SUB-PICOSECOND OPTICAL PULSES AT THE SLS STORAGE RING

Transcription

1 SUB-PICOSECOND OPTICAL PULSES AT THE SLS STORAGE RING G. Ingold, A. Streun, B. Singh, R. Abela, P. Beaud, G. Knopp, L. Rivkin, V. Schlott, Th. Schmidt, H. Sigg, J.F. van der Veen, A. Wrulich, Paul Scherrer Institut, Villigen, Switzerland S. Khan, BESSY, Berlin, Germany Abstract We report on the feasibility and expected performance of a sub-picosecond X-ray source at the Swiss Light Source (SLS), based on the electron-beam slicing method. This method has recently been demonstrated at the Advanced Light Source (ALS), using a bending magnet to produce ultrafast optical pulses [1]. To improve the flux of such a source for user experiments, we have studied its realization both in the hard and soft X-ray regime, employing small gap insertion devices (IDs) installed either in one or in two successive straight sections. The most favourable geometry is one in which the modulator and the radiator are positioned in the same straight section of the storage ring. 1 INTRODUCTION The ID based photon sources [2] at the SLS [3] cover the energy range 10 ev to 40 kev. The energy range 10 ev to 18 kev is covered by undulators, and for photon energies up to 40 kev, a hybrid wiggler is employed. To achieve high flux and brightness at a storage ring of 2.4 GeV, small gap undulators and wigglers have to be installed. In addition, for a versatile photon source the options of a flexible polarisation and a short-pulsed time structure of the photon beam is of central importance to a variety of experiments in magnetism and structural research. The notable features of the ID-based photon sources at the SLS are: ffl For soft X-rays: the use of elliptical twin-undulators with variable linear and circular polarisation, allowing oppositely polarized, rapidly switched ( khz) beams under otherwise identical conditions. ffl For hard X-rays: the use of small gap, short period undulators to extend the high brightness synchrotron radiation (SR) to '18 kev (i.e. into the domain of high energy SR facilities), operating on higher (11th/13th) harmonics. The method to generate sub-picosecond synchrotron radiation as recently demonstrated at the ALS, Berkeley, employs a femtosecond laser to modulate the energy of an electron bunch over the distance of the laser pulse length. The optical laser pulse modulates the energy of a stored electron beam bunch as they co-propagate through the modulator wiggler. In a dispersive section, the modulated electrons become spatially separated from the unperturbed part of the electron bunch ( electron-beam slicing ). The spatially separated electron slice can be used to generate sub-picosecond X-rays in the so-called radiator. The radiator can be a dipole magnet in the arc-sector or an insertion device in a straight section. To enhance the overall efficiency, a laser system with a high power and a high repetition rate is needed. Using state-of-the-art lasers, an upper limit is set on the energy of the storage ring by the requirement that the energy modulation must be several time larger than the energy spread of the electron beam ( E 5 ff E ' 13 MeV at the SLS). To efficiently employ the electron-beam slicing method, the beam energy cannot be much higher than 2.5 GeV. Therefore, for intermediate energy storage rings such as the SLS ( GeV), this technique opens a unique avenue for time-resolved experiments: ffl For the production of short photon pulses, the combined effect of the electron beam slicing method and the use of higher undulator harmonics means that high brightness sub-picosecond X-rays up to 18 kev are accessible at undulator beamlines specially developed at user facilities with a beam energy of about 2.5 GeV. ffl For the conceptual design of an ID-based subpicosecond photon source at the SLS, one can profit from several unique attributes of the facility: Π Low emittance optics Π Large momentum acceptance Π Long straight sections Π Small gap, short period wiggler as a modulator Π Linear/circular polarized twin-undulator as a radiator for soft X-rays Π Small gap, short period in-vacuum undulators as a radiator for hard X-rays 2 LATTICE MODIFICATIONS At the PX-beamline 6S we recently have installed the short period (24 mm) in-vacuum undulator U24. As the (radiator) source for sub-picosecond X-rays, we studied to install small gap (4-6 mm) undulators of this type also in the long straight section 5L (10.8 m). For this purpose, to lower the vertical fi-function from nominally 3 m to» 1 m at the center of the radiator, the optics in the straight section 5L needs to be modified. An additional quadrupole triplet mounted asymmetrically splits the long straight 5L into two short straights of different length with fi y =1.6 m and fi y =0.6 m respectively at the center (Figure 2). The change of the ring optics is confined between sextupole pairs in the arcs adjacent to the straight 5L. Therefore no optical function at any sextupole will be affected. The betatron phase advance introduced by the modification is Φ x =0 horizontally and Φ y = ß vertically ( ß-trick ). Since in a regular (non-skew) lattice all normal multipoles contain only even powers of the vertical coordinate y, the non-linear beam dynamics remains basically unchanged [4]. Marginal deteriorations are due to a slight increase of the integrated sextupole strength and due

