Charged particle accelerator and radiation source

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;
• and
• means 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, wherein
• the 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.

An apparatus for accelerating charged particles, comprising:

• a capillary suitable for forming a plasma channel;
• and
• a laser source arranged to fire a laser pulse through the plasma channel, wherein
• said 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.