EP2380414B1 - Beam tube and particle accelerator having the beam tube - Google Patents

Description

The invention relates to a jet pipe for guiding a charged particle beam and to a particle accelerator with such a jet pipe.

Such a jet pipe is particularly provided in a particle accelerator for charged particles. The charged particle beam may include, for example, electrons, nuclei, ionized atoms, charged molecules or charged molecular fragments. The acceleration of the charged particle beam takes place in a jet-carrying hollow volume, which is enclosed by the jet pipe. The hollow volume is usually evacuated during operation of the particle accelerator. For this purpose, usually associated with the jet pipe vacuum pump system is provided.

The jet pipe, which separates the hollow volume and the charged particle beam from the environment, is electrostatically charged by the accelerating electric field. With increasing field strength of the electric field increases the probability that stray electrons are torn out of the surface of the inner wall of the jet pipe. This process occurs first and preferably on so-called whiskers. Whiskers are acicular single crystals of a few microns in diameter and up to several hundred microns in length, which occur on all, especially on metallic, surfaces. At the tip of a whisker, an elevated electric field occurs. As a result, stray electrons are torn out of the tip of the whisker. The scattered electrons are now accelerated as well as the charged particle beam from the electric field. If such scattered electrons hit the inner wall of the beam tube, secondary electrons are triggered upon impact. The process is self-inflating. Finally, there is a flashover on the inner wall and thus to a Bursting of the charged particles accelerating electric field.

To solve this problem is out of the US 6,331,194 B1 a jet pipe is known in which the particle beam leading hollow volume is directly surrounded by a hollow cylindrical insulating core, which is referred to as a high gradient insulator, HGI. The insulation core comprises a number of thin rings (thickness approx. 0.25 mm) made of a dielectric, which are each provided with a thin metallic conductive layer (thickness approx. 4 * 10 ^-3 mm, ie 40,000 angstroms). To produce the insulation core, the rings are assembled into a hollow cylinder. Under pressure and the influence of temperature, the adjacent metal layers of adjacent rings melt and combine to form metal rings.

The HGI increases the puncture resistance of the jet pipe. If secondary electrons form on the inner wall of the HGI, the adjacent metal rings of the HGI are charged. The electrical charge is thus distributed in each case over all of the secondary electrons directly acted upon metal rings. This leads to a homogenization of the electric charge on the inner wall of the HGI and thus to a reduced tendency for secondary electron multiplication.

The division of the electrical charge on adjacent thin metal rings is a purely capacitive distribution. The principle thus works only for rare and short voltage pulses. Charging of the metal rings is not effectively prevented because the metal rings are embedded in the dielectric of the insulator core and thus the applied charge can flow only slowly over creepage distances. An operation of the linear accelerator with a high rate of acceleration pulses thus leads to an increasing probability of breakdown.

Out US 2 569 154 A For example, a discharge tube for generating an electron beam is known. The discharge tube comprises a tubular insulating jacket immediately surrounding a jet-carrying hollow volume. The insulating jacket is formed from a dielectrically acting carrier substrate, in particular a ceramic. In the carrier substrate, metal layers arranged one behind the other along the axis of the insulation jacket are introduced, which are connected to one another inductively by an electrical conductor. Via the conductor, the layers are galvanically connected between a cathode and an anode, between which a voltage for generating a gas discharge and accelerating the released electrons is applied.

In an alternative embodiment, the discharge tube according to US 2 569 154 A a held in the carrier substrate electrical conductor which is divided into a plurality of conductor loops, which completely surround the circumference of the insulating core at different axial positions and which are electrically connected to form a helical coil with each other.

A cathode ray tube similar in construction to the above-described discharge tube is made US 2 005 021 A known.

Out US 3,761,720 A For example, a high voltage insulator is known for a van der Graaf generator or van der Graaf accelerator. The high-voltage insulator comprises a tubular insulating jacket, which immediately surrounds a hollow volume. The insulating jacket is formed from a dielectrically acting carrier substrate. In the carrier substrate are arranged along the axis of the insulation jacket arranged one behind the other metallic layers which are connected by an electrical conductor with each other resistive (ie via a respective ohmic resistance). The layers are galvanically connected between a live pole and ground via the conductor. For detection and localization of defects of the high voltage insulator a high test voltage is applied over this, under its effect in the hollow volume At possibly existing defect sites electrons are released. Detection and spectral analysis of the Bremsstrahlung generated by these electrons, the defect sites are detected and localized.

