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The 13th harmonic of a Ti:sapphire (Ti:S) laser in the plateau region was injected as a seeding source to a 250-MeV free-electron-laser (FEL) amplifier. When the amplification conditions were fulfilled, strong enhancement of the radiation intensity by a factor of 650 was observed. The random and uncontrollable spikes, which appeared in the spectra of the Self-Amplified Spontaneous Emission (SASE) based FEL radiation without the seeding source, were found to be suppressed drastically to form to a narrow-band, single peak profile at 61.2 nm. The properties of the seeded FEL radiation were well reproduced by numerical simulations. We discuss the future precept of the seeded FEL scheme to the shorter wavelength region.
©2010 Optical Society of America
1. Introduction
Frontiers in optical science in the short-wavelength region have greatly been expanded [1–3] by the advent of intense, single-pass free-electron lasers (FELs) based on a self-amplified spontaneous emission (SASE) scheme [4–6]. Promising applications include determination of precise structure of non-crystallized solid materials [7,8], creation of highly multiply charged atoms extreme states [9,10], and visualization of ultrafast chemical processes [11].
In the SASE-FEL scheme, spontaneous radiation originating from stochastic density modulations in the electron bunch is exponentially amplified along a periodic magnetic field in the undulator to form extremely intense light pulses [1]. However, their temporal profile and frequency-domain spectra are composed of random and uncontrollable spikes, exhibiting shot-to-shot fluctuation originating from the stochastic start-up process intrinsic to spontaneous radiation [12,13]. A seeding scheme for making the SASE-FEL pulses full-coherent and free from spike-noise has been therefore awaited to effectively and widely facilitate scientific investigation of phenomena induced by intense light pulses.
One of the promising approaches for producing full-coherent radiation in the short wavelength region is high-order harmonic (HH) generation [14]. By focusing intense ultrashort laser pulses into a gaseous medium, we can now routinely generate full-coherent HH radiation in the extreme ultraviolet (EUV) region and even in the soft X-ray one by the ionization-recombination processes of bound electrons of atoms or molecules occurring in intense laser fields.
Even though the HH radiation has the advantageous point of full-coherence, its intensity drops drastically in the shorter wavelength region because of the lower conversion efficiency. The most straightforward method for generating intense, full-coherent radiation in the short wavelength region is to amplify the HH radiation by an FEL amplifier, as was demonstrated in the deep ultraviolet region at 160 nm [15]. However this scheme could not work as a seeder in the shorter EUV wavelength region because the intensity of the HH seed light was not high enough to surpass the initial power level of the spontaneous radiation in the SASE-FEL. This intensity limit is possibly due to small laser power, large loss of intensity in transport optics, and lack of the phase matching condition [16]. In the present study, we report for the first time the significant amplification of the seed HH radiation in the EUV region at 61.2 nm by the HH-seeded FEL scheme.
2. Experiment
The experiment was performed at the SPring-8 Compact SASE Source (SCSS) test accelerator operating with the electron beam energy of 250 MeV [2]. Figure 1 shows the configuration for the experiment. The electron beam from a pulsed thermionic electron gun with a CeB6 single crystal cathode [17] was compressed mainly through a velocity bunching process and was accelerated by a C-band high-gradient acceleration system up to 250 MeV. The electron beam was injected into two in-vacuum undulators with a period length of 15 mm.
Fig. 1 Experimental setup. HH radiation is generated by loosely focusing a Ti:S laser (800 nm, 100 mJ, 160 fs FWHM, 30 Hz) in a Xe gas cell (focal length f = 4000 mm), and is separated from the fundamental beam by a SiC mirror. Pt-coated concave mirrors with 8000-mm radius of curvature are used for collimating and focusing HH radiation. The HH radiation is transported by the SiC mirror to the undulator section, overlapped with an electron beam (250 MeV, 300 fs, 30 Hz) spatially and temporally. The seeded FEL is observed by the spectrometer at the end of the beamline. The inset shows the beam profile of HH radiation at the undulator entrance. The spatial profile was measured by a phosphor screen coupled with a multichannel plate (MCP) and CCD camera.
As a seed source, the 13th harmonic of a Ti:sapphire (Ti:S) laser in the plateau region with sufficiently large intensity for the seeding was generated with a loosely-focusing geometry [18], which is a key technique for producing intense HH radiation. The HH source was designed by our energy scaling strategy of Xe HH generation [19]. The pumping laser system, used for generating the HH light, was based on a chirped pulsed amplification of a Ti:S laser (800 nm, 160 fs, 30-Hz). The system was composed of a mode-locked oscillator (TSUNAMI Spectra Physic), which was synchronized to a 238-MHz master clock of SCSS by feedback locking of the cavity length (Lok-to-Clock system, Spectra Physics), a regenerative amplifier (SPITFIRE, Spectra physics) and a 4-pass amplifier. The pump pulse with a pulse energy of 30 mJ was loosely focused by a f = 4000 mm plano-convex lens and delivered into the target chamber through a thin window. We set the focus around the entrance pinhole of the interaction cell that was filled with Xe. The optimized target gas pressure was adjusted to balance between the geometrical phase shift and the harmonic dipole phase. The HH radiation generated in the Xe gas cell was reflected with a SiC harmonic separator mirror [20] set at the Brewster angle (69 degrees) for the Ti:S pump laser. By introducing a pair of Pt-coated, nearly-normal-incidence mirrors with the curvature radius of 8000 mm, the collimation and focusing of the HH radiation were achieved.
