Control of the amplification process in baseline XFEL undulator with mechanical SASE switchers

Control of the amplification process in baseline XFEL undulator with mechanical SASE switchers

Gianluca Geloni, Vitali Kocharyan and Evgeni Saldin European XFEL GmbH, Hamburg, Germany Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany
Abstract

The magnetic gap of the baseline XFEL undulators can be varied mechanically for wavelength tuning. In particular, the wavelength range nm - nm can be covered by operating the European XFEL with the SASE2 undulator. The length of the SASE2 undulator ( m) is sufficient to independently generate three pulses of different radiation wavelengths at saturation. Normally, if a SASE FEL operates in saturation, the quality of the electron beam is too bad for generation of SASE radiation in the subsequent part of undulator which is resonant at a few times longer wavelength. The new method of SASE undulator-switching based on the rapid switching of the FEL amplification process proposed in this paper is an attempt to get around this obstacle. Using mechanical SASE shutters installed within short magnetic chicanes in the baseline undulator, it is possible to rapidly switch the FEL photon beam from one wavelength to another, providing simultaneous multi-color capability. Combining this method with a photon-beam distribution system can provide an efficient way to generate a multi-user facility.

journal:

DEUTSCHES ELEKTRONEN-SYNCHROTRON

Ein Forschungszentrum der Helmholtz-Gemeinschaft

DESY 10-010

January 2010

Control of the amplification process in baseline XFEL undulator with mechanical SASE switchers

Gianluca Geloni,

European XFEL GmbH, Hamburg

Vitali Kocharyan and Evgeni Saldin

Deutsches Elektronen-Synchrotron DESY, Hamburg

ISSN 0418-9833

NOTKESTRASSE 85 - 22607 HAMBURG

thanks: Corresponding Author. Tel: ++49 40 8998 5450. Fax: ++49 40 8998 1905. E-mail address: gianluca.geloni@xfel.eu

1 Introduction

The recent achievement of LCLS LCLS1 , LCLS2 relies on a high-performance beam formation system, which works as in the ideal operation scenario described in the conceptual design report LCLS1 . In particular, the small electron-beam emittance achieved allows saturation within undulator modules, out of the available. A similar scenario is also foreseen for the European XFEL. One has, then, the possibility of taking advantage of a long, unused part of the SASE undulators to upgrade the facility.

In this paper we describe a method for providing simultaneous multi-color capability at three different wavelengths. For the sake of exemplification we will consider radiation around nm, nm and nm, produced by an electron beam with m normalized emittance and nC charge. After integration with a photon-beam distribution system, this method can lead to an effcient multi-user facility.

Figure 1: Design of the undulator system for the three color X-ray source in the case of a long-pulse ( fs) mode operation. Wavelength selection is based on the use of two mechanical SASE switchers. Each SASE switcher consists of a short magnetic chicane and a SASE shutter. The magnetic chicane generates an offset for the SASE shutter installation and additionally washes out the electron beam modulation. The extra path-length is much smaller than the radiation pulse length.
Figure 2: The three modes of operation for the SASE shutters in the baseline undulator.
Figure 3: Sketch of principle of multi-color X-ray pulse generation in the baseline XFEL undulator. Here the SASE shutter is off. Modulation of the electron beam due to the FEL process in the first undulator part is washed out in the magnetic chicane. In the second part of the undulator, the seeded main part of electron bunch reaches saturation with ten GW power level at nm wavelength.
Figure 4: Sketch of principle of multi-color X-ray pulse generation in the baseline XFEL undulator. Here the SASE shutter is on. Modulation of electron bunch due to FEL process in the first part of undulator is washed out in magnetic chicane. In the second part of undulator FEL amplification process start up from shot noise and reaches 0.1 GW power level only. As a result, energy losses and energy spread within the electron bunch are negligible and the electron bunch is still a good ”active medium” for the next color pulse generation in the following undulator parts.

