Dielectric loaded particle accelerator

1. A device for accelerating charged particles, the device comprising:

a dielectric loaded accelerator for accelerating charged particles along a longitudinal axis of the accelerator and towards an outlet of the accelerator, said accelerator comprising;

an elongated body made from a conductive material; and

a dielectric material extending continuously along a length of the accelerator and extending radially inward from an interior surface of the elongated body to a central cavity surrounding said longitudinal axis;

said central cavity being uniform in diameter throughout its length; and

said dielectric material and elongated body being varied in outer diameter along said length of the accelerator such that an electromagnetic mode of the accelerator has a phase velocity that (i) increases towards the outlet of the accelerator and (ii) matches a velocity of the charged particles accelerated along the longitudinal axis.

2. The device of claim 1 , wherein the phase velocity of the accelerator along the longitudinal axis of the accelerator is configured to continuously match the velocity of the charged particles accelerated along the longitudinal axis.

3. The device of claim 1 , wherein the dielectric material has a thickness that decreases monotonically along the longitudinal axis and towards the outlet of the accelerator.

4. The device of claim 1 , wherein the dielectric material has a dielectric permittivity that varies along the longitudinal axis of the accelerator.

5. The device of claim 1 , wherein the dielectric material comprises a plurality of layers.

6. The device of claim 5 , wherein at least two of the plurality of layers comprise different dielectric permittivity.

7. The device of claim 5 , wherein the plurality of layers comprises a dielectric mirror configured to confine electromagnetic fields of a particular frequency within an inner volume of the accelerator.

8. The device of claim 5 , wherein at least one layer of the plurality of layers comprises a material with a tunable dielectric permittivity.

9. The device of claim 8 , wherein the material with the tunable dielectric permittivity comprises a ferro-electric material.

10. The device of claim 5 , wherein at least one layer comprises a conductive element for collecting charge deposited by the charged particles in the dielectric material.

11. The device of claim 1 , wherein the dielectric loaded accelerator comprises a plurality of sections that are joined together along the longitudinal axis.

12. The device of claim 1 , further comprising:

a plurality of couplers positioned at a plurality of positions along the longitudinal axis of the accelerator and configured to couple the dielectric loaded accelerator to a power source.

13. The device of claim 1 , wherein the dielectric material and elongated body are varied stepwise in outer diameter along said length of the accelerator, such that the elongated body and dielectric material transition between regions of constant outer diameter along the length of the accelerator.

14. An X-ray generator comprising:

a target for generating X-rays;

a dielectric loaded accelerator for accelerating electrons along a longitudinal axis of the accelerator and towards the target, said accelerator comprising;

an elongated body made from a conductive material; and

a dielectric material extending continuously along a length of the accelerator and extending radially inward from an interior surface of the elongated body to a central cavity surrounding said longitudinal axis;

said central cavity being uniform in diameter throughout its length; and

said dielectric material and elongated body being varied in outer diameter along said length of the accelerator such that the accelerator comprises an electromagnetic mode with a phase velocity that increases towards the target and matches a velocity of the electrons accelerated along the longitudinal axis; and

an electron source for providing electrons to the accelerator.

15. The X-ray generator of claim 14 , wherein the phase velocity of the accelerator along the longitudinal axis is configured to continuously match a velocity of the electrons being accelerated along the longitudinal axis.

16. The X-ray generator of claim 15 , wherein the dielectric loaded accelerator comprises a dielectric material with a dielectric permittivity that varies along the longitudinal axis of the accelerator.

17. The X-ray generator of claim 15 , wherein the dielectric material has a thickness that decreases monotonically along the longitudinal axis and towards the target.

18. The X-ray generator of claim 14 , wherein the X-ray generator is part of a wellbore tool and the X-ray generator generates X-ray radiation that enters an earth formation.

19. The X-ray generator of claim 18 , wherein the wellbore tool comprises a gamma-ray detector for detecting X-ray radiation scattered from the earth formation.

20. The X-ray generator of claim 14 , wherein the dielectric material and elongated body are varied stepwise in outer diameter along said length of the accelerator, such that the elongated body and dielectric material transition between regions of constant outer diameter along the length of the accelerator.

21. A method for accelerating charged particles along a longitudinal axis of the accelerator, the method comprising:

providing charged particles to a dielectric loaded accelerator comprising:

an elongated body made from a conductive material; and

a dielectric material extending continuously along a length of the accelerator and extending radially inward from an interior surface of the elongated body to a central cavity surrounding said longitudinal axis;

said central cavity being uniform in diameter throughout its length; and

said dielectric material and elongated body being varied in outer diameter along said length of the accelerator such that an electromagnetic mode of the accelerator has a phase velocity that increases along the longitudinal axis of the accelerator and towards an outlet of the accelerator; and

driving the accelerator using a microwave signal to accelerate the charged particles along the longitudinal axis and towards the outlet of the accelerator.

22. The method of claim 21 , wherein the phase velocity of the accelerator along the longitudinal axis continuously matches the velocity of the charged particles being accelerated along the longitudinal axis.

23. The method of claim 22 , wherein the dielectric material has a thickness that decreases monotonically along the longitudinal axis and towards the outlet of the accelerator.

24. The method of claim 22 , wherein the dielectric loaded accelerator comprises a dielectric material with a dielectric permittivity that varies along the longitudinal axis of the accelerator.

25. The method of claim 24 , wherein dielectric material comprises a plurality of layers and at least one of the layers comprises a tunable dielectric permittivity material.

26. The method of claim 25 , further comprising:

applying an electric field to the dielectric loaded accelerator to tune the tunable dielectric permittivity material.

27. The method of claim 21 , wherein the charged particles are electrons and the electrons are accelerated by the dielectric loaded accelerator to a sub-relativistic velocity.

28. The method of claim 21 , wherein the dielectric material and elongated body are varied stepwise in outer diameter along said length of the accelerator, such that the elongated body and dielectric material transition between regions of constant outer diameter along the length of the accelerator.