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Keywords:

  • petawatt femtosecond laser;
  • chirped-pulse amplification Ti:sapphire laser;
  • electron acceleration;
  • proton acceleration;
  • relativistic high-order harmonic generation

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Femtosecond petawatt laser
  5. 3 Particle acceleration
  6. 4 Ultrashort secondary light source generation
  7. 5 Conclusion
  8. Acknowledgement
  9. References
  10. Biographies

The high-power femtosecond laser has now become an excellent scientific tool for the study of not only relativistic laser–matter interactions but also scientific applications. The high-power femtosecond laser depends on the Kerr-lens modelocking (KLM) and chirped-pulse amplification (CPA) technique. An all-Ti:sapphire-based 30-fs PW CPA laser, which is called the PULSER (Petawatt Ultrashort Laser System for Extreme Science Research) has been recently constructed and is being used for accelerating the charged particles (electrons and protons) and generating ultrashort high-energy photon (X-ray and γ-ray) sources. In this review, the world-wide PW-level femtosecond laser systems are first summarized, the output performances of the PULSER-I & II are described, and the future upgrade plan of the PULSER to the multi-PW level is also discussed. Then, several experimental results on particle (electron and proton) acceleration and X-ray generation in the intensity range of mid-1018 W/cm2 to mid-1020 W/cm2 are described. Experimental demonstrations for the newly proposed phenomena and the understanding of physical mechanisms in relativistic and ultrarelativistic regimes are highly expected as increasing the laser peak intensity up to over 1022 W/cm2 ∼1023 W/cm2.

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Femtosecond petawatt laser
  5. 3 Particle acceleration
  6. 4 Ultrashort secondary light source generation
  7. 5 Conclusion
  8. Acknowledgement
  9. References
  10. Biographies

Since the development of the laser in 1960, it has played a critical role in exploring new scientific phenomena and evolving new research topics such as nonlinear optics, laser-plasma physics, ultrafast optics, femtochemistry, and so on. In the laser-science community, one of major research trends is to increase the laser peak power by developing a high-power laser to pursue the deep understanding of nature (and utilization of scientific findings). The development of a high-power laser system historically relies on the short-pulse generation from the laser oscillator and its amplification. The Q-switch technique was first devised for generating a nanosecond laser pulse [1]. The technique was immediately employed to develop high-power Nd:glass lasers for the inertial confinement fusion research. Laser pulses shorter than a nanosecond can be produced in a sophisticated manner. The mode-locking technique is a typical way to generate a shorter (picosecond or femtosecond) laser pulse [2]. Although a picosecond or femtosecond laser pulse offers an advantage in achieving a higher power with a relatively small amount of cost and area, it was not until the chirped pulse amplification (CPA) technique [3] was introduced in 1985 that successful amplification of the picosecond or femtosecond laser pulse was achieved. Before the CPA technique, the direct amplification of an ultrashort laser pulse suffered from the low extraction efficiency and nonlinear effects.

The CPA technique employs a concept of stretching and compressing a laser pulse in the temporal domain before and after the amplification of ultrashort laser pulses. This idea could overcome the low extraction efficiency and reduce the damage risk related to nonlinear effects. In addition, the Kerr-lens mode-locking (KLM) technique [4] could produce stable fs laser pulses from a Ti:sapphire laser and make the CPA technique powerful in building a terawatt- (TW-) or even PW-laser system in the laboratory scale. The first CPA PW laser system was demonstrated by a Ti:sapphire/Nd:glass hybrid laser system in the Lawrence Livermore National Laboratory (LLNL) [5]. Since then, many efforts have been made for building PW-class laser systems that have reduced their pulse durations by few tens of femtosecond. As a result, the first 33-fs, 0.85-PW Ti:sapphire laser system operating at a few shots per hour has been demonstrated by the Advanced Photon Research Center (APRC) in Japan [6].

After the first demonstration of PW-class Ti:sapphire laser system, the race to build a 30-fs level PW or even higher-power laser system was initiated in Europe (France, United Kingdom, Italy, etc.), the United States, China, Japan, and Korea. In 2010, the Advanced Photonics Research Institute (APRI) in Korea demonstrated a 30-fs, 1-PW all Ti:sapphire-based CPA laser system operating at 0.1 Hz [7]. The research team has also completed the second beamline with a higher-peak power of 1.5 PW [8] in 2012. Multiple 100 TW- or PW-class Ti:sapphire laser systems operating in few tens of fs are now operational in Europe, United States, etc. [9-16] as well as Japan and Korea.

