US3887832A

US3887832A - Auto-resonant acceleration of ions

Info

Publication number: US3887832A
Application number: US373089A
Authority: US
Inventors: William E Drummond; Millard L Sloan
Original assignee: ARALCO
Current assignee: ARALCO
Priority date: 1973-06-25
Filing date: 1973-06-25
Publication date: 1975-06-03
Anticipated expiration: 1992-06-03
Also published as: IT1016146B; FR2234732B1; FR2234732A1; DE2430270A1; GB1465298A; CA1004358A

Abstract

Accelerating ions utilizing a relativistic electron beam in a static, inhomogeneous magnetic field, the electron beam resonating in the cyclotron waveguide mode. Accelerating power is efficiently furnished to the driven ion particles by the electron beam through the intermediary of the cyclotron mode, hence causing acceleration to high energy levels of the ions trapped therewith.

Description

O United States Patent 1191 1111 3,887,832

Drummond et a1. 5] June 3 1975 AUTO-RESONANT ACCELERATION OF 3,346,819 10/1967 Birdsall 315/5 x IONS 3,398,376 8/1968 Hirshfield 315/5 X 3,432,722 3/1969 Naydan et a] 315/3 X [75] Inventors: i i Drummond; Millard 3,450,931 6/1969 Feinstein et. a1. 315/5 Sloan, both of Austin, Tex. 3,463,959 8/1969 Jory et a1. 315/5 [73] Assigneez Aralco, Au e 3,489,943 1/1970 Denholm 315/5 [22] Filed: June 25, 1973 Primary Examiner-James W. Lawrence [21] Appl 373 089 Assistant ExaminerSaxfield Chatmon, Jr.

57 ABSTRACT [52] US. Cl. 315/541; 313/63; 315/4; 1

328/233 Accelerating ions utilizlng a relatlvistic electron beam [51] Int Cl l H01 j 25/10 in a static, inhomogeneous magnetic field, the electron [58] Field 658651611 315/315 5.41 5.42- beam resonating in the CYCIOYOH Waveguide 3 3/ 5 Accelerating power is efficiently furnished to the driven ion particles by the electron beam through the [56] References Cited intermediary of the cyclotron mode, hence causing acceleration to high energy levels of the ions trapped UNITED STATES PATENTS therewith 3,072,817 l/1963 Gordon 315/3 3,270,241 8/1966 Vural 315 36 x 19 Claims, 2 Drawing Figures I I I I l ELECTRON; ION l SOURCE l INJEC TOR SHEET 1 gl amm/mg l ELECTRON L 0 v SOURCE INJECTOR I I wwwwwwwnx FIG] ELECTRON BEAMS AND IONS F/GZ ELECTRIC POTENTIAL V TIME FIG. 3

, IONS I r r f I 1 l ELECTRIC I POTENTIAL I TIME i l I I l :a-uEmEnJm ms SHEET CYCLOTRON MODE (UPPER BRANCH) w (FREQUENCY) MODIFIED ELE C TRO MA GNETIC MODES DOPPLER SH/FTED PLASMA MODES CYCLOTRON MODE TO BE USED (LOWER BRANCH) MOD/F /E D E LE C TRO MA ONE 7' l C MODES AUTO-RESONANT ACCELERATION OF IONS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention pertains to the acceleration of ions and more particularly to the acceleration of a large number of ions to high energy levels.

2. Description of the Prior Art Linear accelerators in the recent prior art employ one or both traveling wave techniques and collective acceleration techniques.

Traveling wave accelerators make use of a traveling electromagnetic or electrostatic wave to effect the acceleration of the particles. The operation of a traveling wave accelerator may be best appreciated by viewing a typical accelerator as comprising four basic components: (1) the background medium in which the wave propagates, (2) the wave, (3) the particles to be accelerated, and (4) the power supply for accelerating the particles.