Out WO 2006/043366 A1 For example, a jet pipe for a particle accelerator is known. The jet pipe comprises a tubular insulating jacket, which immediately surrounds a jet-carrying hollow volume. The insulating jacket is formed from a dielectrically acting carrier substrate, in particular a synthetic resin. In the carrier substrate along the axis of the jet tube successively arranged annular acceleration electrodes are introduced, which are connected to each other resistively by an electrical conductor. The conductor is wound helically around the outer circumference of the insulation jacket.

Out US Pat. No. 3,506,865 Another beam tube for a particle accelerator is known. The jet pipe comprises a tubular insulating jacket, which immediately surrounds a jet-carrying hollow volume. Inside or outside of the electrically non-conductive insulation jacket, a conductor is provided which is divided into a plurality of conductor loops, which completely surround the circumference of the insulation core at different axial positions and which are galvanically connected to form a helical coil.

Further jet tubes for particle accelerators, whose beam-guiding hollow volume immediately surrounding the insulating jacket is formed from a dielectrically acting carrier substrate, in which along the axis of the jet tube successively arranged metallic layers are introduced, are made WO 98/33228 A2 and US 5,757,146 known.

The invention is therefore based on the object of specifying a jet pipe, which has a low breakdown probability having. The invention is further based on the object of specifying a particle accelerator with such a jet pipe.

With regard to the jet pipe, the object is achieved according to the invention by the feature combination of claim 1. For this purpose, the jet-guiding hollow volume is surrounded directly by a hollow cylindrical insulating core. The insulating core is formed of a dielectrically acting carrier substrate and an electrical conductor held therein. The conductor is divided into several conductor loops that completely circumscribe the circumference of the insulation core at different axial positions. The individual conductor loops are galvanically connected with each other.

As the electrical conductor, a metal such as copper, gold or the like can be used. As a dielectric, for example SiO ₂ , Al ₂ O ₃ , a polycarbonate, a polyacrylic, a glass or a ceramic can be used.

Metallic layers, e.g. Metal plates are introduced in succession along the beam tube arranged in the dielectric carrier substrate. The metallic layers serve as intermediate electrodes. The metallic layers are electrically connected to each other by the electrical conductor. Thus, the structure essentially corresponds to the aforementioned HGI. Due to the galvanic connection of the metallic layers, any impacting electrons may flow off.

However, a low impedance connection of the metallic layers would result in an inductive particle accelerator with such a beam pipe to a load of the induction generator and thus to a reduction of the acceleration voltage. However, it can be ensured by the electrical conductor guided in conductor loops that the metallic layers are essentially inductively coupled on the jet pipe surface. This is special advantageous in a pulsed operation of the jet pipe. The capacitive coupling of the insulator sections to a near metal electrode is thus achieved. However, possible charges can flow off in a short time (but long with respect to an acceleration period), so that the self-dying breakdown process is suppressed even at high repetition rates.

If secondary electrons now form on the inside wall of the insulator core facing the hollow volume, then a number of adjacent conductor loops are directly and punctually charged with the electric charge of the secondary electrons. The electrical charge is now distributed in the circumferential direction on these conductor loops. Since all conductor loops are galvanically connected to each other, the charge is distributed even on conductor loops that do not come into direct contact with the secondary electrons. The probability of secondary electron multiplication and breakdown of the insulator is thus effectively reduced. A particle accelerator with such a jet pipe can thus be operated at a high rate of acceleration pulses and / or with an increased field energy, without the breakdown probability increasing significantly.

Suitably, the jet pipe is surrounded by a metallic housing. Such a metallic housing can be made, for example, from pipe sections which are sealed against one another and can be evacuated in a simple manner by means of a vacuum pump system in order to provide the spray-conveying evacuated hollow volume. However, the metallic housing can also comprise a device provided for the provision of the accelerating electric field or form part of such a device.

