Method And Device For Producing By Charge-transfer A Beam Of Neutral Particles Or Of Ions Having Multiple Charges

Abstract

A method and device for the production of a beam of neutral particles or of heavy ions charged a plurality of times in which electric charges are neutralized or transferred between the charged particles of an incident beam and a substance traversed by the beam. A substantially homogeneous stream of powder consisting of grains having a diameter of the order of 100 A is caused to pass through the incident beam of charged particles in order to form a target which is traversed by said beam, the rate of circulation being such that each charged particle should meet either one grain or at least a plurality of atoms of one grain.

Description

This invention relates to a method of production of a beam of uncharged particles which consists in neutralizing the ions of an incident beam of charged particles by transfer of electric charges between said ions and the atoms of a substance which is traversed by the beam; the invention is also directed to a device for carrying out said method.

When a beam of I.sup.+ ions passes through a target formed of a substance having a density N.sub.0 and a thickness 1, the charge-transfer collisions between the I.sup.+ ions and the atoms A of the substance neutralize the ions and give rise to neutral or uncharged particles in accordance with the reaction:

I.sup.+ + A -- I + A.sup.+ (1)

the charge-transfer yield, and therefore the proportion of uncharged particles in the output beam, is of maximum value if the density N.sub.0 and the thickness 1 comply with the condition :

N.sub.0 1 = 1/.delta..sub.+.sub.o ( 2)

In formula (2), .delta..sub.+.sub.o designates the electron-capture charge transfer cross-section : the above condition amounts to saying that the mean free path of charge transfer is equivalent to the thickness of the target.

Consideration has been given in the past to a number of different expedients for satisfying condition (2) at least in an approximate manner. A first expedient consists in making use of a very thin diaphragm which is interposed on the path of the beam of ions to be neutralized. However, a simple calculation shows that the diaphragm which satisfies condition (2) has a degree of thinness such that it proves extremely difficult to fabricate and is fragile to such a degree that it is pierced instantaneously if it is subjected to an intense beam of particles. By way of example, should it be desired to convert protons having 5 keV energy (for which .delta..sub.+.sub.o = 10.sup.-.sup.15 cm.sup.2) to uncharged particles H.sup.0 having practically the same energy, it is necessary to have a target consisting of a sheet for which N.sub.0 .sup.. 1 is of the order of 10.sup.15. If the diaphragm is fabricated from carbon-12, the ideal thickness for said diaphragm is of the order of 1 A., with a mass per unit area of approximately 0.02 .mu.g/cm.sup.2. In point of fact, the fabrication of diaphragms having a thickness of less than 100 A is not practicable in the present state of knowledge.

A diaphragm 100 A in thickness (corresponding to N.sub.0 1 = 10.sup.17) would still be acceptable from the point of view of neutralization. However, an intense beam of particles which passes through a diaphragm of this thickness would leave a sufficient proportion of its energy to destroy the diaphragm practically instantaneously. The only solution which remains is to adopt diaphragms having even greater thicknesses which have the disadvantage of attenuating the beam to a very substantial extent and to diffuse this latter.

Another solution which also forms part of the prior art consists in making use of a condensable gas curtain instead of a solid diaphragm and this curtain circulates transversely to the beam at supersonic velocity. The gas curtain is discharged at supersonic velocity from an ejector which must be so designed that the curtain maintains accurate localization in space (to permit adjustment of the thickness 1 and of the density N.sub.0) and condenses on a cold wall. A description of this design concept is given in the communication by R. Geller and F. Prevot (C.R. Acad. Sc. Paris 38, page 1578, 1954) to which reference may usefully be made. This method represents a substantial advance over the previous method. Carbon dioxide gas, water vapors and magnesium are employed in particular to form the condensable gas curtain. But this solution is subject to major difficulties of a technological order when the particle production rate increases : it is in fact necessary to increase the flow rate of the gas which constitutes the curtain progressively as the rate of production of the ions of the incident beam increases since the gas flow rate at supersonic velocity must represent a production rate of particles of the same order as that of the beam in order that each incident ion should find at least one neutral or uncharged atom with which it can carry out a charge transfer. In order to achieve neutralization of a beam representing a charge of several tens of amperes, namely a flux of the order of 10.sup.20 particles per second, and if it is desired to increase the pressure on each side of the curtain from 1 torr to 10.sup.7 torr over a distance of less than 1 m, the cold condensation wall must be capable of ensuring a pumping output of the order of 1,000 million liters per second at 10.sup.-.sup.7 torr, which would represent the equivalent of a large plant within a very small available volume.