The state-of-the-art in traveling wave accelerators may be represented by the accelerator operated at the Stanford Linear Accelerator Center (SLAC). This accelerator has as its background medium a vacuum drawn internal to a waveguide made of highly conductive material. The wave established in the SLAC accelerator is in an electromagnetic vacuum waveguide mode. Electrons are the particles accelerated. The power supply includes roughly one hundred 40 megawatt rf generators producing a net4 X watts of power. The energy imparted to the electrons from the SLAC accelerator is in the range of 40 GeV.

The limiting factor of the traveling wave accelerator just described is the amount of the electric field that can be established and maintained. It may be seen that rf power in great quantities must be available. The energy from the power supply furnishes the energy to the wave, which, in turn, furnishes energy to the particles. In addition to the difficulties of generating such highvoltage rf power, the high voltage causes arcing problems within the waveguide cavity, thereby setting a practical limit to the energy per unit length that can be obtained by the particles.

Collective acceleration utilizes the natural attraction between electrons and positively charged ions. Thus, a relatively dense cloud of electrons can attract and hold a relatively less dense group of ions. If this cloud of electrons is accelerated to high velocity, the electrons will drag the ions with the electron cloud. In this way, the more massive ions can be accelerated to the same velocity as the electrons and because they are much more massive, the ions will have a much higher energy per particle than the electrons. Furthermore, the elec tric field which acts to accelerate the ions is due to the dense cloud of electrons. This collective electric field can be much larger than accelerating electric fields supplied by external sources. Thus, a principal advantage of collective accelerators is that they can have very large accelerating fields.

This collective technique is not an independent technique of particle acceleration, but a technique that may be employed with a linear traveling wave accelerator of the type described previously.

SUMMARY OF THE INVENTION A preferred embodiment of the present invention, in terms of the four basic components set forth above, in-

cludes the following: (1) as the background medium in which the wave propagates, a relativistic electron beam in a spatially decreasing magnetic field internal to a conductive and evacuated waveguide, 2) as the wave, the relativistic beam of the background medium resonating in the beam cyclotron waveguide mode, (3) as the particles to be accelerated, ions, and (4) as the power supply for accelerating the particles, the relativistic electron beam itself, which, in turn, automatically imparts power to the cyclotron wave and hence to the driven ion particles.

Physically, the preferred waveguide configuration may conveniently take the shape of a tubular, flaring waveguide free of internal constrictions. The magnetic field may be established by use of conventional turns of coil along. the length of the flaring waveguide, the coil spacing and current arranged at each successive turn in such a way as to cause the desired decrease in the established magnetic field.

It is therefore a feature of this invention to provide an improved linear'accelerator utilizing both traveling wave and collective techniques.

It is another feature of this invention to provide an improved linear accelerator wherein the medium in which the wave propagates includes a beam of relativistic electrons.

It is still another feature of this invention to provide an improved linear accelerator wherein the medium in which the wave propagates includes a magnetic field gradually decreasing spatially along its length.

It is yet another feature of this invention to provide an improved linear accelerator including a magnetic field and beam of relativistic electrons that allow the operation of the traveling wave in such a manner to eliminate internal constrictions in the waveguide, yet suitably confining the traveling wave, thereby drastically reducing the high voltage limitations prevalent in the prior art accelerators.

It is still another feature of this invention to provide an improved linear accelerator that operates in the beam cyclotron waveguide eig'enmode to achieve particle acceleration to high energies from relatively small energy injection values.

It is yet another feature of this invention to provide an improved linear accelerator operating in such a manner that the power required for acceleration of ions is supplied by the medium, rather than from an external supply independent of the medium, to therefore effectively eliminate the power transfer problems prevalent in the prior art accelerators.

BRIEF DESCRIPTION OF THE DRAWINGS So that the manner in which the above-recited features, advantages and objects of the invention, as well as others which will become apparent, are attained and can be understood in detail, :more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof which are illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the appended drawings illustrate only typical embodiments of the invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

In the Drawings:

FIG. 1 is a schematic of the accelerator of the present invention illustrated in a typical system application.

