EP1400158B1 - Hom damped high-frequency resonator - Google Patents

Abstract

A HOM attenuated high frequency resonator provided with a cylindrical resonator cavity on the outer surface of which are arranged three circular tapered waveguides with two symmetrically arranged ridges each, the cut-off frequency of the waveguide base mode being kept constant over the length of the waveguides by varying the height of the ridges, and the ridge waveguides being provided at their end of the smaller diameter with an impedance transformer each for the broadband adjustment of the coaxial line is to be cost-efficiently manufacturable as a compact structure and is to be of improved attenuation properties while at the same time having, relative to prior art arrangements, a high shunt impedance for the fundamental modes.

Description

The invention relates to a HOM-attenuated high-frequency resonator, comprising a cylindrical resonator cavity, on the lateral surface of which three circular tapered waveguides are arranged with two symmetrically arranged webs, wherein the cut-off frequency of the waveguide fundamental over the length of the waveguide by varying the web height constant is held, and the ridge waveguide at its end with the smaller diameter have a impedance transformer for broadband RF matching of the coaxial line.

In electron storage rings for generating synchrotron radiation, the brilliance of the photon beams depends sensitively on the quality of the stored electron beam. In particular, beam instabilities have a negative influence on the generated brilliance. The beam instabilities are caused by multibunch oscillations, which entail an increase of the energy width (longitudinal vibrations) and the transversal emittance (transverse vibrations).

The Muttibunch vibrations are excited by the interaction of the electron packets with the higher order modes (HOM) of the acceleration resonator. By reducing the impedances of these HOMs below the critical impedance specific to each synchrotron source, the o.g. Instabilities are suppressed.

For the suppression of instabilities, various solutions for resonators are known in the prior art.

For example, in Proc. of the European Particle Accelerator Conference (EPAC 1990), Vol. 1, pp. 149 ; Proc. of the European Particle Accelerator Conference (EPAC 1996), Vol. 1, pp. 148 and ibid Vol. 3, pp. 1976 the resonator at the Laboratorio Nazionale di Frascati, INFN, described in Frascati / Italy. This resonator consists of a bell-shaped resonator cavity with three long rectangular waveguides for HOM attenuation, which are arranged at an angle of approximately 15 degrees to the resonator axis. Due to this geometry and the long truncated cone-shaped jet pipes to adapt the large tube-side diameter of the vacuum chamber of the ring, the installation length in the axial direction is about 2 m. The broadband rectangular waveguides have a transition to a 7/8 "EIA coaxial line to extract the HOM energy, and not least because of the large diameter of the beam tube openings, relatively small values for the HOM impedances can be achieved reduced shunt impedance for the basic mode, resulting in higher operating costs.

Another solution for coupling radially arranged waveguides to the interfering HOMs has been developed for the cavity resonator at the Stanford Linear Accelerator Center, SLAC, Stanford, USA (described, for example, in SLAC-PUB-6129, LBL-30624, BECON). 91, April 1991). This arrangement consists of a resonator cavity with a spherical radial contour and three rectangular wave conductors for HOM attenuation, which are arranged at an angle of approximately 30 degrees to the beam axis on the resonator, but then oriented parallel to the axis and finally bent for space reasons by 180 degrees. The HOM energy is absorbed in ferrite absorbers in the interior of the waveguides. Due to the geometry of the arrangement, the installation length in the axial direction is about 1.8 m.

Both of these resonators are for use in electron-positron storage rings for high energy physics with longer straight sections been developed and therefore suitable for use in synchrotron radiation sources only conditionally.

In Proc. of the European Particle Accelerator Conference (EPAC 1996), Vol. 3, pp. 1940 , and vol. 3, pp 1937 a resonator is described as proposed by the Berlin electron storage ring company for synchrotron radiation mbH. In this case, the high-frequency resonator has a cylindrical resonator cavity, on the lateral surface of which are arranged three circular waveguides for coupling to the HOMs, which are each connected to a broadband transition to a coaxial line (CWCT). With this arrangement, the necessary dimensions, in particular the installation length, can be reduced compared to the prior art. In Proc. of the European Particle Accelerator Conference (EPAC 1998), Vol. 3, pp. 2065 For such an arrangement, a circular waveguide formed as a tapered ridge waveguide with a constant cutoff frequency and a 7/8 "coaxial line impedance transformer is described. As previously mentioned, the dimensions of a HOM attenuated high frequency resonator have been reduced However, the reduction of the shunt impedance of the fundamental mode is relatively large and the damping efficiency for the higher-order modes is insufficient.

