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US3281617A - Plasma ion source having apertured extractor cathode - Google Patents

Plasma ion source having apertured extractor cathode Download PDF

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Publication number
US3281617A
US3281617A US259497A US25949763A US3281617A US 3281617 A US3281617 A US 3281617A US 259497 A US259497 A US 259497A US 25949763 A US25949763 A US 25949763A US 3281617 A US3281617 A US 3281617A
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United States
Prior art keywords
cathode
plasma
ion source
extractor
target
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
US259497A
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English (en)
Inventor
Hedley James David London
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UK Atomic Energy Authority
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UK Atomic Energy Authority
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Filing date
Publication date
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J27/00—Ion beam tubes
    • H01J27/02—Ion sources; Ion guns
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J27/00—Ion beam tubes
    • H01J27/02—Ion sources; Ion guns
    • H01J27/08—Ion sources; Ion guns using arc discharge
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J27/00—Ion beam tubes
    • H01J27/02—Ion sources; Ion guns
    • H01J27/16—Ion sources; Ion guns using high-frequency excitation, e.g. microwave excitation
    • H01J27/18—Ion sources; Ion guns using high-frequency excitation, e.g. microwave excitation with an applied axial magnetic field
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H3/00—Production or acceleration of neutral particle beams, e.g. molecular or atomic beams
    • H05H3/06—Generating neutron beams

