US3634704A

US3634704A - Apparatus for the production of highly stripped ions

Info

Publication number: US3634704A
Application number: US68828A
Authority: US
Inventors: Thomas H Stix
Original assignee: US Atomic Energy Commission (AEC)
Current assignee: US Atomic Energy Commission (AEC)
Priority date: 1970-09-02
Filing date: 1970-09-02
Publication date: 1972-01-11
Anticipated expiration: 1989-01-11

Abstract

Apparatus for producing highly stripped ions having a negatively biased plasma with energetic magnetic-mirror confined electrons that provides a suitable environment for long term ion confinement and for intense multiple ionization of heavy ions, wherein classical rates for ion loss from a negative potential well are low compared to the comparable ion mirror loss rates, and the plasma is maintained by steady electron injection together with cyclotron acceleration for electron trapping and heating.

Description

United States Patent 72] Inventor Thomas H. Stlx Mercer County, NJ.

[21] Appl. No. 68,828

[22] Filed Sept. 2, 1970 [45] Patented Jan. 11, 1972 The United States of America as represented by the United States Atomic Energy Commission [73] Assignee [54] APPARATUS FOR THE PRODUCTION OF HIGHLY STRIPPED IONS 11 Claims. 3 Drawing Figs.

[52] U.S. Cl 310/11, 176/7 [51] Int. Cl H02m 4/20 [50] Field ofSearch 310/10, 11;

[56] References Cited UNITED STATES PATENTS 3,090,737 5/1963 Swartz 176/5 X 3,160,566 12/1964 Dandl et a1. 176/7 3,425,902 2/1969 Consoli et al. 176/7 3,500,077 3/1970 Post 310/11 3,571,734 3/1971 Consoli etal. 328/233 Primary Examiner-D. X. Sliney Altorney- Roland A. Anderson ABSTRACT: Apparatus for producing highly stripped ions having a negatively biased plasma with energetic magneticmirror confined electrons that provides a suitable environment for long term ion confinement and for intense multiple ionization of heavy ions, wherein classical rates for ion loss from a negative potential well are low compared to the comparable ion mirror loss rates, and the plasma is maintained by steady electron injection together with cyclotron acceleration for electron trapping and heating.

103 I03 i MAGNETIC\ i l9 VACUUM CHAMBER EXTRACTION PORT 69 MICROWAVE SOURCE PATENTEDJAM 1 1972 36 4 4 IDETECTOR] ns V I3 I9 55 EXTRACTION VACUUM 1 PORT CHAMBER m T 27 -79 8| VACUUM 1| 38 PUMP A f 67 B 69 MlsCgUORVAEVE l J in 76 gi 78 Fig. 3

l I I Ne N I ZN, ZN,

,P *4 4\ 4 'T 4 T ww z 4 (z) I m L o Fig.2 g 1 INVENTOR THOMAS H.ST|X

MQ/W

APPARATUS FOR THE PRODUCTION OF HIGHLY STRIPPED IONS CROSS-REFERENCE TO RELATED APPLICATIONS This is a continuation-in-part of Ser. No. 856,337, filed Sept. 9, 1969 by Thomas H. Stix.

BACKGROUND OF THE INVENTION Considerable current interest has arisen in the production of far transuranic elements by the fusion of heavy nuclei, as described in Nuclear Phys. 81, i 1966). However, as discussed in IEEE Trans. on Nuc. Sci. NS-l4, No. 3,7 (1967), difficult problems have heretofore been involved in obtaining high-density ion beams with sufficient electron stripping. For example, a proposed technique of injection, acceleration, electron foil stripping and reacceleration has been too intricate for achieving the desired output energy.

In order to overcome the problems and shortcomings of the heretofore known or complicated foil-stripping systems, it has been advantageous to provide a high degree of electron stripping by confinement of heavy ions in the electrostatic potential well of an electron plasma and by exposure of these ions to the energetic electrons of the plasma, as described in copending application Ser. No. 856,337, filed Sept. 9, 1969 by the inventor of this application, who has assigned that application to the assignee of this application. The invention described herein which utilizes electron stripping in an electrostatic potential well, is a continuation-in-part of the abovementioned copending application. As such, this invention advantageously provides an improved system for obtaining a high degree of electron stripping of ions for acceleration of a high-density beam of ions in conventional accelerators, comprising cyclotrons, synchrotons, and Van de Graaf brand electrostatic accelerators, which are described in Particle Accelerators," by Livingston and Blewett, McGraw Hill, Inc., 1962.