In addition, alternative ways to reach PW-class laser systems using techniques such as pure optical parametric pulse amplification (OPCPA) [17] and the hybrid CPA-OPCPA [18] technique are also being developed in Russia and China. Based on the demonstration of PW laser systems, Europe, through the European extreme light infrastructure (ELI) project [19], is now constructing 10-PW-level laser systems, in the Czech Republic, Romania, and Hungary, for high-field science, photonuclear physics, and attosecond science. Furthermore, the construction of a 200-PW laser system based on the OPCPA technique using multiple beamlines is also under discussion in Russia [20]. The output characteristics and applications of most of femtosecond PW-class laser systems across the world are summarized in Table 1.

Table 1. Output characteristics and applications of femtosecond PW-class laser systems across the world
Laser system (Organization)Minimum pulse durationMaximum energyMaximum Peak powerRepetition RateYear constructedApplications
  1. *CPA laser system

  2. OPCPA laser system.

  3. Hybrid CPA-OPCPA laser system.

J-KAREN* [6]33 fs28.4 J0.85 PWSingle shot2003Particle acceleration
(APRC, JAEA)     Relativistic optics
Petawatt laser* [11]29 fs25.8 J0.89 PWSingle shot2007Relativistic laser-plasma
(SIOM)     Particle acceleration
      X-ray generation
PEARL [17]43 fs24 J0.56 PWSingle shot2007Electron acceleration
(IAP)      
HERCULES* [12]30 fs17 J0.3 PW0.1 Hz2008Particle acceleration
(CUOS)     Relativistic optics
Astra Gemini* [13]30 fs15 J0.5 PW0.05 Hz2008Particle acceleration
(RAL)     Coherent X-ray generation
      Laboratory astrophysics
FLAME* [14]20 fs4.4 J0.22 PW10 Hz2009Particle acceleration
(ILIL,INO-CNR)     Relativistic laser plasma
Optical synchronize dual laser beam [9]40 fs1.6 J40 TW10 Hz2010Particle acceleration
(HFL, MBI)25 fs2.5 J0.1 PW10 Hz Relativistic laser plasma
PULSER* [7, 8]30 fs33 J1.1 PW0.1 Hz2010,Particle acceleration
(APRI, GIST) 44.5 J1.5 PW 2012Relativistic laser plasma
      Ultrashort high-energy photon generation
XL-III [18]28 fs32.3 J1.16 PWSingle shot2011Particle acceleration
(IOP)     Relativistic laser-plasma
BELLA*40 fs42.4 J1.06 PW1 Hz2012Electron acceleration
(LBNL)      

When a PW or higher-power fs laser pulse is focused, the laser intensity of 1020 W/cm2 to 1023 W/cm2 can be typically attainable in the focal plane, depending on the focusing optics. As the laser intensity exceeds a value of 1018 W/cm2, then the electric field by the laser intensity becomes greater than 1012 V/m and the quiver speed of electrons under the electric field becomes close to the speed of light in the nonrelativistic limit. The laser intensity beyond 1018 W/cm2 is called the relativistic regime [21]. To date, the highest intensity of 1022 W/cm2 was demonstrated by Bahk et al. by focusing a fs laser pulse down to 0.8 µm with a deformable mirror and a low f/# (f/0.6) focusing mirror [22]. The applications using intense laser pulses to electron acceleration [23], proton acceleration [24], and ultrashort X-ray generation through a high-harmonic process [25] have been intensively investigated in the peak power range of a few tens of terawatt (TW) to 100 TW. PW lasers will be used for several GeV electron generation, 200 MeV proton beam generation, and ultrashort X-ray in the water-window range. Many interesting features including relativistic transparency [26] and radiation friction (or radiation reaction) [27] can be investigated with PW and higher-power laser pulses.