FIG. 2 is an illustration of the magnetic field existing in the accelerator portion of the system illustrated in FIG. 1.

FIG. 3 is an illustration of the trapped ions in a nonaccelerated wave.

FIG. 4 is an illustration of the trapped ions in an accelerated wave in accordance with the present invention.

FIG. 5 is an illustration of the relativistic electron beam eigenmodes as viewed in the laboratory frame for typical values.

DESCRIPTION OF PREFERRED EMBODIMENTS Generally speaking, all traveling wave accelerators make use of a traveling electric or electromagnetic wave propagating in a medium in which the wave velocity increases as the wave moves along the length of the accelerator. Charged particles moving synchronously with the wave are thereby accelerated as the wave velocity increases. In many common conventional traveling wave accelerators, the medium through which the waves are propagated is a passive medium, such as a vacuum waveguide, and the energy imparted by the wave to accelerate the particles is supplied to the wave from external high-frequency, high-power generators.

The accelerator disclosed herein operates on a completely different and novel principle. The medium through which the wave propagates is an active medium, rather than a passive one. This active medium is a rather intense beam of relativistic electrons immersed in a longitudinal static magnetic field surrounded by a conductive boundary. The conductive boundary is, in turn, contained within a vacuum chamber.

The relativistic beam is not only an important part of the background medium for the accelerator, it also plays a dynamical role in the traveling wave. That is, the traveling wave of the accelerator disclosed herein is the resonant cyclotron eigenmode, or cyclotron natural oscillation, of this relativistic beam. There are eight eigenmodes of a cold relativistic beam, including the modified electromagnetic modes and the doppler shifted plasma modes, as well as the cyclotron modes. For purposes herein the cyclotron modes are defined as being the upper hybrid modes. The lower branch of the upper hybrid (cyclotron) mode has the distinct characteristic, unique among all of the relativistic electron beam eigenmodes, of variably resonating in a range from a low phase velocity to a high phase velocity near the speed of light. It has been discovered that the phase velocity of this resonant eigenmode can be made to increase simply by progressively decreasing the magnetic field in which the relativistic beam is immersed. By loading the ions in this wave, then they too may be increased in velocity with the wave to near the speed of light.

For example, to achieve an increasing wave velocity throughout the length of a linear accelerator of the type described herein, magnetic field coils may be provided to establish a large magnetic field at the entrance of the accelerator and decreasing to a small magnetic field at the exit. By proper choice of the magnetic field and its spatial variation, the phase velocity of the resonant cyclotron eigenmode may be varied from a small velocity at the entrance of the accelerator to a velocity of almost the speed of light at the exit.

Ions injected at the entrance of this accelerator at a velocity equal to the local phase velocity of the wave (i.e., the phase velocity of the wave at the entrance) can thus be loaded on the wave to be synchronously accelerated from their original velocityto the high phase velocity of the wave, at the exit of the accelerator.

The injection of the ions at the phase velocity of the wave is readily accomplished since this wave is present at a relatively small velocity at the entrance of the accelerator. The medium of the electron beam and the magnetic field together sustain the wave throughout the length of the accelerator path, thus carrying the ions to the high velocities attained by the wave at the accelerator exit.

A novel characteristic of such an accelerator is that there are no external high-frequency generators required to supply the wave with energy for accelerating the ions. The ions are accelerated by the wave. The wave, in turn, automatically interacts with the background medium (i.e., the relativistic electron beam) and thereby extracts energy directly from the medium. Put simply, the background medium automatically supplies energy to the resonant accelerating field, which, in turn, passes energy on to the ions with which it is in resonance. Hence, this accelerator may be conveniently referred toas the auto-resonant accelerator, or the ARA.

