Abstract A novel prototype of SCRF cavity tuner is being designed and tested at Fermilab. This is a superconducting C-type iron dominated magnet having a 10 mm gap, axial symmetry, and a 1 Tesla field. Inside the gap is mounted a superconducting coil capable of moving ± 1 mm and producing a longitudinal force up to ± 1.5 kN. The static force applied to the RF cavity flanges provides a longterm cavity geometry tuning to a nominal frequency. The same coil powered by fast AC current pulse delivers mechanical perturbation for fast cavity tuning. This fast mechanical perturbation could be used to compensate a dynamic RF cavity detuning caused by cavity Lorentz forces and microphonics. A special configuration of magnet system was designed and tested

A Makulski

E Borissov

FNAL T Khabiboulline

G W Foster

V Kashikhin

Y Pischalnikov

CiteSeerX

ELECTROMAGNETIC SCRF CAVITY TUNER* 
V. Kashikhin#, E. Borissov, G.W. Foster, A. Makulski, Y.Pischalnikov, T. Khabiboulline, FNAL, 
Batavia, IL 60510, U.S.A.
Abstract 
   A novel prototype of SCRF cavity tuner is being 
designed and tested at Fermilab. This is a superconducting 
C-type iron dominated magnet having a 10 mm gap, axial 
symmetry, and a 1 Tesla field. Inside the gap is mounted a 
superconducting coil capable of moving ± 1 mm and 
producing a longitudinal force up to ± 1.5 kN. The static 
force applied to the RF cavity flanges provides a long- 
term cavity geometry tuning to a nominal frequency. The 
same coil powered by fast AC current pulse delivers 
mechanical perturbation for fast cavity tuning. This fast 
mechanical perturbation could be used to compensate a 
dynamic RF cavity detuning caused by cavity Lorentz 
forces and microphonics. A special configuration of 
magnet system was designed and tested. 
 
INTRODUCTION 
 
A superconducting tuner relates generally to 
superconducting magnets and more particularly to radio 
frequency tuners for use in linear particle accelerators. 
Technology in superconducting linear accelerators has 
been substantially improved over 10 years. Niobium 
superconducting radio frequency cavities (SCRF) are the 
main element of linear accelerators. Future linear 
accelerators and colliders ILC [1], TESLA [2], XFEL [3], 
Project-X [4] need to explore high (25 – 35 MV/m) 
electrical field gradient cavities with the possibility of 
cavity tuning during accelerator operation. SCRF cavity 
deviations from a resonant frequency depend on the 
cavity geometric changes caused by manufacturing errors, 
cool down shrinkage, pressure fluctuation of LHe, 
Lorentz forces, and external mechanical vibration. 
There are two types of cavity detuning effects, slow and 
fast. Typically, slow correction of the cavity resonant 
frequency uses a mechanical tuner with a stepper motor 
drive. A drive shaft to the tuner mounted on the low 
temperature cavity is used if the stepper motor is placed 
outside the cryostat at room temperature [5].  It increases 
the heat leak and the inherent backlash of the system. 
Tuners require special reliable stepper motors that operate 
at low temperature to operate within the cryostat [6].  
Complicated mechanics also drive the cost of these 
devices. Piezoelectric or magnetostrictive tuners are used 
for fast cavity tuning to eliminate low and high frequency 
vibration [7]. Nevertheless, at that time no complete 
technological solutions existed for cold SCRF tuners [8].  
So, the ideal tuner should provide simultaneously slow 
and fast tuning with high device reliability at liquid 
helium temperature.  
ELECTROMAGNETIC TUNER  
The G.W. Foster idea to use Lorentz forces between 
two superconducting coils to tune a superconducting 
resonant cavity frequency in slow and fast modes was 
realized in the electromagnetic SCRF tuner device shown 
in Fig. 1.     
 
Electromagnetic Tuner Design 
The electromagnetic tuner includes two sets of 
superconducting coils combined with a ferromagnetic 
yoke in one assembly. The tuner specification given in 
Table 1. 
 