Similar complications would be encountered if, instead of neutralizing an ion beam, it was desired to convert by charge-transfer a beam of weakly charged heavy ions into strongly charged heavy ions. It will be recalled by way of reference that the charge transfer process applies both to the capture of an electron (neutralization) and to the loss of one or a number of electrons when the ion passes through a curtain of material. Since the probabilities of electron captures or losses are essentially a function of the energy of the incident ion, in order to strip the electrons from a weakly charged heavy ion, it is only necessary to accelerate this latter to a velocity which is higher than that of its orbital electrons and then to pass said ion through the curtain of material.

By way of example, there have been obtained nitrogen ion beams of low intensity but strongly charged by passing through a thin diaphragm nitrogen ions of 15 MeV energy and charged once. The composition of the emergent beam was :

ions charged three times : 0.4% " " four " : 6 % namely an average charge " " five " : 45.6 % of 5.5 " " six " : 40 % " " seven " : 7.5 %

The aim of this invention is to provide a method and a device for producing by charge-transfer a beam of neutral particles or of heavy ions charged a plurality of times and capable of receiving intense particle beams while remaining within a technologically feasible range. To this end, the invention proposes a method which is primarily characterized in that a substantially homogeneous stream of powder consisting of grains having a diameter of the order of 100 A forming a target is circulated transversely to the incident beam of charged particles.

The invention also proposes a device for the production of a beam of neutral particles by neutralization of the ions of an incident beam, which makes it possible to carry out the method hereinabove defined, characterized in that said device comprises means for circulating transversely to the beam a substantially homogeneous flow of powder consisting of grains having a diameter of the order of 100 A and forming a target, the flow rate of powder being such that either one grain or at least a number of atoms of one grain each to each charged particle.

In accordance with a particular mode of application of the invention, said means comprise a duct adapted to open into a tube through which the beam passes, a supply which releases an adjustable flow of powder at the upper extremity of the duct and means for creating a vacuum within the tube and the duct.

In accordance with another mode of application, said means for constituting the target comprise an installation for delivering an electric charge to the grains of powder and means whereby an electric field for accelerating the grains towards the beam is produced within a separate zone with respect to the zone through which the beam passes.

Within the foregoing definitions, the term "charged particles" must be interpreted as applying essentially to molecular or atomic ions which can be accompanied by electrons.

The maximum grain diameter of 100 A corresponding substantially to the minimum thickness which can possibly be given to a solid diaphragm is the size which is found in various divided states, some of which do not in any case exist in the dry state and must accordingly be rejected. More precisely, there is found in particular in the grain-size range extending from 20 to 200 A metallic micelles, various aerosols, dust particles of smokes constituted by a dispersion in the colloidal state in a gas as well as charged clusters. The charged clusters, which are agglomerates of neutral molecules which are formed around ions produced by ionizing agents, are provided in comparison with the other substances contemplated above with the property of being charged : this property offers additional possibilities in the field of acceleration and collection but, on the other hand, entails the application of a costly and complex technology.

The powder which is employed must further comply with a certain number of conditions : the constituent material of said powder must not permit outgassing or dissociation by producing gases under the action of the incident particle beam. The grains must not give rise to particles of larger size by flocculation or coalescence at the end of a very short half-life. Among the particles whose use can be contemplated for converting an incident ion beam (nuclei of deuterium or tritium, for example) into a beam of neutral particles, it is possible in particular to mention chromium oxide in the form of powder having a grain size on the order of 40 A and slightly sub-stoichiometric titanium oxide having a grain size on the order of 80 A.

Once the powder has been chosen, three parameters are available for modifying the action of the target of powder on the beam of particles. These are the rate of passage of the grains, the thickness 1 of the target and the density N.sub.0 of the target. The choice will consist of a compromise between requirements which are to some extent contradictory : on the one hand, it is clearly necessary to ensure that the flow rate of grains is such that at least one grain should meet each incident charged particle. In practice, it will be found necessary to have a much higher ratio in order to come close to the optimum value so far as the other conditions are concerned. The ratio N.sub.0 .sup.. 1 must be chosen so as to comply substantially with condition (2) hereinabove. Finally, the target must be such that it does not diffuse the incident beam to an excessive extent. The thickness and the density of the target as well as the sizes of the unitary grains must accordingly be chosen in such a manner as to take into account the energy of the incident particles.

