EP2436240B1 - Cascade accelerator - Google Patents

Description

The invention relates to a cascade accelerator with two sets of each connected in series, connected via diodes in the manner of a Greinacherkaskade capacitors. It further relates to a radiotherapy device with such a cascade accelerator.

In medical radiotherapy, ionizing radiation is used to cure or delay disease progression. As ionizing, high-energy beams mainly gamma radiation, X-rays and electrons are used.

Particle accelerators are commonly used to generate an electron beam either for direct therapy or X-ray generation. In particle accelerators, charged particles are brought to high speeds and thus kinetic energies by electric fields, with the electric fields in some types of accelerators being produced by electromagnetic induction in variable magnetic fields. The particles acquire a kinetic energy that corresponds to a multiple of their own rest energy.

The particle accelerators distinguish between cyclic acceleration particle accelerators such as the betatron or cyclotron and those with straight acceleration. The latter allow a more compact design and also include so-called cascade accelerators (also Cockcroft Walton accelerator) in which by means of a Greinacher circuit, which is repeatedly cascaded, by multiplication and rectification an AC voltage high DC voltage and thus a strong electric field can be generated.

The functioning of the Greinacher circuit is based on an arrangement of diodes and capacitors. The negative half-wave of an AC voltage source charges via a first diode to a first capacitor to the voltage of the AC voltage source. In the subsequent positive half-wave then adds the voltage of the first capacitor to the voltage of the AC voltage source, so that a second capacitor is charged via a second diode now to twice the output voltage of the AC voltage source. By multiple cascading in the manner of a Greinacherkaskade one obtains such a voltage multiplier. The respective first capacitors form a first set of capacitors of the cascade connected directly in series, the respective second capacitors a corresponding second set. The diodes form the cross-connection between the sets.

In such a cascade accelerator, it is possible to achieve comparatively high particle energies in the megaelectron volt range. However, there is the risk of electrical flashovers (breakdown voltage air: 3 kV / mm), especially in the case of cascade accelerators set up under normal atmospheric pressure, whereby the maximum particle energy is undesirably limited.

A cascade accelerator according to the preamble of claim 1 is made EP 0 412 896 A1 known. The electrodes of the capacitors of this cascade accelerator are insulated from one another by a pressurized gas, in particular SF ₆

The invention is therefore based on the object to provide a cascade accelerator having a particularly high achievable particle energy in a compact design.

This object is achieved by a cascade accelerator with a formed by openings in the electrodes of the capacitors of a set to one in the area Acceleration channel directed to the electrode with the highest voltage arranged particle source, wherein the electrodes of the capacitors are insulated from one another with the exception of the acceleration channel with a solid or liquid insulating material.

The invention is based on the consideration that an increase in the energy of the generated particle beam of the cascade accelerator would be possible by increasing the acceleration voltage. In order to minimize the resulting risk of electrical flashover, the distance between the individual capacitor plates of the cascade accelerator could be increased. However, this would contradict a compact design, which is just wanted for the use in the medical field. In order to allow an increase in the acceleration voltage at the same time compact design, the capacitors should therefore be otherwise protected against electrical flashovers. For this purpose, appropriate liquid or solid insulators should be used, which allow a reliable insulation of the capacitor plates. This can be achieved by the interstices of the electrodes is filled to the acceleration channel with a solid or liquid insulating material.

The resulting in a cascade accelerator high voltages should be secured in addition to a corresponding insulation thickness by a corresponding design of the geometry against electrical breakdown. Therefore, voltage generation and particle accelerators should be integrated and the components with particularly high voltage should be accommodated within the smallest possible volume. Since the maximum electric field strength is proportional to the curvature of the electrodes, a spherical or ellipsoid geometry is of particular advantage. In particular, a spherical geometry means a particularly small volume in terms of the maximum possible electric field strength within the insulator and therefore also a particularly small mass. However, in certain designs, deformation towards an ellipsoid may be desired. Thus, advantageously, a plurality of electrodes are formed as concentric hollow ellipsoidal segments spaced apart from the particle source.

