High field $Q$ slope and the effect of low-temperature baking at 3 GHz

A strong degradation of the unloaded quality factor with field, called high field $Q$ slope, is commonly observed above ${\mathrm{B}}_p\cong 100\mathrm{mT}$ in elliptical superconducting niobium cavities at 1.3 and 1.5 GHz. In the present experiments several 3 GHz niobium cavities were measured up to and above ${\mathrm{B}}_p\cong 100\mathrm{mT}$. The measurements show that a high field $Q$ slope phenomenon limits the field reach at this frequency, that the high field $Q$ slope onset field depends weakly on the frequency, and that the high field $Q$ slope can be removed by the typical empirical solution of electropolishing followed by heating to 120°C for 48 hrs. In addition, one of the cavities reached a quench field of 174 mT and its field dependence of the quality factor was compared against global heating predicted by a thermal feedback model.

Figure 1

The photo of FH3A assembled for testing is shown in the top. The sketch of the cavity shape, which is the TESLA long end half cell [13] scaled to 3 GHz, is shown in the bottom. Dimensions are in mm [inches].

Figure 2

FH3C test results after BCP treatment for three different helium bath temperatures, 2.0, 1.8, 1.6 K and after EP treatment and mild baking at ${\mathrm{T}}_b=2.0\mathrm{K}$. Note the increase in the low-field quality factor as expected for lower helium bath temperatures, but similar $Q$ drops. Also, note the absence of the high-field degradation in the test after the EP and mild baking.

Figure 3

FH3D test results at ${\mathrm{T}}_b=2.0\mathrm{K}$ before and after baking at $120°\mathrm{C}$ for 48 hours. No radiation was observed during the measurements.

Figure 4

FH3E test results at ${\mathrm{T}}_b=2.0\mathrm{K}$. Note the characteristic high-field degradation.

Figure 5

FH3A data between ${\mathrm{T}}_b=3.7\mathrm{K}$ and ${\mathrm{T}}_b=1.6\mathrm{K}$ at ${\mathrm{E}}_{\text{acc}}\sim 4.2\mathrm{MV}/\mathrm{m}$. The solid black line was obtained from a least-squares fit with Eq. (1).

Figure 6

FH3A test results at ${\mathrm{T}}_b=2.0\mathrm{K}$ and at ${\mathrm{T}}_b=2.0\mathrm{K}$, ${\mathrm{T}}_b=1.8\mathrm{K}$, and ${\mathrm{T}}_b=1.6\mathrm{K}$ after $120°\mathrm{C}$ baking. Note the characteristic high-field degradation before $120°\mathrm{C}$ baking. $Q_0$ dropped after multiple quenches at 1.6 K as shown by the data measured while lowering the field after quench, indicated by the arrow.

Figure 7

FH3F test results at ${\mathrm{T}}_b=2.0\mathrm{K}$. Note the characteristic high-field degradation.

Figure 8

FH3C, FH3D, and FH3E at ${\mathrm{T}}_b=2.0\mathrm{K}$ are plotted here along with the earlier data reproduced from [15]. ${\mathrm{E}}_{\text{peak}}/{\mathrm{E}}_{\text{acc}}=1.83$ was used for the 3 GHz cavities. Note that the temperature of the helium bath increased from about 1.4 K to about 1.8 K with rf dissipated power during the earlier measurements in 1991.

Figure 9

FH3A, FH3C, and FH3D results before and after mild baking are plotted here. All data is measured at ${\mathrm{T}}_b=2.0\mathrm{K}$. No field emission was observed in any test.

Figure 10

FH3A test results for three different helium bath temperatures, 2.0, 1.8, 1.6 K (symbols) along with results from a thermal feedback model calculation for each temperature with values of the thermal boundary resistance matching the experimental quench fields (solid lines).

Figure 11

3 GHz cavities test results after EP or BCP along with test results for 1.3 GHz and 1.5 GHz single-cell cavities. ${\mathrm{B}}_p/{\mathrm{E}}_{\text{acc}}=4.2\mathrm{mT}/(\mathrm{MV}/\mathrm{m})$ was used for 1.3 GHz (TESLA end cell); ${\mathrm{B}}_p/{\mathrm{E}}_{\text{acc}}=4.5\mathrm{mT}/(\mathrm{MV}/\mathrm{m})$ was used for 1.5 GHz [old Cornell (OC) center cell]. All data is measured at ${\mathrm{T}}_b=2.0\mathrm{K}$. No field emission was observed in any test.