RECENT MEASUREMENTS ON THE SC 325 MHz CH-CAVITY∗

M. Busch†, M. Basten, F. Dziuba, H. Podlech, U. Ratzinger, IAP, Frankfurt am Main, GermanyM. Amberg, Helmholtz-Institut Mainz (HIM), Mainz, Germany

Abstract

At the Institute for Applied Physics (IAP), Frankfurt Uni-versity, a superconducting 325 MHz CH-Cavity has beendesigned and built and extensive tests have successfullybeen performed. The cavity is determined for a 11.4 AMeV,10 mA ion beam at the GSI UNILAC. This cavity consistsof 7 gaps and is envisaged to deliver a gradient of 5 MV/m.Novel features of this structure are a compact design, lowpeak fields, improved surface processing and power cou-pling. Furthermore a tuner system based on bellow tunersattached inside the resonator and driven by a stepping mo-tor and a piezo actuator will control the frequency. In thiscontribution measurements performed at 4.2 K and 2.1 K atthe cryo lab in Frankfurt will be presented.

INTRODUCTION

Currently planned projects like the sc cw Heavy IonLinac at GSI/HIM [1] require compact and efficient cavitiesto deliver beams of high intensity, quality and availabilty.For those applications the superconducting CH-cavity has

Figure 1: Rendering of the cavity with welded helium ves-sel.

already proved to be an appropiate candidate being charac-terized by a small number of drift spaces between neighbor-ing cavities compared to conventional low-β ion linacs [2].Additionally the KONUS beam dynamics, which decreasesthe transverse rf defocusing and allows the development oflong lens free sections, yields high real estate gradients withmoderate electric and magnetic peak fields. At the Insti-tute for Applied Physics, Frankfurt University, a new cavityoperating at 325.224 MHz, consisting of 7 cells, β = 0.16and an effective length of 505 mm (see Table 1) has beendesigned [3] and extensively measured after all fabrication

∗ Work supported by GSI, BMBF Contr. No. 06FY7102† busch@iap.uni-frankfurt.de

and processing steps at Research Instruments [4]. In the fi-nal stage the cavity is being welded to a helium vessel toprovide a closed helium circulation (see Fig. 1).

Table 1: Specifications of the 325 MHz CH-Cavity.

β 0.16frequency [MHz] 325.224no. of cells 7length (βλ-def.) [mm] 505diameter [mm] 352Ea (design) [MV/m] 5Ep /Ea 5Bp /Ea [mT/(MV/m)] 13G [Ω] 66Ra /Q0 1260RaRs [kΩ2] 80

MEASUREMENT SETUP

At the cryo lab of the IAP a measurement setup compris-ing a vertical cryostat has been installed for various test pur-poses (see Fig. 2) allowing power measurements at 4.2 Kand 2.1 K, respectively. In this system the spent Heliumcan be extracted to a recovery system or the cryostat canbe evacuated by a roots pump to reach 2 K. The CH-cavity

Figure 2: Schematic layout of the vertical test environment.

has been provided with four low-temperature probes, 40Thermo-Luminescence-Dosimeter (see Fig. 3) to record

Proceedings of SRF2015, Whistler, BC, Canada MOPB065

New Proposals

B02-Hadron Linacs

ISBN 978-3-95450-178-6

255 Cop

yrig

ht©

2015

CC

-BY-

3.0

and

byth

ere

spec

tive

auth

ors

field emission events and two piezos for microphonic ex-citation and detection.

Figure 3: Setup of the cavity with TLDs and temperatureprobes.

RESULTS

The initial conditioning of low multipacting barriers (s.Fig. 4, bottom, red dotted lines) took several days. Fur-

Figure 4: Multipacting simulations in the region of the cav-ity wall (top). Comparison between measured and simu-lated multipacting events (bottom).

ther barriers in the range between 2.5 MV/m and 5.7 MV/m

could be crossed but still remained softly in repetitive mea-surements (s. Fig. 4, bottom, red straight lines). Simu-lations indicate that in the quarter sections of the tank (s.Fig. 4, top) multipacting is possible depending on the sur-face preparation. Also the simulated values match quitewell with the measured ones between 2 MV/m and 6 MV/m.With a residual pressure of 6 ·10−10 mbar measurements upto the quench of the cavity have been performed. Figure 5depicts the Q vs E curves for three different temperatures.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15108

109

1010

4.2 K 3.5 K 2.1 K

Q0

Ea / (MV/m)

Figure 5: Q vs E curve for different temperatures.

1 2 3 42

4

6

8

10

12

14

16

18

20

22

24 Girder 1 Tank 1 Girder 2 Tank 2 Girder 3 Tank 3 Girder 4 Tank 4 Top Cap Bottom Cap

Dos

is [m

Sv/h

]

Position

Figure 6: Dosis distribution among the TLD.