SUMMARY OF THE DISCLOSURE This invention, which was made in the course of, or under a contract with the United States Atomic Energy Commission, provides a body of substantially charge-neutral plasma with energetic electrons in a compact ion stripper possessing an ion-confining electrostatic potential well. More particularly, the apparatus of this invention provides a substantially chargeneutral plasma with ions confined in an electrostatic potential well wherein the plasma has a strong negative potential with respect to the apparatus walls by means of a negative surface charge in contrast to the strongly negative plasma body described in the above-mentioned copending application. In one embodiment, the apparatus of this invention provides an electrostatic potential well and a low ion temperature in a plasma with energetic electrons confined in a steady-state magnetic-mirror configuration having steady electron injection. In this regard, this invention contemplates a high-potential electrode as the virtual electrode for avoiding the density limitations of the devices known theretofore. Moreover, high voltages need not appear on the external electrodes. Also, permanent magnets as well as normal resistance or superconductors may be used. Additionally, microwave power is transmitted via electromagnetic propagation across a vacuum in accordance with one aspect of this invention. In another aspect, the confinement of the energetic electrons is enhanced by giving a'cylindrical symmetry to the plasma. In addition, the problem of undesirably trapped electrons is reduced by the more favorable shape of the axial potential profile. With the proper selection of components, as described in more detail hereinafter, the desired electron stripping is achieved.

The above and further novel features of this invention will be described in more detail hereinafter in connection with the accompanying drawings, and the novel features will be particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, where like elements have like reference numbers:

FIG. 1 is a partial graphic illustration of one embodiment of an electrostatic potential well;

FIG. 2 is a partial graphic illustration of an embodiment of the electrostatic potential well of this invention;

FIG. 3 is a partial three-dimensional view of one embodiment of the apparatus of this invention.

DETAILED DESCRIPTION OF ONE EMBODIMENT This invention is useful for stripping electrons from ions. As such, this invention is useful in any application where highdensity highly stripped ions are required. For ease of explanation, however, this invention will be particularly described for use in producing a high-density beam of highly stripped heavy ions for acceleration for the production of transuranium elements. In this regard, the linear device of one embodiment of this invention is adapted to be placed at or near the high voltage terminal of a Van de Graaf brand electrostatic accelerator. However, as will be understood from the following description by one skilled in the art, the apparatus of this in vention is useful as an ion source for any of the heretoforeknown conventional accelerators such as those described in the above-mentioned Livingston and Blewett publication.

In understanding this invention, the calculation by Daugherty et al. in Phys. Rev. Letters 20, 369 1968) indicates than an exposure time corresponding to n 1-l0l0"sec.-cm." is required to attain 50 percent removal of electrons for elements in the range Z=2092. Here n,. designates the density of electrons with energy typically in the range 10-20 kev. and 1' is the ion exposure or confinement time. This combination of required density, average electron energy and ion confinement time represents a performance level beyond current achievements for conventional magnetically confined chargeneutral plasmas. In this regard, this invention contemplates the use of an open-ended energetic-electron plasma with electrostatic ion confinement to achieve the ion confinement needed for heavy-ion stripping.

