EP0012054A1 - Ion beam bundling device with two acceleration intervals of differing lengths, working in a broad range of speeds - Google Patents

Abstract

Ion beam grouper-unbundler, comprising two interaction intervals (I1, I2) defined by a sliding tube (22) and highly asymmetrical inlet (20) and outlet (24) conduits, the one of the two intervals offering the ion beam an electric field much lower than that offered by the other interval, the action exerted in said interval by the electric field on the ions then being negligible compared to that which is exerted in the another interval. Application to ion acceleration installations.

Description

The present invention relates to an ion beam grouper-unbundler, at asymmetrical intervals, and operating in a wide speed range. It finds application in ion acceleration installations.

It is known that an ion beam grouper or ungrouper is constituted by a resonant structure supplied by a high frequency or hyper-frequency source, and crossed by an ion beam so that the electric field established in the structure modulates the ion velocity appropriately.

In a consolidator, the modulation on the speed has the effect of accelerating the slow ions more than the fast ions, which allows grouping in packets of small spatial extent at a determined distance from the consolidator; the speeds of the different ions constituting a pack are then distributed over a wider range. Consolidators are used in ion acceleration systems when, for example, time of flight experiments or any injection into a high frequency accelerator is desired.

In an unbundler, modulation has the effect of increasing low speeds and reducing high speeds, which makes it possible to reduce the speed dispersion of the ions. Such an apparatus is used when it is desired to use monoenergetic ions, without particular concern over the width of the packet of ions.

Figures 1 and 2 recall, in broad outline, the principles of construction and operation of these two devices.

On part (a) of FIG. 1 is shown a resonant structure constituted by a wall 2 closed at its two ends by lateral faces 4 and 6 crossed respectively by inlet 8 and outlet 10 conduits of a beam d 'ions 12. The structure shown further comprises a sliding tube 14 connected to the wall 2 by a conductive support 16. This tube slip is separated from the conduits 8 and 10 by two identical intervals I ₁ and 1 ₂ . All these parts are conductive and for example metallic.

Part (b) of this same figure 1 illustrates the electrical diagram of the structure shown in part (a). These two conduits 8 and 10 are connected to ground (or more generally to a reference potential) and the sliding tube 14 is brought to an alternating voltage V due to the high frequency (or hyper-frequency) field prevailing in the structure (this voltage V being counted from said reference potential). The intervals I ₁ and 1 ₂ each have a length 1 and their centers are distant from the length L. The same mean electric field V / 1 therefore prevails in these two intervals.

The diagrams in FIG. 2 illustrate the operation of the device in FIG. 1. The ions of average speed penetrate in the interval I ₁ at the instant t ₀ (part a), the distance z which they travel is plotted on the ordinate as a function of the times on the abscissa. These ions are subjected (b) to an electric field E ₀ during their crossing of the interval I ₁ , this field modifying their speed slightly, (this modification of speed is generally small compared to the speed proper). Faster ions than the previous ones reached the interval I ₁ at time t ₁ before t ₀ . They are subjected to a field E ₁ weaker than E ₀ Conversely, slower ions reach the interval I ₁ only at time t ₂ > t ₀ and the latter see a field E ₂ more intense than E ₀ . The relative magnitudes of the fields are therefore such that the slowest ions will be able to join the fastest ions: this is the grouping operation.

This mechanism obviously assumes that the electric field grows over time appropriately. In practice, we rarely use the ideal modulation which would be linear, but rather a sinusoidal modulation or a sum of sinusoidal modulations much easier to obtain, this modulation being used only on a substantially linear part. The ions which enter the interval II at periods when the field does not exhibit the appropriate variation are obviously not grouped. But, for the others, we observe a grouping at a distance z ₀ of the interval I ₁ .

In an ion decoupler, the mechanism is analogous to the difference that it tends to reduce the energy deficit of slow ions and reduce the excess energy of fast ions. A decoupler placed at a distance z receives ions at time t ' _o and applies a field E' _{0 to them} . Faster ions have reached the interaction interval of the unbundler at time t 'prior to t' ₀ . They are subjected to a field E ' ₁ weaker than E' ₀ . As for the slower ions, which reach the ungrouper at t ' ₂ , they see in the interaction interval a field E' ₂ greater than E ' ₀ . At the exit of the unbundler, the ions are driven at substantially equal speeds, but correlatively, they occupy an extended part of the space.