It may be seen that the advantages of the ARA include the following: (I) A large effective accelerating electric field (on the order of MVs/cm) can be achieved in the background medium of relativistic electrons. This permits the accelerator to be much shorter in length for a specified ion energy than provided by accelerators in the prior art where material electric field strength is the limiting factor. (2) There is no requirement for high-powered external, high-frequency rf generators. (3) The ARA transfers a sizeable fraction of the relativistic electron beam energy to the accelerated ions. In other words, it is relatively highly efficient in converting power to high energy ions. Many electron beam generators are available relatively inexpensively with beam power outputs of approximately 0.5 terrawatts and larger devices have been operated with beam powers of over 10 terrawatts. Used with the ARA, ion beams which are simultaneously high intensity and high energy can be obtained and obtained efficiently.

Now referring to the drawings, and first to FIG. 1, a conceptual layout of the ARA is illustrated. A vacuum chamber 10, evacuated by vacuum pump 12, houses the three principle functioning elements of the system with which the ARA operates. From left to right in the drawing, the first element is electron source 14, which is an electron beam generator for establishing an intense relativistic electro beam. As noted, the prior art is replete with suitable electron sources for generating beams with power outputs of between 0.5 and 10 terrawatts. Since electron source 14 is located within vacuum chamber 10, only the electron beam and the ions to be propagated are present within conductive waveguide boundary 16. Boundry 16 is preferably cylindrical, symmetrical and conductive throughout its length. In some embodiments of the invention, plasma is inserted in chamber 10 via insertion pump 17.

Electron source 14 is connected to ion injector l8, waveguide boundary 16 forming a continuous boundary to enclose both sections to provide a conducting guide through both regions. Magnetic field coils around the waveguide boundary l6 establish a static and uniform, longitudinal magnetic guide field inside the conducting guide through ion injector region 18. In this region, the lower branch of the relativistic electron cyclotron wave, denoted herein as merely the cyclotron wave, is excited at a low phase velocity and the ions are loaded into the traps thereof, this wave being of rather large amplitude.

Ion injector 18 is connected to accelerator region 22, waveguide boundary 16 continuing smoothly and without interruptions or constrictions in the manner illustrated. Note that the waveguide boundary flares out along its length in the accelerator region. Furthermore, magnetic field coils 20 around the boundary in this region gradually increase in spacing to establish a gradually decreasing, longitudinal magnetic field, starting at a very high value at the entrance of the accelerator region and decreasing to a very weak value at the exit. The physical configuration of boundary 16 and the coil spacing combine to cause the entering low phase velocity cyclotron wave with its load of ions to be brought up to a velocity comparable with the speed of light.

As has been previously noted, the magnetic guide field, particularly in accelerator region 22, should be cylindrical and symmetrical. To achieve this configuration, of course, waveguide boundary 16 must be similarly shaped. However, other conditions must also be met.

The electron beam, since it is not charge neutral, sets up a self-generated potential difference from the center of the beam to the conducting walls. If it is envisioned that the anode of the emitting device is electrically connected to the walls so that it resides at the same potential, then the electron beam must have sufficient energy to overcome this potential barrier to permit it to propagate away from the anode region.

Once away from the diode, a proper equilibrium must be assured for propagating the electron beam. Because of the interrelated effects of the self-radial electric field and the magnetic guide field, the beam undergoes rotation as it propagates. To maintain equilibrium, it is required that neither the centrifugal force associated with this rotation nor the electric field itself be able to overcome the self-focusing properties of the self-magnetic field and the guide magnetic field. Finally, stability must also be maintained.

It has been stated that of the eight modes available, the only one with a phase velocity variable from near zero to near the speed oflight, consonant with the equilibrium restrictions on the electron beam system, is the lower branch of the relativistic electron cyclotron, or upper hybrid, mode. This mode exhibits several desirable characteristics. First, the group velocity of this mode is roughly equal to the speed of light. Second, the phase velocity may be varied by merely changing the strength of the magnetic field. Finally, the mode is a negative energy mode for phase velocities greater than zero. That is, processes which would tend to damp ordinary positive energy modes, such as generally exist in accelerators using a passive reactive medium in which the accelerating wave travels, cause the electric field of this mode to grow. Of course, total energy is conserved, the energy source being the active reactive medium associated with this wave, previously referred to as the relativistic electron beam.

