Superconducting CH structure

The superconducting CH structure is a novel multicell cavity for the acceleration of protons and ions in the low and intermediate energy regime. The CH structure is a cross-bar-type cavity; it is the first superconducting low energy multicell cavity operated in an H mode. A superconducting CH-prototype cavity with 19 gaps at $\beta =0.1$ has been built and tested successfully. Maximum surface fields of $25\mathrm{MV}/\mathrm{m}$ and a corresponding effective cavity voltage gain of 3.7 MV have been achieved. In this paper the development and the tests of this new cavity are presented. This includes also tuning and coupling methods as well as a comparison with other low energy cavities.

Figure 1

View into the superconducting CH cavity, $\beta =0.1$, $f=360\mathrm{MHz}$, before welding of the end walls (courtesy ACCEL, Bergisch-Gladbach, Germany).

Figure 2

H-mode drift-tube cavities: rt IH-DTL (217 MHz, left), rt CH-DTL (350 MHz, center) and sc CH-DTL (350 MHz, right).

Figure 3

Effective shunt impedance for different rf structures including the transit time factor $T$ and the synchronous phase $\varphi$ as a function of the particle velocity $\beta =v/c$. The black horizontal bars represent some existing IH-DTLs [13, 21, 22] and the red bars represent the expected shunt impedance of the rt CH cavities for the GSI-FAIR 70 MeV, 70 mA proton linac [57].

Figure 4

2D cut of the realized sc CH-cavity geometry.

Figure 5

Top: Simulated electric gap field distribution for different end-cell lengths and constant $g/L$ ratio of 0.5. Bottom: Simulated electric gap field distribution for different end-cell lengths and variable $g/L$ ratios.

Figure 6

Simulated influence of the girder height on the electric and magnetic peak fields.

Figure 7

Simulated electric gap field distribution for three different girder heights (0%, 23%, and 43% at a constant tank radius $R=140\mathrm{mm}$). The girder height of $0.43R$ corresponds to the prototype geometry. Because of the cavity symmetry only one-half of the length is shown. Each gap voltage corresponds to the local magnetic flux [see Eq. (1)].

Figure 8

Plot of the magnetic field $|B_z|$ along the CH cavity. The field is shown for three different paths along the cavity. The girders support a flat field distribution along a CH cavity and lead to very homogeneous local current densities which reduce the magnetic peak fields.

Figure 9

Electric field along a path perpendicular to the beam axis for different drift-tube wall thicknesses $\Delta R$. The field is normalized to the maximum field for $\Delta R=5\mathrm{mm}$ (prototype geometry). The path distance from the tube ends is 2 mm in all cases. The aperture diameter is 25 mm in all cases, the $g/L$ ratio is 0.5, $L=42.6\mathrm{mm}$.

Figure 10

Electric and magnetic peak field as a function of the stem width $b$ at constant $a$ and $L=42.6\mathrm{mm}$. The peak fields are based on the $\beta \lambda$ definition of the gradient.

Figure 11

The room temperature copper model which has been used to validate the electromagnetic simulations. This photograph shows the model with a graded beta from 0.085 to 0.12.

Figure 12

Comparison between the measured and the simulated field distribution of the CH-copper model as shown in Fig. 11.

Figure 13

Gap and drift-tube lengths which are required to obtain the flat field distribution in Fig. 12.

Figure 14

The picture shows the favored location of the capacitive coupler during investigations with the room temperature copper model.

Figure 15

Comparison between measurement and simulation of the external $Q$-value as a function of the position of the capacitive coupler.

Figure 16

The picture shows the location of the inductive coupler in the room temperature copper model. The external $Q$-value has been varied by changing the loop angle.

Figure 17

Comparison between measurement and simulation of the external $Q$-value as a function of the loop angle.

Figure 18

Drift-tube structure from bulk niobium welded into one pair of girders during fabrication (courtesy ACCEL).

Figure 19

The first low level rf measurement at room temperature shows the mode spectrum between 340 and 570 MHz. The ordinate scale is $10\mathrm{dB}/\mathrm{\text{division}}$, the frequency is swept between 342 and 570 MHz.

