EP2082625B1 - Betatron comprising a contraction and expansion coil - Google Patents

Description

The present invention relates to a betatron with a contraction and expansion coil, in particular for generating X-radiation in an X-ray inspection system.

When checking large-volume items such as containers and vehicles for inadmissible content such as weapons, explosives or contraband, X-ray inspection systems are known to be used. X-rays are generated and directed to the object. The X-radiation attenuated by the object is measured by means of a detector and analyzed by an evaluation unit. Thus, it can be concluded on the nature of the object. Such an X-ray inspection system is for example from the European patent EP 0 412 190 B1 known.

Betatrons are used to generate X-rays with the energy of more than 1 MeV necessary for the test. These are circular accelerators in which electrons are accelerated in a circular path. The accelerated electrons are directed to a target, where they produce a bremsstrahlung upon impact, the spectrum of which depends, among other things, on the energy of the electrons.

One from the published patent application DE 23 57 126 A1 known betatron consists of a two-part inner yoke, in which the end faces of the two inner yoke parts are spaced apart. By means of two main field coils, a magnetic field is generated in the inner yoke. An outer yoke connects the two mutually remote ends of the inner yoke parts and closes the magnetic circuit.

Between the end faces of the two inner yoke parts an evacuated betatron tube is arranged, in which the electrons to be accelerated revolve. The end faces of the inner yoke parts are formed in such a way that the magnetic field generated by the main field coil forces the electrons into a circular path and, moreover, focuses them on the plane in which this circular path lies. To control the magnetic flux, it is known to arrange a ferromagnetic insert between the end faces of the inner yoke parts within the betatron tube.

$$\mathrm{mit}<B\left(r_s\right)>=\frac{1}{\pi \cdot r_s^2}\underset{A}{\iint }B\left(r\right)\mathit{dA}$$

The electrons are injected, for example by means of an electron gun in the betatron tube and the current through the main field coil and thus increases the strength of the magnetic field. The changing magnetic field creates an electric field that accelerates the electrons in their orbit. At the same time, the Lorentz force on the electrons increases with the magnetic field strength equally. This keeps the electrons at the same orbit radius. An electron moves in a circular path when the Lorentz force and the opposite centripetal force are directed towards the center of the orbit. From this follows the Wideroe condition $$\frac{1}{2}\frac{d}{\mathit{dt}}<B\left(r_s\right)>=\frac{d}{\mathit{dt}}B\left(r_s\right)$$

$$\mathrm{With}<B\left(r_s\right)>=\frac{1}{\pi \cdot r_s^2}\underset{A}{\iint }B\left(r\right)\mathit{there}$$

Here, r _{s is} the desired orbit radius of the electron, A is the area bounded by the nominal orbit radius r _s and <B (r _s )> the magnetic field strength averaged over the area A. US 2,738,421 discloses an additional coil to obtain the magnetic field evenly.

The disadvantage of the known betatron is the fact that, for example due to manufacturing tolerances or the scattering of the electron gun, only a small part of the electrons injected into the betatron tube is focused on the desired circular path and thus accelerated to the final energy. This results in a reduced efficiency. In addition, the problem arises of discharging the accelerated electrons, that is to steer from the desired path to the target.

It is therefore the object of the present invention to provide a betatron which does not have the above disadvantages.

This object is achieved according to the invention by the features of claim 1. Advantageous embodiments are given in the dependent claims 2 to 9. Claim 10 relates to an X-ray inspection system using a betatron according to the invention.

A betatron according to the present invention comprises a rotationally symmetrical inner yoke of two spaced-apart parts, an outer yoke connecting the two inner yoke parts, at least one main field coil, a torus-shaped betatron tube arranged between the opposite end faces of the inner yoke parts and at least one contraction and expansion coil ( CE coil), wherein each exactly one CE coil between the end face of a Innenjochteils and the Betatronröhre is arranged and the radius of the CE coil is substantially equal to the nominal orbit radius of the electrons in the betatron tube. Preferably, the betatron additionally has at least one round plate between the inner yoke parts, wherein the round plate is arranged so that its longitudinal axis coincides with the rotational symmetry axis of the inner yoke.