Now referring to FIG. 2, a schematic representation of accelerator region 22 is shown, again the electron beam traveling from left to right. The low velocity cyclotronmode wave with the ions trapped in the troughs of the wave enters the region at the left from ion injector region 18. As the wave propagates through this region, the magnetic field slowly decreases spatially from its initial value B to a final value B. The electron beam, because it tends to be tied to the magnetic field lines, expands in a flux preserving manner. That is, the radius of the electron beam will scale like (B)" The walls of the conducting cavity in which the electron beam propagates are expanded spatially in a similar manner.

Because there is a spatial change in the system, the frequency of the electron cyclotron mode stays fixed at frequency w,,, the initial frequency of the wave as it enters the acceleration region. However, the wave vector changes in order to continue to satisfy the dispersion relation wherein, k the value of the wave vector parallel to electron flow, 0 the speed of light, Q (eB/ymc), B strength of the magnetic field, e absolute value of the electron charge, y electron relativistic factor, and m electron rest mass; and the boundary condition k a 2.4, wherein k the perpendicular wave vector and a radius of the electron beam, where the beam extends essentially out to the wall.

The phase velocity of the cyclotron mode is given by v c [w /(m (1)], wherein the terms are as defined above. Hence, phase velocity scales roughly as 1/8. Thus, by decreasing the magnetic field, the phase velocity of the wave is increased. If this is done in a smooth manner so that the ions are not spilled from the trap by too sudden an acceleration, the ions are accelerated with the trap to a velocity comparable to c.

A significant advantage of the ARA over prior art accelerators is that no external energy is required other than the relativistic electron beam to achieve accelerations of the ions. The magnetic field, being static, can supply no energy to the system. A study of the energetics reveals that the beam electrons supply the required energy to the ions.

If the wave is not accelerated, it may be shown that the ions are symmetrically spaced about the minima of the potential, as indicted in FIG. 3.

Now consider the case where the potential is accelerated in the positive direction, such as shown in FIG. 4. Because of this acceleration, the ions no longer sit symmetrically spaced about the minima of the potential, but rather are displaced backwards in the wave so that they see an effective force which just compensates the acceleration force, as shown in FIG. 4. Moreover, since the ions are displaced backwards, their electric field is also displaced. The rate of energy change of the elec trons can be shown to be equal and opposite to the rate of change of energy of the electric field and ions. Thus, the electrons automatically give up energy to the ions to accelerate them and keep them in the electrostatic trap when that trap undergoes acceleration. The only restriction is that the trap is not accelerated so suddenly as to spill the ions out. A more analytical study of the energetics shows that the electrons, in giving up their longitudinal (or beam) energy, give roughly (c v)/c units of energy to the electron beam transverse oscillation, which constitutes the cyclotron wave, for

every unit given to the ions, wherein c speed of light and v phase velocity of the wave and of the ions trapped therewith. The energy given to the electron cyclotron wave resides essentially in coherent radial oscillation of the beam.

FIG. is an illustration of the eight relativistic electron beam eigenmodes as viewed in the laboratory frame for typical values. The laboratory frame is the reference frame of the accelerator and not of the ion particles. The dispersion relation illustrated relates angular frequency, w, to the longitudinal component of the wave vector, k,,. Six of the eight modes are forward propagating. These include the two modified electromagnetic modes, the two plasma modes (actually, the two Doppler-shifted, lower hybrid modes) and the two cyclotron modes (the two Doppler-shifted, upper hybrid modes). These two cyclotron modes (as well as the plasma modes, for that matter) are further differentiated by designating the higher frequency one the upper branch and the lower frequency one, the lower branch. The ARA makes use of the lower branch. The terms in the ratios that are shown on the drawing have all been previously defined.