The HH radiation was transported through the magnetic chicane part into the undulator section after passing through a second SiC separator mirror. Because the mirror set reflects EUV light above 30 nm the several degrees of HH around 13th are directed to the undurator. The beam size for the HH radiation at the entrance of the first undulator was 0.80 mm and 0.53 mm in the horizontal and vertical directions, respectively (Fig. 1, Inset). The pulse energy of the 13th harmonic was estimated to be 2 nJ/pulse, which is measured and calibrated by the spectrometer and the gas monitor detector [21] at the entrance of the experimental building. By taking into account the optical throughput (~1%) of two Pt-coated mirrors and two SiC separator mirrors, ~200-nJ HH pulse was generated from the gas cell. The resulting peak power was estimated to be 40 kW, assuming the pulse duration of 50 fs [22].
For achieving the seeding, sufficiently large spatial and temporal overlaps between the electron bunch and the HH radiation are required. Moreover, the HH wavelength should be tuned to that of the undulator radiation. First, profile monitoring systems, which simultaneously visualize the spatial profiles of the HH radiation and the optical transition radiation (OTR) from the electron beam, were installed to ensure the spatial overlap in the first undulator. The spatial and angular deviations between the HH radiation and the electron beam were suppressed into the ranges below 100 μm and 100 μrad, respectively, by precisely steering the path of the HH radiation using the two Pt-coated mirrors. Second, a temporal overlap between the electron bunch and the HH radiation was monitored with a streak camera (FESCA-200, Hamamatsu Photonics K.K.) through OTR and the fundamental radiation of the Ti:S laser. The timing was adjusted using a delay system of the Ti:S laser with a step of 50 fs, although the probability of temporal overlap was lowered to a certain extent by a temporal jitter (typically ~1 ps) between the mode-locked oscillator and the rf signal of the accelerator. Third, the wavelength of the undulator radiation was adjusted to that of the HH radiation by monitoring a spectrometer [23] in order to achieve spectral overlap.
3. Results and discussions
Figure 2 shows the recorded spectra of the FEL radiation in fifty successive shots both without (a) and with (b) the HH injection. They exhibit sharp increases in the spectral intensity for several shots shown as red lines in Fig. 2 (b). A threshold level, I th, for selecting these curves was set to three times the average intensity, I ave, of the SASE-FEL radiation in Fig. 2 (a). The difference, I th − I ave, is as large as four times the standard deviation, σ, of the SASE-FEL intensity fluctuation. It can securely be said that the enhancements appeared only in the presence of HH radiation, originating from the seeding effect. Note that the small event-number ratio of the enhancement, which is typically ~10 shots per 1000 shots, is caused by the timing jitter between the seeding laser pulse and the electron bunch.
Fig. 2 Spectra of FEL radiation in fifty successive shots without (a) and with (b) HH injection. The red lines in (b) show profiles that have higher intensities above the threshold level. The inset shows an appearance probability of the high-intensity condition as a function of the deviation of K-value, ΔK=K-1.37944 (lower axis), and the central wavelength of the undulator radiation (upper axis).
Furthermore, we investigated the resonance effect by changing the deflection parameter, K, of the undulator (i.e., the central wavelength of undulator radiation) by varying the undulator gap. The high-intensity pulses are observable only in the vicinity of the conditions with the central wavelength of 61.2 nm, as shown in the inset of Fig. 2 (a). This resonance behavior can be regarded as an evidence of the successful operation of the seeded FEL in this EUV region.
Figure 3 (a) shows typical spectra of the FEL obtained under both seeded and unseeded conditions, as well as the spectrum of the seed HH radiation. Significant improvement of the spectral shapes into those corresponding to a single mode radiation was observed only at the central wavelength of 61.2 nm under the seeded conditions. The spectral intensity required for the seeded conditions was confirmed to be four times higher than the SASE-FEL background. The pulse energy of the seeded FEL is estimated to be 1.3 μJ, compared with 0.7 μJ for unseeded SASE-FEL. This value corresponds to a significantly large amplifying factor of 650 defined by a seed pulse energy of 2 nJ while it is ~twice as large as that of the unseeded SASE-FEL . It should be noted that the spectrum of the seeded radiation is shifted towards the shorter wavelength direction compared with that of the HH radiation by around 0.3 nm.
Fig. 3 Spectra of seeded (red lines) and unseeded (blue lines) conditions, as well as that of HH radiation (green line), given by experiment (a) and simulation (b). The inset of (b) shows intensity growths along the undulator for seeded (red line) and unseeded (blue line) conditions.
