The LEG Project: 2003 to 2010 the beginning of the SwissFEL Project

The electron gun: the key component for beam quality

The electron gun is the most sensitive component for the beam quality in a linear accelerator like SwissFEL. In fact, the electron beam quality can only deteriorate but not improve after being produced by the gun. Since the electron beam quality determines the total length of a FEL, particular effort should be put in the electron gun development (that is to say in the first few meters of the accelerator).

The low emittance gun (LEG) project started in 2003 with the goal of using field emitter arrays (FEA) as a cathode in an electron gun. A diode – RF gun combination was then developed between 2004 and 2006 with the possibility of inserting FEA. The diode pulser was commissioned in 2007 with metallic photo cathodes illuminated by VUV laser pulses. High gradient test as well as beam characterization at 300 – 400 keV was then performed [1,2]. In the meantime FEA development was still going on at PSI Laboratory for micro and nanotechnology [3]. In 2008, a 1.5 GHz two cell cavity [4] was added to the diode together with a complete beam diagnostic line [5] in order to produce and characterize beam around 5 MeV.

In January 2009, the commissioning of the diode – RF gun started. A few months later, we reached the electron beam quality required for the low charge operation mode (10 pC) of the SwissFEL using metallic photo cathode [6].

Performances at 200 pC were however still below SwissFEL requirements [7]. Recently, tests with an FEA as electron source in the diode – RF gun started [8] and opened the door to complete new types of accelerator electron gun. The diode – RF gun test stand enabled us to test many concepts such as the intrinsic emittance reduction by tuning laser wavelength [9], the electrostatic focusing with special diode geometries, or to make breakthrough in maximum diode gradient [8] and to explore new emission scheme with FEA. In general, fruitful experience with high brightness electron beam generation and characterization has been cumulated at the LEG test stand. This experience was then very helpful for the commissioning of the RF photo gun of the SwissFEL injector test facility. The LEG test stand made room for the test of other key components of SwissFEL (namely the RF LINAC module) and stopped operation in fall 2010.

For SwissFEL, PSI is building a 2.5 Cell S-band RF photo injector integrating many advanced features from high brightness electron sources worldwide. This includes innovations developed at the LEG test facility, like emittance minimization by laser wavelength tuning.

[1] T. Schietinger et al., Proceedings of the LINAC Conference, Victoria, Canada, 2008, p. 630.
[2] F. Le Pimpec et al., arXiv:1003.3594 (2010), accepted for publication in JVST A.
[3] E. Kirk et al., J. Vac. Sci. Technol. B 27, 1813-1820 (2009).
[4] K. Li, Thesis, ETH Zurich (No. 18168), 2008.
[5] V. Schlott et al., Proceedings of the DIPAC Conference, Venice, Italy, 2007, p. 198.
[6] R. Ganter et al., Proceedings of the FEL09 Conference, Liverpool, UK, 2009.
[7] R. Ganter et al., Phys. Rev. ST Accel. Beams, Vol. 13, Issue 9 (2010).
[8] M. Paraliev et al., Proceedings of the IEEE International Power Modulator & High Voltage Conference Conference, Atlanta, USA, arXiv:1006.2001, 2010.
[9] C. Hauri et al., Phys. Rev. Lett. 104, 234802 (2010).