It is therefore an object of the invention to provide a low-cost manufacturable HOM attenuated RF resonator with improved damping characteristic at the same time high shunt impedance for the fundamental mode in a space-saving design.

This object is achieved by a HOM attenuated high-frequency resonator of the type mentioned above in that according to the invention the waveguide with two symmetrically arranged webs for adjusting an asymmetry relative to the center plane of the cylindrical resonator cavity in the direction of its longitudinal axis offset on the The outer surface of the resonator cavity are arranged, the waveguide with two symmetrically arranged webs are formed adjustable in their angle to the axis of the cylindrical resonator cavity and the webs of the waveguide protrude into the cylindrical resonator cavity such that the modes of higher order are optimally coupled.

For the efficient reduction of the HOM impedances, the widest possible and low-reflection adaptation of the circular waveguides with the greatest possible coupling is of crucial importance. These effects are being realized by the erfindungsgerriäße solution.

To ensure that both the mid-plane symmetric modes and the antisymmetric modes are efficiently coupled, the circular waveguides are offset relative to the center plane of the cylindrical resonator cavity in the direction of its longitudinal axis.

Since the waveguides are designed to be adjustable in their angle to the axis of the cylindrical resonator cavity, for example by means of rotationally symmetrical UHV flanges are connected to the resonator cavity, the orientation of the waveguide web is selected relative to the beam axis. This makes it possible to selectively optimize the coupling to individual in a specific storage ring particularly disturbing HOM's.

Since the waveguides with their webs protrude deeper into the resonator cavity than the connecting element, for example the mentioned flanges, given, the solution according to the invention ensures that the vacuum transitions and the RF transitions are not realized at the same location. The partial length of the webs of the waveguides projecting into the resonator cavity is varied (in addition to other geometrical parameters) by numerical simulation such that the HOM impedances be minimized above the cut-off frequency (650 MHz) to 3 GHz.

In particular, the adjustment of the angle of the webs of the waveguide with respect to the axis of the cylindrical resonator cavity by means of rotationally symmetrical flanges allows optimization of the coupling of particularly disturbing HOM's.

In one embodiment, it is provided that the ridges of the waveguides are aligned parallel with respect to the axis of the cylindrical resonator cavity, i. the angle of the ridge waveguide to the axis of the cylindrical resonator cavity is 0 degrees. This design is the optimal solution for the case where all HOMs are excited by the electron beam with equal strength. If this is not the case, the adjustability of the orientation of the waveguide webs allows memory ring-specific minimization of the HOM's.

In another embodiment, the circular taped ridge waveguides have a variable ridge height described along the length of the tapered waveguide by the second order polynomial y = 3.6328 + 0.0347513x + 0.000183869x ² , where x is the length (in mm ) of the tapered waveguide and y mean half the distance of the webs (in mm) to each other. This web profile is particularly favorable because the cut-off frequency of the waveguide is kept constant and thus the reflection factor of the tapered waveguide section is minimized in the above frequency range.

A further embodiment provides that the impedance transformer has a section which is designed as a tapered coaxial connection. This allows the use of vacuum RF windows of any type.

In a next embodiment, the resonator cavity has a beam tube opening with nose-shaped extensions for optimizing the shunt impedance of the fundamental mode. This used "nose cone" geometry in the area of the beam tube opening causes a concentration of the accelerating field on the resonator axis, whereby a large shunt impedance is achieved with simultaneously high HOM damping efficiency. By implementing a high shunt impedance, a more energy-efficient acceleration of the electron beam i.V. guaranteed to the prior art.

