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Enhanced Self-Amplified Spontaneous Emission by an Optical Klystron at FERMI

After achievement of the goal parameters for FEL-1 and its successful operation for users’ experiments, the FERMI team has been focused on the study of new FEL configurations with the intent of adding useful capabilities to FERMI FEL-1. From one side, two-color FEL pump – FEL probe experiments have been extensively carried on and widely appreciated by the user community.
More recently, the efforts have been focused on the operation of FEL-1 in the self-amplified spontaneous emission (SASE) mode that is the usual one in all existing short wavelength FELs. It is based on sending a high brightness electron beam through a long undulator to generate high power photons pulses. In this configuration the FEL process starts from the shot noise and the SASE output is characterized by poor degree of longitudinal coherence that is manifested by several spikes both in the spectral and temporal domain. Since FERMI is designed as a seeded FEL, the undulator length does not allow the simple SASE signal to be sufficiently amplified, and powerful FEL pulses can only be generated by mean of an external seed, that also allows the generation of very narrow bandwidth FEL pulses.

 

Figure 1. FERMI FEL sketch including the 3-m-long modulator (planar undulator with a period of 100.3 mm), the dispersive section (three-pole wiggler with an overall length of 700 mm) and six 2.3 m long radiators (Apple II undulator, with a period of 55.2 mm and tunable both in linear and in circular polarization).

In a recent experiment we demonstrated for the first time in a high gain single pass FEL successful enhancement of SASE in the extreme ultra-violet regime, by using an optical klystron undulator configuration. The optical klystron consists of two undulators separated by a dispersive section, that converts the beam-energy modulation induced by the first undulator into a longitudinal density modulation, enhancing therefore the emission in the second undulator. This scheme was implemented in the past in a number of multi-pass oscillator FELs operating from the visible to the UV spectral range, but never in a single-pass high-gain FEL lasing at shorter wavelengths.
Indeed, FERMI is designed to operate in seeded high-gain harmonic generation scheme, but its layout (see fig. 1) is also suitable to implement the high-gain optical klystron scheme by tuning both modulator and radiator at the same frequency and exploiting the dispersive section in between them to enhance the bunching induced by the radiation of the modulator.
By means of the optical klystron scheme, it has been possible to amplify the initial shot noise signal up to a very high power within the undulator length available at FERMI

The optical klystron performance in a high-gain FEL is strongly influenced by the electron beam relative uncorrelated energy spread δ, which has to be much smaller than the FEL parameter ρ. A proper setting of the laser heater system allows suppressing microbunching instabilities and constraining δ more than one order of magnitude less than ρ, with a relevant increment of the efficiency of the optical klystron, especially at short wavelength. Figure 2 shows the FEL enhancement to the pure SASE mode as a function of the dispersive section strength (i.e. the momentum compaction, R56) due to the optical klystron at 43 nm, 32.4 nm and 20 nm, providing from few to 100 μJ per pulse. The optical klystron configuration has been also tested on the FERMI FEL-2 line, whose layout includes two dispersive sections. A fine-tuning of the configuration parameters allowed to strongly enhance the pure SASE emission, obtaining photon pulses of about 100 μJ at 12 nm.
Despite the different spectral and temporal properties, the SASE operation mode at FERMI is a backup solution providing energy per pulse similar to that available in seeded mode, when the seed laser is unavailable. In addition users can benefit by the possibility of having alternatively the FEL in seeded and SASE optical klystron mode, to investigate particular phenomena depending on the longitudinal coherence. On a wider perspective, this result paves the way to the implementation of the optical klystron scheme to existing SASE FEL operating in the X-ray regime.



Figure 2.  SASE FEL relative enhancement due to the optical klystron as a function of the dispersive section strength, i.e. the momentum compaction R56 at 43 nm, 32.4 nm and 20 nm.

Figure 3.  Single-shot FEL spectrum at 32 nm obtained in SASE (on the left) and in HGHG (on the right) mode at FERMI. While the horizontal axis is dispersed in wavelength, the vertical axis represents the vertical distribution of the FEL intensity at the spectrometer CCD. The energy per pulse provided in SASE mode by using the optical klystron technique is about 100 micro-joules and the bandwidth is 3.3 x 10-3.
 

This research was conducted by the following research team:

G. Penco, E. Allaria, G. De Ninno, E. Ferrari, and L. Giannessi
 
  • Elettra – Sincrotrone Trieste, Trieste, Italy
     

Contact persons:
Giuseppe Penco: giuseppe.penco@elettra.eu

Reference

G. Penco, E. Allaria, G. De Ninno, E. Ferrari, and L. Giannessi, “Experimental Demonstration of Enhanced Self-Amplified Spontaneous Emission by an Optical Klystron”, Phys. Rev. Lett. 114, 013901 (2015). doi: http://dx.doi.org/10.1103/PhysRevLett.114.013901

 

Last Updated on Tuesday, 27 January 2015 14:26