Using a fast cool-down scheme to 4 K (> 1 K/min) gradi-ents of up to 8.5 MV/m corresponding to voltages of 4.2 MVcould be achieved. Utilizing a roots pump Helium temper-atures of 3.5 K and 2.1 K, respectively, could be attained.The respective curves yield a gradient of up to 9.5 MV/min case of 3.5 K and 14.1 MV/m at 2.1 K. The quench at thehighest field levels is supposedly due to a thermal defectsince the degradation of the Q-value is still quite low. Alsothe evaluation of the TLD (see Fig. 6) shows only a small,potential field emitting site located somewhere near the bot-tom area of the cavity. To illustrate the enhancement factorof the emitting sites a Fowler-Nordheim plot is shown in Fig.

MOPB065 Proceedings of SRF2015, Whistler, BC, Canada

ISBN 978-3-95450-178-6

256Cop

yrig

ht©

2015

CC

-BY-

3.0

and

byth

ere

spec

tive

auth

ors

New Proposals

B02-Hadron Linacs

0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14-18

-16

-14

-12

-10

-8

-6

-4

-2 Measurement 2014, FN=290 Measurement 2012, FN=520

ln(I/E

p2.5 )

1/Ep / (m/MV)

Figure 7: Fowler-Nordheim plot for two different surfacequalities.

7. The two curves refer to different surface qualities. Theblack curve belongs to a measurement without HPR, theblue one with extensive HPR treatment showing a distinctdifference in emitter activity.

Figure 8: LFD-signature on the VCO-signal (green curve)at 8.5 MV/m gradient pulsed operation.

Furthermore measurements in pulsed mode have beenconducted to study Lorentz-Force-Detuning behaviour. Fig-ure 8 shows an example of the VCO response at a field levelof 8.5 MV/m. The according frequency shift compensatedby the control system is -435 Hz yielding a LFD factor of-6.1 Hz/(MV/m)2 (s. Fig. 9).

CONCLUSION AND OUTLOOK

The cold measurements showed a very promising perfor-mance of the CH-cavity with gradients of 8.5 MV/m at 4 Kand 14.1 MV/m at 2 K for a Q of 1 · 109 and 3 · 109, respec-tively. Next steps are the welding of the helium vessel tothe cavity, a final HPR treatment and tests in a large verticalcryostat at the new cryo-bunker at IAP.

0 10 20 30 40 50 60 70 80-500

-400

-300

-200

-100

0

f [H

z]

Ea2 [(MV/m)2]

Figure 9: Measurement of the LFD effect.

ACKNOWLEDGEMENT

This work has been supported by Gesellschaft fürSchwerionenforschung (GSI), BMBF contr. No. 06F134Iand EU contr. No 516520-FI6W. We acknowledge alsothe support of the European Community-Research Infras-tructure Activity under the FP6 Structuring the EuropeanResearch Area program (CARE, contract number RII3-CT- 2003-506395). This work was (financially) supportedby BMBF 06FY161I, 06FY7102 and by the Helmholtz-InternationalCenter for FAIR within the framework ofthe LOEWE program (Landesoffensive zur EntwicklungWissenschaftlich-Ökonomischer Exzellenz) launched bythe State of Hesse. We’d like to thank our contact personsand the staff at RI for the close collaboration and the kindassistance for the measurements during this project.

REFERENCES

[1] W. Barth, K. Aulenbacher, F. Dziuba, M. Amberg, V.Gettmann, S. Jacke, S. Mickat, A. Orzhekovskaya, H. Podlech,U. Ratzinger "Advanced Superconducting cw Heavy IonLinac R&D", Proceedings of IPAC 2013, Shanghai, China,p. 3770-3772.

[2] H. Podlech, A. Bechtold, H. Liebermann, M. Busch, G.Clemente, H. Klein, U. Ratzinger, R. Tiede, C. Zhang "De-velopment of Room Temperature and Superconducting CH-Structures for High Power Applications", Proc. of the HPPA5workshop, Mol, Belgium, 2007, p. 115-124.

[3] M. Busch, H. Podlech, M. Amberg, F. Dziuba, U. Ratzinger,"Superconducting CH-Cavity development", Proceedings ofIPAC 2010, Kyoto, Japan, p. 753-755.

[4] RI, Research Instruments GmbH, Bergisch Gladbach, Ger-many, http://www.research-instruments.de

Proceedings of SRF2015, Whistler, BC, Canada MOPB065

New Proposals

B02-Hadron Linacs

ISBN 978-3-95450-178-6

257 Cop

yrig

ht©

2015

CC

-BY-

3.0

and

byth

ere

spec

tive

auth

ors