The solution according to the invention, with its space-saving design, allows the use of HOM-damped resonators in most synchrotron radiation sources. The maximum local thermal power densities on the inner surface of the resonator in the transition region between waveguide and resonator wall (with external excitation of the fundamental mode) are about 50% lower when using circular waveguides than with rectangular waveguides. This allows a much simpler design of the cooling water channels .. From manufacturing technology. Point of view, the connection of a circular waveguide with a cylindrical resonator is simpler and less expensive than the connection of a rectangular waveguide with a spherical or bell-shaped design. Compared to the solutions mentioned in the prior art, the production costs are only about 40%. The "nose cone" geometry used for the design of the beam tube opening of the resonator cavity causes - as already mentioned - a high shunt impedance of the fundamental mode with simultaneous efficient HOM attenuation.

The invention will be explained in more detail in the following embodiment with reference to drawings.

Fig. 1

schematische Gesamtdarstellung eines HOM-gedämften HF- Resonators in Strahlrichtung;

Fig. 2

schematische Seitenansicht gemäß Fig. 1; .

Fig. 3

schematisch eine räumliche Schnittzeichnung durch einen HOM- gedämpften HF-Resonator gemäß Fig. 1;

Fig. 4

einen Stegwellenleiter schematisch im Längsschnitt.

Show

Fig. 1

schematic overall representation of a HOM-dimmed RF resonator in the beam direction;

Fig. 2

schematic side view according to Fig. 1 ; ,

Fig. 3

schematically a spatial sectional drawing through a HOM attenuated RF resonator according to Fig. 1 ;

Fig. 4

a ridge waveguide schematically in longitudinal section.

In Fig. 1 is schematically shown an inventive HOM attenuated RF resonator. In a normal conducting 500 MHz acceleration cavity for synchrotron sources in a cylindrical resonator cavity 1 three circular ridge waveguides 2.1; 2.2; 2.3 by means of the flanges F1; F2; F3 arranged. The flanges F1; F2; F3 allow the adjustment of the orientation of the webs of the waveguide 2.1; 2.2; 2.3. Also shown in this figure is the opening for an RF injection element 4, the opening for the tuner 3 and the opening for connection to a measuring loop 5.

In the in Fig. 2 shown schematically side view Fig. 1 is clearly visible the offset of the three arranged on the lateral surface of the resonator cavity 1 in the direction of its longitudinal axis waveguide 2.1; 2.2; 2.3 to each other. Also shown in this figure are the impedance transformers 6.1; 6.2; 6.3. The shown inventive offset of the waveguide 2.1; 2.2; 2.3 to each other causes both the symmetrical with respect to the center plane and antisymmetric modes are coupled out efficiently. Shown is also the jet pipe SR, in which the resonator cavity 1 is fitted.

The Fig. 3 is a schematic spatial sectional drawing of the in Fig. 1 HOM attenuated RF resonator shown. Here is now clearly visible, as each of the two webs S1.1 and S2.1; S1.2 and S2.2; S1.3 and S2.3 of the three waveguides 2.1; 2.2; 2.3 according to the invention protrude into the resonator cavity 1 , that is, the length of the webs S1.1; S2.1; S1.2; S2.2; S1.3; S2.3 is greater than the length of the wall of the waveguide 2.1; 2.2; 2.3. This results in an improved compared to the prior art coupling higher modes. By means of the flanges F1; F2; F3 are the circular waveguides 2.1; 2.2; 2.3 in its orientation with respect to the beam axis adjustable connected to the resonator cavity 1 , whereby a memory ring specific optimization of the coupling of particularly disturbing HOM's is possible. The opening R of the jet pipe SR in the resonator cavity 1 has a "nose cone" geometry, whereby - as already described - a concentration of the accelerating field is realized on the resonator axis.