At a higher intensity of more than 1023 W/cm2, which can be reached with a higher power than 10 PW, the proton motion driven by a laser field becomes relativistic. A laser intensity of ∼1024 W/cm2 is considered as an ultrarelativistic intensity [28]. Many other interesting physical phenomena related with fundamental quantum systems can be treated in this intensity level [29]. Reaching an intensity of ∼1024 W/cm2 requires many technical breakthroughs in the laser field. The backward Raman amplification technique [30] and plasma focusing optics [31] can be applied for this purpose. The investigation on the physics under the ultrarelativistic intensity will allow us to understand fundamental processes in atoms, molecules, plasmas and subatomic entities occurring in an ultrafast timescale. In this review, we first describe the output performance of a 30-fs, PW CPA Ti:sapphire laser system constructed for the research on high-field science at APRI. In the second part of the paper, the laser–matter interaction (mainly particle acceleration and ultrashort X-ray generation) performed with fs high-power laser are described with experimental results. The fs high-power laser will be an essential tool for studying relativistic and ultrarelativistic laser–matter interactions.

2 Femtosecond petawatt laser

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Femtosecond petawatt laser
  5. 3 Particle acceleration
  6. 4 Ultrashort secondary light source generation
  7. 5 Conclusion
  8. Acknowledgement
  9. References
  10. Biographies

The generation of a fs (0.5 ps) laser pulse was first demonstrated by Shank and Ippen in 1974 [32]. A dye gain medium was used to generate the fs laser pulses at that time. Later, many other solid-state femtosecond lasers using external nonlinear cavity, semiconductor saturable absorber, and nonlinear Kerr effect of a gain medium were developed [33-37]. Ti:sapphire laser crystal is advantageous because of the wide emission spectrum that can support several fs laser pulse [38]. The Ti:sapphire became a key crystal in the front-end oscillator for building a PW laser system. In 2003, APRI in Korea started, through the national project of “Construction of the Ultrashort Quantum Beam Facility (UQBF)”, to build a fs, PW Ti:sapphire laser system for research on high-field physics. Through the project, 30- and 100-TW laser systems had been successfully developed in 2004 and 2005, respectively. The PW laser system is extended from the 100-TW laser system, which is now called LiFSA (Light source for Femto/atto-Sciences and Applications). The PW laser system, which is now called PULSER (Petawatt Ultrashort Laser System for Extreme Science Research), produces PW laser pulses [30-fs, 33-J (for beamline I), 39-J (for beamline II)] at a repetition rate of 0.1 Hz (one shot every ten seconds). This was the first demonstration of an all Ti:sapphire-based PW laser system in the world [7, 8]. All the laser system is designed to support multiple laser–plasma interaction experiments at the same time. The PW Ti:sapphire laser system consists of a front-end system including a fs-laser oscillator and pulse stretcher, 100 TW, 1-PW and 1.5-PW laser beamlines. The overall schematic diagram for the PW laser system is shown in Fig. 1.

image

Figure 1. Schematic diagram of the 30-fs, PW CPA Ti:sapphire laser system at APRI.

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Front-end system

A commercial 1 kHz Ti:sapphire laser (Femtopower Compact Pro, Femtolasers GmbH) was used as a front-end system to provide seed pulses for the following amplifiers. The ultrashort laser oscillator operating at 78 MHz employs the dispersion-compensated mirrors to produce 10-fs-level laser pulses. The laser pulse was amplified up to the 1-mJ level in a 1-kHz amplifier system. Two ultrafast Pockels cells (UPCs) operating at 10 Hz and a saturable absorber were implemented to enhance the temporal contrast of the laser pulse. Two UPCs had risetimes of 150 ps and 800 ps, respectively. As a saturable absorber, a colored glass filter (RG850, CVI) [39] was placed after the second UPC. The contrast ratio was improved by more than 5 orders of magnitude with the UPCs and saturable absorber. After the saturable absorber, the laser pulse passed through an aberration-free all-reflective Öffner-triplet-type stretcher [40] with a 1400-groove/mm grating and it was temporally stretched to about 0.9 ns.

