US3679930A

US3679930A - Method for increasing the output of an electron accelerator

Info

Publication number: US3679930A
Application number: US849705A
Authority: US
Inventors: Alex D Colvin; David H Mcnitt; Allen H Turner
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1969-08-13
Filing date: 1969-08-13
Publication date: 1972-07-25
Anticipated expiration: 1989-07-25
Also published as: DK141080B; FR2057691A5; ZA705583B; SE363189B; BE754746A; DK141080C; GB1324850A; NL147578B; DE2040158C3; DE2040158B2; NL7011910A; DE2040158A1

Abstract

A method of increasing the energy output of an electron accelerator by scanning the electron beam produced in the accelerator in a manner so that excessive heat build-ups do not occur at the electron permeable window structure through which the beam passes to exit the accelerator. This method is used in conjunction with an accelerator having at least two elongate, electron permeable windows arranged in a side-by-side orientation and scanning coils allowing the beam to be scanned parallel to at least two, perpendicular axes. The method provides that the beam is scanned along the length of a first of the windows in a first direction and then instantaneously is scanned laterally to the second of the windows. The beam then is scanned along the length of the second of the windows in a second direction 180* from the first direction. A return of the beam to its original position is accomplished as the beam then is scanned laterally and instantaneously a second time such that it again is directed at the first of the windows. Unlimited repetition of these scanning steps is possible.

Description

United States Patent [151 3,679,930 Colvin et al. 51 July 25, 1972 [54] METHOD FOR INCREASING THE Primary Examiner-Rodney D. l3ennett, Jr.

OUTPUT OF AN ELECTR N Assistant Examiner-Brian L. Ribando ACCELERATOR O Att0rneyJohn R. Faulkner and E. Denms OConnor [57] ABSTRACT [72] Inventors: Alex D. Colvin, Livonia; David H. McNltt,

Royal Oak; Allen Turner Ann Arbor A method of increasing the energy output of an electron acan of Mich. celerator by scanning the electron beam produced in the accelerator in a manner so that excessive heat build-ups do not occur at the electron permeable window structure through [73] Asslgnee' Ford Motor Company Dearbom Mlch' which the beam passes to exit the accelerator. This method is [22] Filed; Aug 3 19 9 used in conjunction with an accelerator having at least two elongate, electron permeable windows arranged in a side-by- PP 849,705 side orientation and scanning coils allowing the beam to be scanned parallel to at least two, perpendicular axes. The method provides that the beam is scanned along the length of [52] US. Cl. ..315/21 R, 315/24 3 first of the windows in a first direction and then instantane- [51] Int. Cl. ..H01 j 29/76 ously is scanned laterally to the second of the windows. The [58] Field of Search ..3l5/l8, 24, 26, 31, 21 beam then is scanned along the length of the second of the windows in a second direction 180 from the first direction. A return of the beam to its original position is accomplished as [56] Refe'emes cued the beam then is scanned laterally and instantaneously a second time such that it again is directed at the first of the win- UNITED STATES PATENTS dows. Unlimited repetition of these scanning steps is possible. 3,066,238 11/1962 Arndt ..3l5/22 6 Claims, 4 Drawing Figures METHOD FOR INCREASING THE OUTPUT OF AN ELECTRON ACCELERATOR BACKGROUND OF THE INVENTION Electron beam generators or electron accelerators having accelerating voltages in the order of several million volts conventionally include a long insulating container. This container defines an evacuated chamber through which electrons are accelerated to form a beam by means of a large potential difference existing between an electron gun, including a hot cathode emitter at one end of the chamber, and an anode at the other end of the chamber. The anode includes an electron permeable window through which the beam passes from the chamber onto a substance being irradiated.

Conventional materials for the formation of electron permeable windows are thin metal foils that allow the passage of the electron beam and can be supported in an opening in a container wall so that the vacuum within the chamber is maintained. As the electron beam passes through the metal window, the electrons are scattered to some extent with a consequent heating of the window. If a portion of the metal window is heated beyond a tolerable level as the beam is scanned along the window, this overheating causes oxidation and weakening of the metal and resultant punctures through the metal. The necessary vacuum seal otherwise provided by the window obviously is destroyed upon the formation of punctures. In order to avoid such overheating of the window, the electron intensity of the beam produced must be controlled rigidly, thus causing beam generator operation below the potential output of this machine.

Various steps have been taken to avoid the overheating described above. Cooling means such as blasts of air directed along the window are being used, but they are not sufficient in themselves to dissipate enough heat to prevent adverse heating effects. Also, it has been proposed to prevent excessive localized heating by increasing greatly the size of the window so that the beam may be scanned over a large area.

