First heavy ion beam tests with a superconducting multigap CH cavity

Very compact accelerating-focusing structures, as well as short focusing periods, high accelerating gradients and short drift spaces are strongly required for superconducting (sc) accelerator sections operating at low and medium energies for continuous wave (cw) heavy ion beams. To keep the GSI-super heavy element (SHE) program competitive on a high level and even beyond, a standalone sc cw linac (Helmholtz linear accelerator) in combination with the GSI high charge state injector (HLI), upgraded for cw operation, is envisaged. Recently the first linac section (financed by Helmholtz Institute Mainz (HIM) and GSI) as a demonstration of the capability of 217 MHz multigap crossbar H-mode structures (CH) has been commissioned and extensively tested with heavy ion beam from the HLI. The demonstrator setup reached acceleration of heavy ions up to the design beam energy. The required acceleration gain was achieved with heavy ion beams even above the design mass to charge ratio at high beam intensity and full beam transmission. This paper presents systematic beam measurements with varying rf amplitudes and phases of the CH cavity, as well as phase space measurements for heavy ion beams with different mass to charge ratio. The worldwide first and successful beam test with a superconducting multigap CH cavity is a milestone of the R&D work of HIM and GSI in collaboration with IAP in preparation of the HELIAC project and other cw-ion beam applications.

Figure 1

General cw-linac layout.

Figure 2

CH-cavity test environment at GSI.

Figure 3

Sectional drawing of the 15-gap demonstrator CH cavity.

Figure 4

Radio frequency testing cavity (left) for the rf power couplers at GSI; two couplers could be tested simultaneously (right)—the temperature at the outer coupler surface was measured with sensors.

Figure 5

Layout of matching line to the demonstrator and beam diagnostics test bench; $\mathrm{QT}=\text{quadrupole triplet}$, $\mathrm{QD}=\text{quadrupole}$ duplet, $\mathrm{R}=\text{rebuncher}$, $\mathrm{X}/\mathrm{Y}=\mathrm{beam}$ steerer, $\mathrm{G}=\mathrm{SEM}$-grid, $\mathrm{T}=\mathrm{beam}$ current transformer, $\mathrm{P}=\text{phase}$ probe, $\mathrm{BSM}=\text{bunch}$ shape monitor, $\mathrm{EMI}=\text{emittance}\text{meter}$.

Figure 6

${\mathrm{Ar}}^{9+}$-rf phase scan with rebuncher R2; nominal beam energy (HLI exit) is measured for $1.366\mathrm{MeV}/\mathrm{u}$.

Figure 7

Bunch shape measurement for HLI beam at $1.366\mathrm{MeV}/\mathrm{u}$ (top) and at the same energy for the matched case with rebuncher R1 and R2 (bottom).

Figure 8

First rf acceleration with CH cavity; measured ${\mathrm{Ar}}^{11+}$-phase probe signals from HLI beam at $1.366\mathrm{MeV}/\mathrm{u}$ (top), rf frequency is 108.408 MHz ($\mathrm{T}=9.224\mathrm{ns}$). By acceleration up to the nominal beam energy (bottom), the coarse time of flight between the blue and the red signal is slightly reduced. The time of flight for the fine measurement between the red and the green signal is significantly shifted, according to the beam energy of $1.866\mathrm{MeV}/\mathrm{u}$.

Figure 9

Acceleration of an ${\mathrm{Ar}}^{9+}$ beam; maximum achieved beam energy and transmission as a function of the accelerating gradient.

Figure 10

3D scan of ${\mathrm{Ar}}^{9+}$-beam energy versus accelerating gradient and rf phase.

Figure 11

3D scan of ${\mathrm{Ar}}^{9+}$-beam transmission versus accelerating gradient and rf phase.

Figure 12

Measured width of the bunch length (rf phase scan) for ${\mathrm{Ar}}^{9+}$ beam at an accelerating gradient of $3.5\mathrm{MV}/\mathrm{m}$.

Figure 13

Selected bunch structure for ${\mathrm{Ar}}^{9+}$ at a gradient of $3.5\mathrm{MV}/\mathrm{m}$ for an rf phase of 190°, 200° and 220° according to Fig. 12.

Figure 14

Phase scan of ${\mathrm{Ar}}^{6+}$-beam energy for $3.5\mathrm{MV}/\mathrm{m}$ and $5.5\mathrm{MV}/\mathrm{m}$.

Figure 15

Phase scan of ${\mathrm{Ar}}^{6+}$-beam energy for $5.5\mathrm{MV}/\mathrm{m}$.

Figure 16

Amplitude scan of ${\mathrm{Ar}}^{6+}$-rf phase of 210°.

Figure 17

Transverse ${\mathrm{Ar}}^{9+}$-beam emittance at $1.366\mathrm{MeV}/\mathrm{u}$ (top) and at $1.85\mathrm{MeV}/\mathrm{u}$ (bottom); normalized (horizontal) emittance growth is measured for 15%.

Figure 18

Bunch shape of ${\mathrm{Ar}}^{9+}$-beam emittance at $1.366\mathrm{MeV}/\mathrm{u}$ (top) and fully matched at $1.85\mathrm{MeV}/\mathrm{u}$ (bottom); the FWHM of is measured for 0.5 ns.