Patent Number: 
Section: claims

1. A method of producing electromagnetic radiation, comprising:forming a plasma channel in a capillary;firing a laser pulse through the plasma channel;arranging for a group of charged particles to be injected into a plasma density wake of the laser pulse so as to be accelerated by the wake; andarranging the plasma channel and the firing of the laser pulse such that the wake of the laser pulse exerts a transverse force on the injected group of charged particles that varies periodically as the laser pulse propagates along the channel length, the resulting transverse acceleration of the group of charged particles causing emission of said electromagnetic radiation. 2. (A method according to claim 1, wherein the laser pulse is fired into the plasma channel off-axis so as effectively to cause a periodic transverse deflection of the laser pulse and its wake as the laser pulse propagates along the channel length. 3. A method according to claim 2, wherein the laser pulse is fired into the plasma channel at a position radically separated from a longitudinal central axis of the channel, at an oblique angle to the longitudinal central axis of the channel, or a combination thereof. 4. A method according to claim 1, wherein the plasma channel has a shape, induced by the shape of the capillary, that causes a periodic transverse deflection of the laser pulse and its wake as the laser pulse propagates along the channel length. 5. A method according to claim 4, wherein the shape of the capillary comprises at least one of the following: undulations of substantially sinusoidal longitudinal cross-section, localized deviations in cross-section separated longitudinally, helical deviations in cross-section, and undulations of square-wave longitudinal cross-section. 6. A method according to claim 5, wherein a spatial periodicity in the shape of the capillary varies longitudinally such as to achieve a substantially constant frequency of transverse deflection. 7. A method according to claim 5, wherein a spatial periodicity in the shape of the capillary varies longitudinally such as to achieve a frequency of transverse deflection that substantially varies as the laser pulse propagates down the plasma channel. 8. A method according to claim 1, wherein the step of arranging for a group of charged particles to be injected into a wake of the laser pulse comprises producing a group of charged particles externally of the channel and injecting them into the channel. 9. A method according to claim 1, wherein the step of arranging for a group of charged particles to be injected into a wake of the laser pulse comprises promoting extraction of a group of charged particles from the plasma by the wake of the laser pulse. 10. An electromagnetic radiation source, comprising:a capillary suitable for creating a plasma channel;a laser source arranged to fire a laser pulse through the plasma channel; andmeans for injecting a group of charged particles into a plasma density wake of the laser pulse so that the group is accelerated by the wake, whereinthe laser source and channel are arranged so that in use the wake of the laser pulse exerts a transverse force on the injected group of charged particles that varies periodically as the laser pulse propagates along the channel length, the resulting transverse acceleration of the group of charged particles causing emission of said electromagnetic radiation. 11. An electromagnetic radiation source according to claim 10, wherein:said means for injecting a group of charged particles comprises a longitudinally localized deviation in the cross-section of the capillary that, in use, causes a corresponding deviation in the plasma density, said deviation in the plasma density being such as to cause injection of a group of charged particles from the plasma into a wake of the laser pulse in the region of the deviation in the capillary so that the group is accelerated by the wake. 12. An apparatus for accelerating charged particles, comprising:a capillary suitable for forming a plasma channel; anda laser source arranged to fire a laser pulse through the plasma channel, whereinsaid capillary has a longitudinally localized deviation in its cross-section that, in use, causes a corresponding deviation in the plasma density, said deviation in the plasma density being such as to cause injection of a group of charged particles from the plasma into a wake of the laser pulse in the region of the deviation in the capillary so that the group is accelerated by the wake. 13. An apparatus according to claim 11, wherein said longitudinally localized deviation comprises one of one of the following:a localized change in the cross-sectional area of the capillary;localized increase in the cross-sectional area;an additional section of capillary opening out into, and extending laterally away from, the capillary down which the laser pulse will propagate. 14. An apparatus according to claim 11, further comprising:a gas flow controller arranged to establish a gas flow along said capillary; anda discharge circuit for passing an electric discharge through the gas flow in order to form a plasma channel within the capillary, whereinsaid localized deviation comprises a step change in the capillary diameter. 15. An apparatus according to claim 11, wherein a position of said deviation in the capillary determines a final energy for the accelerated group of charged particles. 16. An apparatus according to claim 11, wherein said capillary comprises at least one further longitudinally localized deviation in its cross-section that, in use, causes at least one corresponding further deviation in the plasma density, each said further deviation in the plasma density being such as to cause injection of a further group of charged particles from the plasma into the wake of the laser pulse in the region of the further deviation in the capillary so that each further group is accelerated by the wake. 17. An apparatus according to claim 16, wherein groups of charged particles injected at different deviations in the capillary are accelerated to different final energies, the positions of the respective longitudinal deviations determining said final energies. 18. An apparatus according to claim 11, wherein said deviation(s) comprise(s) at least one of the following: a helical cross-sectional deviation, and a tapered cross-sectional deviation. 19. An apparatus according to claim 10, wherein said capillary is arranged so that in use the density of the plasma at a given time gradually increases or gradually decreases as a function of position along an extended longitudinal portion of the capillary. 20. An apparatus according to claim 11, wherein said capillary is arranged to that in use the density of the plasma at a given time gradually increases or gradually decreases as a function of position along an extended longitudinal portion of the capillary.