By way of example, if it is assumed that the grains have a diameter of on the order of 80 A and consist of a substance having a molecular mass of 50, it is possible to impart a velocity of 1.4 .times. 10.sup.3 cm/sec to the grains by allowing these latter to fall into empty space over a vertical distance of 10 m. If 100 atoms are permitted for each incident charge particle, a result of N.sub.0.1 = 5 .times. 10.sup.16 is achieved, that is to say a value which is scarcely higher than that provided by a solid diaphragm of very short duration. It is hardly possible for material reasons to increase the height of drop but, if charged grains are employed, it is only necessary to have a potential difference of 10 kV in order to impart a velocity of the order of 10.sup.5 cm/sec to said grains.

A theoretical study which need not be given here permits a determination of the angular diffusion of the incident particles. It is observed that in the case of ions having an energy of more than 1 KeV and grains having a maximum thickness of 100 A, the angular diffusion is limited to a few degrees. On the other hand, electrons are deviated to a much greater extent but this result represents an advantage rather than a disadvantage in the majority of applications, in particular in the case of the particle beams employed in thermonuclear physics in which it is desired to obtain neutral particles unaccompanied by electrons and produced at the same time as ions by many accelerating structures.

It is apparent that the invention provides a method and a device which replaces a static diaphragm having a thickness 1 which is incapable of withstanding an intense ion beam by a target constituted by flowing powder, the grians of which are continuously renewed. Provided that the powder which is selected does not undergo intense outgassing under the action of incident ion beam, there is no restrictive condition similar to that which is attendant upon the use of a condensable gas target.

A clearer understanding of the invention will be gained from the following description of a particular device for carrying out the invention which is given by way of non-limitative example, reference being made to the accompanying drawings, wherein :

Fig. 1 is a sectional diagram of the device ;

Fig. 2 is a view in elevation showing the device of FIG. 1.

The device which is illustrated diagrammatically in FIG. 1 and from the exterior in FIG. 2 comprises a reservoir 10 for receiving the powder. A filling opening 12 fitted with a leak-tight valve 14 serves to introduce the powder therein. The reservoir 10 is connected by means of an adjustable leakage cock 16 to a pumping installation for creating a vacuum therein. The leakage cock 16 permits progressive evacuation of the system which is compatible with the capacity of the pumping installation. This installation can consist solely of a primary pump 18 (shown in FIG. 2). There is also shown in the diagram of FIG. 1 a duct 20 for introducing an inert rinsing gas into the reservoir.

The dimensions of the reservoir 10 are such that this latter ensures a certain degree of self-contained operation and prevents over-frequent opening of the system, the result of which would be to release the vacuum. As a rule, this reservoir will be provided with moisture absorbent, with resistors for heating the walls and with calibrated screens for filtering the powder which escapes therefrom.

The powder 22 which rests on the bottom of the reservoir 10 flows towards a supply tank 24 through a duct 25 fitted with a flow-regulating valve 27, said tank being placed at a lower level and fitted with a slow-agitation device for maintaining the powder in the form of a homogeneous suspension. The tank 24 which is illustrated has a cylindrical shape and is fitted with a rotary stirrer 26 which ensures controlled agitation. Instead of a rotary stirrer, it would be possible to make use of other known types such as those which operate by translational motion ; the transmission can be effected without passing through the wall, for example by electromagnetic means. The stirrer can also be replaced by a sound generator: the major condition to be satisfied consists in preventing any deposit or sedimentation of the powder and in maintaining a substantially homogeneous concentration within the entire tank 24.

The tank 24 communicates through a duct 28 with a vacuum installation which can be the same as that of the reservoir 10. Baffles 30 (shown in FIG. 1) must usually be provided within the tank in order to prevent the entrainment towards the pumping installation of the powder grains which are maintained in suspension by means of the stirrer.