During the injection phase, in which the electrons do not yet move on the desired Sollkreisbahn, the CE coil is energized. This current flow is also called contraction pulse. The magnetic field generated thereby changes the magnetic field between the inner yoke parts such that the Wideröe condition is disturbed and temporarily results in a changed nominal orbit radius. The desired nominal orbit radius lies between the injection radius and the changed nominal orbit radius. The electrons move in a spiral path toward the changed nominal orbit radius until they are at or near the desired nominal orbit radius. At this time, the contraction pulse ends and the electrons are held on the stable circular path with the desired nominal orbit radius and accelerated.

The electron gun, which injects the electrons into the betatron tube, gives the electrons in a funnel-shaped solid angle range with a certain Frequency distribution. By means of the duration of the contraction pulse, it can be set from which part of this solid angle range the electrons are focused on the target circle path. In addition, at the same time installation tolerances of the electron gun can be compensated.

If the radius of penetration of the electrons into the betatron tube is greater than the nominal orbit radius during acceleration, then a smaller nominal orbit radius fulfills the Wideröe condition by the magnetic field of the CE coil. This causes the electrons to travel on a path that tends to the desired orbit radius for the duration of the contraction pulse.

At the end of the acceleration process, the electrons are directed to the target in the discharge phase. For this purpose, the contraction and expansion coil is energized again. The current flow through the CE coil during the ejection of the electrons is also referred to as the expansion pulse. At this time, the main field coils generate a stronger magnetic field than during the injection phase. The material of the yokes and the blanks is located in a non-linear region of the hysteresis curve, which describes the relationship between the exciting magnetic flux and the magnetic flux in the material. The magnetic flux in the material is therefore proportional to the magnetic flux in the air between the inner yoke parts affects the contraction and expansion coil differently than during the injection phase. This leads to a disturbance of the Wideröe condition, which is now met again by a changed nominal orbit radius. The electrons move on a spiral path to the changed nominal orbit radius and hit the target during this movement.

For example, if the target is outside the nominal orbit radius, the magnetic field of the CE coil will alter the magnetic flux such that a larger radius will satisfy the Wideroe condition. The electrons drift outward until they hit the target.

In an advantageous embodiment of the invention, the terminals of a CE coil are connected to a current or voltage source and in at least one Line between the CE coil and the current or voltage source is arranged by a control electronics actuated switch. The switch is, for example, a high-power semiconductor switch such as an IGBT (Insulated Gate Bipolar Transistor). The switch determines both the timing and duration of current flow through the coil. By means of the variation of the duration of the contraction and / or expansion pulse, the amplitude of the maximum coil current and thereby the maximum change of the magnetic field is set. For this purpose, the control electronics are preferably designed such that the switch-on and the switch-on of the switch, so the beginning and duration of the contraction or expansion pulse, are variable.

According to the invention, the same contraction and expansion coil is used both for focusing the electrons on the desired circular path during the injection phase and for discharging the electrons onto the target. Thus, the space requirement is minimized compared to two separate coils, whereby a better insulation of the coil wire can be used. In addition, a power electronics can be saved to power the coils.

In one embodiment of the invention, the betatron has a detector for determining the intensity of the generated X-ray radiation. The detector is preferably connected to the control electronics, so that the switch-on and the switch-on of the switch by means of the control electronics from the output signal of the detector can be determined. The result is a control system that selects the contraction pulse so that the desired radiation intensity is achieved.

Preferably, the opposite end faces of the inner yoke parts are designed and arranged mirror-symmetrically with respect to one another. The plane of symmetry is advantageously oriented so that the rotational symmetry axis of the inner yoke is perpendicular to it. This leads to an advantageous field distribution in the air gap between the end faces, through which the electrons in the betatron tube are held in a circular path.