Table 1: Electromagnetic Tuner Specifications 
Parameter Unit Value 
Maximum field in main coils T 2.1 
Maximum field in moving coil T 1.25 
Maximum field in bucking coils T 0.28 
NbTi superconductor diameter mm 0.26 
Superconductor critical current at 4.2K 
and 3 T field 
A 70 
Main coils current A 33 
Main coil number of turns  300 
Moving coil current A 30 
Moving coil number of turns  80 
Electromagnetic force kN 1.5 
 
 
Figure 1: Electromagnetic Tuner cross-section. 
 ___________________________________________  
*Work supported by Department of Energy, USA.  
#kash@fnal.gov 
 
FERMILAB-CONF-09-165-TD 
   The electromagnetic tuner connected to the SCRF cavity 
(1) through cavity LHe vessel (2) flanges by connection 
elements (7-11). The first – main set of coils (3) generates 
a stationary magnetic field. The second – moving and 
bucking coil set (4) provides the moving coil and cavity 
motion in the longitudinal direction. Bucking coils (5) and 
the iron yoke (6) eliminate the magnet system fringe field.    
Each coil set has current leads are powered from separate 
power supplies as shown in Fig. 2 
The total current of this assembly going through the 
magnet cross-section at any moment of operation is equal 
zero. It eliminates the tuner outer fringe field to an 
acceptable level for SCRF cavity operation. 
 
Figure 2: Tuner scheme. 
DC current powers the tuner main coils Lm. These coils 
produce the radial axial symmetric magnetic field in the 
magnet gap. The moving coil La when excited under 
Lorentz forces moves along z-axis. Fig. 3 shows the field 
flux lines when both coil systems are powered. The yoke 
thickness is chosen to avoid the iron saturation at peak 
currents and eliminate fringe fields in the cavity area. 
 
Figure 3: Magnet cross-section and flux lines. 
 It should be noted that the magnetic field radial 
component and corresponding Lorentz force applied to 
the moving coil is constant and does not depend on the 
moving coil position.  
TUNER TEST RESULTS 
Tuner Fabrication and Mechanical Test 
   The tuner superconducting coils were wound inside 
aluminum alloy bobbins and then vacuum impregnated 
with epoxy. The tuner was assembled within a 3.9 GHz 
cavity as shown in Fig.1.  To simplify the test, a copper 
3.9 GHz RF cavity was used.  This was a mechanical 
prototype of a third harmonic cavity [9]. 
  The mechanical and electrical properties of the cavity 
assembled with the helium vessel were investigated. This 
assembly was cooled down to a liquid Nitrogen 
temperature and the mechanical system spring constant 
(2.3 kN/mm) and the cavity resonance frequency shift 
versus cavity length coefficient (2.2 MHz/mm) were 
measured.  
 
Measurement Stand 
   Cavity-Tuner assembly has been tested at the FNAL/TD 
vertical cryogenic test stand [10]. The instrumentation 
used to test parameters of the tuner were  AC and DC 
programmable power supplies, Network Analyzer, 3.9 
GHz RF power source, an analog phase detector and 
PXI/LabView DAQ (see Fig. 4).  
 
Figure 4: Phase detection scheme. 
Tuner Slow Mode of Operation 
    In a slow mode of operation the tuner force applied to 
the cavity changes  the cavity length and corresponding 
cavity frequency proportionally to the value of currents 
going through the main and active coils (see Fig. 5).  
 
Figure 5: Cavity slow tuning (Im = 30 A, Ia = ± 37 A) . 
   Cavity resonant frequency was measured with the 
Network Analyzer connected through GPIB to PXI DAQ. 
Maximum cavity tuning applied to cavity during 
experiment was ± 650 kHz, with current on the main coil 
Im = ± 30 A and Ia = ± 35 A. To change the tune of the 
cavity within the range of ± 650 kHz, the tuner changes 
the cavity length of ± 300µm and applies a force of ± 680 
N. A saw-tooth current is applied to the active coil (Im = 
30 A) to change the cavity frequency as shown in Fig. 5. 
 
