Abstract
Several beam manipulation methods have been studied and experimentally tested to generate two-color photon beams in free electron laser facilities to accommodate the user requests. We propose to use the interaction of the beam with an oscillating longitudinal wakefield source to obtain a suitable electron beam structure. The bunch generates two subpulses with different energies and delayed in time passing through a magnetic chicane after its longitudinal phase space has been modulated by the wakefield source. According to this approach the power of the emitted radiation is not degraded compared to the monochromatic beam, and the setup in the machine is quite simple because the bunch is manipulated only in the high energy section, where it is more rigid. We present the design applied to SwissFEL. We identified the parameters and the corresponding range of tunability of the time and energy separation among the two subbunches.
Highlights
Free electron laser (FEL) facilities are the highest brightness sources used for scientific applications, opening up atomic imaging at femtosecond down to attosecond time resolution
We propose to modulate the beam longitudinal phase space upstream of BC1 using the longitudinal oscillating wakefield excited by the bunch itself passing through an opportune device, and to use the same compressor to generate the current peaks
With the scheme we propose we can provide 3 kA peak current two sub-bunches with a time delay tunable from 85 fs up to about 380 fs and an energy separation between about 0.6% and more than 1% assuming the constraint we have on the maximum energy gain in the last SwissFEL cavities
Summary
Free electron laser (FEL) facilities are the highest brightness sources used for scientific applications, opening up atomic imaging at femtosecond down to attosecond time resolution. The main advantages of this technique are the large tunability of the time delay (from 0 ps up to few hundreds of fs), the energy separation (up to few tens of percent), and the synchronization of the two photon pulses, albeit the total power is typically strongly reduced (about 1=10 of the full saturation power for the LCLS case [6]) Another scheme, called twin-bunch approach, has been proposed and experimentally demonstrated at optical wavelengths [8] and in the range of X-rays [9]. In contrast to the previous approach based on distinct values of K, in the twin-bunch scheme each subpulse of the electron beam efficiently participates to the lasing process, producing a larger X-ray power, albeit the tunability of the time and energy separation of the two-pulses is limited and the setup of the machine requires a longer procedure compared to the K method. We will discuss the tunability of time delay and energy separation among the subpulses (subsection III C)
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