Further preferably, at least one main field coil is arranged on the inner yoke, in particular on a taper or a shoulder of the inner yoke. As a result, substantially all the magnetic flux generated by the main field coil is passed through the inner yoke. Advantageously, the betatron has two main field coils, wherein a main field coil is arranged on each of the inner yoke parts. This leads to an advantageous distribution of the magnetic flux on the inner yoke parts.

The betatron according to the invention is advantageously used in an X-ray inspection system for security checking of objects. Electrons are injected into the betatron and accelerated before being directed to a target made of tantalum, for example. There, the electrons generate X-radiation with a known spectrum. The X-radiation is directed to the object, preferably a container and / or a vehicle, and modified there, for example, by scattering or transmission attenuation. The modified X-radiation is measured by an X-ray detector and analyzed by means of an evaluation unit. From the result, the nature or content of the object is deduced.

Figur 1

eine schematische Schnittdarstellung durch ein erfindungsgemäßes Betatron,

Figur 2

einen qualitativen Verlauf der Magnetfeldstärke über dem Radius während der Injektionsphase,

Figur 3

einen qualitativen Verlauf der Magnetfeldstärke über dem Radius während der Ausschleusephase und

Figur 4

einen Stromkreis zur Ansteuerung einer CE-Spule.

The present invention will be explained in more detail with reference to an embodiment. Show

FIG. 1

a schematic sectional view through an inventive betatron,

FIG. 2

a qualitative course of the magnetic field strength over the radius during the injection phase,

FIG. 3

a qualitative course of the magnetic field strength over the radius during the Ausschleusephase and

FIG. 4

a circuit for driving a CE coil.

FIG. 1 shows the schematic structure of a preferred betatrone 1 in cross section. It consists inter alia of a rotationally symmetrical inner yoke of two spaced-apart parts 2a, 2b, four optional discs 3 between the inner yoke parts 2a, 2b, wherein the longitudinal axis of the discs 3 corresponds to the axis of rotational symmetry of the inner yoke, an outer yoke 4 connecting the two inner yoke members 2a, 2b a torus-shaped betatron tube 5 arranged between the inner yoke parts 2a, 2b, two main field coils 6a and 6b and one in FIG. 1 not shown control electronics 8. The main field coils 6a and 6b are arranged on shoulders of the Innenjochteile 2a and 2b respectively. The magnetic field generated by them passes through the inner yoke parts 2a and 2b and the region between their opposite end faces, the magnetic circuit being closed by the outer yoke 4. The shape of the inner and / or outer yoke can be selected by the skilled person depending on the application and of the in FIG. 1 specified form differ. Also, only one or more than two main field coils may be present. Another number and / or shape of the blanks is also possible.

Between the end faces of the inner yoke parts 2a and 2b, the magnetic field passes partially through the blanks 3 and otherwise through an air gap. In this air gap, the betatron tube 5 is arranged. It is an evacuated tube in which the electrons are accelerated. The end faces of the inner yoke parts 2a and 2b have a shape selected such that the magnetic field between them focuses the electrons on a circular path. The design of the end faces is known in the art and is therefore not explained in detail. At the end of the acceleration process, the electrons strike a target and thereby generate X-radiation whose spectrum depends, among other things, on the final energy of the electrons and the material of the target.

For acceleration, the electrons are injected into the betatron tube 5 with an initial energy. During the acceleration phase, the magnetic field in the betatron 1 is continuously increased by the main field coils 6a and 6b. This creates an electric field that exerts an accelerating force on the electrons. At the same time, the electrons are forced due to the Lorentz force on a Sollkreisbahn within the betatron tube 5.

