US3406349A - Ion beam generator having laseractivated ion source - Google Patents

Description

Oct. 15, 1968 SWAIN ET AL ION BEAM GENERATOR HAVING LASER-ACTIVATED ION SOURCE 2 Sheets-Sheet 1 Filed June 16, 1965 IIIIIII INVENTORS JAMES E. SWAIN BY JOHN B. TRUHEF? ATTORNEY Oct. 15, 1968 ION BEAM GENERATOR HAVING LASER-ACTIVATED ION SOURCE Filed June 16, 1965 OUTPUT ENERGY L5 JOULES X ALUMINUM o CARBON u COPPER 0 IRON a NICKEL A TANTALUM J. E. SWAIN ET AL 3,406,349

2 Sheets-Sheet 2 2 5 POTENTIAL DIFFERENCE SBBBdINV) .LNBBEJOO 1 llllll 3 4 56789IO (JOULES) .2 .5 LASER OUTPUT ENERGY lllllll I (SBBBdWV) I NVENTORS JAMES E. SwA/A/ BY JOHN B. TRUHER ATTORNEY .LNBBUOO United States Patent 3,406,349 ION BEAM GENERATOR HAVING LASER- ACTIVATED ION SOURCE James E. Swain, Livermore, and John B. Truher, Redwood City, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed June 16, 1965, Ser. No. 464,589 7 Claims. (Cl. 328--233) ABSTRACT OF THE DISCLOSURE An ion source and accelerator for producing ion beams of energies up to several m.e.v. and up to hundreds of amperes, comprising a target of a material to be ionized disposed in the light beam path of a laser, means for concentrating the light beam to impinge the target surface with an energy sufiicient to ionize the target material and produce a plasma, electrode means for extracting the desired ions from the plasma produced by the light beam impinging said target, and electrostatic accelerator means for producing an electric field to accelerate the ions.

The present invention relates to charged particle beam generators, and more particularly, to an ion beam generator having an ion source employing a laser for production of ions and especially adapted for use in combination with electrostatic ion accelerators.

In its broad aspect, the present invention provides an ion source capable of generating heavy, i.e., high atomic or molecular weight, ions for use in an ion gun, which is particularly'suited for use in electrostatic ion beam accelerators. In its specific use, the present laser beam ion generator is adapted to a Van de Graaf generator, to furnish a beam of heavy ions of improved intensities, e.g., at least 300 amperes. Generally the apparatus of the present invention forms a plasma including as its major constituent a selected ion species, by directing a laser light beam on a target body of feed material including molecules of the selected ion species. The ionizing action of the impinging laser beam liberates the molecules to form the plasma by evaporation and/or erosion of the surface of the target. The desired ions are then electrostatically extracted from the plasma and accelerated.

A primary advantage provided by the present ion beam generator is the increased intensity, i.e., current density of the ion beam which may be generated thereby. A primary factor which determines the relative rate at which the ions are produced, i.e., the ion beam current density which is produced is the power level of the light beam impinging on the feed body. Any laser preferably delivering a kilowatt or more of power is suitable for use in the ion source of the present invention and the light output of the laser device is directly determinative of the ion output current levels that may be achieved.

As evidenced by the teachings of the prior art, the production and acceleration of beams of light ions is well known. Machines which yield electrons, protons, deuter ons and a particles at relatively high amperages are numerous and extensively used, especially in nuclear reaction experiments. With the advent of heavy ion accelerators, such as, the Hilac (Heavy Ion Linear Accelerator) at Lawrence Radiation Laboratory, of the University of California, and the accelerator described in US. Patent No. 2,867,748, Jan. 1, 1959, Chester M. Van Atta, the mass range of atomic species useful for nuclear reaction experiments has been extended to about an atomic number of 10. If high energy ion beams of the heavier atomic species are desired, electrostatic type accelerators are the only accelerators thus far known to be available for their ice generation. The electrostatic accelerator, notably the Van de Graaf generator, has many advantages, primarily due to the simplicity and relatively low cost of the instrument. Also, the instrument may be used to accelerate light ions as well as heavy ions without basic changes. However, the most serious limitation of the electrostatic accelerators including the Van de Graaf is the low current intensities of the ion beams issuing therefrom, which ion currents are usually in the range of milliamperes to about 1 ampere.