Definitions

  • This invention relates to ion sources suitable for use in apparatus for carrying out a nuclear reaction by bombarding a target material with high energy ions in an ion beam, the ions being produced as a plasma in an ion source and accelerated across an acceleration gap which contains gas at substantially the same pressure as gas in the ion source.
  • Apparatus of the above type does not require pumping means to keep the gas pressure low in the acceleration gap and as a result the ion source and the acceleration gap can be contained within a common sealed envelope.
  • the apparatus has its prime use as a neutron generator, in which case the favoured nuclear reactions are the DT and DD reactions and a gas pressure controller is provided within the sealed envelope to compensate for gas used in the nuclear reaction or lost by absorption on solid surfaces.
  • Ion sources which have been used up to now consist essentially of two cathodes which face each other from opposite sides of the ion source, a cylindrical anofde positioned between the cathodes with its ends facing the cathodes and a magnet or electromagnet for producing a magnetic field which penetrates the plasma and has the same direction as the axis of the cylindrical anode.
  • the two cathodes generate a potential trough in which electrons can swing to and fro inside the cylindrical anode, and the magnetic field forces the electrons into helical paths and increases the lengths of the paths they travel before capture by the anode.
  • the plasma is an electrically conducting medium, consisting as it does essentially of positive ions and electrons, and electrostatic field lines therefore terminate at its boundary and do not extend into its interior.
  • the magnetic field can penetrate the plasma and therefore applies a constraint to the whole of the plasma thereby exerting its optimum elfect and it has been universally regarded as essential for the constraint to be applied as uniformly as possible within the plasma.
  • the magnetic field has up to now been the only known means for preventing charged particles escaping and diffusing to the walls of the ion source and to constrain the directions of movement of electrons.
  • This magnetic field can be best provided either by a permanent magnet inside the sealed envelope or by an electro-Inagnetic coil positioned outside the sealed envelope.
  • the external arrangement adds to the bulk of the tube and introduces complications into the design of the ion source since special construction of the tube is needed to avoid screening preventing the entry of the magnetic field into the interior of the ion source and the plasma.
  • This invention provides an ion source arrangement which does not require the use of a magnetic field and therefore avoids the above disadvantages.
  • the invention consists in an ion source for generating a plasma, which comprises a cathode, an extractor cathode spaced from the said cathode and having an aperture therein, an anode positioned between the cathode and the extractor cathode and separated by a gap therefrom, and a screen negative electrode positioned about the said cathode and external to the plasma, and extending in a direction towards the extractor cathode to apply an electric field to the gap between the cathode and the anode.
  • the elfect of the screen negative electrode is most unexpected in vieW of the known properties of plasmas.
  • the plasma extends to within microns of the cathode and therefore the electrons emitted by the cathode are swallowed almost immediately by the plasma and should be screened from the field of the screen negative electrode.
  • the anode itself must screen the plasma from any external electric field and it would be expected that the behaviour of the plasma would therefore be determined only by the field exerted by the anode and cathode and that the system would behave simply as an ion source having no magnetic field.
  • the gas pressure required to produce ions in any useful quantity to be about 1000 times the pressure which could be used in the presence of the magnetic field.
  • the ion source can be either the cold cathode type or the hot cathode type. With the col-d cathode type each cathode would be provided with a screen negative electrode.
  • FIG. 1 is a sectional elevation of a neutron generator
  • FIG. 2 illustrates plasma impedance at 20 micron deuterium pressure
  • FIG. 3 illustrates variations in plasma voltage and screen volts
  • FIG. 4 illustrates the relationship of target current to gas pressure
  • FIG. 5 illustrates the relationship between target current and target volts
  • FIG. 6 shows the neutron output as a function of target voltage
  • FIG. 7 shows the neutron yield as a function of screen negative electrode volts.
  • FIGURE 1 an oxide hot cathode at earth potential I mounted on supporting leads 2 is surrounded by a cylindrical screen negative electrode 3 mounted on a connecting lead 4.
  • a cylindrical anode 5 is carried on a support lead 6. All support leads pass through a pinch 7 of a glass envelope 8.
  • An extractor cathode 9 at earth potential is mounted between the anode 5 and a target 10. Leads 11 are connected to the target 10 which consists of tritiated titanium supported on a molybdenum base.
  • a dispenser 12 (described and claimed in my specification No. 850,950) for deuterium is contained in a glass extension 13 and a Pirani gauge 14 in another glass extension 15, the interiors of which communicate directly with the space inside the envelope 8.
  • a transparent plastic container 16 protects the glass envelope 8 and a corona shield 17 covers one end of the container 16.
  • the generator described is more suitable for use as a continuous generator than as a pulsed generator.
  • Ion source variations are shown in FIGURE 2, screen volts varying from -100 to 200 volts and target volts from 50 to 90 kv. It will be seen that up to 1 milli-amp plasma current, the impedance across the plasma, increases at a steady rate. Above 1 ma. the impedance is fairly constant up to 1.7 ma., above 1.7 ma. the impedance falls. Removal of the target volts appears to make no appreciable change in the plasma impedance.
  • FIGURE 3 shows the variation of plasma voltage and screen current with screen volts, for 1 milli-amp plasma current at pressures of 10 to 40 microns. It should be noted that no target volts were applied during the period readings were taken.
  • FIGURE 4 shows variation of target current at 10 to 40 microns pressure.
  • the screen was maintained at 200 volts.
  • FIGURE 5 shows variation of target current with voltage. It will be observed that the chart divides itself into two groups. The three high level runs were obtained when the plasmacurrent was set between 1.5 to 2.0 ma. The lower three curves were taken with the level of plasma current between 1.1 and 1.3 ma. There is a small difference in the rate of slope between the two sections, which cannot readily be explained.
  • FIGURE 6 shows neutron yield as a function of target voltage. Consideration should be given to the fact that the ion beam decreases in diameter, with increasing screen negative electrode volts. It is clearly an advantage to contain the ion source into a diameter equal to that of the target, because of the increased efficiency of the tube. It is possible, however, to increase the screen potential until ultimately the ions are concentrated into a pencil beam, that operates over a small area of the target surface, thereby causing damage.
  • FIGURE 7 shows the neutron yield as a function of screen voltage.
  • the target voltage was 50 kv.
  • the plasma current was set at 1 ma.
  • the gas pressure was 20 microns. It can be seen that the neutron yield rises as the screen negative electrode voltage was varied from 25 to -100 v.
  • a plasma ion source comprising a cathode, an apertured extractor cathode spaced from said cathode, a cylindrical anode located between said cathode and said extractor cathode and separated by gaps therefrom, and a screen negative electrode extending over the gap between said cathode and said anode to apply a constraining field.
  • a plasma ion source comprising a cathode, an apertured extractor cathode spaced from said cathode, a cylindrical anode located between said cathode and said extractor cathode and separated by gaps therefrom, and a cylindrical screen negative electrode extending over the gap between said cathode and said anode, said cathode being located within one end of the cylindrical screen negative electrode and said anode extending within the other end.
  • a neutron generator comprising an envelope enclosing a target containing a hydrogen isotope and adapted to produce neutrons when bombarded with accelerated hydrogen isotope ions and a plasma ion source for providing a beam of said hydrogen isotope ions, said source comprising a cathode, an apertured extractor cathode located between said cathode and said target, a cylindrical anode located between said cathode and said extractor cathode and separated by gaps therefrom, and a screen negative electrode extending over the gap between said cathode and said anode to apply a constraining field.