In an expedient development, the electrical conductor held on the dielectric carrier substrate is connected in a galvanically conductive manner to the metallic housing at at least one point.

In the invention, at least two spaced-apart points of the electrical conductor are galvanically connected to the housing. Thus, there is no potential gradient within the electrical conductor.

The conductor loops can be of annular design and can be galvanically connected to one another by a number of conductor bridges running essentially in the cylinder longitudinal direction.

In an advantageous development, the conductor loops of the electrical conductor but wound in the manner of a helical coil about the central longitudinal axis of the hollow cylindrical insulator core and thus form a helical coil. The conductor acts as an inductance and attenuates high-frequency components of the accelerating electric field.

In an expedient variant, the electrical conductor is embedded in the dielectrically acting carrier substrate. For the production of the insulating core, for example, a mold is provided which has the shape of a hollow cylinder with a cylindrical core to form an annular space. In the annulus, for example, the bent in the manner of a helix electrical conductor is inserted, which consists of a metal wire. Subsequently, the annular space is filled with the dielectrically acting carrier substrate to form the hollow cylindrical insulating core together with the electrical conductor. The dielectric is, for example, a flowable plastic compound, such as a synthetic resin or the like, which solidifies after it has been filled in the mold. But it may also be a powdered dielectric, which is filled as a flowable bulk material in the mold and solidified under temperature and / or pressure application.

In another expedient variant, the electrical conductor is on the inner wall of the hollow cylindrical carrier substrate attached, in particular glued. The electrical conductor can also be imprinted or vapor-deposited.

In another advantageous variant, both the electrical conductor and the dielectrically acting carrier substrate are formed as wire-shaped strips and wound into each other to form the hollow cylindrical insulating core in the form of a double helix. To produce this shape of the insulation core, the two strips are wound, for example, around a cylinder as an assembly aid and then fastened to one another.

All variants described for the production of the hollow cylindrical insulating core are relatively simple and thus cost feasible.

In the final production state, the electrical conductor advantageously completely penetrates the carrier substrate. In other words, both the inner wall and the outer wall of the hollow cylindrical insulating core have a metallically conductive portion. Thus, a large amount of electrically conductive material can be installed in the insulation core, which is suitable for receiving a large amount of electrical charge.

With respect to the particle accelerator, the above object is achieved by the features of claim 6. Thereafter, the particle accelerator comprises a jet pipe according to any one of claims 1 to 5. The particle accelerator can be used for example for research purposes, but also as a medical therapy device. The particle accelerator is designed in particular as a Dielectric Wall Accelerator, DWA, as described in US Pat US 5,757,146 is described in detail.

The particle accelerator can be operated in particular in pulsed operation and based on electromagnetic induction, ie the accelerating electric field is through generates a magnetic flux change around the particle trajectory.

An embodiment of the invention will be explained in more detail with reference to a drawing.

The single FIGURE shows a partial region of a particle accelerator 2 with a section of a jet pipe 4 in a three-dimensional sectional view.

The particle accelerator 2 is designed for example as a linear accelerator, in which the accelerating electric field by a DC voltage or by a pulsating AC voltage (see. Linear accelerator from Wideröe, 1928 ) provided. But it can also be designed as a Dielectric Wall Accelerator.

The jet pipe 4 is shown only schematically as a hollow cylinder. It comprises a tubular metallic housing 5. However, it can also have attachments, for example, not shown in the figure vacuum pumping system. The jet pipe 4 receives a likewise hollow cylindrical insulating core 6. The insulating core 6 in turn directly surrounds a jet-guiding cylindrical hollow volume 8. In the hollow volume 8, a charged particle beam 10 which is indicated only symbolically is guided and accelerated.

The particle accelerator 2 is based on the principle of electromagnetic induction. It generates a symbolically indicated in the figure magnetic field 12 to the particle trajectory, which coincides with the directional arrow for the charged particle beam 10. In the figure, the magnetic field 12 forms closed field lines around the hollow volume 8 or about the particle trajectory of the charged particles 10. By a temporal change of the magnetic flux of the magnetic field 12, an electric field, not shown in the figure, is generated, the charged particle beam 10 in Arrow direction accelerates.