Figure 20

Setup for the frequency and field tuning of the superconducting CH prototype. Ten movable tuners (top left) have been used to tune the cavity.

Figure 21

Comparison between the measured and simulated frequency shift using the tuners shown in Fig. 20. With an identical tuner height of 30 mm for all 10 tuners a frequency shift of almost $-1\%$ of the resonance frequency could be achieved.

Figure 22

Simulated frequency shift as a function of the tuner radius $R_T$ at a constant tuner height of $h=30\mathrm{mm}$.

Figure 23

Using different heights for the tuners, it was possible to change the field distribution in a well-defined way. Here only 3 tuners with a height of 30 mm have been installed at the right side.

Figure 24

This field distribution has been obtained by installing 3 tuners at each cavity end with a height of 30 mm.

Figure 25

Comparison between measurement and simulation of the final field distribution. Ten tuners with heights of 0, 10, and 20 mm have been used. The field flatness is very good and sufficient for most of the applications.

Figure 26

Several temperature sensors have been placed at the cavity to measure the temperature increase due to rf power. Up to 300 W cw has been coupled to the cavity. Sensor 1 is located at the end-cell region, sensor 2 and sensor 3 close to the stem base, and sensor 4 is located at the outer tank wall. The higher the current density, the higher is the obtained temperature.

Figure 27

Experimental setup for the first cryotest of the CH prototype.

Figure 28

Principle of the rf control system to operate the cavity in a closed loop.

Figure 29

Measurement of the transmission (${\mathrm{S}}_{21}$ parameter) as a function of the frequency. The frequency bandwidth is 500 Hz and the $y$ scale is $10\mathrm{dB}/\mathrm{\text{division}}$. At low field levels (forwarded power $\approx \mathrm{mW}$). the cavity showed multipacting as it can be recognized by the flattop of the resonance curve.

Figure 30

The rf signals of the CH cavity at high fields: Reflected signal (green), transmitted (pickup) signal (magenta), and forwarded signal (yellow). The time scale is 100 ms per division.

Figure 31

Commonly used length definitions in a cavity and their illustration with respect to CH cavities.

Figure 32

The intrinsic $Q$-value as a function of the effective accelerating gradient based on the $\beta \lambda$ definition.

Figure 33

The intrinsic $Q$-value as a function of the effective accelerating voltage. The highest voltage which has been achieved is 3.8  MV.

Figure 34

Maximum gap voltage amplitudes as achieved during the test runs.

Figure 35

Measured sensitivity of the resonance frequency against external pressure variations.

Figure 36

Measured frequency shift due to Lorentz force detuning (gradient based on $\beta \lambda$ definition).

Figure 37

Measured x-ray dose along the CH structure (top) and position of the x-ray TLD detectors inside the girders.

Figure 38

Measured (red, blue) and simulated frequency shift during the cooldown. The frequency shift during different cooldowns was very reproducible.

Figure 39

The stress load distribution on the deformed end caps with an applied external force of 4 kN.

Figure 40

Maximum values for the van Mises stress.

Figure 41

Shift in rf frequency due to an external force.

Figure 42

Simulated influence of the external vacuum pressure and of the external force of the tuner on the electric field distribution.

Figure 43

Number of accelerating cells per cavity as a function of $\beta$.

Figure 44

Geometrical factor $G=R_s{Q}_0$ as a function of $\beta$. See the legend in Fig. 43.

Figure 45

$R_a/Q_0$ value as a function of $\beta$.

Figure 46

$R_a{R}_s$ per cell as a function of $\beta$. See the legend in Fig. 45.

Figure 47

Ratio $E_p/E_a$ of the electric peak field and accelerating field as a function of $\beta$. The values are based on the $\beta \lambda$ definition. See the legend in Fig. 45.

Figure 48

Ratio $B_p/E_a$ of the magnetic peak field and accelerating field as a function of $\beta$. The values are based on the $\beta \lambda$ definition. See the legend in Fig. 45.