Each waveguide 2.1; 2.2; 2.3 is - as in FIG. 3 shown - also depending on an impedance transformer 6.1; 6.2; 6.3 assigned. These impedance transformers 6.1; 6.2; 6.3 each have a section 7.1; 7.2; 7.3 , which is designed as a tapered coaxial line. The special design of the waveguide 2.1; 2.2; 2.3 with their penetrating into the resonator 1 each symmetrically arranged two webs S1.1 and S2.1; S1.2 and S2.2; S1.3 and S2.3 are particularly easy to recognize in this sectional drawing.

In the FIG. 4 is one of the three circular waveguides 2 with two symmetrically arranged webs S1; S2 shown in longitudinal section. The distance between the two webs S1; S2 in the waveguide 2 over its length is described, for example, by the polynomial of the second order y = 3.6328 + 0.0347513x + 0.000183869x ² , where x is the length (in mm) of the tapered waveguide 2 and y is half the spacing of the webs (FIG. in mm) S1; S2 mean each other. With this web profile, the cut-off frequency along the waveguide 2.1; 2.2; 2.3 constant and thereby - as already mentioned - the reflection factor of the tapered waveguide section in the frequency range 650 MHz to 3 GHz minimized.

It has been demonstrated numerically that the solution according to the invention permits the realization of an accelerator resonator which ensures the almost complete suppression of multibunch instabilities in modern synchrotron radiation sources of the third generation. In addition, it was shown that the maximum current thresholds in synchrotron radiation sources i.V. be increased to the state of the art for resonators at least by a factor of 2.

Claims (5)

1. HOM attenuated high frequency resonator provided with a cylindrical resonator cavity (1) on the outer surface of which are arranged three circular tapered waveguides (2.1; 2.2; 2.3) with two symmetrically arranged ridges (S1.1 and S2.1; S1.2 and S2.2; S1.3 and S2.3) each, the cut-off frequency of the waveguide base mode being kept constant over the length of the waveguides (2.1; 2.2; 2.3) by varying the height of the ridges, and the ridge waveguides (2.1; 2.2; 2.3) being provided at their end of the smaller diameter with an impedance transformer (6.1; 6.2; 6.3) each for the broadband adjustment of the coaxial line,
   characterised by the fact that
   the waveguides (2.1; 2.2; 2.3) with two symmetrically arranged ridges (S1.1 and S2.1; S1.2 and S2.2; SS1.3 and S2.3) each for adjusting an asymmetry relative to the center plane of the cylindrical resonator cavity (1) are arranged offset in direction of its longitudinal axis on the outer surface of the resonator cavity (1),
   the waveguides (2.1; 2.2; 2.3) with the two symmetrically arranged ridges (S1.1. and S2.1; S1.2 and S2.2; S1.3 and S2.3) each are structured for adjusting their angle relative to the axis of the cylindrical resonator cavity (1) and
   the ridges (S1.1 and S2.1; S1.2 and S2.2; S1.3 and S2.3) of the waveguides (2.1; 2.2; 2.3) protrude into the cylindrical resonator cavity (1) such that the modes of higher order are optimally coupled.
2. HOM attenuated resonator of claim 1,
   characterised by the fact that
   the ridges (S1.1 and S2.1; S1.2 and S2.2; S1.3 and S2.3) of the waveguides (2.1; 2.2; 2.3) are arranged parallel relative to the axis of the cylindrical resonator cavity (1).
3. HOM attenuated resonator of claim 1,
   characterised by the fact that
   the circular tapered ridge waveguides (2.1; 2.2; 2.3) have a ridge height defined over the length of the tapered waverguides (2.1; 2.2; 2.3) by the second order polynomial $$\mathrm{y}=3.6328+0.347513\mathrm{x}+.000183869{\mathrm{x}}^2,$$

   

   
   wherein x is the length (in mm) of the tapered waverguide and y is half the spacing of the ridges (in mm) between each other.
4. HOM attenuated resonator of claim 1,
   characterised by the fact that
   each impedance transformer (6.1; 6.2; 6.3) is provided with a section (7.1; 7.2; 7.3) structured as a coaxial line.
5. HOM attenuated resonator of claim 1,
   characterised by the fact that
   the resonator cavity (1) is provided with a nose cone shaped beam hole (R) for the concentration of the accelerating field on the resonator axis.

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