niobium quarter-wave resonator development for a heavy ion re ...

≪≪≪ Poster TuP67 ≫≫≫
Presented at the
13th Int’l Workshop on RF Superconductivity
15–19 October 2007, Beijing, China
NIOBIUM QUARTER-WAVE
RESONATOR
DEVELOPMENT FOR A
HEAVY ION
RE-ACCELERATOR
W. Hartung, J. Bierwagen, S. Bricker,
C. Compton, T. Grimm, M. Johnson, F. Marti,
J. Popielarski, L. Saxton, R. York
National Superconducting Cyclotron Laboratory
Michigan State University
A. Facco
Istituto Nazionale di Fisica Nucleare
Laboratori Nazionali di Legnaro
E. Zaplatin
Forschungszentrum Jülich

Introduction
The National Superconducting Cyclotron Laboratory
(NSCL) is building a reaccelerator for exotic
ion beams. Stable ions are produced in an ion
source and accelerated in the NSCL coupled cyclotron
facility. The primary beam produces a secondary
beam of exotic ions by particle fragmentation.
The reaccelerator will consist of a gas stopper to
slow down the secondary ion beam, a charge breeder
to increase the charge of the ions by removing electrons,
a radio frequency quadrupole to provide initial
acceleration and focussing, and a superconducting
linac to accelerate the beam to a final energy
of about 3 MeV per nucleon. The superconducting
linac will consist of quarter-wave resonators
(QWRs) optimised for β = 0.041 and β = 0.085.
This poster covers the RF design and prototyping
of the β = 0.041 QWR. The β = 0.085 QWR has
already been prototyped.

Experiments
with
fast RIBs
Coupled Cyclotron Facility
Reaccelerator Layout

Cavity Design
The quarter-wave resonators developed by Legnaro
for ALPI and PIAVE are the basis for the design
of the QWR for the reaccelerator. A slightly larger
aperture is used for the reaccelerator cavities. Another
differences is separation of the cavity vacuum
from the insulation vacuum to reduce particulate
contamination of the cavity surfaces.
• Shorting plate is formed from sheet Nb (3 mm
thick) (similar to Argonne design) instead of
being machined.
• Tuning plate (1.25 mm thick) is slotted to reduce
the tuning force (similar to TRIUMF design).
• Probe couplers are attached to the bottom flange
(same as β = 0.085 prototype).

Selected QWR Parameters
Optimum β 0.042
Resonant frequency f 80.5 MHz
Design E p
16.5 MV/m
Design B p
29.2 mT
Design V a
0.45 MV
R s /Q
433 Ω
Geometry factor
15.4 Ω
Operating temperature 4.5 K
Design Q 0 5 · 10 8
Nominal OC diameter 180 mm
Nominal IC diameter 60 mm
Nominal height (λ/4) 931 mm
Active length
95 mm
Aperture
30 mm
OC = Outer Conductor; IC = Inner Conductor
ANALYST was used for numerical calculations and
optimisation of the RF parameters.

0.2
0.4
0
3181007−006
V a
[MV]
0.6
0 0.1 0.2 0.3 0.4 0.5
β
Dependence of accelerating voltage on beam
velocity

Fabrication of Prototype Cavity
• Sheet Nb of thickness 2 mm and RRR ≥ 250
was used.
• The tip of the center conductor and the beam
tubes were machined from solid Nb.
• Nb tuning plate on bottom in Nb-Ti to stainless
steel flange.
• Forming was done at NSCL and in local area;
electron beam welding was done with an industrial
company.
• Indium joints were used to seal the bottom flange.
Knife-edge seals were used for beam tube flanges.
• c. 150 µm etch (BCP).
• High-pressure rinse with ultra-pure water for about
80 minutes.

Electron beam welding of β = 0.041 QWR

Nb parts for β = 0.041 QWR (top) and inside view
of completed cavity (bottom)

Etching (left) and rinsing (right) of β = 0.041
QWR

0
Ref − S 21
phase (degrees)
0.5 1 1.5
3181007−005
2
0 50 100 150
z (arbitrary units)
Bead pull for β = 0.041 QWR

RF Test on β = 0.041 QWR
• Vertical test with cavity immersed in liquid helium.
• Multipacting barriers at low field, were able to
get through them rapidly.
• Observed x-rays at high field (9 R/hour at max
field); did some RF conditioning, but not able
to eliminate the field emission.
• Did not observe thermal breakdown.
• Max fields: E p ≈ 80 MV/m, B p ≈ 140 mT,
V a ≈ 2.2 MV.

Preparing for RF test of the β = 0.041 QWR

Q 0
10 8 10 9 10 10
4.6 K
4.2 K
2.0 K
0 20 40 60 80
E p
[MV/m]
3181007−004
RF test of β = 0.041 (September-October 2007)

Frequency Issues
• Measured resonant frequency at 4.5 K was too
high by 225 kHz; need to correct this with production
cavities.
• Tuning range and tuning force need to be checked
and compared with predictions.
Structural Analysis
• Structural analyses of the QWR are being done
with ANSYS.
• Stiffening methods are being explored with the
aim of minimising frequency shifts due to pressure
fluctuations and microphonics.

Conclusion
• A β = 0.041 QWR has been fabricated and
tested.
• RF test results exceeded the design goals by a
comfortable margin.
• Multipacting and field emission were observed,
but they did not prevent the cavity from reaching
the design goals.
• Future steps: production β = 0.041 and β =
0.085 cavities for the NSCL reaccelerator.