The acceleration of the electrons is repeated periodically, resulting in a pulsed X-radiation. In each period, the electrons are injected into the betatron tube 5 in a first step. In a second step, the electrons are accelerated by an increasing current in the main field coil 6a and 6b and thus an increasing magnetic field in the air gap between the inner yoke parts 2a and 2b in the circumferential direction of their circular path. In a third step, the accelerated electrons are ejected to generate the X-radiation on the target. This is followed by an optional pause before electrons are again injected into the betatron tube 5.

erfüllt, ist der stabile Sollbahnradius, auf dem die Elektronen umlaufen.For the orbit of the electrons in the betatron tube 5, the above-mentioned Wideröe condition applies, which results from the fact that the centripetal force outweighs the Lorentz force. The radius r _s that satisfies the equation $$\frac{1}{2}\frac{d}{\mathit{dt}}<B\left(r_s\right)>=\frac{d}{\mathit{dt}}B\left(r_s\right)$$

is satisfied, is the stable nominal orbit radius on which the electrons revolve.

The electron gun emits the electrons with a known aperture angle, the distribution of the electrons usually not being constant over this aperture angle. In addition, the electron gun injects the electrons at an injection radius r _l deviating from the nominal path radius r _s . It is therefore necessary to first transfer the electrons from the injection radius r _l to the nominal orbit radius r _s . Serve the two contraction and expansion coils 7a and 7b, which are arranged between the end faces of the Innenjochteile 2a and 2b and the betatron tube 5. The CE coils are in FIG. 1 indicated by three spiral turns, but any other configuration is possible. The radius of the CE coils 7a and 7b is substantially equal to the nominal track radius r _{s of} the electrons in the betatron tube 5. Due to the spatial extent of the CE coils 7a and 7b, their outer edges extend slightly beyond the nominal track radius r _s . The exact size and positioning of the CE coils is left to the person skilled in the art. However, it is the condition that the inner radius of the CE coils 7a and 7b is larger than the outer radius of the blanks 3, so that the magnetic field generated by them also passes through parts of the area outside of the blanks 3.

The central axes of the CE coils 7a and 7b coincide with the rotational symmetry axis of the inner yoke. Due to this arrangement and the size of the CE coils 7a and 7b, the magnetic field generated by them passes through a circular area whose radius is greater than the radius of the round blanks 3 and approximately in the range of the nominal orbit radius r _s .

FIG. 2 shows qualitatively the course of the magnetic field B shown in solid lines over the radius, starting from the rotational symmetry axis of the inner yoke, and the injection radius r _{l of} the electrons. Due to the magnetically active material of the blanks 3 results in an approximately constant magnetic field within the blanks 3. The magnetic field is significantly lower in the air outside the blanks and also falls off with increasing radius. In the illustrated magnetic field, the in FIG. 2 Plotted nominal orbit radius r _s the Wiederöe condition.

If a current, the so-called contraction pulse, is impressed into the CE coils 7a and 7b, the result is qualitatively the in FIG. 2 Dashed line B '(r) of the magnetic field strength over the radius as a superposition of the magnetic fields of the main field coils 6a, 6b and the CE coils 7a, 7b. In the case of this resulting magnetic field, the changed nominal track radius r _s ' fulfills the Wiederöe condition. It follows that the electrons are drawn in a spiral path from the injection radius r _l to the changed nominal path radius r _s '. In this case, the electrons, for example, depending on their angle of entry into the betatron tube 5, pass the desired nominal orbit radius r _s at different times. The electrons that are at the end of the contraction pulse on or near the desired nominal orbit radius r _s , are accelerated in the following on this radius.

By choosing the end time of the contraction pulse can thus be selected from which part of the opening angle of the electron gun are the electrons, which are accelerated to the desired final energy. Thus, the intensity of the X-ray radiation generated by the betatron 1 can be maximized and regulated.

At the end of the acceleration process, the main field coils 6a and 6b generate the qualitatively in FIG. 3 shown in solid magnetic field B (r), whose course substantially from the magnetic field FIG. 2 equivalent. However, due to the higher current through the main field coils 6a and 6b, the magnetic field is much stronger. In addition, the material of the yokes and / or blanks is in a non-linear region of the hysteresis curve. When the CE coils 7a and 7b are energized with the so-called expansion pulse, the result in FIG FIG. 3 Superimposed magnetic field B "(r) shown in dashed lines, starting from this superimposed magnetic field, the changed nominal orbit radius r _s " fulfills the Wiederöe condition. It follows that the electrons drift in the direction of the changed nominal track radius r _s "on a spiral path from the nominal path radius r _s valid during the acceleration During this drift movement, the electrons strike the target and thereby generate X-radiation.