As an illustrative example, consider a cylinder of plasma 1 cm. in radius centered inside a long, metal, cylindrical shell of larger radius and immersed in a conventional static axisymmetric magnetic mirror field with B=3,000 gauss in the central region. We assume an average density of 10 cm. and an electron temperature of 10 kev. We assume further that the ions in the plasma are ions of uranium each with net charge Z=46 and with average energy per unit charge of ev. (kT,=Z ev.). Finally, we assume that by means of an electron sheath on the surface of the plasma, the space potential within the plasma is depressed by about 1,000 volts below the potential of a cylindrical metal shell surrounding the plasma. The RMS ion Larmor radius is 1.1 cm. and the ion containment is thus electrostatic, as understood from the electrostatic potential well described in the above-mentioned copending application. To estimate the rate of ion leakage over the potential energy barrier of the herein described well, it is noted that for a Maxwellian distribution of energies, the fraction, F, of ions with kinetic energy greater than E is approximated by the asymptotic expression where 0=2E/(kT,). Thus approximately 1 out of 5,900 ions in a kT,=l00Z ev. distribution will have an energy exceeding 1,0002 ev. and can escape over the energy barrier. Now a uranium ion with kinetic energy of 1,000Z=46,000 ev. would normally lose its excess energy by collisions with the thermal ions in about 1.0 milliseconds. A crude detailed balance argument would then require that the truncated energy distribution be refilled at this same rate. Refilling the tail of the distribution pushes these ions over the energy barrier and the characteristic loss time for a U* ion selected at random would be 5,900Xl.0 =6 seconds. This ion loss time is considerably longer than the time required to strip uranium in this plasma down to Z=46.

The rate of ion loss is however very sensitive to the ion temperature and we may examine briefly the processes of ion heating and cooling. lens in the specific plasma come slowly to temperature equilibrium with the electrons. in 6 seconds, for example. the U temperature rise due to ion-electron collisions would be 80 ev. per unit charge. Additional energy is gained by the ions in the stripping process. For example, at each ionization step the potential energy of the ion is changed by the amount e (the freed electron acquires e in potential energy). Thus the total energy acquired by an ion is 22 (r,) where r, is the position at which the jth ionization occurred. However, it will be seen below that the ion-confining potential well of this invention tends to have steep sides and a very flat bottom with the consequence here that the potential energy acquired during ion stripping is not large. It is estimated, for example, that an initially neutral test atom would gain a total amount of energy of order kT, in this fashion. On the other hand, ion loss provides a powerful cooling process. For example, with the barrier height equal to 1,000 volts, the loss of only 18 percent of ions with an initial temperature of 200 ev. per unit charge would drop the temperature of the remaining ions to 100 ev. per unit charge.

For illustrative purposes a model plasma is chosen with a single ion species (U but it is clear that the same type of analysis may be applied to ions of different mass and different charge states including the interesting case of He. The important qualitative points for any such plasma are that the ions acquire energy slowly clue to ion-electron collisions and due also to the multiple ionization process which takes place within the electrostatic potential well of this invention, and that ion-ion collisions tend rather quickly to randomize the distribution of this energy. Some ions escape over the electrostatic potential barrier but each carries away a significantly above average amount of energy with the result that the temperature of the remaining ions is lowered and their confinement time in the electrostatic well is correspondingly lengthened.

Now considering the spatial variation of the electron and ion densities n, (r) and n, (r) respectively, and of the electrostatic potential, (1) (r), and taking first a cross section of the model plasma perpendicular to the magnetic field, Poissons equation is as follows:

Assuming symmetry in the azimuthal direction and at this stage neglecting the slow axial variation, the electron density function n, (r) is determined by the electron injection and loss process, while the total number of ions can be varied by changing n, The detailed spatial variations of the ions and of the electric potential are allowed to come to a self-consistent equilibrium. Extrapolation of the forms of the above ion distribution, n(r,Z)=n(O,Z) exp (ZeqS/kT,), would indicate incidentally that the most highly stripped ions lie deepest in the potential well. A qualitative examination of the Poisson equation above shows Debye shielding of the electrons by the ions, Zn,(r)==n,,( r),from r=0 out to that radius, r,,, at which the total number of available ions is almost exhausted. The potential variation in this range, r r,,, can be found in an approximate manner by equating the right-hand side of the Poisson equation to zero, i.e., =(kT,/Ze) in [n (r)/Zn,,,,0)]. Then, in a region of a few Debye lengths [ADi '-1\T,41rZe n,(r around r==r the ion density drops to almost zero. The electrostatic potential outside of r, then rises in accordance with the variation of the electron space charge, n, (r), for r r,,, reaching the potential 4),, at the wall.

A similar analysis yields the axial variation. Integration of Poissons equation over the cross section of the plasma and the sheath shows E, at the wall proportional to the net line charge, e(ZN,-N,.). Now one can assume that an electron sheath between r,, and r is of uniform density and thickness. One can designate the constant wall potential by 5 and neglect small radial variations in order to write the space potential within the plasma simply as d)(z). Integration of E, between r, and r then gives the approximate solution to Poisson's equation with axial (z-dircction) variation.