A particularly simple construction which combines the advantages of an ellipsoid geometry with the simple generation of stress within a Greinach cascade is possible in that the electrodes designed as hollow ellipsoidal segments are each half-hollow ellipsoids, i. that is, separation at the equator of the respective hollow ellipsoid so that the resulting multiple layers of semi-hollow ellipsoids form the two sets of capacitors needed for the Greinach cascade. The acceleration channel then advantageously leads through the vertex of the respective semi-hollow ellipsoid, whereby a particularly simple geometry is achieved.

In a further advantageous embodiment, the respective diodes are arranged in the region of a great circle of the respective semi-hollow ellipsoid. Namely, when the half-hollow ellipsoids each form the two sets of capacitors connected in series, the diodes each connect half-hollow ellipsoids on alternating hemispheres. The diodes can then be arranged for an especially simple construction within an equatorial section.

In order to achieve a particularly high stability of the cascade accelerator against breakdown, a uniform voltage gradient along the acceleration path, d. H. be provided between the individual electrodes of the Greinacherkaskade. This can be achieved by a plurality of electrodes are equidistant from each other. Since the electrodes of each set have a linear voltage increase, this results in a practically linear increase in the voltage along the acceleration channel.

In a further advantageous embodiment, the particle source is a cold cathode. Electrodes of a cold cathode are unheated and remain so cold during operation that no glow emission takes place on them. This allows a particularly simple construction of the cascade accelerator.

The acceleration channel allows the particle stream to be extracted from the cascade accelerator. In order for the acceleration channel also to resist the tangential electric fields without breakdown, the acceleration channel should comprise a cylindrical wall coated with diamond-like carbon and / or oxidized diamond. These materials are able to withstand these comparatively high voltages.

Advantageously, such a cascade accelerator is used in a radiotherapy device.

The advantages achieved by the invention are in particular that in a cascade accelerator based on a Greinacherkaskade by embedding particle source and / or electrodes in a solid or liquid insulating material, a particularly high acceleration voltage for the acceleration of charged particles can be generated. In addition, when forming the electrodes in a spherical or ellipsoidal geometry, a particularly compact design is possible and the two capacitor sets of the Greinacher circuit are additionally used as concentric potential equilibration electrodes for the electric field distribution around the particle source and high voltage electrode. Such a cascade accelerator enables a particularly high voltage with a particularly compact design, as is particularly desirable in medical applications.

FIG 1

eine schematische Darstellung eines Schnitts durch einen Kaskadenbeschleuniger, und

FIG 2

eine schematische Darstellung einer Greinacherschaltung.

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

FIG. 1

a schematic representation of a section through a cascade accelerator, and

FIG. 2

a schematic representation of a Greinacherschaltung.

Identical parts are provided in both figures with the same reference numerals.

The cascade generator 1 after the FIG. 1 has a first set 2 and a second set 4 of semi-hollow spherical electrodes. These are arranged concentrically around a particle source 6.

Through the second set of electrodes 4 leads an acceleration channel 8, which is directed to the particle source 6 and allows extraction of the particle stream 10, which emanates from the particle source 6 and receives a high acceleration voltage from the hollow-spherical high-voltage electrode 12.

In order to prevent internal breakdown of the high voltage from the high voltage electrode 12 to the particle source 6, the particle source 6 can be completely embedded in a solid or liquid insulating material 14, so that the space between high voltage electrode 12 and particle source 6 except for the acceleration channel 8 with the insulating material 14 is completed. As a result, particularly high voltages can be applied to the high-voltage electrode 12, which results in a particularly high particle energy. In addition, the electrodes or the capacitor plates of the electrodes can be insulated from one another substantially up to the acceleration channel 8 by the solid or liquid insulating material 14.

The voltage generation of the high voltage on the high voltage electrode 12 is done by means of a Greinacherkaskade 20, which serves as a circuit diagram in the FIG. 2 is shown. At the input 22 an AC voltage U is applied. The first half-wave charges the capacitor 26 to the voltage U via the diode 24. At the following half cycle of the alternating voltage, the voltage U from the capacitor 26 is added to the voltage U at the input 22, so that the capacitor 28 is now charged via the diode 30 to the voltage 2U. This process is repeated in the subsequent diodes and capacitors, so that in the in FIG. 2 shown circuit total at the output 32, the voltage 6U is achieved. The FIG. 2 also clearly shows how each of the first set 2 of capacitors and the second set 4 of capacitors is formed by the illustrated circuit.

