Resonator for electron spin resonance experiments

A resonator for electron spin resonance experiments has two cylindrical elements of the same length arranged with their axes parallel to each other and with their end faces in the same plane so that they partly merge with each other. Devices are coupled to each of the two elements for exciting a degenerate TM.sub.010 mode and a degenerate TE.sub.111 mode. In one area of the resonator whose center is at the intersection point of the two planes of symmetry of the resonator, one of which is the intersection plane of the two elements and the other containing the axes of the two elements, the two modes have a mostly vanishing electric field and, for the most part, magnetic fields perpendicular to each other. In this area, there is, at least in one end face of the resonator, an aperture for inserting a test piece.

The invention is a resonator for electron spin resonance experiments in 
which two different wave modes of the same frequency can be excited whose 
magnetic fields build up approximately perpendicularly to each other in 
one particular area of the resonator in which the electric field 
practically vanishes, and which has devices for exciting the two modes and 
an aperture for inserting a test piece into the above-mentioned area. The 
centre of this aperture is at the intersection point of the two orthogonal 
planes of symmetry of the resonator. 
With electron spin resonance experiments, just as with nuclear spin 
resonance experiments, it is necessary to separate the field required for 
exciting the electron spin resonances from the field produced by the 
excited electron spin. This is attained by having orthogonality between 
the excitation field and the field to which the resonance signal receiver 
devices respond. Whereas orthogonal coils are used for this purpose for 
nuclear spin resonance, cavity resonators must be used for electron spin 
experiments because of the frequencies in the GHz range. These cavity 
resonators must be constructed so that they permit, at the same frequency, 
excitation of two different wave modes whose flux lines build up mostly 
perpendicularly to each other so that the desired decoupling is attained. 
In addition, there must be an area within the resonator in which the 
electric field vanishes, if possible, so that the magnetic field is as 
uniform as possible and a considerable volume of the test piece is 
subjected to an alternating magnetic field which has the same intensity at 
all points. 
A resonator well-known from U.S. Pat. No. 3,609,520 has two cavity 
resonators made up of rectangular waveguides. Their longitudinal axes 
bisect each other in the middle and the long sides of the one are parallel 
to the longitudinal edges of the other. The two cavity resonators are 
excited in the TE.sub.102 mode via windows in one of their end walls. 
In the case of this well-known resonator, the orthogonality of the magnetic 
fields is dependent, to a large degree, on the accuracy with which the two 
waveguide sections are orthogonally aligned with each other. In spite of 
this, the slightest disturbance of the symmetry can lead to the excitation 
of modes which disturb the orthogonality of the fields. Therefore, 
additional measures are necessary in order to guarantee the desired 
decoupling of the modes, i.e. the fitting of conducting bars at the limits 
of the commom cavity area (mode fences). Apart from the difficulties which 
this well-known resonator has with regard to its construction and tuning, 
it has a particular disadvantage in that one of the two waveguides lies in 
the direction of the d.c. magnetic field which has to be applied for 
electron spin resonance experiments. Therefore, this resonator can only be 
used in field magnets with very large air gaps. This again makes it 
impossible to produce a uniform magnetic field in the central area of the 
test piece of the required intensity for good resolution. 
From both U.S. Pat. No. 3,609,520 (FIG. 3) and German DE-OS No. 19 49 944 
resonators are known which consist of square waveguides so that two 
TE.sub.01n modes whose electric fields are perpendicular to each pair of 
opposite sides of the resonator can be excited in the one resonator. In 
the case of this kind of cavity resonator, the desired orthogonality of 
the modes is dependent on the walls of the cavity resonator being exactly 
perpendicular to each other. The slightest asymmetry, which can also be 
conditional on the tuning elements, causes coupling between the two modes. 
In addition, with this resonator, it is difficult to adjust the resonance 
frequency of one of the modes independently of the resonance frequency of 
the other mode, as in necessary in many experiments. This also applies if, 
in the case of the excited mode TE.sub.10n, the index n is greater than 2 
and the cavity resonator has a configuration with one or several bends in 
it, as is the case with several of the DE-OS 19 49 944 resonators. 
In contrast to this, the basis of the invention was to develop a resonator 
for electron spin resonance experiments of the type mentioned in the 
introductory paragraph so that it meets the great requirements made of 
such a resonator and, in addition, is easy to manufacture. 
