The 1 PW/0.1 Hz laser beamline in SULF facility Download: 843次
1 Introduction
Significant advances in ultraintense and ultrashort laser technology have led numerous laboratories around the world to develop table-top PW-class laser systems as a means of investigating laser–matter interactions in relativistic regime[1, 2]. The repetition rate of PW-class femtosecond lasers is an important issue for their practical applications. And the development of repetitive PW-class lasers has attracted a great attention in recent years. For example, in the United States, the BELLA PW laser working at 1 Hz and another 0.85 PW laser operating at 3.3 Hz were reported in 2017[3, 4]. In Japan, the J-KAREN-P laser with PW-level peak power and 0.1 Hz repetition rate was reported in 2018[5]. In South Korea, the
The Shanghai Superintense Ultrafast Laser Facility (SULF) is a large-scale scientific project located in Shanghai, China[14–16]. The project was formally launched and funded in July 2016, which aims to generate the most powerful laser pulse with peak power up to 10 PW. In 2018, amplified pulse energy of 339 J from Ti:sapphire amplifiers was achieved in the 10 PW-class laser prototype, which can support a peak power of 10.3 PW with a compressed pulse duration of 21 fs[17]. However, the 10 PW-class laser prototype can only operate at one shot every 3 h. Since then, great efforts have been devoted to the upgrade of the laser repetition rate. The SULF will finally consist of two laser beamlines, the SULF-10 PW beamline operating at one shot per minute and the SULF-1 PW beamline operating at 0.1 Hz repetition rate. The SULF will provide repetitive PW-level and 10 PW-level laser pulses for scientific researches on materials dynamics under extreme conditions, ultrafast sub-atomic physics and big molecule dynamics, and extreme-fast chemistry[16, 18]. The layout of the SULF is illustrated in Figure
In this paper, the recent progress on the
2 Laser designs of the SULF-1 PW beamline
The SULF-1 PW beamline is a typical double-CPA system, which consists of two complete CPA stages linked by a nonlinear temporal filter. The schematic diagram of SULF-1 PW beamline is shown in Figure
A commercial Ti:sapphire CPA laser (Coherent, Astrella) is used as the first CPA stage, which can deliver sub-40-fs pulses with 7 mJ energy at 1 kHz repetition rate. Initial seed pulses with energy of 3.9 mJ are injected into the temporal filter, which is based on the techniques of cross-polarized wave generation (XPWG) and femtosecond optical parametric amplification (OPA). This novel temporal filter mainly includes three parts: (1) a stage of XPWG, which can generate high-contrast and broadband seed pulses with several microjoule energy; (2) a stage of second harmonic generation (SHG) pump laser, which can provide pump laser with
The second CPA stage consists of a stretcher, a regenerative amplifier (RA), a pre-amplifier, two power amplifiers, a final amplifier, an achromatic relay-imaging system and a grating compressor. Cleaned pulses from the temporal filter are temporally stretched to
An achromatic image relay system is designed and installed between the Final Amp and the grating compressor, which can expand the beam size from
After the achromatic image relay system, laser pulses are transported from the ground floor to B2 floor by a large-aperture periscope. The grating compressor is installed on the B2 floor, which consists of a large-dimension vacuum chamber, two large-aperture mirrors and four gratings. As Figure
3 Characteristics of the SULF-1 PW beamline
The output energy of the Final Amp is measured by an energy meter (Gentec-EO, QE95). Under pump energy of 110 J, the average output energy is 50.8 J which corresponds to an extraction efficiency of 39.1%. The low efficiency is probably caused by the energy loss in the 100 mm diameter Ti:sapphire crystal. Without the pumping, we have found that the incident seed energy decreases from 7.8 to 7.3 J after passing through the Ti:sapphire crystal once. It indicates that the single-pass energy loss of the large-aperture Ti:sapphire crystal is as large as 6.4%. The energy loss is caused by both the reflection of the end faces and the absorption of this large-aperture Ti:sapphire crystal. The Final Amp is operated near the saturation region and pumped by four Nd:glass lasers independently. A total of eight pump beams are delivered to the Ti:sapphire crystal, which can significantly reduce the effect of pump energy fluctuation by averaging. Then stable output energy can be obtained after the Final Amp. The measured energy fluctuation is about 1.2% (std) for 36 successive laser pulses, which is shown in Figure
The output laser has a near flat-top spatial profile. The inhomogeneity of the spatial profile is caused by the Nd:glass pump lasers, which are under in situ tests, and their near-field profiles are still under improvement.
