Cavity resonator

A cavity resonator serves to generate magnetic dipole transitions in a sample, for instance for carrying out electron spin resonance measurements. The natural frequency of the cavity resonator (10) is influenced by a reduction in length of the E flux lines. To this end, conductive areas (20) are arranged at the points where the E flux lines are encountered. This results in an increase of the space factor, related to an unchanged specimen volume, and thus in an improvement of the measuring sensitivity.

BACKGROUND OF THE INVENTION 
The present invention starts out from a cavity resonator for generating 
magnetic dipole junctions in a test specimen, the test specimen being 
located within a resonator area exhibiting a high density of magnetic flux 
lines and the cavity resonator being excited by an oscillation mode with 
closed electric field lines. 
In analytic measuring techniques, a number of methods have been known for 
investigating the properties of materials by exciting magnetic dipole 
junctions in test specimens of the said materials. The most widely used 
methods of this type are the nuclear magnetic resonance (NMR) and the 
electron spin resonance (ESR) methods. 
For generating one of these resonance phenomena it is necessary that the 
test specimen to be investigated be simultaneously exposed to a radio 
frequency field and a constant magnetic field directed vertically thereto. 
The test specimen is introduced into a system capable of resonating and 
positioned at a point where magnetic flux lines (H) propagate at a high 
density or intensity. 
FIELD OF THE INVENTION 
There have been known from the literature resonant systems of the most 
different configurations, for use with different frequency ranges. As is 
generally known, such resonant systems may consist of a connection of 
individual discrete components, the most widely employed connection of 
this type being the series resonant circuit or the parallel resonant 
circuit comprising one capacitor and one coil each. In such arrangements 
it is possible to achieve a clear distribution of the capacitive and/or 
conductive components of the resonant system, with respect to the 
structure. In the higher frequency ranges, such as the very-high frequency 
range, it has been known to use resonant arrangements designated for 
instance as line resonators, strip lines, or the like. These resonant 
arrangements also permit the capacitive and inductive portions to be 
clearly distributed by giving specific geometric areas of the arrangement 
the capacitive or inductive property, respectively, of the resonant 
arrangement provided, however, that such areas must be clearly determined 
for different frequencies and geometrically definable. Finally it has been 
known in connection with super-high frequencies, for instance in the 
microwave range, to use cavity resonators as resonant systems. These 
cavity resonators distinguish themselves from the two structures described 
before in that there exists a fixed integral relation between the 
dimensions of the resonator and the employed wave length. In addition, it 
is no longer possible to obtain a clear distribution of the inductive and 
capacitive areas of the resonant system as the system may be excited in 
different modes, different modes of the first magnitude being possible in 
the same cavity resonator. In a cylindrical cavity resonator, for example, 
a H.sub.011 or a H.sub.111 oscillation may be excited independently of 
each other, and in addition higher magnitudes of the two modes are also 
possible. However, the capacitively or inductively active areas vary 
depending on the node of oscillation, the magnitude and the frequency 
employed. 
It is a common problem in such investigations using magnetic resonance to 
give the measuring instruments used the highest possible sensitivity. The 
sensitivity achievable is proportional to the product of the quality of 
the resonant system used by the space factor, i.e. the portion of the 
volume included by the resonant system which is occupied by the test 
specimen to be investigated. However, the test specimen arranged within a 
resonant system cannot be increased in size at desire to increase the 
space factor because it would then get into the zones of the electric 
field lines (E) which would again lead to losses and, thus, to a loss in 
quality. 
From an article by Froncisz and Hyde entitled "The Loop-Gap Resonator: A 
New Microwave Lumped Circuit ESR Sample Structure" published in the 
Journal of Magnetic Resonance, volume 47, pages 515 to 521 (1982) a 
resonant structure for carrying out electron spin resonance experiments 
has been known which is based on the before-described mechanism of the 
strip line. For, this known arrangement uses two barrel-type half shells 
separated by two narrow slots. The inductive portion of this structure is 
formed by the half shells while the capacitive portion is formed by the 
slots. However, it is a drawback of this known structure that the geometry 
of the half shells and the slots determine the resonance frequency of the 
structure so that highest demands must be placed upon the production of 
this structure. The slot widths used in such structures are considerably 
smaller than 1 mm which gives rise to problems not only as regards the 
exact positioning of the half shells, but also as regards the defined 
surface treatment of this arrangement, for instance by galvanizing. 