In its essence, the method is based on controlling the amplification process in the baseline XFEL undulator with the help of SASE switchers constituted by mechanical shutters to be installed at the position of a weak magnetic chicanes at specific locations down the SASE undulator. A sketch of the setup is presented in Fig. 1. Three different modes of operations are foreseen, based on a long ( fs) pulse, depending on what shutters are off and on (see Fig. 2). In the first part of the undulator the beam undergoes the SASE process in the linear regime. After that, it passes through the first switcher, which consists of two devices. First, a weak magnetic chicane creating a transverse offset for the electrons and washing out the electron beam modulation. Second, a mechanical shutter, which has two positions: ”on” when absorbing SASE radiation and ”off”, when SASE radiation from the first undulator passes unperturbed to the second undulator. The electron bunch passes through the magnetic chicane, and the beam modulation is washed out. Note that in the chicane, the electron bunch is also delayed with respect to the radiation but the extra-path length can be chosen small enough to provide a shift of fs, much shorter than the long electron bunch ( fs rms). If the shutter is off (see Fig. 2 top and Fig. 3), at the position of the second undulator almost all the ”fresh” (washed out) bunch is seeded by the radiation coming from the first part, and the seeded main part of electron bunch reaches saturation with ten GW power level at nm wavelength. In this case, energy losses and energy spread within the electron bunch are important, and the bunch cannot further be used for generation of high intensity SASE radiation at wavelength comparable to nm. If the shutter is on (see Fig. 2 middle and bottom, and Fig. 4) instead, a fresh electron beam will enter the second part of the undulator, and will start radiate according to the usual SASE process. The second part of the undulator is short enough so that the SASE amplification process ends, once more, in the linear regime. In this case energy losses and energy spread within the electron bunch are negligible and the electron bunch is still a good ”active medium” for the next color pulse generation in the following undulator parts. These parts are tuned to a different wavelength, namely nm and nm. When the second shutter is off (see Fig. 2 middle), the seeded SASE process reaches saturation at nm. When the second shutter is on, instead, one has saturation in the final part of the undulator only at nm (see Fig. 2 bottom). The distribution of photons can be achieved on the basis of pulse trains, thereby allowing many users working in parallel at different wavelengths. For the temporal structure of the radiation produced at the European XFEL tdr-2006 , this means that we have no restrictive requirement on the switching time of the mechanical shutter, which should be simply shorter than msec, a very suitable condition for mechanical system.

  Units
Undulator period mm 47.9
Undulator length m 256.2
Undulator segment length m 5.0
Intersection length m 1.1
Number of segments - 42
K parameter (rms) - 1.97-2.96
m 17
Wavelength nm 0.1 - 0.2
Energy GeV 17.5
Charge nC 0.25
Bunch length (rms) m 10.0
Normalized emittance mm mrad 0.4
Energy spread MeV 1.5
Table 1: Parameters for the pulse mode used in this paper. The undulator parameters are the same of those for the European XFEL, SASE2, at 17.5 GeV electron energy.

In the next section we present a feasibility study of the method, and we make exemplifications with the parameters of the SASE2 line of the European XFEL (see Table 1). With the help of this scheme it will be possible to provide in parallel X-rays at three different wavelengths around 0.2 nm, 0.15 nm and 0.1 nm. In the following section we will see how, combining this method with a photon-beam distribution system, one can provide an efficient way to generate a multi-user facility.

2 Feasibility study

In the following we describe the outcomes of computer simulations using the code Genesis GENE .

2.1 Both shutters off

First we consider the case when both shutters are off and the first and second parts of the undulator are tuned at nm, Fig. 2 top.

The power and spectrum after the second part of the undulator are shown in Fig. 5 and 6.

Figure 5: Beam power distribution at the end of the second part of the undulator after cells ( m+ m). The first shutter is off.
Figure 6: Spectrum at the end of the second part of the undulator cells ( m+ m). The first shutter is off.
Figure 7: Electron beam energy loss (left) and induced energy spread (right) at the entrance of the third part of the undulator. The first shutter is off.

In this case the SASE process reaches saturation in the second undulator part. The electron beam energy loss and induced energy spread are severe, and prevent the beam to undergo the SASE process again in the following undulator parts, Fig. 7.

2.2 First shutter on

Subsequently, we studied the case when the first shutter is on and the second is off, corresponding to the situation in Fig. 2 middle. The presence of the shutter prevents the seeding process in the second undulator part. As a result the SASE process at nm is far from saturation, and the beam can be used to produce radiation at nm in the third (6 segments, m) and fourth part (6 segments, m) of the undulator. In this case, the properties of the electron beam at the entrance of third part of the undulator are summarized in Fig. 8.

Figure 8: Electron beam energy loss (left) and induced energy spread (right) at the entrance of the third part of the undulator. The first shutter is on.

The power and spectrum after the fourth part of the undulator are shown in Fig. 9 and Fig. 10.