While it is true that a larger window has a larger tolerable beam current intensity, prior art arrangements having increased window sizes have had severe disadvantages and limiting factors. This is because thethin window materials require mechanical support structures of an exceedingly complex nature if these windows have large areas. These support structures cause gaps in the window area through which the beam can pass and a resultant wastage of beam current when the beam impinges on the impermeable support structure. It is because of this that conventional windows have a relatively large length compared to their width. Window lengths, however, also are limited to support requirements as attested to by the fact that accelerators with windows more than 72 inches long are not known and many facilities have become unmanageable with windows more than 48 inches long.

The necessarily narrow configuration of beam aperture windows also limits the efiectiveness of various multi-dimension beam scanning methods designed to prevent overheating. Such methods do not restrict beam movement to a single dimension, that is, along the length of the window, but rather provide for additional beam scanning at least along the transverse dimension of the window. Narrow window shapes greatly impede beam movement back and forth along the length of the window, regardless of transverse beam manipulations, without subsequent heating of already heated locales and the consequent danger of overheating.

It is an object of this invention to provide a method for increasing the output of an electron beam generator by reducing the risk of window overheating due to the passage therethrough of a high current intensity beam. This method takes advantage of a relatively large window area that is a composite of plural windows, and provides for beam scanning over the entire composite area without undue wastage of beam current due to impingement of the beam on impermeable structure.

SUMMARY OF THE INVENTION This invention provides a method for increasing the output of an electron accelerator by effecting this efiicient distribution of available ionizing energy from the electron beam produced in the evacuated container of the accelerator and directed toward at least a pair of elongate, electron permeable windows arranged in a side-by-side, spaced apart configuration in the wall of the container. This method includes scanning the beam over a predetermined time period along the length of a first of the windows and then instantaneously directing the beam from the first window to the second of the windows. The beam then is scanned over a predetermined time period along the length of the second of the windows and then instantaneously directed from the second window to the first window. The beam scanning along the length of the first window is in a direction 180 from the direction of the beam scanning along the length of theother window. These steps may be repeated as a cycle as many times as desired.

DESCRIPTION OF THE DRAWINGS FIG. I is an isometric view illustrating schematically 'an electron accelerator that may be used to practice the method of this invention;

FIG. 2 is a view taken along the line of sight identified by the arrow 2 in FIG. 1;

FIG. 3 is a schematic representation of circuitry utilized in conjunction with theaccelerator of FIG. 1 and;

FIG. 4 is a composite representation of six electrical signals generated by the circuitry of FIG. 3, these signals being plotted against time.

DETAILED DESCRIPTION OF THE INVENTION Referring now in detail to the drawings, and in particular in FIGS. 1 and 2 thereof, the numeral 10 denotes generally an accelerator useful in the practice of the method of this invention. This accelerator includes an elongate container 12 defining an internal chamber 14. Proximate one end wall 16 of container I2 is a hot cathode emitter 18. Remote from end wall 16, container 12 includes a tapered portion 20 terminating in an end wall 22. End wall 22 has formed therethrough a pair of elongated apertures in which are mounted electron permeable, spaced apart window members 24 and 26v As is conventional, window members 26 may be constructed of such electron beam permeable material as thin metal foil. The window members are substantially rectangular and are spaced apart with their longitudinal axes lying substantially parallel to one another. Windows 24 and 26 are mounted in the apertures formed in the container end wall 22 such that they provide an air tight seal of these apertures so that a vacuum may be maintained within chamber 14. Windows 24 and 26 are included in the anode structure of the accelerator. A great potential difference exists between this anode structure and cathode 18 such that electrons emitted by the cathode form an electron beam directed at the windows 24 and 26.

Control of the direction of this electron beam is provided by four

electromagnetic scanning coils

28, 30, 32 and 34 positioned 90 apart about the outer periphery of the container 12.

As illustrated, coils 28 and 30 are positioned to scan an electron beam emitted by cathode 18 in directions parallel to the Y-axis shown in FIG. 2, that is, parallel to the longitudinal axes of windows 24 and 26.

Coils

32 and 34 are positioned to scan the electron beam parallel to the X-axis shown in FIG. 2, that is, transverse to the longitudinal axes of the windows 24 and 26.

