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Rf contact for superconducting resonators
NUCLEAR
INSTRUMENTS
AND
METHODS
]4I (I977) 57-59;
©
NORTH-HOLLAND
PUBLISHING
CO.
RF CONTACT FOR S U P E R C O N D U C T I N G RESONATORS M. G R U N D N E R ,
H. L E N G E L E R *
a n d E. R A T H G E B E R
Kernforschungszentrum Karlsruhe, lnstitut fiir Experimentelle Kernphysik, 7500 Karlsruhe, Postfach 3640, W. Germany Received 28 October 1976 A n rf contact used for assembling s u p e r c o n d u c t i n g resonators f r o m different sections is described. Its p e r f o r m a n c e s with respect to quality factors, peak electric a n d m a g n e t i c fields a n d v a c u u m - t i g h t n e s s against He-lI are described.
1. I n t r o d u c t i o n
At Karlsruhe a superconducting rf particle separator is currently under construction which will be used for particle separation at CERN, Geneva1). Its two deflectors are uniform periodic waveguides and are operated in a rc/2-standing wave mode at S-band (2865 MHz). Each deflector has a length of 2.74 m and contains 104 cells (fig. 1) which are machined out of solid Nb and electron beam welded around their outer circumference. In order to reach the design values for the Q-factor (Qo = 5 x 108) and the deflection field (E o = 2 MV/m corresponding to magnetic and electric peak fields of 31 rnT and 11 MV/m resp.) the deflectors are submitted to a high temperature treatment at UHV-conditions. As the UHV-furnace available allows only annealing of sections with a length of about 60 cm we decided to assemble each deflector from five sections and we are therefore faced with the problem of rf contacts between sections. For the deflection mode the peak fields are located at the iris roundings, therefore it was decided to place the rf contacts at the outer diameter.
a) the joint-cell is made "field-free", i.e. the rf contact is not exposed to currents and fields; in this case a low quality rfjoint is possible, b) one allows fields and currents in the joint cell up to a given level and develops an rf joint supporting these currents and fields. The first possibility has been applied, for instance, at the superconducting electron linac developed at HEPL, Stanford2). The fields at the joints are kept below 1% of the maximum field in the field full cells and a simple indium joint between two Nb-contact surfaces provides a satisfactory rf contact. Although our working mode (n/2-mode) also allows this possibility because every second cell is in principle field-free Indium
/ N b -joint
h= 26,25
ol ~/A
,42¢1 s°~-
* On leave o f absence from C E R N .
2b : IZZ~
ct
0
~
g
,YX/3
~
~elding'
E
~d
E
t
~,
2 . R e q u i r e m e n t s for r f c o n t a c t s in s u p e r c o n d u c t i n g resonators
Ideally an rf contact (joint) for a superconducting resonator should fulfill the following requirements: it should support for the working mode sufficiently high field levels without deterioration of the Q-values; it must be UHV-tight at room temperature and at He-temperature (at our working temperature 1.8 K, this means vacuum-tightness against superfluid He-ll); its properties should not be affected by repeated cooling cycles; its mounting should be easy and an exchange should be possible. For the choice of the rf contacts in a periodically loaded waveguide one has two possibilities.
v
~:oolmg channel
0 ~c
Fig. 1. (a) G e o m e t r y o f a n o r m a l cell and a joint cell for the diskloaded deflector. Some field lines o f the deflecting m o d e are indicated. (b) Schematic layout o f one deflector assembled f r o m five sections. A: rf contact, B: rf coupling, C: rf probing, D, E: frequency tuner, F: m a t c h e d b e a m tube.
58
M. GRUNDNER et al.
we have tried to develop an rfjoint withstanding high fields and currents. The following arguments led to this choice. In a periodically loaded resonator with N cells, N + I different modes can be excited in a given frequency band and with the help of the measured dependence of the Q-factor on the different modes one can localize cells with an increased surface resistance3). Each of the N + I modes has a characteristic field distribution and the fields inside the joint cells are normally different from zero. Therefore, for the application of the cell localization method a high quality joint supporting non-zero field and current levels is necessary because otherwise the Q-mode dependence would be dominated by the rfjoint. There exists a possibility for separating at low momenta particles with a single deflector (one cavity separation) but one has to work in a mode different from the n/2-mode4). In this case the joint cells are exposed to nearly the maximum field level, and an rf joint withstanding such high field levels is necessary.
3. Therfcontact For the development of an rf contact we have always tried to separate the functions of rf contacting and vacuum-tightness. Tests have been performed with carefully machined and flat Nb surfaces, Nb surfaces of different shapes (triangular and rounded), anodised Nb surfaces (i.e. Nb surfaces covered with ~ 1000,~ of Nb2Os) which were clamped together. Also specially shaped Pb and N b rings have been tried out. The different joints either did not give sufficiently high Q-factors or produced grooves on the contact surfaces
I
3dur
Fig. 2. Rf contact and clamping system.