2 to a deviation of Φ y from ß for off-momentum particles (j p/pj 6%). The locally modified optics allows the installation of two small gap insertion devices, either two undulators ( Two- Straight Scenario, Figure 1 and 2) or one wiggler and one undulator ( One-straight Scenario, Figure 3 and 4). Figure 1: Schematic view: lattice modification and 2- straight scenario. A sizeable dispersion function ( ) is necessary to achieve a transverse spatial separation (5 ff) of the energymodulated electron slice at the radiator. Because of the small vertical beam size, a vertical -function will be implemented. 2.1 Two-Straight Scenario Two undulators (U24, U17) in section 5L cover the photon range 2-18 kev. Laser modulation of the electron beam occurs in the modulator (revolver-type wiggler W61/W90) installed in section 4S, immediately upstream from the radiator straight 5L (Figure 1 and 2). A revolver-type modulator is proposed to ensure the highest horizontal flux density for the Material Science (MS) beamline at 4S using the small gap wiggler W61 (under construction) when the sub-picosecond X-ray facility is not operational. Operating the laser on the 2nd harmonic in resonance with the modulator wiggler W90 has the advantage that the regular MS-beamline operation is not much affected by the subpicosecond operation in section 5L. Four skew quadrupoles in the arcs adjacent to 4S are used to generate a vertical asymmetric -bump by coupling the horizontal in the arc to the vertical plane at the location of the modulator [4]. An effective dispersion ef f =8.5 mm and 0 ef f ' 0 at the radiator is generated by a vertical betatron oscillation of the energy modulated electrons induced by the skew quadrupoles. The principle is explained in Figure 2. For electrons with an energy modulation 0.5%, a vertical spatial separation of 43 μm is achieved. The vertical beam size is 8 μm. To keep the emittance coupling low (' 0.5%), various options for creating the asymmetric -bump are under study. 2.2 One-Straight Scenario The first radiator (undulator U24) in Figure 1 is replaced by a small gap wiggler as the modulator. The modulator and the radiator are now installed in the same straight section. This scheme, shown in Figure 3 and 4, provides Figure 2: The modulator (in 4S, with asymmetric -bump) and the radiator(s) (in 5L, with mini-beta lattice) are installed in two successive straight sections. Solid curve: closed vertical dispersion bump of the stored beam. Dashed curve: vertical dispersion of the sliced beam. Figure 3: Schematic view: 1-straight scenario with vertical chicane at the wiggler (modulator). the smallest temporal stretching of the electron bunch slice (due to the non-isochronicity of the SLS storage ring) and thus the shortest X-ray pulses. We estimate that instead of ps the photon pulses can now be as short as 0.1 ps using a 50 fs laser. In addition, the modulator wiggler (W110) is now resonant to the laser fundamental (800 nm), since a simultaneous operation with a second beamline is not required. The closed vertical -bump around the modulator is generated by a vertical 4-dipole chicane, providing =8.8 mm and 0 =0 for the 5ff-separation (48μm) of the short photon pulses. The vertical displacement ('8 mm) Figure 4: Mini-beta lattice with chicane for pure spatial beam separation. The modulator W110 and the radiator U17 are installed in the long straight section 5L. Solid curves: vertical beta function and vertical dispersion of the stored beam. Dotted curves: horizontal beta function and vertical dispersion of the sliced beam.