The tank 24 supplies by means of a variable-flow valve 32 a vertical duct 34 for the acceleration of the powder grains. The length of this duct is determined as a function of the velocity to be imparted to the grains in that portion of their trajectory in which they constitute the target. The duct 34 has its opening in a charge-transfer cell 35 traversed by the tube through which the incident ion beam 36 passes. A branch pipe 38 is taken off the duct 34 and connected to a pumping installation which serves to produce a vacuum therein. So far as concerns the transfer cell through which the ion beam passes, said cell is connected at 40 to a pumping installation which can be common with that of the powder-recovery chamber 42.

The chamber 42 in which the powder collects under the action of gravity, is connected by means of the duct 44 to a vacuum-producing installation which comprises not only a primary pump (not shown), but also a secondary vacuum pump 46 (ion pump, for example). The powder which collects at the bottom of the chamber can be withdrawn through a duct 48 and regenerated in order to permit re-use or else is finally removed, depending on requirements.

The charge-transfer cell 35 which is illustrated diagrammatically in FIGS. 1 and 2 corresponds to the case of an incident beam 36 made up of ions and electrons such as the beams produced by an accelerator of the "Pleiade" type, a description of which may be found in the article by T. Consoli entitled "H.F. Fields and Plasma Accelerators" (B.I.S.T. No. 102, March, 1966, pages 35 to 48), to which reference may usefully be made. With this type of beam, it is necessary to ensure that the transfer cell 35 in which the incident ions (hydrogen or deuterium nuclei) yield their charge to the powder is placed in an intense magnetic field produced by coils 49. In this case, the beam 50 which is delivered from the cell is essentially composed of neutral particles derived from the incident ions, a high proportion of the electrons having been diffused by the powder and retained by the collimator 52.

The powder is brought up to speed solely under the action of gravity within the vertical pipe 34 which is illustrated in FIGS. 1 and 2. Other solutions can be employed, especially when it is sought to attain a high velocity. In the case of batch operation, the action of periodic jets of fluid or of impulses produced by a piston may be added to the action of gravity. A further solution consists in charging the powder grains (for example by causing these latter to pass through a low-energy electron cloud which yields negative charges to said grains or by subjecting these latter to electron bombardment which causes the appearance of positive charges). It is also possible to make use of charged clusters. In this case, the grains can be accelerated by an electric field at the outlet of the tank 24. The collection of the powder can also be obtained by means of a localized electric field in the chamber 42.

Finally, the grains of substance can be accelerated by employing the radiation pressure of a laser beam disposed above the powder jet.

It is readily apparent that the invention is not limited to the embodiment hereinabove described and illustrated by way of example and accordingly extends to alternative forms of either all or part of the arrangements described which remain within the definition of equivalent means.

Claims

1. A method of production of a beam of neutral particles or of heavy ions charged a plurality of times in which electric charges are neutralized or transferred between the charged particles of an incident beam and a substance traversed by the beam, the steps of passing a substantially homogeneous stream of powder consisting of grains having a diameter of the order of 100 A through the incident beam of charged particles, forming a target of the powder which is traversed by said beam, the thickness of the target being equivalent to the mean free path of charge transfer and adjusting the rate of circulation of the powder whereby each charged particle meets either one grain or at least a plurality of atoms of one

2. A device for the production of a beam of neutral particles or heavy ions charged a plurality of times by neutralization of charged particles of an incident beam or addition of charges to said particles, wherein said device comprises a tube, means for passing a charged particle beam through said tube, and means for circulating transversely to the beam passing through said tube a substantially homogeneous flow of powder consisting of grains having a diameter of the order of 100 A and forming a target, the thickness of the target being equivalent to the mean free path of charge transfer, the flow rate of powder being such that each charged particle

3. A device according to claim 2, wherein said target forming means includes a vertical duct opening into said tube through which the beam passes and a supply which releases an adjustable flow of powder at the

4. A device according to claim 2, wherein said means for forming said target includes means for giving an electric charge to the grains of powder and means for creating an electric field for accelerating the

5. A device according to claim 4, wherein said device includes means for producing an electric field within a localized zone outside the beam and the grains which have passed through the beam entering said zone, said

6. A device according to claim 4 including irradiation means producing

7. A device according to claim 2, wherein said device includes a laser in which the beam radiation pressure accelerates the grains of said powder.

8. A device according to claim 2, the powder being selected from the group consisting of chromium oxide grains having a grain diameter on the order of 40 A and of slightly substoichiometric titanium oxide grains having a particle diameter on the order of 80 A.