US2809322A - Neutralization of ion plasma oscillations in electron discharge tubes - Google Patents

Description

1957 K. G.'HERNQVIST I NEUTRALIZATIQN 0F ION PLASMA OSCILLATIONS m ELECTRON DISCHARGE TUBES Filed Feb. 1954 SQURCE BY ATTORNEY t n ed States. Patent 9 NEUTRALEZATION FF ION PLASMA OSCILLA- TION S IN ELEQTRQN DISCHARGE TUBES Karl G. Hernqvist, Princeton, N. 5., assign-or to Radio Corporation of America, a corporation of Delaware Application February 1, 1954, Serial No. 407,287 8 Claims. (Cl. 315-538) This invention relates to neutralization of ion plasma oscillations, and particularly to means for generating a uniform unmodulated high density beam of charged particles such as an electron beam.

It is known to generate a conical electron beam of high density at the apex by thermally emitting electrons from a concave cathode, thru a concentric accelerating grid, and thru an aperture in an anode to a beam utilizing device. The beam utilizing device, for example, may be a klystron, a magnetron, a traveling Wave tube or an impedance transformer.

The electrons in the beam repel each other and resist being concentrated into a dense beam at the apex. To make a very dense beam, the mutual repulsion of the electrons must be overcome. This may be done by enclosing the beam in a region substantially free of an electric field and having a potential greater than the ionization potential of the residual gas. This potential may be in the order of to volts. Positive ions are formed by collisions of the electrons with residual gas atoms, and since the ions are in a field-free region, they remain to form a positive ion plasma which neutralizes the mutual repulsion of the electrons in the beam. This is known as space charge neutralization.

It has been found that in a space charge neutralized beam, stray electrons strike an electrode in contact with the ion plasma, and thru the effects of secondary emission, the ion plasma is caused to oscillate at a frequency in the order of 0.5 to 10 megacycles depending on the ion density. The ion plasma oscillates at a frequency which varies directly as the square root of the ion density. Under the equilibrium conditions existing, the ion density is equal to the electron density. Therefore the ion plasma oscillates at a frequency proportional to the square root of the electron density. The ion plasma oscillates at a natural frequency according to the formula:

f= 2.15k1/ L Mw V where f is frequency in megacycles, k is a geometry factor which is equal to the number 1 for plane parallel oscillations of the plasma and is equal to l/ /2 for a cylindrical plasma column, Ia. is electron density in terms of milliamperes of anode current per square centimeter of efifective anode area, Va is anode voltage and M is the molecular weight of the residual gas molecules. The number 2.15 is a constant including such factors as electron charge, electron mass, dielectric constant, etc.

The ion plasma oscillations tend to cause a modulation of the electron beam passing thru the aperture in the anode to the utilization device. It is therefore a general object of this invention to provide means for neutralizing the undesired modulation of a beam of charged particles such as an electron beam caused by the ion plasma oscillations.

It is another object to provide novel means to generate an unmodulated electron beam of high density.

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In one aspect, the invention comprises a vacuum tube having a concave cathode, an accelerating grid concentric therewith, and an apertured anode constructed to cooperate with the grid to form a field-free region. A positive ion plasma is formed in the field-free region to provide space charge neutralization which allows the electron beam to be highly concentrated in passing thru the aperture in the anode. An undesired modulation of the electron beam passing thru the aperture in the anode results from oscillations of the ion plasma. This modulation of the electron beam is neutralized by taking a signal from the anode, passing it thru an amplifier and a phase adjuster, and applying the signal in proper phase to the accelerating grid. In this way, the modulation of the electron beam where it passes thru the aperture in the anode to the utilization device is substantially eliminated.

These and other objects and aspects of the invention will be apparent to those skilled in the art from the following more detailed description of the invention taken in conjunction with the appended drawing, the single figure thereof showing one embodiment of the invention.