Both in a grouper and in a degrouper, the intervals where the ions undergo the action of the electric field must be short enough so that the transit time of the ions is less than the half-period of the field. If v is the speed of the ions and T the period, we must therefore have:

By way of explanation, it can be indicated that the voltages usually encountered in ion beam groupers are defined by two imperatives: the speed modulation provided to the beam must be low compared to the speed of said beam, and the voltage d acceleration must be great in front of the natural fluctuations of the beam. In practice, voltages of the order of a few tens of kilovolts or less are used.

As for the voltages used in the unbundlers, they are of the order of magnitude of the dispersion in beam energy and can be between 10 kV and 100 kV approximately.

On these devices, see the article by E.L. HUBBARD et al entitled "Heavy-Ion Linear Accelerator" published in the review "The Review of Scientific Instruments", vol. 32, No. 6, June 1961, page 621 and the article by JS SOKOLOWSKI et al entitled "Status Report on Stanford's Superconducting Heavy Ion Linear Project" published in the journal "IEEE Transactions on Nuclear Science", vol NS-24, N ° 3, June 1977, page 1141, and finally the article by B. CORK entitled "Proton Linear Accelerator Injector for the Bevatron" and published in the review "The Review of Scientific Instruments" vol. 26, No. 2, February 1955, page 210.

These generalities having been recalled, it is possible to define the invention in relation to the prior art. The structure shown in Figure '1 is, among the known structures, that which most closely resembles the invention. We can consider that it is formed of a first part constituted by the tube 16, the faces 4 and 6 and the wall 2, this part being equivalent to a resonant line in X / 4, if λ is the wavelength of the electromagnetic field introduced into the structure and of a second part constituted by the intervals Il and I ₂ which are zones of a capacitive nature.

The advantage of such a structure is that it has a small footprint (less than λ / 4) whereas for structures with a single interaction interval, the dimensions are of the order of λ / 2, which becomes prohibitive for working frequencies lower than 100 MHz (the half-wavelength is then equal to 1.5m).

This structure of the prior art requires that the actions exerted by the electric field on the ions in the two interaction intervals are cumulative. This implies that the ions travel the distance L separating these two intervals in a time which is a odd multiple of the half-period T of the field. This structure of the prior art therefore only works correctly if the ions are driven at a speed close to 2L / T (or a multiple of this speed).

This constraint imposed on the speed of the ions is troublesome in most applications of groupers and unbundlers. Indeed, these devices are generally used in installations comprising, for example and successively, an ion source, an injector, a first accelerator (of the Van de Graaf type for example), a second accelerator (of the linear type for example ). However, in such installations, it is frequent to have to vary the energy of the ions, which amounts to modifying their speed, or to pass from one type of ions to another with constant energy, which involves another change in their speed.

It is therefore not possible to use the devices described in all cases and they must be modified in their size as required, which is inconvenient.

Admittedly, devices with a single acceleration interval are known which do not have this drawback by the very fact of the uniqueness of this interval. But as has already been pointed out above, these devices have a major drawback because of their bulk which is all the greater the lower the frequency.

The subject of the invention is precisely a grouper-unbundler which simultaneously remedies these two drawbacks. To this end, the grouper-unbundler of the invention is of the type which includes two intervals and, as such, it benefits from the advantage offered by this family of devices, namely a small footprint. Furthermore, the grouper-unbundler of the invention does not have the disadvantage of having a speed range close and this thanks to an original arrangement of the two interaction intervals.

The grouper-unbundler of the invention can therefore operate in a wide range of speeds while having a small footprint.

This double aim is achieved by the use of two intervals which are no longer symmetrical, as in the prior art, but on the contrary very asymmetrical, one of them being the seat of an electric field which is weak compared to the field that reigns in the other. The contribution of this interval in the speed modulation process is then negligible compared to that of the other interval. It follows that the condition on the cumulative nature of the two actions exerted by the field in the two intervals disappears and with it, the constraint on the travel time of the ions from one interval to another.

More specifically, the present invention relates to an ion beam grouping-ungrouping unit, of the type which includes a resonant structure supplied by a high frequency or hyper frequency generator, this structure comprising a cylindrical wall closed by two lateral faces traversed respectively by an inlet duct and a beam outlet duct, and comprising a sliding tube disposed between said ducts and defining with the inlet duct a first interval and with the outlet duct a second interval, the ion beam being introduced into this structure through the inlet duct, first undergoing, in the first interval, a first action from the electric field which prevails there, then traversing the sliding tube and finally undergoing in the second interval a second action on the part of the electric field which reigns there and leaving the structure by the outlet conduit; this consolidator-unbundler is characterized in that the two intervals defined by the sliding tube and the inlet and outlet conduits are highly asymmetrical, one of the two intervals providing the ion beam with an electric field much lower than that offered by l 'other interval, the action exerted in said interval by the electric field on the ions, then being negligible compared to that which is exerted in the other interval.