Considering that such components are usually galvanized in almost 
completely assembled state so as to obtain the best possible conditions 
for optimum reproducibility, it is not possible in practice to obtain 
perfect galvanization results in the gaps between the two half shells if 
the distance between the latter is considerably smaller than 1 mm. On the 
other hand, however, the geometry of the slot is of extraordinary 
importance as regards the developing resonance frequency. 
Now, it is the object of the present invention to provide a resonant 
structure for generating magnetic dipole junctions in a test specimen 
which on the one hand offers a high product of quality by space factor, 
and on the other hand can be produced in a reproducible manner by the 
usual methods. 
SUMMARY OF THE INVENTION 
According to the present invention, this object is achieved in that the 
cavity resonator comprises electrically conductive means in such an 
arrangement that electric flux lines (E) are short-circuited at least over 
part of their length. So, contrary to the known arrangement described 
before, the element determining the basic frequency in the cavity 
resonator of the invention consists of a usual, for instance cylindrical, 
cavity resonator in which the electric flux lines are "shortened" to 
reduce its natural frequency. Such shortening of the electric flux lines, 
or reduction of the natural frequency of the cavity resonator leads on the 
other hand to a considerably higher space factor as the large-volume 
resonators normally required for low frequencies can now be considerably 
smaller in size so that an unchanged volume of the test specimen will 
result in a considerably higher space factor. This is a particular 
advantage in cases where only small quantities of the matter to be 
investigated are available so that if such small quantities were employed 
in combination with a large-volume resonator extremely unfavorable space 
factors would be obtained. 
Considering that the conductive means used for shortening the electric flux 
lines provoke only a relative change of the natural frequency of the 
resonator, the exact positioning of such conductive means is not as 
critical in this arrangement as in the known arrangement in which the 
basic frequency is determined by the employed strip lines themselves. 
A preferred improvement of the invention uses a cylindrical cavity 
resonator of the oscillation mode H.sub.01n, wherein "n" is an integer. 
This known mode of oscillation exhibits a maximum of H lines and a minimum 
of E lines in the cylinder axis. So, this resonator arrangement is 
particularly suited for carrying out a great number of different 
experiments, for instance such in which the test specimen is 
simultaneously heated or exposed to rays of the most different types. 
A particularly advantageous effect is obtained in this arrangement by the 
fact that shortening of the electric field lines is achieved by a toroidal 
structure in which electrically conductive and non-conductive areas are 
provided in alternate arrangement. So, the E flux lines can be shortened 
and the natural frequency of the resonator can be reduced at desire within 
a very broad range by a corresponding adjustment of the percentage of the 
conductive areas relative to the total circumference of the torus. 
A particularly advantageous arrangement is achieved if the electrically 
non-conductive sectors of the torus are designed to give low dielectric 
losses, for instance made of quartz. This latter material is one which 
offers particularly low electric losses so that its presence in an area of 
high E line density can be regarded as uncritical. 
In a further preferred improvement of the invention, the torus is formed in 
part by barrel-shaped conductive films separated by slots. The slots 
define the area of the remaining E lines as the conductive films carry 
only currents. So, the natural frequency of the resonator can be adjusted 
by corresponding adjustment of the slot widths. 
A particularly advantageous effect can be achieved in this arrangement if 
the before-mentioned slots are covered up by additional barrel-shaped 
conductive films, because in this case the reduction of the natural 
frequency of the cavity resonator is no longer exclusively determined by 
the width of the slots in which non-homogeneous electric fields develop; 
rather, this arrangement creates additional areas between the conductive 
films in which homogeneous E lines develop so that the natural frequency 
can be varied in an easily reproducible manner by changing the width of 
the overlapping areas of the conductive films. 
In a further improvement of the invention, the continuous barrel-shaped 
electrically conductive films are replaced by several sections extending 
in the axial direction of the cavity resonator, and the additional 
conductive films covering up the slots are preferably formed as one piece. 
As a result of this arrangement, the currents flow in the conductive films 
on the one hand in circular direction, i.e. in the neighboring areas, and 
on the other hand in the axial direction, i.e. via the additional, 
preferably one-piece conductive films. This physical distribution of the 
currents flowing in conductive films provides an additional means for 
varying the frequency in a reproducible manner. 
If the non-conductive areas in the toroidal arrangement or arrangements are 
made of quartz glass, at least one of the said quartz sectors may, in a 
preferred improvement of the invention, be extended in length, for 
supporting the torus arrangement on an end face of the resonator. 