  Tuner Fast Mode of Operation 
    To study parameters of fast tuner capabilities we run the 
cavity in a continuous wave operation mode with small 
power signal from the 3.9 GHz source. Dynamic cavity 
tune effect has been measured using the standard 
technique of measuring phase shift between forward and 
transmitted power (see Fig. 4). A well established 
technique for cavity Lorentz force compensation used the 
method of exiting mechanical motion into the cavity from 
the tuner before the RF pulse in one of the mechanical 
cavity resonances [11]. We measured (see Fig. 6) the 
cavity-tuner system response to the different frequencies 
of sine wave currents used to drive the moving coil.     
 
Figure 6: Cavity-tuner system transfer function. Active 
coil exited with sine wave current drive signal. Frequency 
of drive signal swept from 20 Hz up to 700 Hz. 
 
 
Figure 7: Tuner fast mode of operation. The active coil 
current is (f = 200 Hz  and Ia = 2A), Im = 30 A . 
  One can see that the cavity-tuner system has mechanical 
resonance frequencies of 95 Hz, 130 Hz, 165 Hz, 200 Hz, 
and 550 Hz.  The response of the cavity when tuner 
(active coil) is driven with the sine wave current at       
200 Hz  (Ia = 2A, Im = 30A) is presented in Fig. 7.  The 
stroke of active coil motion is close to 100 µm. The stroke 
at least an order of magnitude larger than could be 
achieved with piezo or magnetostrictive tuner. 
 
CONCLUSION 
The novel SCRF Electromagnetic Superconducting 
Tuner was designed, fabricated, and tested at Fermilab. 
This tuner has following advantages over other tuner 
types: 
- simple configuration and low cost 
- large and linear tuning force 
- combined in one device slow and fast tuning mode of  
operation 
- reliable parts and components 
- fast response 
- regulated form of tuning force 
- very low fringe field. 
The next step in the Electromagnetic Tuner R&D will 
be to update this design for 1.3 GHz SCRF cavities used 
in most future accelerators.  
 
REFERENCES 
[1] “International Linear Collider Reference Design Report”, 
http://www.linearcollider.org/cms/?pid=1000025.  
[2] “TESLA Design Report”, Hamburg, DESY 2001-11, 2001. 
[3] "The Technical Design Report of the European XFEL", 
Hamburg, DESY 2006-097,  
        http://xfel.desy.de/tdr/tdr. 
[4]   "Project X", Fermi National Accelerator Laboratory, 
        http://projectx.fnal.gov. 
[5]   C. Joshi, et. al., "Progress in the Development of SRF 
Cavity Tuners Based on Magnetic Smart  Materials", PAC 
2001, Chicago, p. 487. 
[6]   A. Mavanur, et. al., "Magnetostrictive Tuners for SCR 
Cavities", PAC 2003, p.1407. 
[7]   A. Mavanur, et. al., "Magnetostrictive Fast Tuners for SRF 
Cavities", IEEE Trans. on Applied Superconductivity, 
VOL. 15, NO. 2, June 2005, p.2405. 
[8]   C. Pagani, et. al., "Improvement of the Blade Tuner Design 
for Superconducting RF Cavities", PAC 2005, Knoxville, 
Tennnessee, p. 3456. 
[9] N. Solyak, et. al., "The progress in Developing 
Superconducting Third Harmonic Cavity", MOPLS104, 
EPAC 2006, Edinburgh, Scotland. 
[10] R. Rabehl et al, “New Facility for Testing LHC HTS Power 
Leads” IEEE Trans. Appl. Supercond. 15:1504-1507, 2005 
[11] M. Liepe, et al “Dynamic Lorentz Force Compensation with 
Fast Piezoelectric Tuner”, PAC2001, Chicago, USA, pp 
1074-1076 
 
 


ELECTROMAGNETIC SCRF CAVITY TUNER*

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ELECTROMAGNETIC SCRF CAVITY TUNER*

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