Figure 9: Beam power distribution at the end of the fourth part of the undulator. The first shutter is on.
Figure 10: Spectrum at the end of the fourth part of the undulator. The first shutter is on.

In this case the SASE process reaches saturation in the second undulator part. The electron beam energy loss and induced energy spread are severe, and prevent the beam to undergo SASE process again in the last undulator part.

2.3 Both shutters on

Finally, we study the case when both shutters are on, Fig. 2 bottom. In this case, the SASE process can start from shot noise in the fifth part of the undulator (16 segments, m), at nm, because the presence of both shutters prevent saturation before, and the beam quality is preserved up to the entrance of the last undulator part, see Fig. 11.

Figure 11: Electron beam energy loss (left) and induced energy spread (right) at the entrance of the fifth part of the undulator. Both shutters are on.

The power and spectrum after the fifth part of the undulator are shown in Fig. 12 and 13.

Figure 12: Beam power distribution at the end of the fifth part of the undulator. Both shutters are on.
Figure 13: Spectrum at the end of the fifth part of the undulator. Both shutters are on.

In this case the SASE process reaches saturation in the fifth undulator part.

3 Photon distribution

Figure 14: Proposed SASE undulator beam line for multi-color mode operation. A photon beam distribution system based on movable multilayer X-ray mirrors. Distribution of photons is achieved on the basis of pulse trains and it is possible to serve simultaneously ten user stations with one train per second repetition rate at three different wavelengths. In this case each SASE shutter should be operated with one Hz repetition rate for a single on-off-on cycle only.

As said before, the distribution of photons is done on the basis of pulse-trains. The two mechanical shutters need to operate at a frequency of Hz for a single on-off-on cycle with switching time of less than ms (the temporal delay between two consecutive trains). Consider a temporal interval of second, i.e. trains of electron bunches. During the first ms, the first three trains of electron bunches are let through with both shutters off (see Fig. 2 upper part). Therefore, three trains of radiation at nm are produced. Then, the first shutter is switched on in less than ms, i.e. in the interval between the next two trains. During the next ms, three trains of electron bunches produce radiation at nm. Finally, the second shutter is switched on during the interval between the two following trains, and other four trains of radiation are produces during the final ms, this time at nm. Once separate color pulses are obtained in this way, they can be distributed to different users. Combining this method with a photon-beam distribution system based on movable multilayer X-ray mirrors, as discussed before in OUR1 , can provide an efficient way to generate a multi-user facility. This option, exemplified in Fig. 14, is not specific for the European XFEL and may be applied for LCLS and other XFELs.

Figure 15: Possible extension of the number of user stations which can operate simultaneously at three different wavelengths at the European XFEL. The present XFEL layout enables to accommodate two long undulators behind SASE2, for spontaneous emission in parasitic mode of operation. One may exploit these beamlines and distribute the undulator modules of SASE2 respectively inside the SASE2, U1, and U2 tunnels. Distribution of photons is achieved on the basis of pulse trains. Two mechanical SASE switchers in the first and second parts of the SASE2 undulator operate with repetition frequency of Hz for a single on-off-on cycle.

An option for the distribution of photons specific for the European XFEL may also be considered. The original layout of the European XFEL includes two long undulators for spontaneous emission behind SASE2. These two undulators use the spent electron beam of SASE2. We speculate on the possibility of distributing the SASE2 undulator modules in three parts (14 cells, 12 cells and 16 cells), tuned at three different wavelengths as shown before, and of installing the second and the third part inside the U1 and U2 tunnels instead of the spontaneous emission undulators. The idea is sketched in Fig. 15. Combining this re-installation with mechanical SASE switchers for control of the FEL amplification process can provide an efficient way to generate a multi-user facility. The two mechanical shutters and the magnetic chicanes would be installed as shown in Fig. 15. Different colors may then be transported to different experimental halls. In principle then, at each experimental hall one may still take advantage of the multi-user scheme in Fig. 14.

Figure 16: Design of undulator system for generating in parallel three color X-ray pulses. Three different radiation wavelengths can be horizontally separated in baseline undulator and serve three independent beam lines.
Figure 17: Scheme for separating the first radiation wavelength with respect to the other within undulator system. Two X-ray mirrors can be installed within additional magnetic chicane at the second undulator exit.
Figure 18: Baseline layout of the SASE 2 beam lines. It is possible to distribute the three-color photon beam among three independent beam lines. Distribution of photons is achieved on the basis of the horizontal separation of different colors in the baseline undulator.