Alternatively, it is of course also possible to arrange the sectors of the 
torus on a quartz tube which extends right to an end face of the cavity 
resonator. 
If measurements are to be carried out on test specimens which are to be 
heated simultaneously, a particularly favorable effect is achieved if the 
said conductive films are provided on the inner and outer faces of a 
multi-walled Dewar of the type known in connection with measurements 
carried out by means of magnetic resonance and at varying ambient 
temperatures. 
Other advantages of the invention will become apparent from the 
specification and the attached drawing which show and describe certain 
examples of embodiments of the invention.

DETAILED DESCRIPTION OF THE DRAWING 
In FIG. 1, a cylindrical cavity resonator 10 can be seen which comprises an 
opening 11 for a test specimen 12. The opening 11 is arranged in an end 
face 13 of the resonator 10. The shell of the resonator 10 is designated 
by 14. 
The cylindrical cavity resonator shown in FIG. 1 oscillates in the 
H.sub.011 mode, which means that the magnetic flux lines (H) are axially 
directed along the cylinder axis Z and the shell 14, to close in the 
radial direction in the areas of the end faces 13. In contrast, the 
electric flux lines (E) extend circularly around the axis Z of the 
resonator 10. The density distribution of the E flux lines over the axis Z 
is approximately sinoidal which means that the electric field strength is 
equal to 0 in the area of the end faces 13, and reaches its maximum at 
half the height of the resonator. 
Consequently, the test specimen 12 is located in area where the H flux 
lines reaches their maximum, but where the electric field strengths are 
relatively low. 
In the first example shown in FIG. 2, a toroidal arrangement of barrel 
shape is arranged within the resonator 10. The said arrangement is 
subdivided into conductive sectors 20 and non-conductive sectors, for 
instance quartz sectors, 21. In the example shown in FIG. 2, four 
conductive sectors 20 and four quartz sectors 21, all of equal width, are 
provided. One of the said quartz sectors 21 is extended in length right to 
an end face 13 of the resonator 10, for mounting the torus on the said end 
face. 
However, it goes without saying that this type of mounting has been 
described by way of example only and that other means, for instance 
several such extensions or a common quartz tube, may serve the same 
purpose. 
By the arrangement shown in FIG. 2, the E lines are short-circuited in the 
areas of the conductive sectors 20 so that they can propagate only in the 
quartz sectors 21 in the manner indicated in FIG. 1. As a result, the E 
lines are shortened by half their length in the example shown in FIG. 2. 
This reduction in length of the E lines in an area of high intensity 
results in a reduction of the natural frequency of the cavity resonator 
10. Thus, a reduction in size of the resonator by a factor of, for 
example, 5 or 10 can be achieved as compared with the usual resonator 
without such an arrangement, which in turn increases the space factor 
correspondingly without any change of the volume of the test specimen 12. 
In the example shown in FIG. 2, D designates the diameter of the resonator 
10 while d designates the diameter of the toroidal arrangement. The 
overall length of the cavity resonator 10 is designated by L, that of the 
torus by 1. In a typical test setup, the following approximate dimensions 
were used. 
D=20 to 40 mm 
d=5 to 15 mm 
L=20 to 40 mm 
l=5 to 10 mm 
These dimensions are, however, to be understood as examples only which 
relate only to specific frequency ranges. They are merely meant to 
illustrate the relationship between the dimensions as they exist in 
certain test setups and do not in any way restrict the scope and 
importance of the present invention. 
In the second example shown in FIGS. 3a and b, the toroidal arrangement for 
shortening the E flux lines comprises two conductive films 30, 31 
separated by slots 32, 33. The conductive films 30, 31 have the shape of 
half a cylinder shell. The width of the slots 32, 33 is designated by 
.delta.. As related to the dimensions given above by way of example, 
.delta. may be equal to 1 mm. 
In the example shown in FIG. 3, two conductive films 30, 31 with two slots 
32, 33 are used only, but it is of course also possible to use more film 
sectors with a correspondingly higher number of slots, or else only one 
film which is then almost fully closed, except for a single slot. 
FIG. 3b illustrates the E flux lines encountered in one of the slots 32, 
33. Due to the finite thickness of the conductive films 30, 31 and the 
before-mentioned width .delta., inhomogenous E flux lines, for instance in 
the form indicated in the drawing, are obtained. If extremely high demands 
are placed on the reproducibility of this section, this may give rise to 
problems as any variations of the geometry of the slot will lead to 
variations in the inhomogenous E field and, thus, to variations in the 
reduction of the frequency because the reduction in length of the E lines 
is no longer clearly defined. 
