Development of superconducting crossbar-$H$-mode cavities for proton and ion accelerators

The crossbar-$H$-mode (CH) structure is the first superconducting multicell drift tube cavity for the low and medium energy range operated in the $H_{21}$ mode. Because of the large energy gain per cavity, which leads to high real estate gradients, it is an excellent candidate for the efficient acceleration in high power proton and ion accelerators with fixed velocity profile. A prototype cavity has been developed and tested successfully with a gradient of $7\mathrm{MV}/\mathrm{m}$. A few new superconducting CH cavities with improved geometries for different high power applications are under development at present. One cavity ($f=325\mathrm{MHz}$, $\beta =0.16$, seven cells) is currently under construction and studied with respect to a possible upgrade option for the GSI UNILAC. Another cavity ($f=217\mathrm{MHz}$, $\beta =0.059$, 15 cells) is designed for a cw operated energy variable heavy ion linac application. Furthermore, the EUROTRANS project (European research program for the transmutation of high level nuclear waste in an accelerator driven system, 600 MeV protons, 352 MHz) is one of many possible applications for this kind of superconducting rf cavity. In this context a layout of the 17 MeV EUROTRANS injector containing four superconducting CH cavities was proposed by the Institute for Applied Physics (IAP) Frankfurt. The status of the cavity development related to the EUROTRANS injector is presented.

Figure 1

360 MHz prototype CH cavity implemented in the horizontal cryomodule with a picture of the piezo tuning device.

Figure 2

Test of the piezo tuning system compensating frequency changes at 4 K.

Figure 3

Design of the superconducting 325 MHz CH cavity.

Figure 4

Simulation of the electric field distribution for different end stem geometries.

Figure 5

Simulated tuning range of the four static tuners.

Figure 6

Structural ANSYS simulation of a bellow tuner model (top) and deformation of the bellow tuner depending on several forces (bottom) [17].

Figure 7

Bellow tuner prototype of niobium with one and a half segments (left) and related results from measurements at room temperature in comparison to the simulations (right).

Figure 8

Preliminary layout of the 217 MHz CH cavity for the cw heavy ion linac for an injection energy of 1.4 AMeV.

Figure 9

Installation study of the designed cw linac in parallel to the GSI UNILAC.

Figure 10

Reference design of the 600 MeV EUROTRANS proton driver linac.

Figure 11

Layout of the 17 MeV EUROTRANS front end.

Figure 12

Layout of the first superconducting EUROTRANS CH cavity (simulation with Microwave Studio [18]).

Figure 13

Simulated tuning range of ten hypothetic tuners operated in parallel and electrical peak fields appearing at the tuner surface.

Figure 14

Simulation of the electric field distribution before and after the optimization (above), related gap and drift tube length for the flat field (bottom).

Figure 15

Effective voltage on beam axis and related energy gain per gap for a synchronous particle of the first CH cavity.

Figure 16

First superconducting CH cavity optimized for the EUROTRANS front end.

Figure 17

Transverse beam envelopes for the DTL part of the EUROTRANS injector.

Figure 18

Transverse emittance projections at the input and output of the EUROTRANS DTL part for 5 mA.