According to a first variant, the grouper-unbunder of the invention is characterized in that the interval with weak action has a length such that it is traversed by the ion beam in a large time in front of the half resonance period of the structure.

According to a second variant, the grouper-unbunder of the invention is characterized in that the inlet and outlet conduits have different sections, the sliding tube having a flared shape passing from a small section equal to that of the 'one of the conduits with a strong section equal to that of the other conduit.

According to a third variant, the two preceding arrangements are combined.

• - la figure 1 représente une structure résonnante de groupeur-dégroupeur selon l'art antérieur,
• - la figure 2 est un schéma explicatif du fonctionnement d'un groupeur-dégroupeur,
• - la figure 3 représente un schéma électrique d'une structure résonnante de groupeur-dégroupeur selon une première variante de l'invention,
• - la figure 4 représente, en coupe, une structure correspondant à ce schéma,
• - la figure 5 représente schématiquement une structure résonnante de groupeur-dégroupeur selon une seconde variante de l'invention,
• - la figure 6 représente schématiquement une structure résonnante du groupeur-dégroupeur selon une troisième variante de l'invention,
• - la figure 7 représente un mode particulier de réalisation d'un groupeur-dégroupeur conforme à l'invention.

In any case, the characteristics and advantages of the invention will appear better after the description which follows, of embodiments given by way of explanation and in no way limitative. This description refers to drawings in which:

• FIG. 1 represents a resonant grouper-unbundler structure according to the prior art,
• FIG. 2 is an explanatory diagram of the operation of a grouper-unbundler,
• FIG. 3 represents an electrical diagram of a resonant grouper-unbundler structure according to a first variant of the invention,
• FIG. 4 represents, in section, a structure corresponding to this diagram,
• FIG. 5 schematically represents a resonant grouper-unbundler structure according to a second variant of the invention,
• FIG. 6 schematically represents a resonant structure of the grouper-unbundler according to a third variant of the invention,
• - Figure 7 shows a particular embodiment of a grouper-unbundler according to the invention.

Figures 1 and 2 relate to the prior art and have already been described. In the other figures relating to the present invention, it is assumed, by way of illustration, that the first interval crossed by the ion beam is the predominantly acting interval, the second interval occurring negligibly. But it goes without saying that this order could be reversed, the beam being able to penetrate first in the interval with negligible action. Furthermore, in the description which follows, it is a question of a "grouper", but it goes without saying that the structures described can also function as a grouper.

Figures 3 and 4 firstly illustrate a first variant of the invention. In the diagram of FIG. 3, there is an inlet conduit 20, a sliding tube 22 and finally, an outlet tube 24. An ion beam 26 successively passes through these three elements which are brought to respectively equal potentials at O, V and O. The real structure is represented in FIG. 4. It is supplied by a source 28, high frequency or microwave, the means for coupling this source to the resonant structure not being shown explicitly, because they are well known to those skilled in the art.

comme dans l'art antérieur.The inlet conduit 20 and the sliding tube 22 define a first interval I ₁ , of length 1 ₁ which is the seat of an electric field of average value V / 1 ₁ . The length 1 ₁ is chosen to be short enough so that the transit time of the ions is less than the half-period T / 2 of the field. So we have: 1 ₁ <

as in the prior art.

The sliding tube 22 and the outlet conduit 24 define a second interval I ₂ , of length 1 ₂ much larger than 1 ₁ .

comme l'est 1₁, de sorte que le temps de transit des ions à travers cet intervalle peut atteindre ou même dépasser la période T du champ. Celui-ci change donc de direction au cours de ce transit, de sorte que son action nette sur les ions est très faible.This difference between the lengths 1 ₁ and 1 _{2 has} two consequences: the first is that the mean electric field V / 1 ₂ which prevails in the second interval is much weaker than the mean field V / 1 ₁ which prevails in the first; the second is that the length 1 ₂ is no longer small in front

as is 1 ₁ , so that the transit time of the ions through this interval can reach or even exceed the period T of the field. It therefore changes direction during this transit, so that its net action on the ions is very weak.