100-TW laser beamline

To achieve an energy for 100 TW peak power, the stretched laser pulse was amplified in the preamplifier and two power amplifiers. The preamplifier was pumped by a 120-mJ, 532-nm laser pulse and the single-pass absorption of the pump pulse was about 92%. The pulse energy was amplified up to 50 mJ in the preamplifier. The first power amplifier contained a Ti:sapphire gain medium that had dimensions of 30 mm diameter and 20 mm thickness. The first power amplifier was pumped by four frequency-doubled 1-J nanosecond Nd:YAG lasers. The output energy was amplified up to 1.8 J in the first amplifier. The second power amplifier contained a Ti:sapphire gain medium with dimensions of 50 mm diameter and 20 mm thickness. The final output energy after the second power amplifier reaches 4.5 J.

The gain narrowing effect was compensated for with an acousto-optic programmable dispersive filter (AOPDF) [41]. The additional contrast ratio enhancement technique using the gain control method were also tested and applied for the improvement by a factor of 2.6 [42]. Figures 2a and b show the spectral profiles and spatial beam profiles at several positions in the laser system. When the laser amplified from the second power amplifier was compressed by a grating pulse compressor, the 100-TW laser pulse was obtained. The grating pulse compressor had a line density of 1480 lines/mm and a typical throughput efficiency of 74%, yielding the output energy of 3.3 J. The compressed laser pulse was measured with the spectral phase interferometry (SPIDER) technique [43]. The minimum pulse duration of the compressed laser pulse was about 30 fs, which was only 1.1 times larger than the transform-limited pulse duration of 27 fs. The beam size of the 100 TW laser was about 70 mm.

image

Figure 2. Laser spectrums (a) and beam profiles (b) at oscillator, preamplifier, and second amplifier.

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1-PW laser beamline (PULSER-I)

A laser pulse from the second power amplifier was delivered to the final booster amplifier for the generation of PW laser pulses. Four frequency-doubled Nd:glass lasers (Continuum, Inc.) were used to pump the final booster amplifier. Each Nd:glass laser system produces 30-ns-long laser pulses at 527 nm with three beams at a repetition rate of 0.1 Hz. Each beam delivers an energy of 8.5 J and all pump lasers deliver a total energy of 100 J. The pump beam has a flat-top beam profile, obtained by compensating for thermally induced birefringence. The use of the flat-top beam profile allows us to amplify the IR laser pulse strongly at a high repetition rate without incurring any optical damage. In this case, neither an image relay nor a beam-shaping homogenizer, such as a microlens array or diffractive optical element, was used. A Ti:sapphire crystal with a 100-mm diameter and 25-mm thickness was used as a gain medium. The output energy and spatial beam profile from the 1 PW booster amplifier are shown in Fig. 3a.

image

Figure 3. (a) Output energy from the 1 PW booster amplifier (inset is the output beam profile at 47-J output energy). (b) Pulse profile reconstructed from the SPIDER measurement (red dashed profile is the transform-limited pulse profile).

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The parasitic lasing can be generated when the gain in the transverse direction reaches the parasitic lasing threshold [44]. The parasitic lasing was suppressed by using an index-matching fluid with an absorption dye [13] and the accurate control of the time delay between the pump and IR pulses. The maximum output energy of 47 J was obtained at 96 J pump energy. Serious parasitic lasing was observed when a pumping energy greater than 96 J was used. The amplified laser pulse was compressed by a grating compressor consisting of four 1480-groove/mm gold-coated holographic gratings (Jobin Yvon, Inc.). The sizes of the first and fourth gratings are 390 mm × 275 mm, and the sizes of the second and third gratings are 565 mm × 360 mm. To avoid the optical damage on the grating surface, the amplified laser pulses were expanded to a 200-mm diameter through an achromatic beam expander to reduce the 0.1 J/cm2 of the average input fluence on the first grating compressor [45, 46]. After the grating compressor with the 74% of the compression efficiency, the laser pulses had a 34-J energy and 30-fs duration, yielding PW laser pulses at 0.1 Hz repetition rate. To maintain the efficiency of the pulse compressor stable, a radio-frequency plasma cleaning system has been used to remove deposited carbon on the grating surface without damaging the grating optics. The reconstructed temporal profile of the PW laser pulse is shown in Fig. 3b.

