Plasma source apparatus and methods for generating charged particle beams

1. A plasma source apparatus for generating a beam of charged particles, the apparatus comprising:

a plasma chamber provided with an inlet for the ingress of gas and an aperture for the extraction of charged particles from the plasma chamber;

a radio frequency (RF) plasma generation unit for generating a plasma inside the plasma chamber, the radio frequency plasma generation unit comprising first and second resonant circuits each tuned to resonate at substantially the same resonant frequency, the first resonant circuit comprising a first antenna and a first, RF power source adapted to drive the first resonant circuit at substantially its resonant frequency, and the second resonant circuit comprising a second antenna, whereby in use an RF signal is induced in the second antenna by the first resonant circuit due to resonant coupling, the second resonant circuit being configured to apply the induced RF signal to the plasma chamber to generate a plasma therein; and

a particle accelerating unit for extracting charged particles from the plasma and accelerating the charged particles to form a beam, the particle accelerating unit comprising a second power source configured to apply potential between the plasma chamber and an accelerating electrode, the region between the plasma chamber and the accelerating electrode constituting an acceleration column;

wherein the second power source is adapted to output a high voltage relative to that output by the first, RF power source.

2. A plasma source apparatus according to claim 1 , wherein at least the plasma chamber, acceleration column and the second antenna are disposed within a housing, the apparatus further comprising a pump for evacuating the housing such that the second antenna is substantially electrically insulated from the first antenna in use.

3. A plasma source apparatus according to claim 1 , wherein the first, RF power source is adapted to output a signal with a power amplitude of up to 300 W, preferably up to 100 W, more preferably up to 50 W.

4. A plasma source apparatus according to claim 1 , wherein the second power source is adapted to output a DC signal, preferably having a voltage magnitude in the range 10 kV to 200 kV, more preferably 25 kV to 175 kV, still preferably 60 kV to 150 kV.

5. A plasma source apparatus according to claim 1 , wherein the second resonant circuit has a sufficiently high quality factor such that the voltage of the induced RF signal is higher than the voltage output by the first, RF power source.

6. A plasma source apparatus according to claim 1 , wherein the second resonant circuit is configured to apply the induced RF signal across the plasma chamber between electrodes in contact with the interior of the chamber, the electrodes preferably being formed by electrically conductive portions of the walls defining the plasma chamber, separated by an insulating portion thereof.

7. A plasma source apparatus according to claim 1 , further comprising a controller for modulating the RF signal output by the first power source.

8. A plasma source apparatus according to claim 7 , wherein the controller is further adapted to control the amplitude of the RF signal between a first amplitude at which the power of the generated beam is suitable for materials processing, and a second amplitude at which the power of the generated beam is suitable for imaging a workpiece, the first amplitude being greater than the second amplitude.

9. A materials processing tool comprising a plasma source according to claim 1 .

10. A materials processing tool according to claim 9 , wherein the tool is one of:

an electron beam welding tool, the beam of charged particles being adapted for the welding of materials; an additive layer manufacturing tool, the beam of charged particles being adapted for treatment of a powder material, preferably the fusion thereof; a curing tool, the beam of charged particles being adapted for curing of a workpiece; a cutting tool, the beam of charged particles being adapted for cutting of materials; a melting or evaporation tool, the beam of charged particles being adapted for melting and/or evaporation of materials; a gas treatment tool, the beam of charged particles being adapted for the treatment of gaseous substances, preferably combustion fumes a sterilisation tool, the beam of changed particles being adapted for the sterilisation of solids or liquids; or a drilling tool, the beam of changed particles being adapted for the drilling of a workpiece.

11. A method of generating a beam of charged particles, comprising:

admitting gas to a plasma chamber, the plasma chamber having an inlet for the ingress of gas and an aperture for the extraction of charged particles from the plasma chamber;

generating a plasma inside the plasma chamber using a radio frequency (RF) plasma generation unit comprising first and second resonant circuits each tuned to resonate at substantially the same resonant frequency, the first and second resonant circuits comprising first and second antennas, respectively, by using a first, RF power source to drive the first resonant circuit at substantially its resonant frequency such that an RF signal is induced in the second antenna by the first resonant circuit due to resonant coupling, the second resonant circuit applying the induced RF signal to the plasma chamber to generate a plasma therein; and

extracting charged particles from the plasma and accelerating the charged particles to form a beam by using a second power source to apply potential between the plasma chamber and an accelerating electrode, the region between the plasma chamber and the accelerating electrode constituting an acceleration column;

wherein the voltage output by the second power source is high relative to that output by the first, RF power source.

12. A method according to claim 11 , wherein the first, RF power source outputs a signal with a power amplitude of up to 300 W, preferably up to 100 W, more preferably up to 50 W.

13. A method according to claim 11 , wherein the second power source outputs a DC signal, preferably having a voltage in the range 10 kV to 200 kV, more preferably 25 kV to 175 kV, still preferably 60 kV to 150 kV.

14. A method according to claim 11 , wherein the gas type, dimensions of the plasma chamber and, if applied, the magnetic field strength are selected in combination such that the plasma lifetime within the chamber, once RF excitation is stopped, is less than 10 microseconds, preferably less than 1 microsecond.

15. A method of modifying a workpiece using a beam of charged particles generated in accordance with claim 11 .

16. A method according to claim 15 , wherein the beam of charged particles is used to one of:

weld the workpiece;

fuse the workpiece, wherein the workpiece comprises a powder material

cure the workpiece;

cut the workpiece;

melt and/or evaporate the workpiece;

treat a gaseous workpiece, preferably combustion fumes

sterilise a solid or liquid; or

drill the workpiece.

17. A method of imaging a workpiece during a process of modifying the workpiece using a beam of charged particles generated in accordance with claim 11 , comprising, in either order:

using the beam of charged particles to modify the workpiece; and

using the beam of charged particles to image the workpiece

wherein, between the two steps, the amplitude of the RF signal is modulated from a first amplitude, at which the generated beam power is suitable for modifying the workpiece, to a second amplitude, at which the generated beam power is suitable for imaging the workpiece, or vice versa, the first amplitude being greater than the second amplitude.