Search

The design of an electron gun capable of producing beams with an emittance one order of magnitude lower than current technology would reduce considerably the cost and size of a free electron laser radiating at 0.1 nm. An electron source based on field emission is an attractive alternative for a high brightness source: thousands of high current density beams (10^8 A/cm^2) can be produced and individually focused. The development of a gun based on field emission cathodes implies many innovations in regards to current state of the art guns. The field emitter source will be inserted in a high-gradient diode configuration coupled with a radiofrequency (RF) structure. In the diode part a high electric field (several hundreds of MV/m) will limit the degradation of beam emittance due to space-charge effects. This first acceleration will be obtained with high voltage pulses (~500 kV in 500 ns) synchronized with the low voltage pulses applied between the tips and the extracting gate. This diode part will then be followed by an RF accelerating structure in order to bring the electrons to relativistic energies.

The design of an electron gun capable of producing beam emittance one order of magnitude lower than current technology would reduce considerably the cost and size of a free electron laser radiating at 0.1 nm. Field emitter arrays (FEAs) including a gate and a focusing layer are an attractive technology for such high brightness sources. Electrons are extracted from micrometric tips thanks to voltage pulses applied between the gate and the tips. The focusing layer should then reduce the initial divergence of each emitted beamlet. This FEA will be inserted in a high-gradient diode configuration couplesd with a radiofrequency (RF) structure. In the diode part the high electric field (several hundreds of MV/m) will limit the degradation of emittance due to space-charge effects. The first acceleration will be obtained with high voltage pulses (about one megavolt in a few hundreds of nanoseconds) synchronized with the low voltage pulses applied to the FEA (200-300 V in less than one nanosecond at a frequency lower than a kilohertz). This diode part will then be followed by an RF accelerating structure in order to bring the electrons to relativistic energies.

The development of a new electron gun with the lowest possible emittance would help reduce the total length and cost of a free electron laser. Recent progress in vacuum microelectronics makes field emitter arrays (FEAs) an attractive technology to explore for high brightness sources. Indeed, several thousands of microscopic tips can be deposited on a 1 mm diameter area. Electrons are then extracted by a first grid layer close to the tip apex and focused by a second grid layer one micrometer above the tip apex. In order to be a good candidate for a low emittance gun, field emission cathodes must provide at least the peak current, stability and homogeneity of current state of the art electron sources. Smaller initial divergence should then be achieved by the focusing grid and the intrinsic properties of the field emission process. Another important aspect for improving electron guns is to preserve the emittance during beam acceleration.