Finally, similarly as in OUR0 we remark that the main difficulty concerning the distribution of photons consists in the separation of the three colors. Once this task is performed, mirrors in the photon beam transport system can be used to distribute the three-color photon beam among three independent user beam lines. As in OUR0 we propose to separate the three colors already in the undulator with the help of x-ray mirrors. The idea is sketched in Fig. 16, Fig. 17 and Fig. 18. The three colors can be separated horizontally in two stages by installing the two-mirror setup sketched in Fig. 17 after the undulator parts producing a given color, as specified in Fig. 16. Note that the installation of these setups also requires the presence of a weak magnetic chicane in order to create an offset in the electron trajectory to accomodate the mirrors. The horizontal offset should be chosen small enough to account for the presence of the spontaneous radiation absorbers in the vacuum chamber, which limits the effective aperture to a circle of mm diameter. The horizontal offset may be therefore chosen to be around mm, which is enough for separating the two-color pulses: in fact, at the position of the optical station the FWHM beam size is less than a millimeter. Additionally, mirrors can also be used to generate a few rad deflection-angle, which is not important within the undulator but will create further a small extra-separation of a few millimeters at the position of the experimental station, as shown in Fig. 18.

4 Conclusions

We presented a novel method to control the SASE amplification process in the baseline XFEL undulator with the help of mechanical SASE switchers. After the lasing of LCLS LCLS2 , a new scenario where the beam formation system works as in the ideal case has become reality. This allows for a reduction of the gain length in the SASE process and for exploitation of the extra-available undulator modules. In particular, we show how it is possible to accommodate three FELs, lasing at three different wavelengths within the foreseen undulator length for the SASE2 beam line at the European XFEL. The scheme makes use of mechanical switchers capable of switching on and off the SASE process at a given particular wavelength. Three possible configurations of two switchers allows for separate production of each of the three wavelengths. The switchers should work on the basis of a train of pulses, with a frequency of Hz for an on-off-on cycle, and with a switching time of less than ms. In this way, simultaneous operation at three different wavelength is possible. Distribution of the photons to different stations is discussed.

5 Acknowledgements

We are grateful to Massimo Altarelli, Reinhard Brinkmann, Serguei Molodtsov and Edgar Weckert for their support and their interest during the compilation of this work.

References

  • [1] J. Arthur et al. Linac Coherent Light Source (LCLS). Conceptual Design Report, SLAC-R593, Stanford (2002) (See also http://www-ssrl.slac.stanford.edu/lcls/cdr).
  • [2] P. Emma, First lasing of the LCLS X-ray FEL at 1.5 , in Proceedings of PAC09, Vancouver, to be published in http://accelconf.web.cern.ch/AccelConf/
  • [3] M. Altarelli, et al. (Eds.) XFEL, The European X-ray Free-Electron Laser, Technical Design Report, DESY 2006-097, Hamburg (2006).
  • [4] S Reiche et al., Nucl. Instr. and Meth. A 429, 243 (1999).
  • [5] G. Geloni, V. Kocharyan and E. Saldin, ”Scheme for simultaneous generation of three-color ten GW-level X-ray pulses from baseline XFEL undulator and multi-user distribution system for XFEL laboratory”, DESY 10-006 (2010).
  • [6] G. Geloni, V. Kocharyan and E. Saldin, ”Scheme for femtosecond-resolution pump-probe experiments at XFELs with two-color ten GW-level X-ray pulses”, DESY 10-004 (2010).
Comments 0
Request Comment
You are adding the first comment!
How to quickly get a good reply:
  • Give credit where it’s due by listing out the positive aspects of a paper before getting into which changes should be made.
  • Be specific in your critique, and provide supporting evidence with appropriate references to substantiate general statements.
  • Your comment should inspire ideas to flow and help the author improves the paper.

The better we are at sharing our knowledge with each other, the faster we move forward.
""
The feedback must be of minimum 40 characters and the title a minimum of 5 characters
   
Add comment
Cancel
Loading ...
41075
This is a comment super asjknd jkasnjk adsnkj
Upvote
Downvote
""
The feedback must be of minumum 40 characters
The feedback must be of minumum 40 characters
Submit
Cancel

You are asking your first question!
How to quickly get a good answer:
  • Keep your question short and to the point
  • Check for grammar or spelling errors.
  • Phrase it like a question
Test
Test description