The third example shown in FIG. 4 eliminates this disadvantage which may 
possibly be encountered in extreme cases, by using a structure comprising 
two pairs of conductive films provided in coaxial arrangement. The inner 
two conductive films 40, 41 are separated by gaps 42, 43 and enclosed by 
another pair of conductive films 44, 45 which is again separated by gaps 
46, 47. As can be seen in FIG. 4, the gaps 42, 43, 46, 47 are considerably 
wider than the gaps 32, 33 in FIG. 3, the width .DELTA. of the gaps used 
in FIG. 4 being for instance, .DELTA.=3 mm or above. 
The reduction in length of the electric flux lines is achieved in the 
example shown in FIG. 4 mainly by the films 40, 41, while the films 44, 45 
serve substantially to cover up the gaps 42, 43. This is possible because 
the gaps 46, 47 are set off by 90.degree. relative to the gaps 42, 43 so 
that the gaps 42, 43 are overlapped over an angle .phi..sub.1 of almost 
180.degree.. 
The example shown in FIG. 5a, 5b, although using basically the same 
structure with inner films 50, 51 separated by gaps 52, 53 and outer films 
54, 55, differs from the example shown in FIG. 4 in that the gaps 52, 53 
are overlapped by the films 54, 55 only over a smaller angle .phi..sub.2. 
The effects achieved by the structure shown in FIG. 5a are illustrated in 
FIG. 5b. As can be seen, the overlapping portions of the film 55 and the 
inner films 50, 51 form between them spaces 52, in which homogenous E 
lines develop, contrary to the example shown in FIG. 3 where an 
inhomogenous propagation of the E flux lines was encountered. So, the 
effective reduction in length of the E flux lines can be adjusted very 
efficiently and in a reproducible manner by varying the angle by which the 
outer films overlap the gaps between the inner films, i.e. the angles 
.phi..sub.1 and .phi..sub.2 in FIGS. 4 and 5. For, any tolerances in the 
overlapping width are much less critical as regards the natural frequency 
of the cavity resonator 10 than any tolerances in the gap width .delta. 
according to FIG. 3. 
The fifth example shown in FIG. 6 illustrates on the one hand a manner of 
fastening the conductive films and, on the other hand, another structure 
of the conductive films. 
In FIG. 6, the cavity resonator 10 is provided with a conventional Dewar 60 
extending in the axial direction through the cavity resonator 10. The 
Dewar 60 is of the double-walled type, the outer wall 61 and the inner 
wall 62 being indicated in the drawing. The heating agent, for instance 
gaseous nitrogen which is supplied via an inlet 63 and discharged by an 
outlet 64, flows through the space between the outer wall 61 and the inner 
wall 62. An opening 65 which is closed about the lateral surface area 
passes through the outer wall 61 and inner wall 62 and serves to introduce 
a test specimen 12. The outer wall 61 of the Dewar 60 is provided with a 
pair of conductive films 66, 67 corresponding in shape and function to the 
conductive films 54, 55 of FIG. 5. In contrast, the inner wall 62 is 
provided with two axially spaced pairs of conductive films, i.e. one pair 
68, 69 and, at a certain distance therefrom, another pair 70, 71. The 
pairs 68, 69 and 70, 71 are of strip-like shape, corresponding to half the 
shell of a very flat cylinder. The reduction in length of the E flux lines 
is effected by the said pairs 68, 69 and 70, 71 in the respective areas. 
The characteristic feature of the structure according to FIG. 6 is to be 
seen in the fact that the currents flowing in the films flow in the area 
of the films 68, 69 and 70, 71 along circular lines, but are also capable 
of flowing from the latter via the outer films 66, 67 in axial direction 
so that a propagation of the said currents occurs in the axial direction 
and vertically thereto. This "diversion" of the currents in the said films 
provides an additional possibility of adjustment for the reduction of the 
natural frequency as it varies the effective reduction in length of the E 
lines. 
The outer wall 61 and the inner wall 62 of the Dewar 60 shown in FIG. 6 are 
of course of finite thickness so that the conductive films 66 to 71 may 
also be arranged on the inner and outer faces, respectively, of one of 
these walls.