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Plasma & Fusion (AREA)
  • Electron Sources, Ion Sources (AREA)
  • Physical Vapour Deposition (AREA)
US259497A 1962-02-20 1963-02-19 Plasma ion source having apertured extractor cathode Expired - Lifetime US3281617A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB6503/62A GB976664A (en) 1962-02-20 1962-02-20 Improvements in or relating to ion sources

Publications (1)

Publication Number Publication Date
US3281617A true US3281617A (en) 1966-10-25

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US259497A Expired - Lifetime US3281617A (en) 1962-02-20 1963-02-19 Plasma ion source having apertured extractor cathode

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US (1) US3281617A (fr)
DE (1) DE1220940B (fr)
FR (1) FR1348192A (fr)
GB (1) GB976664A (fr)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3619684A (en) * 1969-04-28 1971-11-09 Philips Corp Ion source
US4206383A (en) * 1978-09-11 1980-06-03 California Institute Of Technology Miniature cyclotron resonance ion source using small permanent magnet
US20040146133A1 (en) * 2002-01-23 2004-07-29 Ka-Ngo Leung Ultra-short ion and neutron pulse production
US20090230314A1 (en) * 2005-08-05 2009-09-17 Ka-Ngo Leung Gamma source for active interrogation

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2831134A (en) * 1953-04-10 1958-04-15 Philips Corp Extraction probe for ion source
FR1268230A (fr) * 1959-09-25 1961-07-28 Siemens Ag Source d'ions à choc d'électrons pour filtres électriques de masse
US3090882A (en) * 1960-04-13 1963-05-21 Rca Corp Electron gun

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3124711A (en) * 1959-05-05 1964-03-10 Reifenschweiler

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2831134A (en) * 1953-04-10 1958-04-15 Philips Corp Extraction probe for ion source
FR1268230A (fr) * 1959-09-25 1961-07-28 Siemens Ag Source d'ions à choc d'électrons pour filtres électriques de masse
US3090882A (en) * 1960-04-13 1963-05-21 Rca Corp Electron gun

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3619684A (en) * 1969-04-28 1971-11-09 Philips Corp Ion source
US4206383A (en) * 1978-09-11 1980-06-03 California Institute Of Technology Miniature cyclotron resonance ion source using small permanent magnet
US20040146133A1 (en) * 2002-01-23 2004-07-29 Ka-Ngo Leung Ultra-short ion and neutron pulse production
US6985553B2 (en) * 2002-01-23 2006-01-10 The Regents Of The University Of California Ultra-short ion and neutron pulse production
US20090230314A1 (en) * 2005-08-05 2009-09-17 Ka-Ngo Leung Gamma source for active interrogation
US7596197B1 (en) * 2005-08-05 2009-09-29 The Regents Of The University Of California Gamma source for active interrogation

Also Published As

Publication number Publication date
FR1348192A (fr) 1964-01-04
DE1220940B (de) 1966-07-14
GB976664A (en) 1964-12-02

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