3 of the stored beam in the modulator W110 with respect to the radiator U17 is used to separate and block the unwanted modulator radiation. Minimizing the photon background is critical for creating a clean image of the source (see section 3.3). Wiggler W110 Modulator Wiggler One Straight 3 SUB-PICOSECOND X-RAY SOURCE 3.1 Modulator and Radiator The following parameters, summarized in Figure 5, 6 and 7, are used to predict the characteristics of the subpicosecond X-ray source: ffl One-straight scenario (5L): Ti:sapphire laser: 800 nm (fundamental), 50 fs [FWHM], 1 mj/pulse, 5 W, 5 khz (10 khz); modulator W110: period 110 mm, gap 7.5 mm, peak field 2.5 T, K-parameter 25.6, length 2m; radiator U17: gap 4 mm, period 17 mm, peak field 1.0 T, energy range 5-18 kev ( harmonic); slicing efficieny (400 bunches): 5.5 E-9 (5 khz) (the slicing efficiency is defined as the number of electrons with E 0.5% / second/ beam current (400 ma)). ffl Two-straight scenario (4S / 5L): Simultaneous operation of the material science (MS) beamline 4S requires that the laser is in resonance with the second harmonic of the modulator wiggler. Ti:sapphire laser: 400 nm (2nd harmonic, conversion efficiency 30-50%), 50 fs [FWHM], 1 mj/pulse, 2 W, 2 khz (5 khz); modulator W90 (revolver type W61/W90): period 90 mm, gap 7.5 mm, peak field 2.35 T, K-parameter 19.7, length 2m; (wiggler W61, under construction, for regular operation at the MS-beamline (4S), period 61 mm, gap 7.5 mm, peak field 1.95 T, K-parameter 11.0, length 2 m); radiator U24: gap 5 mm, period 24 mm, peak field 1.10 T, energy range 2-8 kev (1.-7. harmonic); radiator U17: gap 4mm, peak field 1.0 T, energy range 5-18 kev ( harmonic); slicing efficiency (single bunch): 7.5 E-7, slicing efficiency (400 bunches): 4.8 E-9 (5 khz). 3.2 Laser Initial experimental experience with the sub-picosecond X-ray source can be gained with a reliable commercial system. Commercial systems offer average output powers of typically 1 W independent of the repetition rate (100 khz / 10 μj - 10 Hz / 100 mj). We expect that a 50 fs Ti:sapphire laser system based on the chirped pulse amplification with an average power of 3 W (1 mj/pulse, 3 khz) will commercially be available at the end of year Simultaneously a system delivering pulses of 50 fs and an energy of 1 mj scalable in repetition rate (10 khz) will be developed at the Combustion Research Laboratory at PSI. Such a system could be based on the Coherent RegA9050 system that provides 50 fs pulses with energies in the microjoule range and a repetition rate variable up to 250 khz. Here the switching is performed acoustooptically (cavity dumper). No further electro-optic switching unit is then needed for further amplification. The am- length L W 2 m period length W 110 mm no. periods M W 18 max. field B T (Laser, 1 st Harm.) mag. gap 7.5 mm K-value 25.6 Wiggler W90 Two Straights length L W 2 m period length W 90 mm no. periods M W 22 max. field B T (Laser, 2nd Harm.) mag. gap 7.5 mm K-value 19.7 Figure 5: Modulator parameters. Radiator Undulator - hard X-rays - U17/U24, 'multi-undulator' type U17 undulator (in-vacuum) U24 undulator (in-vacuum) type hybrid hybrid magn. material Sm 2Co 17 NdFeB magn. length LU 2 m 2 m no. periods MU magn. gap gu 4 mm 5 mm max. field B 0;U 0.92 T 1.10 T K-value KU energy range 5-18 kev 2-10 kev no. harmonics polarization linear linear Figure 6: Radiator parameters. plification of the μj pulses to the mj range will be based on the chirped pulse amplification technique [5], using an asymmetric grating pair pulse stretcher, prior to the RegA9050 regenerative amplifier, followed by a doublepass Ti:sapphire amplifier and a grating-pair pulse compressor (Figure 8). A schematic view of the amplifier is shown Figure 9. For the case that a double-pass is not sufficient to saturate the amplifier, a multi-pass amplifier in a butterfly configuration will be used. Assuming saturated amplification, the stability of the amplified femtosecond pulse will be that of the pump laser. The design is scalable to higher repetition rates by incorporating future developments in pump laser technology and additional amplifier stages. The problem of thermal lensing by the high average powers absorbed in the Ti:sapphire crystals can be solved by cryogenic cooling of the amplifier rods [6].

4 Laser: Ti:Sapphire 1 st Harmonic wavelength L energy/pulse AL pulse length L no. optical cycles ML repetition rate fl 800 nm 1.0 mj 50 fs khz 5-10 khz (phase 1) (phase 2) E: modulator W MeV (= 5 E ) energy modulation 2nd Harmonic wavelength L energy/pulse AL pulse length L no. optical cycles ML repetition rate fl E: modulator W90 energy modulation 400 nm 1.0 mj 50 fs khz 2-5 khz conv. e.: 30-50% Figure 10: Calculated momentum and spatial distribution of the electron bunch following interaction with the laser in the modulator. An energy modulation 0.5% is required. (phase 1) (phase 2) 12 MeV 3.3 Sub-Picosecond Synchrotron Pulses Figure 7: Laser parameters. Figure 8: Femtosecond Ti:sapphire laser system. The simulated slicing efficiency 5 E-9 for a 5 khz laser system is in good agreement with the analytical estimate given in [1]. Using this value, the intensity of the subpicosescond X-ray pulses listed in Figure 12 is calculated from the values given in the 3rd column, which are valid for the regular multibunch operation (400 ma, 500 Mhz) of the SLS storage ring. To obtain the values in the 3rd column of Figure 12, we earlier have studied the reduction of brightness due to undulator phase errors, the emittance and the energy spread of the electron beam for harmonic number 1-13 [2] (Figure 11). The on axis flux density F 00 := d3 N/dt(d!=! d j =0 [photons/sec/mrad 2/0.1% bw] has been calculated numerically including (i) phase errors in a realistic undulator field, (ii) electron beam emittance ( =4.8 nm-rad, x =2.6 m, y =1.2 m, coupling 1%) and (iii) energy spread ( E/E=0.09%). Thepnatural brightness p is de L= ( 0 = = L; fined as Bn =F00 /2 2 and is the wavelength of the X-rays and L the length of the undulator.). ) The laser slicing efficiency is determined by the number of electrons per bunch with an energy modulation E 0.5% after the laser interaction in the modulator. By nur ~ ~x; t dt e ~v? E merical integration we calculate E ~ ~x; t the where ~v? is the electron transverse velocity and E electrical field of the laser. The waist of the laser beam at the center of the modulator is typically 200 m (H) x 200 m (V). = ( ) ( ) = 2 2 Imaging will be used to separate the short photon pulses. We follow the concept adopted at the SIM-beamline 11M for time-resolved photoelectron-microscopy on magnetic materials [7] at the SLS (section 4). A focussing mirror having demagnification 1/M, located behind the double crystal monochromator (Si(111)), images the two parallelly displaced source points (see Figure 2 and 4) for the sliced and unperturbed electrons in the radiator onto the intermediate image plane. At the intermediate focus the images =M ' m. A pair of slits, are separated by 0 displaced 5 y above the optical axis, will select the subpicosecond X-ray pulses and block the long-pulse background. A mechanical chopper (1-5 khz) in front of the slits is used to match the time structure of the X-ray beam to the laser system (Figure 13). = Figure 9: Schematic view of the double two-pass Ti:sapphire 10 khz amplifier. P: polarizer; FR: Faraday rotator. 2 22