Each in the FIG. 2 interconnected electrodes of two capacitors are now in the cascade accelerator 1 after the FIG. 1 each formed as a semi-hollow spherical shell concentric. In each case, the voltage U of the voltage source 22 is applied to the outermost shells 40, 42. The diodes for forming the circuit are arranged in the region of the great circle of the respective semi-hollow sphere, ie in the equatorial section of the respective hollow spheres.

A ball capacitor with inner radius r ₀ and outer radius r ₁ has the capacity $$C=4\mathit{\pi \epsilon }\frac{r_0{r}_1}{r_1-r_0}\mathrm{,}$$

The field strength at radius r is then $$e=\frac{r_0{r}_{1}U}{\left(r_1-r_0\right){\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="imgf0001.png"\; state="\{\{state\}\}">r</figure-callout>}}^2}\mathrm{,}$$

This field strength is quadratically dependent on the radius and thus increases strongly towards the inner electrode.

mit minimaler maximaler Feldstärke ist.Characterized in that in the cascade accelerator 1, the electrodes of the capacitors of the Greinacherkaskade 20 are inserted as intermediate electrodes at a well-defined potential, the field strength distribution over the radius is linearly aligned because for thin-walled hollow spheres, the electric field strength is approximately equal to the flat case $$e=\frac{U}{\left(r_1-r_0\right)}$$

with minimum maximum field strength.

Due to the additional use of the two capacitor sets 2, 4 of a Greinacherkaskade 20 as concentric Potentialequilibrierungselektroden for electric field distribution in a substantially completely encapsulated in a solid or liquid insulating material 14 high voltage electrode 12 and particle source 6 a particularly high acceleration voltage in a cascade accelerator 1 is achieved. At the same time, the design is very compact, which allows a flexible application, especially in radiotherapy.

LIST OF REFERENCE NUMBERS

1

cascade generator

2

first sentence

4

second sentence

6

particle

8th

accelerating channel

10

particle

12

High-voltage electrode

14

insulating material

20

Greinacher cascade

22

voltage source

24

diode

26, 28

capacitor

30

diode

32

output

40, 42

extreme shells

r ₀

inner radius of a ball capacitor

r ₁

outer radius of a ball capacitor

U

tension

Claims (8)

1. Cascade accelerator (1) which has two sets (2, 4) of respectively series-connected capacitors (26, 28) connected up via diodes (24, 30) in the manner of a Greinacher cascade (20), and an acceleration channel (8) formed by openings in the electrodes of the capacitors of a set (4) and directed at a particle source (6) arranged in the region of the electrode with the highest voltage (12), characterized in that the electrodes are insulated from one another, except for the acceleration channel (8), by a solid or liquid insulating material (14).
2. Cascade accelerator (1) according to Claim 1, in which a plurality of electrodes are designed as hollow ellipsoidal segments arranged concentrically around the particle source (6) in a fashion separated from one another.
3. Cascade accelerator (1) according to Claim 2, in which the respective hollow ellipsoidal segment is a hollow half ellipsoid, and the acceleration channel (8) is guided through the vertex of the hollow half ellipsoid.
4. Cascade accelerator (1) according to Claim 3, in which the respective diode (24, 30) is arranged in the region of a great circle of the respective hollow half ellipsoid.
5. Cascade accelerator (1) according to one of Claims 1 to 4, in which a plurality of electrodes are spaced apart equidistantly from one another.
6. Cascade accelerator (1) according to one of Claims 1 to 5, in which the particle source (6) is a cold cathode.
7. Cascade accelerator (1) according to one of Claims 1 to 6, in which the acceleration channel (8) comprises cylindrical wall that is coated with diamond-like carbon and/or oxidized diamond.
8. Beam therapy device having a cascade accelerator (1) according to one of Claims 1 to 7.

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