The drawing shows a vacuum tube together with feedback means to neutralize modulation of the electron beam caused by ion plasma oscillations. The cathode 5 is in concave form (a segment of a sphere), and is heated by heater Wire 6 connected to a source of heater current 7. An accelerating grid 8 is arranged concentrically with but spaced from the cathode 5. A two-part anode 9 and 9' includes a conical cavity 10, a radio frequency resonator cavity 11, and a second conical cavity 12. The accelerating part 9 and the collector part 9' of the anode are electrically insulated from each other by insulation 9". The cavities in the anode 9 and the space surrounding the cathode 5 and the accelerating grid 8 are enclosed by glass ends 13 and 14 sealed to the accelerating anode 9 and the collector anode 9', respectively. The space within the tube is evacuated to a pressure in the order of between 10* and 10 millimeters of mercury.

The cathode 5 is maintained at a negative potential in the order of from 250 to 1000 volts negative with respect to the accelerating grid 8 and the anode 9, 9' by means of a battery 15.

A small resistor 16 is connected from the anode 9 to ground. Anode 9' is connected directly to ground. The anode 9 is connected to the input of an amplifier and phase adjuster 17. The output of the amplifier 17 is connected to the accelerating grid 8, and thru a resistor 18 to ground.

Radio frequency energy, in the order of hundreds or thousands of megacycles from a source 29 is applied thru a coaxial line 21 to the resonant cavity 11 in the anode 9. The frequency of source 20 is the same as the resonant frequency of the cavity. The radio frequency energy from source 20 which is applied to the tube reacts upon the electron beam passing thru the resonant cavity 11 to provide an impedance transformation so that the energy passes out of the glass seal 14 to a Waveguide (not shown).

The impedance transforming device is an example of one utilization device which utilizes the concentrated electron beam. Other utilization devices such as klystrons, magnetrons, and traveling wave tubes may be employed to receive the electron beam from the electron gun 5, 8, 9.

The amplifier and phase. adjuster 17 may be any suitable circuits operative at frequencies in the order of 0.5 to 10 megacycles, such as the amplifier shown on page 435 and the phase adjuster shown on page 949 of Termans Radio Engineers Handbook, first edition, 1943.

In operation, electrons are thermally emitted from the concave cathode 5 and are attracted toward and thru the accelerating grid 8 and to the conical cavity portion 12 of the anode 9 because the grid 8 and anode 9, 9 are 7 anode 9.

It is possible for the electron beam from cathode .5 to converge to form a very dense beam at the aperture 22 because the mutual repulsion of the electrons in the beam is neutralized by the presence of a positive ion plasma in the confined region formed by the accelerating grid 8 and the walls of the conical cavity 10 in anode 9. This region is substantially free of any electric field that would tend to move the positive ions from the region 'to an electrode surface. The positive ion plasma is formed 7 by electrons in the beam which strike residual gas molecules.

The positive ion plasma in the region between accelerating grid 8 and the conical cavity 10 in anode 9 has a tendency to oscillate at a frequency determined by the density of the ion plasma. The oscillations of the ion plasma'are usually caused by stray electrons which strike the walls of cavity 10 and produce low energy secondary emission from the walls of cavity 10 back into the ion plasma. The ion plasma oscillation causes a modulation 7 of the electron beam passing thru the aperture 22 in'the anode 9 V 'beam may be at a frequency somewhere between 0.5 and This undesired modulation of the electron 10 'megacycles, and it has a disturbing effect upon the proper operation of the utilization device. f

The modulation of the electron beam by' the ion plasma is reflected in an oscillating partioning of the number of electrons striking the wall of the conical cavity 10 of the anode 9 compared with the number Striking the wall of cavity 12 in anode 9'. This modulation is translated to an alternating current voltage across resistor 16 which is connected from the anode 9 to ground; The voltage across resistor 16 is applied thru the amplifier and phase adjuster 17 to the accelerating 'grid 8 in proper phase and magnitude so that the variation in the number of electrons V striking the walls of cavity 12 is substantially neutralized.