Again letting the ions reach a self-consistent equilibrium, (zF i( p i l(Z)( i l and net negative charge required to drop the space potential from (la down to (0). When, at z=z,,, the mirror confinement factor in exp H (z) finally reduces N e (z) to this required value for net charge, ion shielding is no longer demanded. Further reduction of N (z)gcm gla gallowsdgtz) to rise and drives N.-(z) rapidly toward zero. Axial'profiles of potential and density are sketched in FIG. 1.

In the sample plasma described above, ,,-(O)=1,000 volts is much smaller than kT,./e=l0,000 volts so that the electrostatic potential energy factors in exp H(z) never deviate far from each other, i.e., the electron density distribution is almost the same as it would be for a charge-neutral plasma. Such deviation as does appear occurs in the vicinity of the end electron sheath at z=z,,. The thickness of this sheath is determined primarily by the requirement that electrons falling out through the end sheath still be confined by mirror forces, that is, that Regarding the question of plasma stability, relevant experimental data do not exist and inferences can only be drawn from theoretical calculations. Such calculations indicate two modes of plasma instability which may affect the plasma described in this invention. These modes are the shortwavelength diocotron mode and the long-wavelength twofluid diocotron mode. At full electron density the q E m g/(9 2 figure for the above-described plasma (i.e., electron density of 10 cm., magnetic field of 3,000 gauss in the central region of the mirror configuration) is 0.1 l at which value the shortwavelength diocotron growth rate is still small. Thus this mode should not be the cause of adverse effects to the plasma under the conditions of operation of the invention. Further, theoretical calculations show that the long-wavelength two-fluid diocotron mode will not be unstable provided that the radial electron density distribution is over the cross section of the plasma and sheath, out to the wall. Moreover, stability is still maintained even if there is a small vacuum gap between the wall (assumed to be a perfect conductor) and the sheath. Typical values for the maximum allowable width for this gap would lie in the range of 0.4 to 1.6 mm. for plasmas used in the manner described for this invention. The calculation furthermore shows that irregularities between the wall surface and the magnetic lines of force can lead to instability of this same two-fluid diocotron mode, acting in such a way as to produce local increases in the thickness of the vacuum gap. From consideration of this problem it is inferred that the theoretical tolerance for such mechanical and magnetic deviations from pure axial symmetry will allow irregularities in size up to about one-fifth of the above-mentioned allowable widths for the vacuum gap, i.e., in the range of 0.1 to 0.3 mm. Reducing these ideas to practice, the tolerance on surface irregularities and magnetic nonuniformities and misalignment calls for careful machining of the cylindrical metal wall immediately surrounding the plasma together with careful design and placement of the coils used to produce the magnetic field. Simultaneously, the shape of the radial electron density profile can be adjusted advantageously to achieve plasma stability by changes in the end-injection apparatus, to be described in the next few paragraphs. Typical changes include changes in the mode of operation or in the location, size or shape of bias-voltages on the electron injection apparatus or the introduction of additional electrodes near said injection apparatus in order to modify the velocity and spatial distribution of the end-injected electron beam.

Finally, consider the formation of the plasma. The plausible criterion that q 0.1 implies unusually careful control over the original admission and reemission of neutral molecules into the plasma volume. Experiments on the minimum B geometry Interem device at ORNL indicate MHD stability of a hot electron plasma with a reduced density of cold plasma background. Unique to the proposed stripped-ion source of this invention, however, is the need to maintain the space potential of the mirror-confined hot electron plasma at a negative value which is considerably larger in magnitude than the classical ambipolar potential. Electron injection is therefore demanded and the need to overcome the repulsive negative potential in fact requires injection from an electron gun.