1.5-PW laser beamline (PULSER-II)

The second PW beamline, as shown in Fig. 1, was pumped by four 15-ns frequency-doubled Nd:glass laser systems (Atlas+, THALES) delivering a maximum output energy of 30 J with two beams at 0.1 Hz. The total energy of 120 J from eight beamlines was irradiated on a 100-mm diameter Ti:sapphire crystal. In this case, an image relay system was also designed and installed in order to magnify the pump beam size to 65 mm on the Ti:sapphire crystal with a good spatial profile. The average pump fluence was 1.7 J/cm2 per face. The amount of 92% of pumping energy was absorbed in a Ti:sapphire crystal. The transverse gain, which is responsible for the parasitic lasing, could be controlled by carefully manipulating the time delay between the pump pulse and input seed pulse [47, 48]. Precise control of the time delay between the pump and input seed pulses makes it possible to use the total pump energy of 120 J. A maximum amplified energy of 60.2 J was obtained with slightly better extraction efficiency at a proper time delay condition (See Fig. 4a). The amplified laser pulse was compressed by an identical grating compressor used in the 1-PW beamline. Thus, the final output energy and pulse duration were 44.5 J and 30.2 ±1.8 fs (mean ± standard deviation for 30 successive measurements), respectively, providing 1.5-PW laser pulses. The shot-to-shot fluctuation of the output energy from the 1.5-PW laser beamline was measured. The energy fluctuation was tested under both single-shot and 0.1-Hz operation modes (see Fig. 4b).

image

Figure 4. (a) Output energy from the 1.5-PW booster amplifier (inset is the output beam profile at 47-J output energy). (b) Shot-to-shot energy stability (first nine shots were measured under single-shot operation conditions and the other shots were under 0.1 Hz operation conditions).

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The upgrade of the PW laser system to the multipetawatt level is planned. The technique of amplifying a broadband laser pulse generation will be investigated for generating a 20-fs 80-J laser pulse. The sophisticated techniques including optical parametric chirped pulse amplification (OPCPA) [49-51] and crosspolarized wave (XPW) generation [52] will be tested and implemented for the generation of a multi-PW 20-fs laser pulse [53] with a contrast ratio better than 1013.

Focusability and contrast ratio

An adaptive optics (AO) system was installed for the wavefront correction of PW laser pulses. A wavefront sensor using the shearing interferometric technique [54] was placed beside the PW pulse compression chamber for the wavefront measurement. A bimorph deformable mirror [55], which has a clear aperture of 70 mm, was installed in between the PW booster amplifier and the achromatic beam expander. A precorrected wavefront was incident on the PW pulse compressor. The residual wavefront aberration of 0.097 µm in the root-mean-square (rms) value was obtained after correction for Zernike coefficients of the 10th radial order Zernike polynomials [56]. The Strehl ratio calculated from the wavefront measurement is about 0.67, which means that the focal spot is close to the diffraction-limited one. The sizes of a focal spot formed by an f/16 focusing optic were 20 µm and 20 µm in the horizontal and vertical directions, respectively, yielding a focused intensity of 3.3 × 1020 W/cm2. This implies that a focused intensity of about 1 × 1022 W/cm2 is feasible when an f/3 focusing optic is used. Figure 5 shows the uncorrected wavefront aberration of the 1.5 PW laser pulse (a), the corrected wavefront aberration of the laser pulse (b), the bimorph deformable mirror used (c), and the obtained focal spot with the deformable mirror.

image

Figure 5. Measured wavefront aberrations of 1.5-PW laser pulse before (a) and after (b) correction. (c) 70-mm clear aperture bimorph deformable mirror with 32 electrodes. (d) Three-dimensional plot of the focused laser spot (f/16 focusing optic was used to obtain the focal spot).

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We also measured the temporal change in the wavefront aberrations of a 100-TW laser pulse during the laser operation [57]. The magnitude for the wavefront aberration was directly proportional to the pump power. The defocus and astigmatism were the most dominant wavefront aberration modes and their temporal changes were measured with the first and the second power amplifiers under different condition from single shot to 10-Hz operations. The Zernike coefficient for defocus changed as much as 0.25 µm within 10 s after switching to 10-Hz operation, showing a focus shift of 1.4 mm to the beam propagation direction with a 1-m focus lens. The changes came from a weakening of defocus in the two power amplifiers, in which the thermal loading was mitigated more at 10-Hz operation than at single-shot operation.