This task is solved according to the invention in that the resonator has 
two cylindrical elements of the same length with their axes parallel to 
each other and with their end faces in the same plane so that they partly 
merge with each other, in that the devices for exciting the two modes are 
coupled to the two elements in such a way that they excite a degenerate 
TM.sub.010 mode and a degenerate TE.sub.111 mode in each of the elements 
and in that the aperture for inserting a test piece is located in one of 
the end faces of the resonator and of the two planes of symmetry, one is 
the intersection plane of the two elements and the other contains the axes 
of the two elements. 
The modes excited in the two elements of the resonator have an electric 
field whose axial component vanishes at the cylindrical resonator walls. 
Therefore, with the degenerate wave mode resulting from the combination of 
the two elements, the axial component of the electric field is 
approximately zero in the area of the plane of intersection also. The 
magnetic fields of the two wave modes build up vertically to each other in 
the area of the plane of intersection, with the result that, here, the 
high degree of decoupling and required uniformity of the magnetic fields 
are attained. By coupling the two cylindrical elements, an extraordinarily 
stable field configuration is obtained which is not disturbed to any 
significant extent by inserting a test piece through the aperture provided 
in this part of one of the end faces of the resonator. The manufacture of 
cylindrical resonators and combining them in an arrangement according to 
the invention presents no particular difficulties and this can be done 
with a very high degree of precision, which is of very great importance 
for the practical use of such a resonator. 
The device for exciting the degenerate TM.sub.010 mode can comprise two 
waveguides fed in phase opposition which are coupled, with the long sides 
parallel to the end faces of the resonator in the centre plane of the 
resonator, to the elements by means of windows which run parallel to the 
long sides. Feeding these parallel waveguides in phase opposition can 
easily be done by means of a decoupled branch to whose arms the two 
waveguides are connected. This arrangement makes possible not only simple 
completely symmetrical excitation of the degenerate TM.sub.010 mode, but 
at the same time guarantees compensation of the primary fields from the 
windows, because of the symmetry of the arrangement. With the well-known 
resonators, the wave modes are usually only excited at one single point of 
the resonator, with the result that the primary fields from the exciter 
element are superposed on the excited wave mode and, thus, lead to errors 
of symmetry and to disturbances. 
The device for exciting the degenerate TE.sub.111 mode is formed by a 
waveguide coupled to the two elements, with the long sides perpendicular 
to the end faces of the resonator being symmetrical to the intersection 
plane. This coupling is again completely symmetrical to the two 
cylindrical elements of the resonator, which again leads to perfect 
compensation of the primary fields. A further advantage of this 
arrangement is that the full cross-section of the waveguide arranged 
symmetrically to the intersection plane can open into the resonator. This 
brings about very strong coupling of the waveguide with the result that it 
is possible to fit this waveguide with a shorting plunger at the end away 
from the resonator with which it is possible to influence the resonance 
frequency of the resonator. The adjustment possible in this way is 
approximately .+-.5% of the average resonance frequency of the resonator. 
In order to connect or disconnect power, a further line, for example a 
coaxial line, can be connected to the waveguide by means of a coupling 
loop. It is advisable to fit a tube whose diameter is small in relation to 
the length of a free wave with the resonance frequency of the resonator to 
the end face of the resonator with the test piece aperture so that it 
surrounds the test piece aperture. This prevents high-frequency power 
discharging from the test piece aperture as a wave entering the 
electrically conducting tube is strongly damped because of the small 
diameter. In spite of these measures, however, slight disturbances in the 
symmetry of the fields in the resonator cannot be avoided. However, they 
can be compensated by having a compensating aperture concentric with the 
test piece aperture in the end face opposite the end face with the test 
piece aperture and by connecting an electrically conducting tube to this 
as well. Then, it is possible to fit tuning elements in the tube connected 
to the compensating which then permit compensation of the errors of 
symmetry caused by the insertion of a test piece. A preferred type of the 
invention has the feature that the second tube connected to the 
compensating aperture has a larger diameter than the first tube connected 
to the test piece aperture and a sliding metal bush whose inside diameter 
is the same as the diameter of the first tube is fitted in the second 
tube. Errors of symmetry caused by the insertion of the test piece, too, 
can be compensated easily by sliding along the bush.