In-gr°°ve~.1 Nb-ring
i i
es~.~ ~
........ ~o.s
Fig. 3. Nb joint ring for the superconducting deflectors. Two holes are drilled across the "lips" in order to avoid air pockets behind the contact surfaces. Dimensions in ram.
necessitating a new machining after each application. Finally a Nb ring with specially shaped "lips" (fig. 2) was developed such that the clamping produces a slight shearing movement between the lips and the Nbcontact surfaces. This movement tears the oxyde layer of the Nb surfaces and insures a good metallic contact. A metallographic study on dismantled rf connections showed that a clamping force producing grooves with a depth smaller than 0.06ram gave satisfactory results. In this case the ridges of the lathing grooves were slightly bent and the joint region showed many point-like contacts. Vacuum-tightness is obtained with the help of two indium wires of 1.5 mm thickness wich are situated at a slightly bigger diameter. The clamping system is shown in fig. 2. It is split up into four parts in order to insure a uniform clamping force and in order to avoid a tilting of the clamping rings. The M6 bolts are tightened in two steps with a one-day interval and with a typical torque of 1 0 0 k p ' c m . After final clamping the lips of the Nb ring are compressed by 0.55 mm. The height of the Nb-ring is fixed by the condition that the joint cell has to have the same height as the normal (welded) cells. A special problem arises in the case of the deflecting mode. Being a dipole mode it has to be stabilized in azimuth. This is obtained by giving the cells a slightly elliptical cross-section (fig. l). As for manufacturing reasons the contact ring is made circular and as one has to choose its inner diameter in such a way that the lips are not protruding anywhere inside the cell region the joint causes a frequency shift of the joint cell. A computation shows that this shift, which is about - 13 M H z for every joint cell, has a negligible influence on the field distribution inside the deflector.
RF CONTACT
4. Results Rf joints of the new type have been used by now successfully in numerous cold experiments on deflector sections and test-cavitiesS"6). During a cold test on two combined sections this type of joint has successfully supported three cooling cycles followed by a 430 b test run at design field without showing any deterioration of the Qo-factor and the peak fields. No increase in electron intensity due to multipacting or field emission was experienced and no vacuum leak against the He-li bath surrounding the deflector was observed. We have also tested that the same joint can be used twice after the lips are reopened mechanically to their initial width. In one experiment three joints with identical dimensions could be used successively at the same location without a remachining of the contact surfaces. The field level at the joint cells is influenced by frequency differences between sections. A computer program has been written which allows to calculate the field level at the joint cell produced by such frequency errors, The results of these computations were checked by perturbation measurements and applied to the joint measurements. It was shown that the design value for the Q-factors and the peak fields can be reached reliably with the new type of joint up to field levels at the joint cell of at least 22%. We are able to tune the different sections within _+ 100 kHz to the same frequency. This corresponds to a field level at the joint cell below 10% and thus allows a reliable
59
operation in the working mode. The Q-mode distribution method for localizing cells with increased surface resistance has been applied successfully many times while one or more rf joints were mounted in the deflectors. For field levels at the joint above 60% (of E 0 = 2 MV/m) the design values can no longer be reached safely, Therefore, one may be forced to reduce the field levels for one cavity operation. We would like to thank Mr. R. Dittmann, Mr. W. Barth, Mr. Koppitz and Mr. Schaufelberger for their help in the design and fabrication of the rfjoints, Dr. G. Dammertz for various computations concerning the field distributions in multicelT resonators. References 1) H. Lengeler, W. Bauer, A. Citron, G. Dammertz, M. Grundner and E. Rathgeber, Proc. 1973 Int. Conf. on Instrumentation for high energy physics, Frascati (1973) p. 716. z) M.S. McAshan, H.A. Schwettmann, U Suelzle and J. P. Turneaure, HEPL-report, 665 Stanford (1972). 3) G. Dammertz, L. Husson, H. Lengeler and E. Rathgeber, Nucl. Instr. and Meth. 118 (1974) 141. "~) P, Bernard, H. Lengeler and J. CI. Prelaz, Proc. Int. Conf, on High energy accelerators, CERN (1971) p. 269. 5) W. Bauer, A. Citron, G. Dammertz, M. Grundner, L. Husson, H. Lengeler and E. Rathgeber, Proc. 1975 Particle Accelerator Conf., Washington, IEEE Trans. Nucl. Sci. NS-22, no. 3, (1975) 1144. 6) A. Citron, G. Dammertz, M. Grundner, L. Husson, P. Kneisel, H. Lengeler and E. Rathgeber, presented at the Applied Superconductivity Conf. Stanford (1976).