In contrast thereto, ion currents poduced by an electrostatic accelerator utilizing the ion source of the present invention can be up to at least more than two orders of magnitude greater. As a consequence the time required to produce nuclear reaction products in reasonable quantities can now be run in greatly diminished times which contributes to extraordinarily greater yields when isotopes of very short half life are the desired products. More important still, reactions which formerly were never carried out, because the time required was of impractical length, can now be performed in reasonably short periods. For example, various products having a short lifetime could never be made in reasonable quantities by the apparatus of the prior art because such products disintegrated more rapidly than they could be accumulated in amount sufficient even for experimental purposes.

Another advantage of the present ion beam generator is that only relatively small ion generating portions of the ion source need be positioned in the accelerator itself which facilitates its incorporation within various accelerator systems. The auxiliary equipment of the ion source, i.e., the laser light source, electrical power supplies and monitoring equipment, can be arranged externally, requiring only a light path be established between the laser source and feed material.

Accordingly, a primary object of the present invention is to provide an ion source capable of providing an ion current pulse output of up to or exceeding 300 amperes.

More particularly, an object of the present invention is to provide an ion source which is especially suited for use in conjunction with ion accelerators and moreespecially electrostatic accelerators.

Another object of the present invention is to provide an ion source capable of supplying heavy element ions at high current intensities, particularly atomic species having an atomic weight greater than ten.

A still further object of the present invention is to provide an improved Van de Graaf generator having a high current output of heavy ions.

Other objects and advantages will become apparent upon consideration of the following description taken in conjunction with the drawings, of which:

FIGURE 1 is a schematic representation of the ion source of the present invention;

FIGURE 2 is a cross-sectional view of a Van de Graaf generator, incorporating a laser beam ion generator in accordance with the invention;

FIGURE 3 is graphical representations illustrating the dependence of ion output current on the accelerating voltage used to extract the ions emitted from the target; and

FIGURE 4 is graphical representations illustrating the average ion current generated from various target materia s, by a single laser beam light pulse.

Basically the ion source of the present invention comprises laser means, which is energized to generate a beam of coherent light and an emitter target body comprised of a material containing an element corresponding to a selected ionic species upon which the beam of coherent light is directed to vaporize and ionize said element. The ions created by the action of the laser beam form a plasma adjacent to the target. The invention further contemplates means for extracting selected ions from the plasma and accelerate these ions along a defined path to provide a collimated ion beam.

More specifically, referring now to FIGURE 1. there is shown an emitter target 11, which is comprised of a body of, preferably, solid material which contains, as high a concentration as possible of an atomic or isotopic species of which an ion beam is to be formed. In principle there is no significant limit to the species of ion which can be produced. The ion source of the invention will be particularly useful for producing ion beam pulses of elements and isotopes above atomic weight ten since prior devices are not especially effective in the range. However, lower atomic weight ions can likewise be generated in the event an effective emitter target can be provided. Any material susceptible to vaporization and ionization by the lower beam should be suitable. It is not necessary that the material be solid. The emitter 11 may be, for example, a pool of liquid, such as mercury, if there are provided reasonable safeguards, such as baffles, to prevent undesirable splattering or the fluid might be absorbed into a porous ceramic or porous metal body on which a surface layer of the liquid is retained. The target 11 is disposed to receive a high intensity coherent beam of light 12, directed from a laser light source 13. The laser light source may be any conventional unit, preferably having an output of about one kilowatt or more. Since the number of ions emitted from the target is proportional to the laser output, other parameters being equal, the use of a relatively high-powered laser unit will, of course, be reflected in a marked increase in the ion output of the present ion source.