Since, as noted, the wave can gain a large amount of energy, practical operations must ensure that this energy, which resides essentially in the ordered radial motion of the electron beam, must not be so large that the beam radial modulation is of the order of the unperturbed beam radius. Furthermore, conservation of energy restricts the total flow of output power in the ions to less than the total flow of power input by the electron beam. Finally, it must be ensured that the electrostatic potential associated with the wave, does not grow large enough to trap the electrons and thus destroy the reactive medium in which the wave propagates. These requirements, along with the equilibrium constraints, all serve to put certain restrictions on beam energies, magnetic field strength, and ion current and energy, in order for the accelerator to operate properly.

To consider two practical examples, first consider the acceleration of ions to l Bev energy (v 0.860) using electron beam parameters of 10 Mev, 100 K amps current which are available from typical electron diode devices. Further, consider using 100 Kev ion injection into a peak magnetic field of 100 KG. In order to achieve a 10 increase in ion energy, the phase velocity needs to increase by a factor of 57, with a 347 factor decrease in B. The radius of the electron beam and wall increase like l/ V? Choosing the following set of parameters: initial value of a (radius of electron beam, where beam extends essentially out to the wall) 1.9 cm; final value of a 35 cm; initial value of B (magnetic field) 100 KG; final value of B 288 gauss; I (electron current) 100 K amp and 'y (relativistic factor) 20, the maximum ion current achievable is calculated to be 181 amps. The value of the effective accelerating electric field over most of the accelerator is 600 kV/cm. Allowing the achievement of 181 amps, l Bev ion acceleration is accomplished in a 15 meter length.

Now consider ion injection energy l Mev. A decrease in magnetic field of a factor of 110 is required to achieve a 1 Bev energy. Choosing the following set of parameters: Initial value of a 2.1 cm; final value of a 22 cm; initial value of B 100 KG; final value of B 910 gauss; 1,, 100 K amps; and 'y 20, the maximum ion current achievable is now calculated to be 726 amps. The value of the effective accelerating electric field is now 1.9 Mv/cm, thereby achieving acceleration to 1 Bev ion energy in a little over 4.6 meters of length. The dramatic increase in performance (i.e., larger ion output energy and accelerating fields resulting in a shorter length accelerator) clearly illustrates the desirability of a large injection energy and very strong magnetic fields.

While a particular embodiment of the invention has been shown, it will be understood that the invention is not limited thereto, since many modifications may be made and will become apparent to those skilled in the art. For example, it has been assumed that the medium in which the traveling wave is established is a relativistic beam. A beam of lesser velocity, however, also may be advantageously used for acceleration in the environment of a static, but decreasing, magnetic field in the manner described heretofore. As a further example, the operation of the accelerator in the presence of a background gas or plasma may be advantageous, as it is well known from the literature that higher electron beam currents may be propagated in a waveguide with a residual ionized background gas than can be propagated in a vacuum. Such plasma may be inserted, for instance, by insertion pump 17, or may be allowed to remain present through only partial evacuation of chamber 10 by vacuum pump 12. In such case, the presence of plasma aids the propagation of the electron beam.

What is claimed is:

1. A linear accelerator for accelerating injected ions, comprising a longitudinal waveguide creating a conductive boundary,

a vacuum chamber enclosing said waveguide,

means for establishing a longitudinal static magnetic field in said waveguide,

said magnetic field being large at the entrance of said waveguide and decreasing to a small valve at the exit of said waveguide,

means for injecting ions into said waveguide,

means for establishing an intense beam of electrons immersed in said magnetic field,

said electron beam and magnetic field sustaining a traveling wave that is the lower branch of the upper hybrid resonant cyclotron eigenmode of said beam, the phase velocity of said wave increasing in such a manner as to accelerate the injected ions trapped in said wave.

2. A linear accelerator as set forth in claim 1, wherein said vacuum chamber permits the presence of plasma for aiding the propogation of said electron beam.

3. A linear accelerator as set forth in claim 1, wherein said conductive boundary includes a tubular waveguide flaring from a small dimension at its entrance to a large dimension at its exit.

