multipacting in the EP-only test. Pf and Pt are the forward and transmitted rf powers, respectively. The MP started to appear at an $E_{\mathrm{acc}}$ of $9.3\mathrm{MV}/\mathrm{m}$ ($t=0.3\min$ in this plot). The first three sudden drops of Pt ($t<5\min$) are due to the MP-induced thermal breakdown with no X-rays. After these events, X-rays were detected and we performed rf conditioning increasing the Pf stepwise. The MP was thoroughly conditioned and disappeared at an $E_{\mathrm{acc}}$ of $10.4\mathrm{MV}/\mathrm{m}$ ($t=28\min$). The X-ray detector was located to the side of the vertical test dewar, within 1 m from the cavity.

Figure 7

(a) 644 MHz cavity assembled with the rigid steel bars (arrows) clamped to the ends of the cavity to mimic the presence of the helium jacket. (b) Cavity assembled on the test stand with the mechanical speaker (arrow) in place, with accelerometers (white cables) attached to the cell equators. (c) details of the mechanical speaker coupled to the cavity test stand.

Figure 8

(a) The spectrogram with the mechanical mode scan. Sinusoidal vibrations were excited and its frequency was swept from 20 to 200 Hz. The color map represents the sideband power in units of dBc. (b) The integrated spectrum. The dominant electromechanical resonance is at 96 Hz, with a response at least 15 dB stronger than any other mode.

Figure 9

Displacement oscillations at the 95 Hz mechanical mode. These are measured from the accelerometers attached on the equators and aligned to the cavity axis (longitudinal direction). The phase advances are 28° from Cell #1 to Cell #2, $-4°$ from Cell #2 to Cell #3, 52° from Cell #3 to Cell #5.

Figure 10

(a) Spectrogram of the decay time measurement of the dominant 95 Hz mechanical sideband mode. This mode was excited with the mechanical speaker, which was shut off at $t=8.25\mathrm{s}$. The color map represents the sideband power in the unit of dBc. (b) The decay curve measured at 95 Hz. The decay rate of the mechanical mode energy is $-14\mathrm{dB}/\mathrm{s}$.