An optical lens 14 is positioned between laser 13 and emitter target 11 to intercept and concentrate the light beam 12 onto an appropriate surface of emitter 11. It has been found, in practice, that the number of ions ejected from target 11 increases to a maximum if the laser beam 12 is focused at a focal point 16 somewhat outwardly of the surface of target 11, rather than on the target surface 17 itself. Maximum ion production occurs if thefocal point 16 is about A" away from the surface 17 of target 11. Although a convex lens 14 is shown other optical focusing lenses or a metallic reflector lens system and an optical window may be substituted therefor. In a typical construction emitter 11 is disposed within the closed end region of an elongated tabular envelope 18. Envelope 18 may be made of an insulator such as glass with lens 14 supported in a hermetic mount (not shown) or in an opaque envelope used in the same manner or provided with a light pervious window disposed externally to admit laser light beam 12. The en velope is evacuated by a vacuum pump 19 coupled thereto. A filter grid 21 of spaced parallel conductors is disposed transversely across envelope 18 in spaced relation to emitter 11, and accelerating grid electrode 22 is disposed in spaced parallel relation to grid 21 distally of emitter 11. The grid 21 and electrode 22 may simply be supported by attachment to the glass or ceramic wall of the envelope 18, or alternatively by stand-off insulators or the like. A direct current (DC) power supply 23 is arranged with the negative terminal 24 connected to accelerating electrode 22. The positive terminal 26 of supply 23 is connected to emitter 11 and in common to filter grid 21 which is mounted in envelope 18 between target 11 and accelerating electrode 22 across the line of sight path therebetween at a distance of up to a few centimeters from accelerating electrode 22. Filter grid 21 forms a boundary to the space between emitter 11 and grid 21 occupied by the plasma. Being positively charged with respect to electrode 22, grid 21 attracts and collects electrons from within the plasma. Positive ions are extracted from the plasma by the negative potential between emitter 11 and electrode 22 to escape through the grid 21 by virtue of their higher momenta and are then accelerated toward electrode 22 by the negative electric field gradient established between electrode 22 and filter grid 21. A parallelly connected resistor 27 and monitoring oscilloscope 28 serially connected between negative terminal 24 of supply 23 and electrode 22 afford means for measuring the ion current intercepted by electrode 22, such ion current being proportional to the total generated ion current. The principal portion of the ion current passes unimpeded through the open end of envelope 18 and is available for use thereafter. The vacuum pressure to which envelope 18 is evacuated depends on the voltage impressed between grid 21 and electrode 22. For a potential diiference of a few thousand volts between grid 21 and electrode 22, the pressure should be reduced to at least 10- mm. Hg.

As illustrated in FIGURE 1, electrode 22 furnishes the initial accelerating potential for the ions liberated from emitter 11. Electrode 22 is generally apertured to allow the ions to pass through into a beam tube or the like affixed to the end of envelope 18 distal emitter 11. However, a high accelerating voltage can be impressed on electrode 22 to accelerate the ions to energies useful for example, in bombardment experiments, or the study of nuclear characteristics of materials. If desired, a target may be disposed at or in front of electrode 22 in which case electrode 22 may be of solid discoidal construction and removably sealed to envelope 18.

As noted supra, the region between grid 21 and electrode 22 preferably is maintained at a low pressure with a sufficiently large distance between these elements to prevent arcing therebetween. It has been found, however, that arcing or glow discharge between these electrodes produces ions having higher ionization states, e.g., doubly, triply, and more highly ionized ions. Hence, by adjusting conditions so that arcing will occur between grid 21 and electrode 22, e.g., by increasing the voltage between these elements or decreasing the distance therebetween, ions of higher ionization states can be generated. Moreover, it is theorized that arcing also increases the intensity of the primary ion beam by causing the ionization of neutral atoms and molecules liberated from the target 11 by the laser light beam 12.

Example A Q-spoiled laser 13, comprising a 6" x /z" Brewster angle ruby rod, pumped by two 6" linear Xenon flash lamps furnished a light pulse output of 0.2 joules in 70 nanoseconds. The light beam 12 was directed at a nickel target 11 and focused to a focal point 16 one-quarter inch from the target surface 17 by a lens 14 having a focal length of 1.25 inch. Voltage supply 23 established a potential difference of 3,000 volts between grid 21 and electrode 22. Grid 21 was a 120 mesh 27% transmitting nickel grid. Electrode 22 was of solid construction to enable collection thereon of the entire nickel ion beam generated. The distance between grid 21 and emitter 11 was set at /2". The peak ion current incident on the electrode 22 was measured by means of a (ohm) resistor 27 and oscilloscope 28 and found to be 14 amps per light pulse.