In accordance with this invention, a simple electrostatic gun with a cathode, at z=z which is outside of a magnetic mirror described in more detail hereinafter, is biased at (z,,.)'-(0) to bring electrons into the central region of the magnetic mirror. However, the scattered electrons tend to become trapped in, fill up, and thus eliminate the ion confining potential maximum between z=z, and z=z,,, (See FIG. 1). These undesirably trapped electrons, however, are ejected by frequent pulsing of the cathode up to Alternatively, the problem of electron trapping outside of 2,, is almost eliminated with traveling-wave electron acceleration or magnetostatic (p. B) acceleration, which is also described in more detail hereinafter. Magnetostatic acceleration has the advantage of supplying electrons that reach z= already with considerable perpendicular energy. The principles are illustrated in FIG. 2,

With any type of injection the electrons must be trapped inside the above-mentioned magnetic mirrors. Microwave excitation applied around z=0 at the local electron cyclotron frequency can impart additional perpendicular energy to transiting electrons so as to cause them to be mirror confined. From another point of view, cyclotron acceleration at z=0 and the subsequent collisional scattering cause extensive diffusion of the electron distribution function in velocity space and thus allow the configuration space density at z=0 considerably to exceed the density ofthe injected beam.

In summary, immersion in a plasma of kT,,=lO kev. electrons will cause; 50 percent stripping of the electrons from heavy nuclei (e.g., uranium) in a time 1 5 /n,, seconds cm. It is possible, therefore, in accordance with this invention, to use a negatively biased mirror-confined hot electron plasma for this purpose. To this end, a 1,000-volt negative potential well in the plasma space potential ensures classical electrostatic c onfinement of the heavy ions (kT IZ s11 Q9e \j for times considerably longer than T. Debye shielding by the ions will cause such a three-dimensional potential well to be almost flat-bottomed with the more highly stripped ions in the center of the well. Except for thin electron sheaths on its radial and end surfaces, the plasma cloud is essentially charge neutral. Advantageously, the desired plasma is maintained by steady injection of electrons followed by their cyclotron acceleration and mirror trapping.

Referring now to FIG. 3, the apparatus of this invention, comprises along, metal, cylindrical shell 11 having an ion extraction port 13 at end 15 of shell ll, and a particle source 17 at the other end 19 of shell 11. Advantageously, the particle source 17 comprises a gas source 21 having an inlet conduit 23 and a valve 25 for admitting gas from gas source 21 into cylindrical shell 11 while vacuum pump 27 maintains chamber 29 in cylindrical shell 11 at a low pressure. Also, cathode 31 is energized by a current flow from an energy source 33, illustrated as a battery for ease of explanation. Magnetic filed coils 3S and 37, advantageously comprising normal resistance, magnetic mirror coils and advantageously designed, constructed and placed so as to produce a static magnetic field in chamber 29 of the desired uniformity and symmetry and with the desired shape and relative intensity for the two magnetic mirrors, are energized from other sources, 38 and 38', illustrated like source 39 as batteries for ease of explanation. Additionally, an open-ended cylindrical tube, 43, constructed of high-conductivity metal and carefully machined and located so as to reduce the irregularities between the inside tube surface and the magnetic lines of force to within the tolerance limits prescribed by the plasma instability criteria, is placed inside chamber 29, spaced from the inside surface of shell 11 around plasma 53 by suitable insulating means 55, and connected electronically to one terminal of energy source 39, illustrated as a battery for ease of explanation. Cathode 31 is connected to the second terminal of energy source 39 which biases said cathode with respect to the tube 43 advantageously to achieve the desired spatial and velocity distribution of the end-injected electron beam. Cathode 31 is of such design, shape and location also so as achieve said desired distribution of the end-injected electron beam.

Two separate arrays of slots, 63 and 65, are cut into the metal open-ended cylindrical tube 43 in order to transmit microwaves from separate microwave sources 67 and 69 into chamber 29. Microwave sources 67 and 69 are connected to

suitable energy sources

76 and 78, illustrated as batteries for ease of explanation, and furnish microwave power to chamber 29 to accelerate both the electrons from cathode 31 and the electrons in plasma 53 which lies along the axis of shell 11 in chamber 29 and which is formed from gas admitted into chamber 29 from gas source 21. The two arrays of slots, 63 and 65 are connected to the two microwave power sources, 67 and 69 by