In the practice of the method of this invention, the

scanning coils

28, 30, 32 and 34 have the current passing therethrough and the resultant magnetic field produced varied such that the electron beam extending the length of chamber 14 has its direction varied in a particular manner. The particular scanning pattern that results provides that the current intensity of the beam is maximized without danger of overheating of the windows 24 and 26. The manner in which the electron beam is controlled may be appreciated by reference to H6. 2. Let it be assumed that the electron beam initially is directed by the scanning coils such that it impinges upon window 26 at point 36. This point 36 is chosen arbitrarily as the scanning of the electron beam in a closed path as explained below is a continuous operation such that any point along the closed path may be considered a starting or finishing point.

The current passing through

scanning coils

32 and 34 is maintained at a constant level such that no transverse movement parallel to the X-axis of the beam occurs. Simultaneously, the current passing through coils 28 and 30 is varied such that the beam is canned over a predetermined period of time along the broken line 38 as indicated by the arrows on the line. Asthe beam reaches the end of window 26 remote from point 36, the current passing through

coils

32 and 34 instantaneously is varied to a second constant level. This instantaneous variance produces an instantaneous movement of the beam along the straight line 40 in a direction parallel to the X- axis. This instantaneous movement of the beam from window 26 to window 24 precludes the wastage of'beam energy due to impingement of the beam on the portion of end wall 22 located between windows 24 and 26.

With the current in

coil

32 and 34 maintained at the second constant level such that the beam impinges on window 24, the current in beams 28 and 30 is varied such that the beam is scanned over a predetermined period of time along the length of window 24 as indicated by the broken line 42 and in a direction indicated by the arrows on line 42. It easily may be appreciated that the direction in which the beam is scanned along the window 24 is 180 from the direction along which the beam is scanned over the window 26. When the beam scanning along window 24 reaches the end of this window, an instantaneous variance in the current passing through

coils

32 and 34 causes an instantaneous movement of the beam along the line 44 such that the beam is directed from window 24 to window 26 and the starting point of the scanning cycle point 36. As with the previous transverse scanning parallel to the X-axis, this movement from window 24 to window 26 occurs instantaneously and thus prevents the wastage of beam energy.

As explained above, it is by control of the current passing through the scanning coils that the beam scanning steps of the method of this invention are accomplished. The current through the coils is determined by the operation of the circuitry shown schematically in FIG. 3 of the drawings. It may be seen that a pulse generator 46 is connected electrically by means of a signal transmitting means 48 to a square wave generator 50 that sends output signals along a pair of signal transmitting means 52 and 54 to a Y-axis driver 56 that in turn emits along signal transmitting means 58 the final current signal that is directed to coils 28 and 30. As explained above, the current in these coils determines beam movement along the Y-axis.

The signal transmitted to the Y driver 56 along signal transmitting means 52 and 54 also is transmitted along signal transmitting means 60 and 62, respectively, to the X driver 54 that produces current signals that are directed to coils 32 and 34 by means of signal transmitting means 66 and 68, respectively. The X driver is so termed because

coils

32 and 34 determine beam scanning movement parallel to the X axis of FIG. 2.

As readily may be appreciated from the description of function set forth below, thepulse generator 46, square wave generator 50, Y driver 56 and X driver 64 are conventional electrical circuits that quite easily may be constructed by one having ordinary skill in the art. The signal transmitting means 48, 52, 54, 58, 60, 62, 66 and 68 may be such conventional electrical components as conductive wire leads and thus constitute no part of the instant invention.

Referring now to FIG. 4 of the drawings, the plot 4a represents the voltage signal emitted by pulse generator 46 along signal transmitting means 48 to square wave generator 50. It may be seen that this signal is merely a periodic pulse having a constant frequency. The output signal voltage of the square wave generator 50 along the signal transmitting means 52 and 54 are represented by the plots 4b and 4c, respectively. It may be seen that plots 4b and 4c comprise square wave 180 out of phase and having a frequency the same as the frequency of the pulses of plot 40.

Plot M represents the saw toothed current signal emitted by the Y driver 56 that receives voltage signals 4b and 4c. Current wave 4d is directed to both coils 28 and 30. that are connected in parallel, via the signal transmitting means 58. As the current 4d progresses from a peak to a valley, magnetic fields given off by coils 28 and 30 caused the electron beam produced in the accelerator 10 to be scanned in a first direction along the length of one of the windows 24 or 26. As the current 4d then progresses from a valley to a peak, the electron beam is scanned in a second direction, opposite to the first direction and along the length of the other of the windows.