4. A linear accelerator as set forth in claim 1, wherein said means for establishing the magnetic field includes a plurality of magnetic field coils successively spaced further apart starting from the vicinity of the entrance of said waveguide and concluding in the vicinity of the exit of said waveguide.

5. A linear accelerator as set forth in claim 1, wherein said waveguide is substantially free of internal constrictions.

6. A linear accelerator for accelerating injected ions, comprising a longitudinal waveguide creating a conductive boundary,

a vacuum chamber enclosing said waveguide,

means for establishing a longitudinal static magnetic field in said waveguide,

said magnetic field being large at the entrance of said waveguide and decreasing to a small value at the exit of said waveguide,

means for injecting ions into said waveguide,

means for establishing an intense beam of relativistic electrons immersed in said magnetic field,

said electron beam and magnetic field sustaining a traveling wave that is the lower branch of the upper hybrid resonant cyclotron eigenmode of said relativistic beam, the phase velocity of said wave increasing in such a manner as to accelerate the injected ions trapped in said wave.

7. A linear accelerator as set forth in claim 6, wherein said vacuum chamber permits the presence of plasma for aiding the propogation of said electron beam.

8. A linear accelerator as set forth in claim 6, wherein said conductive boundary includes a tubular waveguide flaring from a small dimension at its entrance to a large dimension at its exit.

9. A linear accelerator as set forth in claim 6, wherein said means for establishing the magnetic field includes a plurality of magnetic field coils successively spaced further apart starting from the vicinity of the entrance of said waveguide and concluding in the vicinity of the exit of said waveguide.

10. A linear accelerator as set forth in claim 6, wherein said waveguide is substantially free of internal constrictions.

11. In combination with an electron beam generator for establishing an intense relativistic beam, and

means for exciting a traveling wave that is the lower branch of the upper hybrid resonant cyclotron eigenmode of said relativistic beam and for loading ions to be accelerated on said wave, the improvement in a linear accelerator comprising a longitudinal waveguide for creating a conductive boundary, the entrance of said waveguide connected to receive the ion-loaded, relativistic beam, an evacuated chamber enclosing said waveguide, means for establishing a longitudinal static magnetic field within said waveguide, said magnetic field being large at the entrance of said waveguide and decreasing to a relatively small value at the exit of said waveguide, said magnetic field sustaining a traveling wave that is the lower branch of the upper hybrid resonant cyclotron eigenmode of said relativistic beam, the phase velocity of said relativistic beam accelerating at a rate to carry the injected ions therewith.

12. A linear accelerator as set forth in claim 11, wherein said evacuated chamber permits the presence of plasma for aiding the propogation of said electron beam.

13. A linear accelerator as set forth in claim 11, wherein said conductive boundary includes a tubular waveguide flaring from a small dimension at its entrance to a large dimension at its exit.

14. A linear accelerator as set forth in claim 11, wherein said means for establishing the magnetic field includes a plurality of magnetic field coils successively spaced further apart starting from the vicinity of the entrance of said waveguide and concluding in the vicinity of the exit of said waveguide.

15. A linear accelerator as set forth in claim 11, wherein said waveguide is substantially free of internal constrictions.

16. The method of accelerating ions, which comprises the steps of creating in an evacuated environment a longitudinal static magnetic field along a path, said magnetic field graduating from a large value at the entrance of said path and decreasing spatially to a relatively small value at the exit of said path,

establishing an intense beam of electrons into the entrance of said path,

establishing in the medium of said magnetic field and said beam of electrons a traveling wave that is the lower branch of the upper hybrid cyclotron mode of said beam, and

injecting ions to be accelerated onto said traveling wave to effect the loading of said ions about the minima of the potential of said wave, the phase velocity of said wave increasing in such a manner as to accelerate the injected ions trapped in said wave.

17. The method of accelerating ions as set forth in claim 16, wherein said ions are injected at substantially the same speed as the initial speed of said wave.