The resonator shown in the drawing has a central block 1, containing two 
cylindrical cavities 2 and 3 indicated by the dashed lines in FIG. 1 which 
partly merge into each other and form the cylindrical elements of the 
resonator. These cavities are closed off at their ends by covers 4 and 5 
which are fastened to the block 1 by means of screws 6. The middle of one 
of the covers 4 is fitted with a tube 7 which surrounds the test piece 
aperture 8 in the cover. The centre of the test piece aperture is situated 
at the intersection point of the two planes of symmetry 9 and 10 of which 
one 9 coincides with the plane of intersection of the two elements of the 
resonator and the other 10 runs through the axes 11 of the two cavities 2 
and 3 forming the elements of the resonator. The other cover 5 has a 
compensating aperture 12 concentric with the test piece aperture 8. The 
compensating aperture has a greater diameter than the test piece aperture 
8 and is also surrounded by an electrically conducting tube 13. This tube 
13 is also fitted with a bush 14 made of electrically conducting material 
and its inside diameter is the same as that of the test piece aperture 8. 
The bush 14 can be slid along the tube 13 of the compensating aperture 12 
by means of bolts 15 which are screwed radially into the bush and drive 
axial guides 16 into the tube 13. 
Two waveguides 17 and 18 are connected to the side of the centre block 1 
which runs parallel to the plane of symmetry 10 connecting the axes 11 so 
that their long sides are parallel with the end faces of the resonator 
formed by the covers 4 and 5. In addition, their centre planes 
perpendicular to the long sides run through axes 11 of the elements of the 
resonator. These two waveguides 17 and 18 are coupled to the cavities 2 
and 3 of the resonator by windows 19 shown in FIG. 5 and which extend 
parallel to the long sides of the waveguides 17 and 18. The two waveguides 
17 and 18 are connected to the arms 19 and 20 of a modified magic T 21 
whose E arm 22 can be connected to a source applying excitation power 
whilst the H arm 23 can be terminated in a characteristic impedance 24 
shown by a broken line in FIG. 1. A further waveguide 25 is let into the 
side of the central block 1 opposite the waveguides 17 and 18 but is 
arranged so that its centre plane parallel to the long sides coincides 
with the plane of intersection 9 of the resonator. The full cross-section 
of this waveguide 25 opens into the resonator. At the end away from the 
resonator, the waveguide 25 is terminated by a shorting plunger 26 which 
can be moved, by means of a regulating screw 27, in the longitudinal 
direction of the waveguide 25. A coaxial line 28 is connected to one of 
the narrow sides of the waveguide 25 and its neutral conductor forms a 
coupling loop penetrating into the waveguide 25. At its free end, the 
coaxial line 28 can be fitted with a standard coaxial connector 30 for 
connection to a receiver. Of course, a receiver can also be fitted 
directly to the coaxial line. 
TM.sub.010 wave modes are excited in cavities 2 and 3 via waveguides 17 and 
18. As FIG. 4 shows, top left, the TM.sub.010 wave mode has closed 
circular B lines whereas the E lines run perpendicular to the end faces of 
the cavities. The electric field reaches a maximum in the centre of the 
cavity and vanishes at the cavity cylinder wall. 
As a result of feeding power into the E arm waves in phase opposition in 
the connected waveguides 17 and 18 are excited by the magic T 21 and in 
turn these waves excite waves in phase opposition in the elements of the 
resonator formed by the cavities 2 and 3 (FIG. 5). The two TM.sub.010 wave 
modes combine in that area of the resonator surrounding the intersection 
point between the two planes of symmetry 9 and 10 to form a B field whose 
flux lines run parallel to the end face of the resonator and parallel to 
the plane of intersection 9 and have a high density in this area, whereas 
the electric field in this area is approximately zero. This field extends 
thus in the system of co-ordinates shown in FIG. 4 in the direction of the 
y-axis. 
If the resonator is set up in a d.c. magnetic field B.sub.0 whose flux 
lines run parallel to the plane of symmetry 10 which cuts the two axes 11 
of the elements of the resonator, i.e. along the z-axis in FIG. 4, then, 
in a test piece in that area of the resonator that surrounds the 
intersection line between the two planes of symmetry 9 and 10, an 
alternating magnetic field will be excited extending along this 
intersection line or the x-axis shown in FIG. 4. Two TE.sub.111 wave modes 
are excited in the cylindrical elements of the resonator by this and the 
behaviour of their flux lines is shown in FIG. 4, top right. This wave 
mode has magnetic flux lines which have a component parallel to the 
cylinder wall, whereas the electric field mostly runs parallel to the 
diameter of the resonator. The electric field vanishes at those points on 
the periphery of the resonator where the magnetic flux lines reaches a 
maximum. The design of the resonator according to the invention 
necessitates alignment of the TE.sub.11 wave modes so that the magnetic 
field is at its maximum in the area surrounding the x-axis, whilst the 
electric field vanishes in this area. As shown in FIG. 6, this field is 
decoupled from the resonator with the aid of the waveguide 25, whose long 
sides are perpendicular to the covers 4 and 5 of the resonator, i.e. run 
parallel to the x-axis with the result that the magnetic flux lines of the 
TE.sub.111 mode run parallel to the long sides of the waveguide 25, as do 
the magnetic flux lines 1 of the TE.sub.10 mode excited in the waveguide. 