X driver 64 receives each of the signals 4b and 4c and has output current signals 4e and 4f that are transmitted to coils 32 and 34 by means of conductive leads 66 and 68, respectively. The square wave current values of the plots 4e and 4f are l out of phase and have positive durations corresponding 7 to the durations of the current 4d along either -a positive or a negative slope. It thus readily may be appreciated that the current in

coils

32 and 34 remains constant during the periods when the current represented by the plot 4d is proceeding either from a peak' to a valley or from a valley to a peak. Whenever the current of the plot 4d reaches a transition point (peak or valley) the current in

coil

32 and 34 undergoes an instantaneous change resulting in instantaneous movement of the electron beam parallel to the X-axis and from one of the windows 24 or 26 to the other. This arrangement thus insures that no transverse scanning of the beam will occur while longitudinal scanning of the beam proceeds.

It thus may be seen that this invention provides a method of enhancing the output of an electron accelerator by making possible the generation of an electron beam of high intensity without danger of localized overheating of the beam permeable window structure through which the electron beam exits the accelerator structure. This method provides for beam scanning between plural, electron permeable windows without the wastage of beam energy due to impingement of the beam on electron beam impermeable structure.

We claim:

l. A method of effecting the efficient distribution of available ionizing energy from an electron beam produced in an evacuated container and directed toward at least a pair of elongate electron permeable windows arranged in a side-byside spaced apart configuration in a wall of said container, said method comprising the steps of: scanning said beam over a predetermined time period along the length of one of said windows, instantaneously directing said beam from said one window to the other of said windows, scanning said beam over a predetermined time period along the length of said other window, and instantaneously directing said beam from said other window to said one window.

2. The method of claim 1, further comprising the step of: repeating the steps recited in Claim 1.

3. The method of claim 1, wherein beam scanning along the length of said one window is a first direction and beam scanning along the length of said other'window is in a second direction opposite to said first direction.

4. A method for increasing the output of an electron accelerator capable of producing an electron beam within an evacuated container, said beam being directed at a pair of elongate electron permeable windows arranged in a side-byside spaced apart configuration in a wall of said container, said accelerator including a first pair of beam scanning coils positioned on opposite sides of said electron beam for scanning said beam along a path parallel to a first axis and a second pair of beam scanning coils positioned on opposite sides of said electron beam for scanning said beam along a path parallel to a second axis, each one of said second pair of coils being spaced 90 from each one of said first pair of coils such that said second axis is normal to said first axis, said method comprising the steps of: establishing a first varying current condition in said first pair of coils and a first constant current condition in said second pair of coils, thereby scanning said beam in a first direction parallel to said first axis along the length of one of said windows; instantaneously establishing a second constant current condition in said second pair of coils, thereby directing said beam in a path parallel to said second axis to the other of said windows; and establishing a second varying current condition in said first pair of coils while maintaining said

Claims

1. A method of effecting the efficient distribution of available ionizing energy from an electron beam produced in an evacuated container and directed toward at least a pair of elongate electron permeable windows arranged in a side-by-side spaced apart configuration in a wall of said container, said method comprising the steps of: scanning said beam over a predetermined time period along the length of one of said windows, instantaneously directing said beam from said one window to the other of said windows, scanning said beam over a predetermined time period along the length of said other window, and instantaneously directing said beam from said other window to said one window.

3. The method of claim 1, wherein beam scanning along the length of said one window is a first direction and beam scanning along the length of said other window is in a second direction opposite to said first direction.

4. A method for increasing the output of an electron accelerator capable of producing an electron beam within an evacuated container, said beam being directed at a pair of elongate electron permeable windows arranged in a side-by-side spaced apart configuration in a wall of said container, said accelerator including a first pair of beam scanning coils positioned on opposite sides of said electron beam for scanning said beam along a path parallel to a first axis and a second pair of beam scanning coils positioned on opposite sides of said electron beam for scanning said beam along a path parallel to a second axis, each one of said second pair of coils being spaced 90* from each one of said first pair of coils such that said second axis is normal to said first axis, said method comprising the steps of: establishing a first varying current condition in said first pair of coils and a first constant current condition in said second pair of coils, thereby scanning said beam in a first direction parallel to said first axis along the length of one of said windows; instantaneously establishing a second constant current condition in said second pair of coils, thereby directing said beam in a path parallel to said second axis to the other of said windows; and establishing a second varying current condition in said first pair of coils while maintaining said second constant current condition in said second set of coils, thereby scanning said beam in a second direction parallel to said second axis.

5. The method of claim 4, wherein said first direction is opposite to said second direction.

6. The method of claim 4, further including the further steps of instantaneously re-establishing said first constant current condition in said second set of coils, thereby redirecting to said one window said beam in a path parallel to said second axis and terminating the second varying current condition in said first pair of coils.