It is known to use free electron laser (FEL) radiation sources to produce radiation of a desired wavelength, in which an electron beam comprising a periodic sequence of electron bunches is passed through an undulator to generate the radiation. Such sources can be used to produce radiation in a range 4 nm to 25 nm, for example extreme ultra-violet (EUV) radiation.
In known FEL radiation sources, ions are produced from residual gas in the electron beam through collisional ionization. An expected rate of ion generation in EUV FEL sources with a beam energy of a few hundred MeV and a beam current of a few tens of mA is such that, in the absence of counter-measures, the electron beam could be fully neutralized (e.g. ion charge matches electron charge per meter of electron beam) in less than 10 seconds. As a result, emittance of electron bunches at the undulator could significantly exceed 10 mm mRad and conversion efficiency would be expected to drop more than ten times, thus effectively disabling the operation of the EUV FEL.
In order to counteract ion concentration build-up several strategies have been suggested, including placing extraction electrodes along the electron beam or implementing a beam current pattern with short additional gaps between selected pairs of electron bunches (also referred to as clearing gaps). The short additional gaps can also be thought of as representing missing bunches in the sequence of electron bunches. The use of clearing gaps is intended to give ions, otherwise trapped in the potential of the bunched electron beam, some time to drift away. The use of clearing gaps according to known suggestions may not completely remove trapped ions but may allow mitigation of fast ion instability which can be detrimental for FEL operation.
Known FEL sources include LINACs for accelerating (and decelerating) electron bunches before (and after) they pass through the undulator. Energy recovery LINACs can be used, which are usually designed to operate with a balanced cavity load close to zero (e.g. currents in accelerating and decelerating beams match, and energy extracted and deposited upon acceleration and deceleration almost match).
The use of clearing gaps between electron bunches may distort operation of energy recovery LINACs. In the case of a single-pass LINAC system it has been suggested in G. H. Hoffstaetter et al, Nuclear Instruments & Methods in Physics Research A, 557 (2006), 205-212 to match clearing gaps (e.g. trains of missing bunches) in accelerating and decelerating electron beams within LINAC modules in a single pass LINAC. The matching of clearing gaps in a single pass LINAC may be relatively straightforward, given that each electron bunch circulates once around a loop and only passes through the single LINAC once when it is accelerating and once when it is decelerating, and may be achieved by a requirement that missing bunches are provided at a suitable regular rate.
It is an aim of the present invention to provide an improved, or at least alternative, FEL radiation source and method of operation of such a source that can provide for reduction of ion build-up.