18. The method of accelerating ions, which comprises the steps of creating in an evacuated environment a longitudinal static magnetic field along a path, said magnetic field graduating from a large value at the entrance of said path and decreasing spatially to a relatively small value at the exit of said path,

establishing an intense relativistic beam of electrons into the entrance of said path,

establishing in the medium of said magnetic field and said relativistic beam of electrons a traveling wave that is the lower brance of the upper hybrid cyclotron mode of said relativistic beam, and

19. The method of accelerating ions as set forth in claim 18, wherein said ions are/injected at substantially the same speed as the initial speed of said wave.

Claims

1. A linear accelerator for accelerating injected ions, comprising a longitudinal waveguide creating a conductive boundary, a vacuum chamber enclosing said waveguide, means for establishing a longitudinal static magnetic field in said waveguide, said magnetic field being large at the entrance of said waveguide and decreasing to a small valve at the exit of said waveguide, means for injecting ions into said waveguide, means for establishing an intense beam of electrons immersed in said magnetic field, said electron beam and magnetic field sustaining a traveling wave that is the lower branch of the upper hybrid resonant cyclotron eigenmode of said beam, the phase velocity of said wave increaSing in such a manner as to accelerate the injected ions trapped in said wave.

6. A linear accelerator for accelerating injected ions, comprising a longitudinal waveguide creating a conductive boundary, a vacuum chamber enclosing said waveguide, means for establishing a longitudinal static magnetic field in said waveguide, said magnetic field being large at the entrance of said waveguide and decreasing to a small value at the exit of said waveguide, means for injecting ions into said waveguide, means for establishing an intense beam of relativistic electrons immersed in said magnetic field, said electron beam and magnetic field sustaining a traveling wave that is the lower branch of the upper hybrid resonant cyclotron eigenmode of said relativistic beam, the phase velocity of said wave increasing in such a manner as to accelerate the injected ions trapped in said wave.

11. In combination with an electron beam generator for establishing an intense relativistic beam, and means for exciting a traveling wave that is the lower branch of the upper hybrid resonant cyclotron eigenmode of said relativistic beam and for loading ions to be accelerated on said wave, the improvement in a linear accelerator comprising a longitudinal waveguide for creating a conductive boundary, the entrance of said waveguide connected to receive the ion-loaded, relativistic beam, an evacuated chamber enclosing said waveguide, means for establishing a longitudinal static magnetic field within said waveguide, said magnetic field being large at the entrance of said waveguide and decreasing to a relatively small value at the exit of said waveguide, said magnetic field sustaining a traveling wave that is the lower branch of the upper hybrid resonant cyclotron eigenmode of said relativistic beam, the phase velocity of said relativistic beam accelerating at a rate to carry the injected ions therewith.

16. The method of accelerating ions, which comprises the steps of creating in an evacuated environment a longitudinal static magnetic field along a path, said magnetic field graduating from a large value at the entrance of said path and decreasing spatially to a relatively small value at the exit of said path, establishing an intense beam of electrons into the entrance of said path, establishing in the medium of said magnetic field and said beam of electrons a traveling wave that is the lower branch of the upper hybrid cyclotron mode of said beam, and injecting ions to be accelerated onto said traveling wave to effect the loading of said ions about the minima of the potential of said wave, the phase velocity of said wave increasing in such a manner as to accelerate the injected ions trapped in said wave.

18. The method of accelerating ions, which comprises the steps of creating in an evacuated environment a longitudinal static magnetic field along a path, said magnetic field graduating from a large value at the entrance of said path and decreasing spatially to a relatively small value at the exit of said path, establishing an intense relativistic beam of electrons into the entrance of said path, establishing in the medium of said magnetic field and said relativistic beam of electrons a traveling wave that is the lower brance of the upper hybrid cyclotron mode of said relativistic beam, and injecting ions to be accelerated onto said traveling wave to effect the loading of said ions about the minima of the potential of said wave, the phase velocity of said wave increasing in such a manner as to accelerate the injected ions trapped in said wave.