Fig. 4. Shot-to-shot energy fluctuation of the amplified pulses. The inset shows the beam profile measured at the output of the Final Amp.
Gain narrowing and gain redshift are common problems in high-peak-power Ti:sapphire CPA lasers, which can narrow the spectrum and hence lengthen the compressed pulse duration[25]. In the SULF-1 PW beamline, gain narrowing and redshift are suppressed by shaping the spectrum in the RA and controlling the gain in multi-pass amplifiers, which have been demonstrated to be effective methods for realizing broadband spectrum output[20, 21]. The evolution of the spectra in the amplifier chain is measured by a spectrometer (Ocean Optics, USB2000
Fig. 5. Measured spectra after the RA (black thin solid line), the Pre-Amp (red dashed line), the Power Amp 1 (green dot-dashed line), the Power Amp 2 (blue dotted line) and the Final Amp (magenta thick solid line).
Though the spectrum determines the FTL pulse duration, the actually achievable pulse duration is generally determined by the dispersion of the laser system. Before the measurement of pulse duration, the EAM is inserted between the Final Amp and the achromatic image relay system, which can decrease the energy of the pulses to be measured. The pulse duration is measured by a Wizzler (Fastlite). The typical pulse duration is 29.6 fs, which is shown in Figure
To characterize the focused peak intensity of SULF-1 PW beamline, an OAP with 5655 mm focal length is used for the laser focusing and the study of laser wakefield acceleration (LWFA). The long focal length of the OAP is generally adopted in LWFA, which can be helpful for the self-guiding of the laser pulse and can stabilize the generation of the electron beams. Without pumping the Final Amp, the alignment of OAP and the measurement of the focal spot are done under
Fig. 7. The focal spot measured in SULF-1 PW beamline (a) before and (b), (c) after optimization of the grating compressor by using $f/26.5~\text{OAP}$ .
Apart from the focused peak intensity, pulse temporal contrast is also of crucial importance, especially for laser–solid interactions. The temporal contrast is measured by a commercial third-order cross-correlator (Amplitude, Sequoia). Without pumping the Final Amp, the measurement is done under
Last but not least, the laser pointing stability is also crucially important for laser–matter interaction, especially for investigations requiring repeatability and reproducibility. Laser pointing fluctuation generally results from thermal drift, random vibration and air turbulence. The temperature of the laboratory is kept at 22
Moreover, the laser beam path is covered to avoid air turbulence. Apart from the passive methods above, a beam stabilization system (New Focus, GuideStar) is also used after the Pre-Amp to further improve the pointing stability. After the compressor, the pointing stability is measured under
4 Commissioning of SULF-1 PW beamline
The SULF-1 PW laser beamline is now in the phase of commissioning, and the long-term reliability of the laser system will be checked by carefully increasing the pulse peak power, which can reduce the risk of optical damage. Meanwhile, preliminary applications on particle acceleration and secondary radiation have been implemented based on the SULF-1 PW beamline.
First, an experiment on LWFA was done under
After daily operation under
5 Conclusions
In conclusion, basic features of the
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Article Outline
Zongxin Zhang, Fenxiang Wu, Jiabing Hu, Xiaojun Yang, Jiayan Gui, Penghua Ji, Xingyan Liu, Cheng Wang, Yanqi Liu, Xiaoming Lu, Yi Xu, Yuxin Leng, Ruxin Li, Zhizhan Xu. The 1 PW/0.1 Hz laser beamline in SULF facility[J]. High Power Laser Science and Engineering, 2020, 8(1): 010000e4.