INSTRUMENTS
AND
METHODS
]4I (I977) 57-59;
©
NORTH-HOLLAND
PUBLISHING
CO.
RF CONTACT FOR S U P E R C O N D U C T I N G RESONATORS M. G R U N D N E R ,
H. L E N G E L E R *
a n d E. R A T H G E B E R
Kernforschungszentrum Karlsruhe, lnstitut fiir Experimentelle Kernphysik, 7500 Karlsruhe, Postfach 3640, W. Germany Received 28 October 1976 A n rf contact used for assembling s u p e r c o n d u c t i n g resonators f r o m different sections is described. Its p e r f o r m a n c e s with respect to quality factors, peak electric a n d m a g n e t i c fields a n d v a c u u m - t i g h t n e s s against He-lI are described.
1. I n t r o d u c t i o n
At Karlsruhe a superconducting rf particle separator is currently under construction which will be used for particle separation at CERN, Geneva1). Its two deflectors are uniform periodic waveguides and are operated in a rc/2-standing wave mode at S-band (2865 MHz). Each deflector has a length of 2.74 m and contains 104 cells (fig. 1) which are machined out of solid Nb and electron beam welded around their outer circumference. In order to reach the design values for the Q-factor (Qo = 5 x 108) and the deflection field (E o = 2 MV/m corresponding to magnetic and electric peak fields of 31 rnT and 11 MV/m resp.) the deflectors are submitted to a high temperature treatment at UHV-conditions. As the UHV-furnace available allows only annealing of sections with a length of about 60 cm we decided to assemble each deflector from five sections and we are therefore faced with the problem of rf contacts between sections. For the deflection mode the peak fields are located at the iris roundings, therefore it was decided to place the rf contacts at the outer diameter.
a) the joint-cell is made "field-free", i.e. the rf contact is not exposed to currents and fields; in this case a low quality rfjoint is possible, b) one allows fields and currents in the joint cell up to a given level and develops an rf joint supporting these currents and fields. The first possibility has been applied, for instance, at the superconducting electron linac developed at HEPL, Stanford2). The fields at the joints are kept below 1% of the maximum field in the field full cells and a simple indium joint between two Nb-contact surfaces provides a satisfactory rf contact. Although our working mode (n/2-mode) also allows this possibility because every second cell is in principle field-free Indium
/ N b -joint
h= 26,25
ol ~/A
,42¢1 s°~-
* On leave o f absence from C E R N .
2b : IZZ~
ct
0
~
g
,YX/3
~
~elding'
E
~d
E
t
~,
2 . R e q u i r e m e n t s for r f c o n t a c t s in s u p e r c o n d u c t i n g resonators
Ideally an rf contact (joint) for a superconducting resonator should fulfill the following requirements: it should support for the working mode sufficiently high field levels without deterioration of the Q-values; it must be UHV-tight at room temperature and at He-temperature (at our working temperature 1.8 K, this means vacuum-tightness against superfluid He-ll); its properties should not be affected by repeated cooling cycles; its mounting should be easy and an exchange should be possible. For the choice of the rf contacts in a periodically loaded waveguide one has two possibilities.
v
~:oolmg channel
0 ~c
Fig. 1. (a) G e o m e t r y o f a n o r m a l cell and a joint cell for the diskloaded deflector. Some field lines o f the deflecting m o d e are indicated. (b) Schematic layout o f one deflector assembled f r o m five sections. A: rf contact, B: rf coupling, C: rf probing, D, E: frequency tuner, F: m a t c h e d b e a m tube.
58
M. GRUNDNER et al.
we have tried to develop an rfjoint withstanding high fields and currents. The following arguments led to this choice. In a periodically loaded resonator with N cells, N + I different modes can be excited in a given frequency band and with the help of the measured dependence of the Q-factor on the different modes one can localize cells with an increased surface resistance3). Each of the N + I modes has a characteristic field distribution and the fields inside the joint cells are normally different from zero. Therefore, for the application of the cell localization method a high quality joint supporting non-zero field and current levels is necessary because otherwise the Q-mode dependence would be dominated by the rfjoint. There exists a possibility for separating at low momenta particles with a single deflector (one cavity separation) but one has to work in a mode different from the n/2-mode4). In this case the joint cells are exposed to nearly the maximum field level, and an rf joint withstanding such high field levels is necessary.
3. Therfcontact For the development of an rf contact we have always tried to separate the functions of rf contacting and vacuum-tightness. Tests have been performed with carefully machined and flat Nb surfaces, Nb surfaces of different shapes (triangular and rounded), anodised Nb surfaces (i.e. Nb surfaces covered with ~ 1000,~ of Nb2Os) which were clamped together. Also specially shaped Pb and N b rings have been tried out. The different joints either did not give sufficiently high Q-factors or produced grooves on the contact surfaces
I
3dur
Fig. 2. Rf contact and clamping system.