5 N hν F 00 B n ffl E/E [kev] 1) 2) E E E-1 8.7E E E E-1 5.0E E E E-1 3.3E E E E-2 2.5E E E E-2 2.0E E E E-2 1.6E E E E-2 1.4E-1 N Phase Phase Beam ALL Error Error Size Factors 1)+2) 3), ),4 0 5) 1)-3), 5) 1 2.4E E-2 1.7E E E-2 2.8E E E-2 1.0E E E-2 4.6E E E-2 2.4E E E-3 1.4E E E-3 8.7E-5 Figure 13: Short pulse separation: imaging of the source. slicing the intrinsic time structure of the storage ring (20 ps (rms) pulses spaced by ns) will be used. Figure 11: SLS U17 undulator: brightness reduction factors. Flux: F [photons/sec/0.1% bw] N hν F F F [kev] [20ps, 500MHz] [20ps, 5kHz] [100fs, 5kHz] E E E E E E E E+9 2.3E E E+9 8.0E E E+8 3.3E E E+8 1.3E E E+7 4.8E+4 Figure 14: Photon beam separation: parallel vs. beams. tilted Brilliance: B [photons/sec/mm 2 /mrad 2 /0.1% bw] N hν B B B [kev] [20ps, 500MHz] [20ps, 5kHz] [100fs, 5kHz] E E E E E E E E E E E E E E E E E E E E E+9 Figure 12: Average flux and brightness of the subpicosecond X-ray pulses for the U17 undulator (integrated over 0.25 mrad (H) x 0.05 mrad (V)). 4 PICOSECOND SYNCHROTRON PULSES We also studied the two-straight scenario for the straight sections 10S and 11M, where the elliptically polarizing twin-undulators UE56 for beamline 11M would be used as radiators. A similar scheme as shown in Figure 2 would again be applicable. Using a 5 khz laser, for the energy range kev a flux and brightness of ( )E+6 and ( )E+10 for sub-picosecond, linear polarized photons is expected. For circularly polarized photons the subpicosecond flux and brightness would be ( )E+5 and ( )E+10 respectively. However, time scales of interest for magnetic studies are rather in the pico- and nanosecond range, where instead of The novel feature of beamline 11M is the concept used for helicity switching. A chicane with 7 dipoles creates a small (± 0.5mm) parallel displacement of the electron beam in the horizontal direction which is opposite for both undulators. The optical paths for the two photon beams with opposite circular or linear polarization overlap almost completely. A single set of relatively small mirrors is used to accept and guide both photon beams. By imaging the two source points with a demagnifying (4:1) optics, both photon beams are again separated at the intermediate focus (Figure 14). At this location a mechanical chopper alternatively blocks one of the two photon beams. The two photon beams can be overlapped on the sample using a single refocussing mirror. 5 SUMMARY We propose to develop at the SLS a facility for subpicosecond X-ray pulses based on electron-beam slicing method with the characteristics of U17 as the radiator. The undulator will likely be the one planned for the micro- XAFS/diffraction beamline. The facility will enable timedependent studies of the structural dynamics of condensed matter and photochemistry and will complement time resolved studies in the pico- and nanosecond range planned at the photoelectron-microscopy beamline for soft X-rays.

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