An X-ray detector not shown in the figures detects the intensity of the generated X-ray radiation and regularly transmits information about the intensity to the control electronics 8. This evaluates the intensity and determines therefrom the duration and the time of the contraction and expansion pulses for the next period the electron acceleration.

The FIG. 4 shows an example of a circuit for energizing the CE coil 7a, which is identical to the CE coil 7b transferable. The CE coil 7a is connected to a voltage source 11 via a switch 9 which can be activated by the control electronics 8. Optionally, multiple CE coils are connected via one or more switches to a common voltage source. Further alternatively, each CE coil is connected via a separate switch to a voltage source associated with the CE coil.

Claims (10)

1. Betatron (1), in particular in an X-ray system, having

   - a rotation-symmetrical inner yoke composed of two spaced-apart parts (2a, 2b),

   - an outer yoke (4) connecting the two inner yoke parts (2a, 2b),

   - at least one main field coil (6a, 6b),

   - a torus-shaped betatron tube (5) arranged between the opposite end faces of the inner yoke parts (2a, 2b),

   - at least one contraction and expansion coil (CE coil; 7a, 7b), wherein in each case exactly one CE coil (7a, 7b) is arranged between the front end of an inner yoke part (2a, 2b) and the betatron tube (5), and the radius of the CE coil (7a, 7b) is substantially identical to the desired predetermined orbit radius of the electrons in the betatron tube (5),

   - an electron cannon which injects electrons at an injection radius into the betatron tube (5), and

   - control electronics (8) which control the power supply to the CE coil (7a, 7b),

   characterized in that the control electronics (8) are configured to supply power to the CE coil (7a, 7b) during the injection phase such that a temporarily changed predetermined orbit radius results, wherein the desired predetermined orbit radius lies between the injection radius and the changed predetermined orbit radius.
2. Betatron (1) according to Claim 1, characterized in that the opposite end faces of the inner yoke parts (2a, 2b) are configured and arranged mirrorsymmetrically with respect to one another.
3. Betatron (1) according to one of Claims 1 or 2, characterized in that at least one main field coil (6a, 6b) is arranged on the inner yoke, in particular on a tapering or a shoulder of the inner yoke.
4. Betatron (1) according to Claim 3, characterized by two main field coils (6a, 6b), wherein one main field coil (6a, 6b) is arranged on each of the inner yoke parts (2a, 2b).
5. Betatron according to one of Claims 1 to 4, characterized by at least one round plate (3) between the inner yoke parts (2a, 2b), wherein the round plate (3) is arranged such that its longitudinal axis coincides with the axis of rotational symmetry of the inner yoke.
6. Betatron (1) according to one of Claims 1 to 5, characterized in that the ports of a CE coil (7a, 7b) are connected to a power or voltage source (11) and a switch (9), in particular an IGBT (insulated gate bipolar transistor), which is actuatable using the control electronics (8), is arranged in at least one line between the CE coil (7a, 7b) and the power or voltage source (11).
7. Betatron (1) according to Claim 6, characterized in that the control electronics (8) are configured such that the turn-on point and the turn-on duration of the switch (9) are variable.
8. Betatron (1) according to Claim 7, characterized by a detector for ascertaining the radiation intensity generated by the betatron (1).
9. Betatron (1) according to Claim 8, characterized in that the detector is connected to the control electronics (8), and the switch-on point and the switch-on duration of the switch (9) are ascertainable from the output signal of the detector using the control electronics (8).
10. X-ray system for security checks of objects, having a betatron (1) according to one of Claims 1 to 9 and a target for generating X-radiation and an X-ray detector and an evaluation unit.

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