The hollow-cylindrical insulation core 6 is formed from a dielectrically acting carrier substrate 14 and from an electrical conductor 16 held therein. The electrical conductor 16 is divided into several, around the circumference of the insulating core 6 seen from its central longitudinal axis 18 forth at different positions circulating conductor loops 20. The conductor loops 20 are galvanically connected to one another and thus form a helical coil.

In the dielectrically acting carrier substrate 14, metallic layers, for example metal plates, can be introduced one behind the other along the axis of the jet tube (not shown here). In this case, the dielectric carrier substrate has a structure as shown in FIG US 6,331,194 B1 shown. The metallic layers are connected to each other by the circulating conductor loops 20. Due to the galvanic connection of the metallic layers, any impacting electrons may flow off.
For the production of the insulating core 6, for example, the electrical conductor 16 is bent in the manner of a helical coil and secured to the inner wall of the hollow cylindrical carrier substrate 14. However, the electrical conductor can also be printed onto the inner wall of the hollow-cylindrical carrier substrate 14 by means of a metallically conductive paste, as is used for printing printed conductors on printed circuit boards.

The two ends of the helical electrical conductor 16 are connected via electrically conductive connections 22 to the jet pipe 4 or its metallic housing 5 and thus to the basic potential of the particle accelerator 2.

The hollow volume 8 is evacuated during operation of the particle accelerator 2.

Scattering and secondary electrons that were released by the accelerating electric field from the jet tube wall meet on impact with the insulation core 6 on one or more conductor loops 20 of the electrical conductor 16 and charge them. As a result of the galvanic connection of the conductor loops 16 with one another, the charge of the secondary electrons is distributed in the direction of the central longitudinal axis 18 along the electrical conductor 16. In this way, the risk of secondary electron multiplication and hence the breakdown probability of the particle accelerator 2 is low. Thus, the particle accelerator 2 can be operated at a high accelerating electric field strength and operates at a high rate of acceleration pulses.

The design of the electrical conductor 16 in the manner of a coil also high-frequency alternating electric fields are filtered.

LIST OF REFERENCE NUMBERS

2

particle Accelerator

4

lance

6

insulating core

8th

void volume

10

charged particle beam

12

magnetic field

14

carrier substrate

16

electrical conductor

18

central longitudinal axis

20

conductor loop

22

electrically conductive connection

Claims (6)

1. Beam tube (4) for guiding a charged particle beam (10), comprising a hollow cylindrical isolation core (6) which directly encloses a beam-guiding cavity (8) and is formed from a dielectrically acting carrier substrate (14), and comprising a metal housing (5) enclosing the isolation core (6), wherein metal layers are placed in succession along the axis of the beam tube (4) in the carrier substrate (14),
   characterized in that the isolation core (6) is furthermore formed from an electrical conductor (16) held in the dielectrically acting carrier substrate (14), wherein the conductor (16) is divided into a plurality of conductor loops (20) which extend fully around the circumference of the isolation core (6) at different axial positions and are DC-connected to one another, wherein the conductor (16) is DC-connected, in particular in an end-side manner, to the housing (5) at at least two spaced-apart points, wherein the metal layers are DC-connected to one another by the electrical conductor (16) and wherein the metal layers are substantially inductively coupled by the electrical conductor, which is guided in conductor loops, on a beam tube surface.
2. Beam tube (4) according to Claim 1, wherein the conductor loops (20) form a helical coil.
3. Beam tube (4) according to Claim 1 or 2, wherein the conductor (16) is embedded in the carrier substrate (14).
4. Beam tube (4) according to one of Claims 1 to 3, wherein the conductor (16) passes entirely through the carrier substrate (14).
5. Beam tube (4) according to one of Claims 1 to 4, wherein the conductor (16) and the carrier substrate (14) are wire-shaped and are wound as a double helix.
6. Particle accelerator (2), in particular a linear accelerator, having a beam tube (4) according to one of Claims 1 to 5.

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