More extensive data were obtained with an un-Q- spoiled laser 13 having an output of 1.5 joules in the form of a light pulse of a duration of about 200 microseconds. The data are summarized in the graphs of FIG- URES 3 and 4. FIGURE 3 illustrates the dependence of the peak current on the grid collector voltage at an output of 1.5 joules. Graph a indicates the total effective current including arcing and ion current components at electrode 22 under arcing conditions of operation, while graph b represents an estimate of the minimum ion current component incident thereon. The graphs a-f of FIGURE 4 show the total peak current incident on electrode 22 for a variety of target materials, i.e., aluminum, carbon, copper, iron, nickel, and tantalum. The potential difference between the grid 21 and electrode 22 was maintained at 5 kv. The ion current component of the total current is at least 40%. A typical installation wherein an ion source in accord with the invention is illustrated in FIGURE 2. In FIGURE 2 is shown in cross section a portion of the potential spheres of a Van de Graaf generator. The accelerator depicted is of the type wherein positively and negatively charged spheres form an electric field therebetween to accelerate an ion beam. As will be apparent from the discussion below, the inventive principle applies equally well to a single potential sphere Van de Graaf accelerator where the ions are accelerated by the electric field between the potential sphere and ground. A more detailed description of Van de Graaf generators as well as other electrostatic generators is found in Chapter 4 of Applied Nuclear Physics, Pollar and Davidson, 2nd Edition, John Wiley and Sons, New York, N.Y., 1951. For present purposes Van de Graaf generators may be basically provided as follows:

A first potential sphere 31 in FIGURE 2 is mounted at one end of a tubular support 32. A conventional belt charge transfer mechanism is mounted with tubular support 32 and transfers positive charge from charge inducing means (not shown) as in conventional practice to sphere 31 via insulating belt 33. Brush collectors 34 pick up the positive charge from the belt 33 and conduct it to sphere 31. Simultaneously, the negative charge is removed from sphere 31 by spray point 36 which is conductively communicated between the inside surface of sphere 31 and that portion of belt 33 from which the positive charge is to be removed. Positively charged member 37 attracts electrons from the sphere 31 to the spray point 36, wherefrom they are removed by the moving belt 33. A second potential sphere 38 is mounted in spaced relation to the first sphere 31 and is charged to an opposite polarity in a manner similar to sphere 31, by conventional means (not shown). It is noted, however, that sphere 38 can be referenced to ground potential or eliminated entirely. For the acceleration of positive ions sphere 31 need only be referenced to a negative potential source and a corona discharge established between such source and sphere 31.

Accelerating tube 39 defines an elongated evacuated volume 41 through which the ion beam from an ion source is directed to be accelerated. A first end 42 of accelerating tube 39 is hermetically joined to sphere 31. Similarly, the other end 42 of accelerating tube 39 is hermetically joined to sphere 38. The surface of each sphere, 31 and 38, in hermetic communication with the interior volume 41 of tube 39 define respectively apertures 44 and 46 to provide communication of the interior 41 of accelerator tube 39 with the respective interiors of spheres 31 and 38.

To establish a uniform voltage gradient between spheres 31 and 38, doughnut-shaped conductors 47 are mounted at regular intervals about tubular member 39 in coaxial relation thereto. The doughnut-shaped conductors 47 provide the uniform potential gradient between the spheres 31 and 38 by providing a conducting path for a corona discharge from one sphere to the other, e.g., from sphere 31 to sphere 38. In order to confine the ions near the center of the tubular member 39, drift tube 48 is electrically connected to each doughnut-shaped conductor 47. The cylindrical conductors 48 are coaxially aligned in spaced apart relation within tube 39. The confinement and acceleration of the particles directed through tube 39 is accomplished by the action of the electric field existing in gaps 49 between adjacent tubular conductors 48.