Inthe example of the invention shown and described, the

amplifier and phase adjuster 17 should preferably include an odd number of vacuum tubes so that there is a ISO-degree phase reversal in the amplifier. At an instant of time when an excess number .of electrons strike the Walls of conical cavity 10, the input to'the amplifier 17 is relatively negative and an amplified positive signal is.

applied to the accelerating grid 8 to increase the number of'electrons in the beam. In this way, the modulation of the'electron beam and the modulation of the number of electrons reaching collecting anode .9 is substantially V eliminated. A phase adjuster is desirable in the box 17 so that adjustments may be made for minor phase shift in the complete feed-back loop.

It will be understood to those skilled'in the art that the neutralizing feed-back signal from the amplifier 17 may be applied to the cathode 5 rather-than .to the accelerating grid 8. The feed-back may be applied in any suitable manner so as to effect a neutralizing modulation 7 of the number of electrons entering the region formed oscillations from said beam, and -means to feed said V 4 signal voltage back to said source in phase opposition to substantially'cancel said modulation of the beam.

2. A vacuum tube system comprising, a cathode, a control electrode and an anode, means to bias said control electrode and said anode at a positive potential relative to saidcathode, said control electrode and said anode being formed to define a space charge neutralized region thru which an electron beam passes and in which ion plasma oscillations are created which modulate said beam, means to derive an alternating signal voltage representa tive of said ion plasma oscillations from said anode, and means to feed said signal voltage across said cathode and control electrode in phase opposition to substantially cancel said modulation of the beam.

3. A vacuum tube system comprising a vacuum tube having electrodes to generate an electron beam and having members forming a space charge neutralized region thru which said beam passes and in which ion plasma oscillations are created which modulate said beam, means to derive a signal voltage representative of said ion plasma oscillations from said beam, and means to feed said signal voltage back to said source in phase opposition to substantially cancel said modulation of the beam.

4. A vacuum tube having .a first region wherein an electron beam is generated, a second space charge neutralized region thru which said beam passes and in which ion plasma oscillations are created which modulate said beam, and a third region wherein said beam is utilized, means to derive a signal voltage representative of said ion plasma oscillations from said beam in said third region, and means to'feed said signal voltage back to said' first region in phase opposition to substantially cancel said modulationof thebeamin'said third region.

5. A-vacuum tube system comprising an evacuated envelope having a concave cathode for producing a beam of electrons, a concentric grid, and an anode, said anode having a conical Wall at the apex of which the electrons from said *cathode are concentrated, said grid and said anode forming a space charge neutralized region therebetween in which ion plasma oscillations are created which modulate said beam, and feed-back means coupling energy r'epr'esentativeof said oscillations from said anode to said grid. a

'6. A vacuumtube'system comprising an evacuated envelope having a concave cathode for producing-a beam of electrons, 'a'concentric grid, an accelerating anode, and a collector anode, said accelerating anode having an aperture through which the electrons from said cathode pass to said collector anode, saidgrid and said. accelerating anode forming a space charged neutralized region therebetween-in which ion plasma oscillations are created which modulate said beam and cause a current partition between said accelerating anode and said collector anode, and feed-back means coupling energy representative of said oscillations from said accelerating anode to said grid.

7. A vacuum tube system as defined in claim 6, and in addition, radio frequency utilization means coupled to said electron beam at a region within said accelerating anode.

8. Avacuum tube'system as defined in claim 6, and in addition, a resonant cavity coupled to said electron beam and located between said accelerating anode and said collector anode, and a radio frequency'source coupled to said resonant cavity.

References Citedin the file of thispatent UNITED STATES PATENTS 7 2,440,089 a Haetr r. 20, .1948 2,455,269 Pierce Nov. 30,1948 2,484,643 Peterson. Oct. 11,1949 2,494,721 Robertson Jan. 17, 1950 2,595,698 Peter May 6,1952 2,622,225 Linden Dec. 16, 1952 2,767,259 Petcr. Oct. 16, @1956

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