waveguides

79 and 81. The arrays of slots, 63 and 65 are formed of slots having at their alternate ends respectively, short azimuthal slots 87 forming with slots 85 a'rectangular-shaped meander path 91 around the circumference of metal tube 43. This permits the microwaves from sources 67 and 69 efficiently to communicate with good matching through tube 43 with the plasma 53 in chamber 29 providing a long slot antenna that is folded back and forth on tube 43 into meander path 91. In this regard, when the length of slots 85 equals half the wavelength of the exciting frequency, the vacuum fields in chamber 29 are suggestive of the fields of a IE cavity in the region between the axis and field maximum, However, an important property of the coupling device of this invention provided by slotted tube 43 is that tube 43 provides such field configurations independently of the diameter of tube 43 and without meeting the conducting sheet boundary conditions of cavity resonators, which require large volumes at low frequencies. Moreover, the slotted tube 43 provides a broadband coupling device, whereby for a given choice of dimensions, the operating frequency can be varied over several octaves.

As described in Princeton University Report MATT-Q44, pp. 164 et seq., a collisionless steady-state plasma showing a low level of fluctuations has been generated in a mirrorgeometry magnetic field, by applying radiofrequency power to a coupling device (e.g., 1.5 cm. diameter) like the one described above, which is referenced as coupling device 57. In this regard, the mode of plasma generation has been carried out with modest power levels (=50 watts) in the 500-mc./sec. to 10 gcJsec. band, in magnetic mirror fields, such as mirror field 92, from 0.2 to 2.3 kilogauss. Moreover, these plasmas, like the plasma referred to above as plasma 53, have had elec tron densities of 10 to 10 particles/emf", electron temperatures of 10 to 30 ev. and ion temperatures around to ev., at various gas pressures as low as 10"" torr. For the actual experiments reported, the magnetic field configuration had a mirror ratio of 2.6:1, and at a maximum mirror field of 2.3 kilogauss, plasma production occurred near the electron cyclotron frequency, w but a range of a few percent in the magnetic field produces plasma near w Also, electron cylcotron acceleration due to the RF excitation in the inhomogeneous magnetic field was observed along with the described actual depositing of RF energy into the plasma 53.

Measurements of the electron density as a function of the applied H.F. power level at constant neutral-gas pressure shows that at values as low as 50 W., the plasma density reaches a saturation value, the plasma frequency at saturation being almost equal or a little above the applied frequency. These measurements also showed the value of the static magnetic field at which the plasma was produced as a function of the applied H.F. power level.

In the operation of the above-described example, the magnetic field produced by

coils

35 and 37 connected to energy sources 38 and 38' is similar to the field whose profile is sketched in FIG. 2. The cathode 31, located at z in FIG. 2, supplies electrons to the region underneath microwave slot array 63, located at z, in FIG. 2. Microwave source 67 is tuned approximately to the electron cyclotron frequency corresponding to the magnetic field at said location 2,, and electrons in this vicinity gain energy by cyclotron acceleration. Magnetic mirror forces on these electrons accelerate them toward the weak-magnetic-field region between the two magnetic mirrors, i.e., toward the location ofz=0 on FIG. 2. The energy that the electrons received from their cyclotron acceleration at z, helps them overcome the potential barrier depicted y -(z) in FIG, 2, said potential barrier being due to the excess ofelectrons in the region between and z in FIG. 2. Electrons which reach the vicinity of z=0 are further accelerated by the electromagnetic field from microwave slot array 65 energized by microwave source 69 which is tuned approximately to the electron cyclotron frequency corresponding to the magnetic filed at said location z=0. This additional cyclotron acceleration further increases the energy and the magnetic moment of said electrons in the vicinity of z=0 and enhances the probability of their being magnetically trapped between the two magnetic mirrors, i.e., of being trapped between the positions z=z,,, and z=z,,,. The electrons thus trapped contribute to the negative excess of charge in this region and thus to said potential barrier (z). Said potential d (z), while tending to act as a barrier for electrons (whose electric charge is negative) simultaneously tends to act as a well for ions (whose electric charge is positive) and thus tends to confine said ions. Therefore, ions formed of the uranium gas introduced into chamber 29 from gas source 21 are advantageously confined in said potential well and produce, together with the electrons magnetically trapped in the same region, a body of plasma, 53, which is substantiallychargeneutral. The excess electrons tend to lie in a thin sheath, 99, at the radial and axial boundaries of surfaces of the plasma, 53, and it is primarily said excess electrons lying in said sheath which produce the ion confining electrostatic potential well. During the period of their confinement, said confined ions undergo successive stages of multiple-ionization and thus become stripped of a major fraction of their electrons, said stripping and ionization being caused by the exposure of said ions to the advantageously energetic electrons of plasma 53. Said electrons have been made energetic by acceleration in the microwave field inside cylindrical tube 43 due to microwave power supplied by microwave power sources 67 and 69, as described above. Thus the apparatus 101 of this invention provides a suitable environment for long-time ion confinement as well as intense multiple ionization of the heavy uranium ions whose loss from the described negative potential well is low compared to comparable ion mirror loss rates.