For more quantitative investigation, we performed numerical simulations using a 3D FEL simulator, SIMPLEX [24]. The results of the simulations are shown in Fig. 3 (b), where the spectra, as well as intensity growing along the undulator, are presented. The intensities of the HH radiation and the SASE-FEL radiation in the simulations are set to be the respective values in our experiment. The spectral profiles obtained by the simulations are in good agreement with the experimental results, while the intensity enhancement of the seeded conditions with respect to that in the unseeded conditions in the experiment is about one fifth of that in the simulation. This discrepancy can mainly be attributed to the insufficient spatial envelope matching of the HH radiation along the undulator in the experiment.
We also found that the 0.3 nm wavelength shift identified in the experiment (Fig. 3 (a)) was reproduced in the simulation shown in Fig. 3 (b). This wavelength shift in the laser power amplification process over the undulators can be explained by a slight bunch compression inside the undulator. That is, the energy-chirp in the electron bunch and the finite dispersion of the undulator cause a longitudinal compression of the micro-density modulation period of the electron bunch, resulting in the shift of the radiation spectrum towards the shorter wavelength side.
4. Prospect for the seeded FEL scheme into the soft and hard x-ray region
Finally, we discuss the possibility of extending the seeded FEL scheme to the shorter wavelength regions. Figure 4 summarizes (i) the intensity thresholds of the HH source required for the seeded FEL operation and (ii) the output intensities after the FEL amplification, as a function of the photon energy. In this simulation, the energy and the peak current of the electron beam were fixed to be 2 GeV and 1.4 kA, respectively, while the undulator parameters were varied. The intensity thresholds were determined so as to keep the mode numbers below 1.5 with which the intensities per mode are maximized. The results of this simulation show clearly that the present HH-seeded FEL scheme can readily be applied to the wavelength region down to ~10 nm, where the HH radiation with sufficiently high intensity (>30 nJ/pulse) has already been achieved [25]. In the shorter wavelength region, that is, in the soft X-ray region below several nanometers, the conventional HH generation methods without the phase matching could provide only weak intensity of the order of sub-picojoules per pulse because of the very low conversion efficiency smaller than 10−10 [26]. To overcome this ionization-induced phase mismatch, quasiphase matching (QPM) [27–29] or non-adiabatic self-phase matching (NSPM) [30] has been proposed for efficient harmonic generation from ionized gases. Here, the scalability of HHG is of paramount importance for the development of seeding harmonic sources. The scalability of QPM to higher photon energies is limited by the structure of modulation periods. Also, the controllability of the phase matching condition is unsatisfactory for QPM. Since, as for NSPM, phase matching is fulfilled automatically, external human control is impossible. On the other hand, an alternative possible approach to extending the harmonic cutoff is to use a longer-wavelength laser for the driving field [31].We recently increased the conversion efficiency of HH by three orders of magnitude [32,33] to sub-nanojoules per pulse in the water window wavelength region (2~4 nm) by combining near-infrared (IR) laser with the phase matching technique in a neutral gas medium. This generation scheme with IR pulses has attracted much attention [34,35] as a promising route for the scaling of harmonic output power in the water window region. From the rule of output scalability of HHG [32], intensity required for exceeding the threshold level of a few nanojoules per pulse can be obtained by upgrading the IR laser system so that its pulse energy becomes several tens millijoules combined with the long-focusing geometry.
Fig. 4 The intensity threshold of the seeding source (green line) and output intensity after FEL amplification (red line), as a function of photon energy (lower axis) and wavelength (upper axis). The inset shows the simulated spectra of the seeded (red line) and the unseeded (blue line) conditions at a wavelength of 4 nm. The intensity of HH radiation, 2.6 nJ/pulse, corresponds to the threshold level required for operating seeded FEL.
As shown in the inset of Fig. 4, the present simulations show that an injection of this HH source into the FEL amplifier dramatically enhances the HH intensity in the 2~4 nm region by five orders of magnitude, while keeping a single-mode, narrow-band profile. Further extension of the wavelength of the seeded FEL radiation into the region below 1 nm can be achieved by combining the high-gain harmonic generation (HGHG) scheme [36].
5. Summary
The 13th harmonic of a Ti:S laser in HH radiation was significantly amplified with the SCSS test accelerator employed as an FEL amplifier. The temporal and spatial overlap of the electron beam and the HH radiation, as well as the adjustment of the wavelength of the undulator radiation to that of the HH radiation, were precisely tuned for achieving the seeded FEL operation. The spectral narrowing and the wavelength shift observed in the seeded conditions agreed with the results obtained by the numerical simulations. It was shown that the present HH-seeded FEL scheme could readily be applied to the wavelength region down to ~10 nm, and further into the water window wavelength region.
Acknowledgements
This study was partially supported by the X-ray Free Electron Laser Utilization Research Project of the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT), a grant for Extreme Photonics Research by MEXT, and a grant for the President's Discretionary Fund of RIKEN.
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