In-gr°°ve~.1 Nb-ring
i i
es~.~ ~
........ ~o.s
Fig. 3. Nb joint ring for the superconducting deflectors. Two holes are drilled across the "lips" in order to avoid air pockets behind the contact surfaces. Dimensions in ram.
necessitating a new machining after each application. Finally a Nb ring with specially shaped "lips" (fig. 2) was developed such that the clamping produces a slight shearing movement between the lips and the Nbcontact surfaces. This movement tears the oxyde layer of the Nb surfaces and insures a good metallic contact. A metallographic study on dismantled rf connections showed that a clamping force producing grooves with a depth smaller than 0.06ram gave satisfactory results. In this case the ridges of the lathing grooves were slightly bent and the joint region showed many point-like contacts. Vacuum-tightness is obtained with the help of two indium wires of 1.5 mm thickness wich are situated at a slightly bigger diameter. The clamping system is shown in fig. 2. It is split up into four parts in order to insure a uniform clamping force and in order to avoid a tilting of the clamping rings. The M6 bolts are tightened in two steps with a one-day interval and with a typical torque of 1 0 0 k p ' c m . After final clamping the lips of the Nb ring are compressed by 0.55 mm. The height of the Nb-ring is fixed by the condition that the joint cell has to have the same height as the normal (welded) cells. A special problem arises in the case of the deflecting mode. Being a dipole mode it has to be stabilized in azimuth. This is obtained by giving the cells a slightly elliptical cross-section (fig. l). As for manufacturing reasons the contact ring is made circular and as one has to choose its inner diameter in such a way that the lips are not protruding anywhere inside the cell region the joint causes a frequency shift of the joint cell. A computation shows that this shift, which is about - 13 M H z for every joint cell, has a negligible influence on the field distribution inside the deflector.
RF CONTACT
4. Results Rf joints of the new type have been used by now successfully in numerous cold experiments on deflector sections and test-cavitiesS"6). During a cold test on two combined sections this type of joint has successfully supported three cooling cycles followed by a 430 b test run at design field without showing any deterioration of the Qo-factor and the peak fields. No increase in electron intensity due to multipacting or field emission was experienced and no vacuum leak against the He-li bath surrounding the deflector was observed. We have also tested that the same joint can be used twice after the lips are reopened mechanically to their initial width. In one experiment three joints with identical dimensions could be used successively at the same location without a remachining of the contact surfaces. The field level at the joint cells is influenced by frequency differences between sections. A computer program has been written which allows to calculate the field level at the joint cell produced by such frequency errors, The results of these computations were checked by perturbation measurements and applied to the joint measurements. It was shown that the design value for the Q-factors and the peak fields can be reached reliably with the new type of joint up to field levels at the joint cell of at least 22%. We are able to tune the different sections within _+ 100 kHz to the same frequency. This corresponds to a field level at the joint cell below 10% and thus allows a reliable
59
operation in the working mode. The Q-mode distribution method for localizing cells with increased surface resistance has been applied successfully many times while one or more rf joints were mounted in the deflectors. For field levels at the joint above 60% (of E 0 = 2 MV/m) the design values can no longer be reached safely, Therefore, one may be forced to reduce the field levels for one cavity operation. We would like to thank Mr. R. Dittmann, Mr. W. Barth, Mr. Koppitz and Mr. Schaufelberger for their help in the design and fabrication of the rfjoints, Dr. G. Dammertz for various computations concerning the field distributions in multicelT resonators. References 1) H. Lengeler, W. Bauer, A. Citron, G. Dammertz, M. Grundner and E. Rathgeber, Proc. 1973 Int. Conf. on Instrumentation for high energy physics, Frascati (1973) p. 716. z) M.S. McAshan, H.A. Schwettmann, U Suelzle and J. P. Turneaure, HEPL-report, 665 Stanford (1972). 3) G. Dammertz, L. Husson, H. Lengeler and E. Rathgeber, Nucl. Instr. and Meth. 118 (1974) 141. "~) P, Bernard, H. Lengeler and J. CI. Prelaz, Proc. Int. Conf, on High energy accelerators, CERN (1971) p. 269. 5) W. Bauer, A. Citron, G. Dammertz, M. Grundner, L. Husson, H. Lengeler and E. Rathgeber, Proc. 1975 Particle Accelerator Conf., Washington, IEEE Trans. Nucl. Sci. NS-22, no. 3, (1975) 1144. 6) A. Citron, G. Dammertz, M. Grundner, L. Husson, P. Kneisel, H. Lengeler and E. Rathgeber, presented at the Applied Superconductivity Conf. Stanford (1976).