The regions 51 and 52 defined by the respective spheres 31 and 38 are at the potential to which the respective sphere is raised. The ion source of the invention is disposed within the field free region 51. More particularly, the charged particle source comprises emitter target material 53 which furnishes selected ions for the formation of the ion beam. In one embodiment, target 53 is in the form of a thin wire, which is stored on a spool 54 carried, e.g., by bearing mounts (not shown) and advanced on command by friction spools 56 provided with motor or manual drive means (not shown) as the tip 57 of the wire is eroded away to form plasma by a laser light beam 58 that is directed to impinge thereon.

When sphere 31 is of a design not required to be evacuated, laser target 53 and friction drive spools 56 are hermetically enclosed in a tubular vessel 59 similar to envelope 18 which is closed at one end 61 and hermetically joined to the inner surface of potential sphere 31 at the opposite end 62 in axially aligned relation to aperture 44, as by direct connection or by flanged joint (not shown) to thereby communicate with interior volume 41 of ac celerating tube 39. Accelerating tube 39 and tubular vessel 59 are then evacuated jointly by pumps (not shown).

Towards generating charged particles, a laser light source 63 mounted externally to sphere 31, generates a laser light beam 58 which is directed through an open space within support column 32 to impinge on target 53. For example, column 32 is provided with a light pervious window 66 at some distance outside sphere 31 and a prism 67 mounted adjacent thereto in the open space of column 32 to reflectively direct the light beam 58 toward the emitter tip 57. A lens 68 mounted in the wall of vessel 59 in the path of light beam 58 concentrates the light beam onto target tip 57 where it causes the emission of ions and electrons with finite velocities from the target surface. For optimum results, the lens 68 is positioned to have its focal point at a distance of about A" from the surface of target 53.

The emitted ions and electrons form a moving plasma 69 proximate the surface of target 53, the plasma 69 moving away from the target surface. Towards forming a beam of ions, a filter grid 70 is mounted in spaced relation to target 53 in path of the moving plasma 69. As noted hereinbefore, the electrons of plasma 69 are separated from the positive ions which are to be formed into an ion beam by charging both target 53 and grid 70 positively. More particularly, a potential generator 71 has a positive terminal 73 connected to target 53 and grid 70, and a second less positive terminal 72 to an accelerating electrode 74. Power supply 71 shown schematically in FIG- URE 2 is disposed outside the main portion of the Van de Graaf generator and furnishes a multi-kev. potential difference between positive terminal 73 and terminal 72. The wire connections are made through the interior of the support column 32 to target 53, grid 70, electrode 74.

A grid mesh, highly transmissive accelerating electrode 74 is preferably of a convex construction and is disposed to point its distended central position toward the grid 70. The high current ion beam has a tendency to diverge rapidly due to the mutual repulsion of the charged particles. This tendency to diverge is counteracted by the axially converging electric field generated by the convex shape of the accelerating electrode 74. Accelerating electrode 74 serves the further purpose of accelerating the positive ions which pass through grid 70 towards accelerating tube 39. The beam is further concentrated and focused by a focusing electrode 76 disposed between the accelerating electrode 74 and aperture 44 and electrically connected to the negative terminal 77 of potential generator 71. The voltage delivered to focusing electrode 76 is negative with respect to accelerating electrode 74 and positive with respect to potential sphere 31.

In operation, the ions liberated from target 53 by laser beam 64 are formed into an ion beam and preaccelerated through the full potential difference between the grid 70 and potential sphere 31. The preaccelerated ion beam enters through aperture 44 into accelerator tube 39 and is accelerated through the full potential difference between the potential spheres 31 and 38, the accelerated beam is then directed to impinge upon a target 78 disposed at the end of the tube 39. Although target 78 is shown in FIG- URE 2 as being disposed at aperture 46 of sphere 38, it could be positioned inside sphere 38 or at any point along tube 39.

It is to be understood that the present ion source is also adaptable to be used in Van de Graaf generators having a single potential sphere. In such devices the support column 33 generally serves to house the beam tube as well as the belt charging mechanism. Or, as is the case in machines having a potential sphere comprised of two joined spherical shapes, belt charging mechanism transfers the charge to one section of the joined spheres within the support tube provided therefor, while the beam tube is integral with the support for the other half of the joined spheres.