In review of the above, and in one example, the apparatus 101 of this invention advantageously comprises a shell 11 forming an evacuated container having a controlled source 17 of neutral gas particles and a tube 43 forming an open-ended metal cylinder in chamber 29 either as part of or located within the evacuated container, coils 35 and 37 forming a magnetic-mirror field in the metal cylinder, means 103 for aligning this magnetic field approximately coaxially with the metal cylinder, means forming an injector 105 to inject electrons into a region 107 that is interior to the open-ended metal cylinder and which is between the two magnetic mirrors formed by the magnetic-mirror field, means 109 for the heating and magnetic trapping of electrons injected into region 107 by the electron injector 105, and means 111 for forming from the neutral gas particles a column 113 of plasma 53 in the magnetic-mirror field from the particles, the plasma being negatively biased with respect to the metal cylinder in order to form an electrostatic potential well 115 for confining the ions of the plasma 53 at sufficient density and for a long enough period of time for the multiple ionization of the ions in the plasma 53.

In this example, the average electron density is 10"cm, the electron temperature is 10 kev., and the uranium ions, each with net charge z==46, have an average energy per unit charge of 100 ev. due to the above-described ion-loss cooling process for an electron sheath 99 at boundary 75 of plasma column 113 depressed by about 1,000 volts below the potential of the metal shell 1 l, e.g., by a suitable connection to shell 11. Since the RMS ion Larmor radius is less than that of shell 11 and tube 43, the desired electrostatic ion containment and production is provided and the ions are extracted from port 13 in the form of a high-density beam 118 of uranium ions.

While the above has described one example for producing a high-density beam of uranium ions, it will be understood that the system of this invention can be used for forming a highdensity beam of any other ions by using a neutral input gas of the desired atoms.

It is also understood that the described magnetic mirror means may also comprise permanent magnets or supercon' ducting windings of sufficient strength for the appropriate size.

This invention has the advantage of efficiently producing a high-density beam of ions by the use of an electrostatic poten tial well. As such this invention provides an efficient source of heavy ions for a wide variety of uses. In one embodiment, the ions are produced for acceleration by a conventional accelerator for the production of transuranium elements.

It is understood that alternative methods of electron end-injection may be used in this invention, replacing the scheme described above using cathode 31, microwave slot array 63, microwave power source 67 and the magnetic field profile drawn schematically in FIG. 2. One such alternative method uses an electron gun located at 2 in FIG. 1 together with the magnetic field profile sketched in FIG. I. Said electron gun is generally biased to the approximate potential of plasma 53, as indicated by the dotted line in FIG. 1, said bias giving electrons from said gun sufficient energy to reach the interior of plasma 53, where they are trapped by microwave fields from microwave slot array 65. From time to time the bias on said gun is temporarily changed so that the potential profile approximates the solid curve, (z), in FIG. 1, and electrons undesirably trapped in the potential maximum in the neighborhood of z,,, are released.

A second alternative method of end-injection would utilize a traveling-wave electron gun located at z,,., FIG. 1, in place of a conventional electron gun. The use of said traveling-wave gun, biased at the potential of the metal tube 43, may be expected advantageously to reduce the number of electrons undesirably trapped in the vicinity of z,,,, FIG. 1.