The whole cross-section of the waveguide 25 with its long side 
perpendicular to the covers 4 and 5 opens into the resonator. In this way, 
extremely strong coupling of the waveguide to the resonator is achieved 
and, consequently, the impedance of the waveguide at the transition point 
with the resonator influences the resonance frequency of the resonator in 
the TE.sub.111 mode. This fact is used in the example given to change the 
resonance frequency in the TE.sub.111 mode by shifting the short-circuit 
26 fitted in the waveguide 25. In the case of the types of resonators 
constructed according to the invention, it was possible to alter the 
resonance frequency by .+-.5% of the average resonance frequency. 
Therefore, it is possible to establish frequency coincidence of the 
excitation and detection modes, which otherwise could be disturbed by 
insertion of the test piece, particularly if a quartz Dewar is used for 
tempering the test piece. 
As shown in FIG. 4, the test piece is inserted into the resonator along the 
x-axis. The test piece aperture 8 is provided in the cover 4 for this 
purpose. The conducting tube 7 surrounding the test piece aperture 8 
prevents any power from radiating from this aperture. It has been 
discovered, however, that the arrangement of the test piece aperture can 
disturb the symmetry of the modes in the resonator. These disturbances 
can, however, be compensated by a compensating aperture 12 in the opposite 
side. At the same time, the effect of this aperture in the end face of the 
resonator can be used to remove disturbances in the electrical 
orthogonality caused by asymmetries in the manufacturing process. The bush 
14 fitted in the tube 13 in the compensating opening 12 for this purpose 
just has to be simply moved to bring about optimum decoupling of the 
excitation and detection sides. 
In the case of one of the types of resonator constructed according to the 
invention, the cylindrical cavities 2 and 3 had a diameter of 
approximately 24 mm. The height of the cavities was also approximately 24 
mm. With the TM.sub.010 and TE.sub.111 modes, the resonator had an average 
resonance frequency of approximately 9.5 GHz. Using the shorting plunger 
26, it was possible to change the frequency of the resonance of the 
TE.sub.111 wave mode by more than 1 GHz. With critical coupling the 
resonator had an energy factor of approximately 3.times.10.sup.3. Even if 
non-cylindrical solid-state test pieces were inserted and quartz Dewars 
used, reproducible decoupling values of -80 to -88 dB were obtained. The 
measured limit for decoupling was around -90 dB which was because of the 
high radio interference level of the microwave bridge used for the 
measurements and which could not be easily removed. Particularly important 
is also the fact that using the resonator according to the invention also 
permits a reduction of the signal/noise ratio of the recorded spectrum by 
1 to 2 orders of magnitude. 
Summarizing, it should be stated that the invention has created a resonator 
which permits reproducible decoupling of the detection side from the 
excitation side around -85 dB to -90 dB. One of the reasons for this is 
that not only the excited modes but also the primary fields used to excite 
these modes are compensated independently of the frequency. 
Exact agreement between the resonance of the excitation field and the 
resonance of the detecting field is made possible by just the one existing 
control element--the shorting plunger. 
There is also just one compensating element with which residual errors of 
symmetry resulting from the manufacture of the test head or caused by 
insertion of the test piece to be examined can be removed. 
The area available for exciting the test piece where there is a practically 
uniform field is big enough to accommodate test piece of considerable size 
and devices for tempering the test piece. 
The resonator according to the invention takes up only the minimum amount 
of space so that if typical iron magnets are used to produce the d.c. 
magnetic field an air gap 50 mm wide is adequate, such as is available 
with conventional magnets. 
The relatively low energy factor of the resonator makes it possible to 
carry out pulse experiments without any signal falsification caused by 
long rise times of the resonator. In spite of this, a spectrometer fitted 
with this resonator has a high degree of sensitivity because the resonator 
permits the insertion of relatively large test pieces (space factor 
.eta.).