Field emission ion source neutron generator
A well logging tool includes a neutron generator having an ion source cathode comprising a nano-structure field emission (FE) array for producing a charged particle current routed through an ionizable gas, thus generating ions by electron impact ionization of the gas. The nanostructures can be provided by bundles of silicon nanotips grown on a substrate. The FE array can be provided on an axially facing substrate located co-axially in an elongate housing, with the charged particle current directed from the centrally located FE array to a co-axial annular anode and having axial and radial directional components.
1. A neutron generator comprising:
a housing defining an ion source chamber containing an ionizable gas;
an anode in the ion source chamber, wherein the anode is radially offset from a longitudinal axis of the ion source chamber; and
a field emitter device comprising a field emitter array and an embedded extraction grid, the field emitter array comprising a plurality of field electron emitters, the field electron emitters comprising a plurality of nanotips, the embedded extraction grid comprising a plurality of openings corresponding to positions of the plurality of nanotips and each nanotip extending beyond a surface of the extraction grid thru a corresponding opening of the plurality of openings, the embedded extraction grid configured to control electron emissions by the field electron emitters based at least in part on a voltage applied to the embedded extraction grid; and
wherein the neutron generator is configured so that a flow of electrons produced by the field electron emitters is routed through the ionizable gas to produce ions independent of a magnetic field from a magnet, and
wherein the field emitter array is configured to supply the flow electrons large enough to enable sub-microsecond ignition of the ionizable gas.
2. The neutron generator of claim 1 , wherein each nanotip comprising an elongate filament which is aligned with the longitudinal axis of the ion source chamber.
3. The neutron generator of claim 1 , wherein the housing is elongate.
4. The neutron generator of claim 1 , further comprising a substrate with a substrate surface that is perpendicular to the longitudinal axis-of the ion source chamber, wherein the field emitter array is attached to the substrate.
5. The neutron generator of claim 1 , wherein the anode comprises an annular anode element that is co-axial with the longitudinal axis of the ion source chamber.
6. The neutron generator of claim 4 , wherein the substrate surface is substantially circular.
7. The neutron generator of claim 1 , further comprising a control arrangement to apply an ion source voltage pulse between a field emitter array substrate and the anode to accelerate emitted electrons in a radial path toward the anode and ionize the ionizable gas by electron impact.
8. The neutron generator of claim 7 , wherein the ion source voltage pulse has an amplitude of 500 V or less.
9. The neutron generator of claim 8 , wherein a turn-on/turn-off delay for an ion source comprising the field emitter array and the anode is less than 1 μs.
10. The neutron generator of claim 1 , wherein the electron current is greater than or equal to 1 A/cm 2 .
11. The neutron generator of claim 1 , wherein the field electron emitters comprise multiple nanotips each having a length between 0.2 and 100 nanometers and wherein the field electron emitters have a nanotip density between 10,000 and 1,000,000 nanotips per square centimeter.
12. The neutron generator of claim 1 , further comprising:
a target rod; and
a frustoconical insulator extending between the ion source chamber and the target rod.
13. A method comprising:
lowering a neutron generator into a borehole, wherein the neutron generator comprises a housing that defines an ion source chamber, and wherein the neutron generator further comprises a field emitter device comprising a field emitter array and an embedded extraction grid inside of the housing, the field emitter array comprising a plurality of field electron emitters, the field electron emitters comprising a plurality of nanotips, the embedded extraction grid comprising a plurality of openings corresponding to positions of the plurality of nanotips and each nanotip extending beyond a surface of the extraction grid thru a corresponding opening of the plurality of openings, the embedded extraction grid configured to control electron emissions by the field electron emitters based at least in part on a voltage applied to the embedded extraction grid;
delivering an ionizable gas to the ion source chamber;
applying an ion source voltage pulse between the field emitter array and an anode in the ion source chamber, wherein the anode is radially offset from a longitudinal axis of the ion source chamber; and
producing an electric field that attracts electrons emitted from the field emitter array towards the anode through the ionizable gas independent of a magnetic field from a magnet, wherein the electrons emitted from the field emitter array provide sub-microsecond ignition of the ionizable gas.
14. The method of claim 13 , wherein the ion source voltage pulse has an amplitude of 500 V or less.
15. The method of claim 13 , wherein a turn-on/turn-off delay for ionization of the ionizable gas in the ion source chamber is less than 1 μs.
16. An apparatus comprising:
a body; and
a neutron generator housed by the body, wherein the neutron generator comprises,
a housing that defines an ion source chamber containing an ionizable gas;
an anode in the ion source chamber, wherein the anode is radially offset from a longitudinal axis of the ion source chamber; and
a field emitter device comprising a field emitter array and an embedded extraction grid, the field emitter array comprising a plurality of field electron emitters, the field electron emitters comprising a plurality of nanotips, the embedded extraction grid comprising a plurality of openings corresponding to positions of the plurality of nanotips and each nanotip extending beyond a surface of the extraction grid thru a corresponding opening of the plurality of openings, the embedded extraction grid configured to control electron emissions by the field electron emitters based at least in part on a voltage applied to the embedded extraction grid;
wherein an electron current produced by the field electron emitters is routed through the ionizable gas independent of a magnetic field from a magnet, and
wherein the field emitter array is configured to supply a flow of electrons extracted by the embedded extraction grid that is large enough to enable sub-microsecond ignition of the ionizable gas.
17. The apparatus of claim 16 , further comprising a substrate with a substrate surface that is perpendicular to the longitudinal axis of the housing, wherein the field emitter array is attached to the substrate.
18. The apparatus of claim 16 , wherein the anode comprises an annular anode element that is co-axial with the longitudinal axis of the housing.
19. The apparatus of claim 16 , wherein each nanotip comprising an elongate filament which is aligned with the longitudinal axis of the housing.
20. The apparatus of claim 16 , further comprising a control arrangement to apply an ion source voltage pulse between the field emitter array and the anode.
21. The apparatus of claim 16 , wherein the field electron emitters comprise multiple nanotips each having a length between 0.2 and 100 nanometers and wherein the field electron emitters have a nanotip density between 10,000 and 1,000,000 nanotips per square centimeter.