It is further understood that additional electrodes, biased at selected potentials with respect to metal tube 43, and placed in the vicinity of cathode 31, or in the vicinity of either of the two guns described in the two preceding paragraphs, may be used to improve the performance of the apparatus, specific ally to achieve the desired spatial and velocity distribution of the end-injected electrons.

in still another embodiment of the invention, an electron detector 117 located in chamber 29 or in the vicinity of the extraction port 13, FIG. 3, is used to measure the transmission of electrons from cathode 31 or from the above-described electron guns in order to determine the magnitude'of the potential difference between the plasma 53. and the surrounding metal tube 43, and to determine the efficacy of the electron-trapping mechanism. In a variation of this embodiment, the information derived from this detector or from a similar detector is used to control the voltages on cathode 31, or on the electron guns just described, or to control the differential magnitude of the magnetic field in the vicinity of point z FIG. 2, compared to the magnetic field in the vicinity of point z,,,, FIG. 2, in order to optimize injection into and trapping of electrons in the plasma 53.

What is claimed is:

1. Apparatus for providing a source of multiply-ionized ions, comprising means forming an evacuated container having a controlled source of neutral gas particles, means forming an open-ended metal cylinder located within said evacuated container, two magnetic mirror means forming a magneticmirror means field in said metal cylinder, means for aligning said magnetic field approximately coaxially with said metal cylinder, means forming an electron injector to inject electrons into the region that is interior to said open-ended metal cylinder and which lies between the two magnetic mirror means forming said magnetic-mirror fields, and an energy source means connected to said cylinder for the heating and magnetic trapping of electrons injected into said region by said electron injector for forming from said neutral gas particles a column of plasma in said magnetic-mirror field from said particles, said plasma being negatively biased with respect to said metal cylinder in order to form an electrostatic potential well for confining the ions of said plasma at sufficient density and for a long enough period of time for the multipleionization of said ions in said plasma.

2. The invention of claim 1 in which electromagnetic energy from a microwave energy source is transmitted into the interior of the open-ended metal cylinder for the purpose of forming said plasma and trapping and heating the electrons from said electrons from said electron injector and from said gas particles in the region inside the metal cylinder and between the two magnetic mirrors.

3. The invention of claim 1 in which relative alignment and freedom from irregularities are provided between the inner surface of said metal cylinder and the adjacent magnetic lines of force of said magnetic mirror field.

4. The invention of claim 1 in which the design, construction and operation of said electron injector achieve an electron distribution in the body of the plasma which is approximately constant and uniform throughout the cross section of the interior region of the metal cylinder.

5. The invention of claim 1 in which an electron detector intercepts electrons leaving said plasma.

6. The invention of claim 2 in which microwave energy for the injection, trapping and heating of the electrons is coupled into said electrons through slots in the wall of the open-ended metal cylinder.

7. The invention of claim 6 in which the slots from an approximately rectangular meandering array.

8. The invention of claim 6 in which separate arrays of slots are used in different locations of the metal cylinder.

9. The invention of claim 6 in which microwave energy at two frequencies is transmitted into the interior of said openended metal cylinder.

10. The invention of claim 1 in which the ions of said plasma are cooled by the ejection of a portion of said ions from said plasma.

11. The invention of claim 1 in which the ions are ejected from said apparatus through an ejection port, thus forming an external beam of multiply igniz d i ons

Claims

1. Apparatus for providing a source of multiply-ionized ions, comprising means forming an evacuated container having a controlled source of neutral gas particles, means forming an open-ended metal cylinder located within said evacuated container, two magnetic mirror means forming a magnetic-mirror means field in said metal cylinder, means for aligning said magnetic field approximately coaxially with said metal cylinder, means forming an electron injector to inject electrons into the region that is interior to said open-ended metal cylinder and which lies between the two magnetic mirror means forming said magnetic-mirror fields, and an energy source means connected to said cylinder for the heating and magnetic trapping of electrons injected into said region by said electron injector for forming from said neutral gas particles a column of plasma in said magnetic-mirror field from said particles, said plasma being negatively biased with respect to said metal cylinder in order to form an electrostatic potential well for confining the ions of said plasma at sufficient density and for a long enough period of time for the multiple-ionization of said ions in said plasma.

9. The invention of claim 6 in which microwave energy at two frequencies is transmitted into the interior of said open-ended metal cylinder.

11. The invention of claim 1 in which the ions are ejected from said apparatus through an ejection port, thus forming an external beam of multiply ionized ions.