To adapt the present invention to use in conjunction with electrostatic accelerators of the type having a charged plate rather than a closed spherical shell as a high voltage terminal, the most negative terminal of the power source employed in forming the ion beam must be connected directly to the high voltage terminal of the accelerator. At the same time care must be taken that points connected to the positive terminal do not exceed the maximum supportable voltage, since these points are at a higher potential than the high voltage accelerator terminal itself.

Throughout the description the problem of separating one particular ionic species from a beam composed of a plurality of positive ion species has not been dealt with. Often it is not necessary to single out one ionic species, as for example in experiments where the desirable product is easily separated from by-products formed from other accompanying ions. For those reactions or experiments, however, where the purity of the ion beam is of importance, the beam may be split by well known techniques, e.g., in accordance with the teachings of the mass spectroscopy art by passing the ion beam through a magnetic field perpendicular to the field lines thereof. In view of the numerous possible embodiments of the present invention which will be apparent from the teachings of the invention, we pray the present invention be limited only by the following claims.

What is claimed is:

1. Apparatus for generating and accelerating a high intensity ion beam current, comprising,

(a) an evacuated accelerating chamber, said accelerating chamber adapted to be directionally permeated by a high intensity first electric field,

(b) an emitter body including a target element disposed in said accelerating chamber in coextensive alignment with said first electric field,

(c) enclosure means circumscribing said target body in spaced relation thereto, said enclosure means defining a hermetically sealed region communicating with the interior defined by said accelerating chamber in coextensive alignment with said first electric field, said enclosure means defining window passive to light in juxtaposed relation to said target body,

(d) laser light source having an output of at least one kilowatt to furnish an energetic beam of coherent photons,

(e) optical means for focusing said beam of photons onto a point proximate the surface of said emitter means, effective to impinge upon said surface a beam of photons of an energy density sufficient to vaporize and ionize said elemental species,

(t) a conductive grid disposed transverse to the principal axis of said enclosure means in spaced relation to said target body,

(g) an accelerating electrode having a structure passive to said ions disposed transverse to the first electric field in said accelerating chamber in spaced relation to and on the side said grid' distal said target body; and

(h) a voltage source having a first and second output terminal, said first terminal positive with respect to said second terminal, said first output terminal being electrically connected to said target body and said grid, said second terminal being connected to said accelerating electrode to thereby establish a second electrostatic field to accelerate said ions of said element from said plasma along a path between said target and said accelerating electrode, said first electric field in said accelerating chamber being established by a first charged conductive shell terminal, said accelerating chamber being joined 'at a first end to said shell and extending radially away therefrom, said shell further defining an aperture within the region where said accelerating chamber and shell are joined, said aperture being axially aligned with said accelerating chamber, said target body, grid, and accelerating electrode disposed within the volume defined by said shell in axial alignment with said aperture.

2. Apparatus of claim 1 further defined in that accelerating electrode has a convexly curved structure, said electrode being disposed with its outer portions flaring uniformly and axially away from said grid.

3. Apparatus of claim 1 in which said first charged conductive terminal is spherical and said apparatus includes a second conductive spherical shell charged opposite said first spherical shell, said conductive spherical shell being joined to said accelerating chamber at the end thereof distal said first spherical shell.

4. Apparatus of claim 3 further defined in that said second spherical shell defines an aperture aligned with said second electric field established between said first and second spherical shells to provide communication between the volume defined by said accelerating chamber and a portion of interior of said sphere, said portion of the interior of said sphere being adapted to be evacuated and contain a beam target in transverse relation to said second electric field.

5. The apparatus of claim 1, further defined in that the distance between said surface of said emitter and said point upon which said beam of photons is focused is approximately inch.

6. The apparatus of claim 1, further defined in that said emitter is comprised of a high Z metal, and in that said laser has an output energy between about .2 and .3 joules.

7. The apparatus of claim 1 wherein said conductive shell terminal is spherical.

References Cited UNITED STATES PATENTS 3,294,970 12/1966 Jenckel. 3,360,733 12/1967 Vali et al. 328233 ROBERT SEGAL, Primary Examiner.

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