These two peculiarities of the field - weakness of its intensity and alternation during the crossing of ions - contribute to make its action negligible compared to that which is exerted in the first interval. It follows that, in such a grouper, the speed of the ions is no longer subject to the condition mentioned above in connection with the prior art and which relates to the time taken by the ions to pass from an interval to l other, the instant when the ions enter the interval I ₂ becoming in effect indifferent.

FIG. 5 illustrates a second variant of the invention. The device shown also comprises an inlet duct 30, a sliding tube 32 and an outlet duct 34. However, this outlet duct has a larger section than the inlet duct. The sliding tube then has a flared shape which constitutes a transition between the inlet and outlet conduits.

où I₀ est la fonction de Bessel modifiée de première espèce et d'ordre 0 et β le rapport de la vitesse v des ions à celle de la lumière et λ la longueur d'onde du champ dans le vide. Si l'on désigne par x la quantité

, le développement en série de I ₀ est : 1 +

+ ...In the variant shown, the intervals I ₁ and I ₂ have the same length 1, = 1 ₂ . The average electric field V / 1 ₂ which prevails between the sliding tube 32 and the outlet pipe 34 is therefore the same as that which prevails between the inlet pipe 30 and the sliding pipe 32. But this is the field which prevails in an area distant from that crossed by the ion beam. The field which acts on the ions is in fact different from that which prevails at the level of the outlet conduit. If we assume that the structure is of revolution around the beam axis, the field E ₀ on this axis is connected to the field E _a at radius a, by the classical relation:

where I ₀ is the modified Bessel function of the first kind and of order 0 and β the ratio of the speed v of the ions to that of light and λ the wavelength of the field in vacuum. If we denote by x the quantity

, the serial development of I ₀ is: 1 +

+ ...

The field E ₀ on the axis is therefore less than the field E _a (which is on average equal to V / 1 ₂ ) and can be significantly less than this field. For example for = 0.01, λ = 6m (f = 50MHz) and a = 10cm, we have I ₀ ≃ 28.

In this variant, the second interval I ₂ therefore plays a negligible role compared to that played by I ₁ , because the field on the y axis is much weaker than in the first. This already erased role played by the interval 1 ₂ can be further reduced in a third variant, if this interval is lengthened, as shown in FIG. 6, until it has a long length 1 _{2 in} front of vT. Then, as in the variant illustrated in FIGS. 3 and 4, the field acts in directions which vary during the crossing of the ions, which reduces its net action.

It goes without saying that the structures which have just been described may be of revolution or have other shapes, for example parallelepipedic.

It also goes without saying that a wide variety can be adopted for the resonant cavity itself. of shapes. It can be of the a / 4 line type as in FIG. 4, but also with a helical or spiral support, the latter variety being illustrated in FIG. 7.

The structure shown in this figure comprises a cavity 40, inlet 42 and outlet 44 conduits, a sliding tube 46, a spiral conductor 48 forming self-inductance; the spaces between the tube 46 and the conduits 42 and 44 constitute capacitive zones. This arrangement makes it possible to significantly reduce the size of the structure.

In all cases, the cavity can be tunable by variations of certain dimensions.

Claims (4)

₁ . Ion beam grouper-ungrouper, of the type which includes a resonant structure supplied by a high-frequency or hyper-frequency generator, this structure comprising a cylindrical wall closed by two lateral faces crossed respectively by an inlet duct and a beam outlet duct, and comprising a sliding tube disposed between said ducts and defining with the inlet duct a first interval and with the outlet duct a second interval, the ion beam being introduced into this structure by the inlet duct, first undergoing in the first interval a first action from the electric field prevailing there, then traversing the sliding tube and undergoing, in the second interval, a second action from the electric field which reigns there and leaving the structure by the outlet pipe, characterized in that the two intervals defined by the sliding tube and the inlet and outlet pipes are strongly dissymmetrical, one of the two intervals providing the ion beam with an electric field much lower than that offered by the other interval, the action exerted in said interval by the electric field on the ions then being negligible before that which is exercised in the other interval. 2. Grouper-unbundler according to claim 1, characterized in that the inlet and outlet conduits have different sections, the sliding tube having a flared shape passing from a small section equal to that of one of the conduits to a strong section equal to that of the other conduit. 3. Grouper-unbundler according to une.quelconque of claims 1 and 2, characterized in that the interval with weak action has a length such that it is traversed by the beam of ions in a large time in front of the half resonance period of the structure. 4. Grouper-unbundler according to any one Claims 1 to 3, characterized in that the structure is of the quarter-wave line, or spiral, or helix, or localized self-inductance type.

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