The contrast ratio is an important parameter that should be carefully considered in the laser–matter interaction experiments. The contrast ratio for the PW laser pulse was measured with a third-order autocorrelator. Figure 6 shows the contrast ratios for 100 TW and PW laser pulses with and without contrast enhancement techniques. As shown in Fig. 6a, the amplified spontaneous emission level at 200 ps before the main pulse was measured to be 10−7 for 100-TW and PW laser pulses without contrast enhancement techniques. As mentioned before, the temporal contrast ratio was improved by implementing UPCs and SA in the front-end system. Figure 6b shows the contrast ratio when the contrast enhancement techniques were applied. The contrast ratios at 500 ps and 50 ps before the main pulse are about 2.3 × 10−12 and 4.8 × 10−10. Detailed information on several peaks observed before the main pulse are given in [8]. For experiments in which a high contrast laser pulse is required to several ps before the main pulse, a double plasma mirror (DPM) system was designed and installed after the pulse compressor. The output performances for the PW double plasma mirror, in terms of surface fluence and reflectivity, were close to those developed for 100-TW laser pulses [58]. The overall output performance of 100-TW, 1-PW, and 1.5-PW laser beamlines in the ultrashort quantum beam facility are summarized in Table 2.

Table 2. Specifications of 100-TW, 1-PW, and 1.5-PW laser beamlines in the UQBF
Image
image

Figure 6. Temporal contrast ratios for 100-TW and PW laser pulses without (a) and with (b) contrast enhancing devices (two ultrafast Pockels cells and saturable absorber). Without devices, ASE of the 100 TW laser pulse was further amplified in the temporal ranges of –550 to –200 ps. With devices, no clear evidence for the amplification of ASE was observed from the measurement, which meant the ASE intensity was still below the instrument detection limit.

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Operation mode

The laser system can be operated in several operation modes. The temporal contrast and peak power level can be chosen for various kinds of laser–matter interaction experiments. Various contrast ratio levels, such as 10−6, 10−8, 10−10, and 10−12, can be achieved with and without UPCs and SA in the laser system. Each beamline can be used independently and simultaneously in the experiments. Beam-switching chambers were constructed for easy and quick switch from one beamline to the other. Moreover, 1-PW and 1.5-PW laser pulses can be focused together in a same target chamber for the photon-particle experiments. The various operation modes and switching chambers make the laser system flexible for the applications in the laser–matter interaction experiments.

3 Particle acceleration

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Femtosecond petawatt laser
  5. 3 Particle acceleration
  6. 4 Ultrashort secondary light source generation
  7. 5 Conclusion
  8. Acknowledgement
  9. References
  10. Biographies

One of the major applications using a fs PW laser system is to accelerate charged particles (electron and proton). In this section, we review the current status of electron and proton acceleration and briefly introduce the experimental results obtained with fs PW laser pulses.

Electron acceleration

The first electron acceleration using the laser-induced wakefield was proposed by Tajima and Dawson in 1979 [59]. This acceleration scheme is known as the laser wakefield acceleration (LWFA). Many results on electron acceleration using plasma beat wave, self-modulated LWFA, and LWFA [60-62] with high-power lasers were reported to produce electrons having several tens of MeV. Later, a new acceleration scheme in a blow-out (or bubble) regime was proposed by Pukhov and Meyer-ter-Vehn in 2002 [63]. The bubble scheme could accelerate electron beams to a few hundred MeV to GeV levels. Three major results for accelerating electrons in the bubble regime were reported in 2004 [64-66]. In the electron acceleration, physical parameters such as the strength of the normalized vector potential, dephasing length, depletion length, and gas density profile should be optimized to produce high-energy electron beams. Despite the great potential of laser-driven electron beams, the instabilities in terms of electron energy, beam pointing, charge, divergence, and spatial beam profile are challenging issues while the use of laser-driven electron beams is considered in the engineering field. Several ideas using a capillary and multiple acceleration stages have been proposed and tested to achieve high-energy and stable electron beams [67, 68]. Recently, the ascent of electron acceleration beyond 100 GeV is being designed by the International center for Zetta-/Exawatt Science and Technology (IZEST) [69].

Using the LiFSA system, the electron acceleration by the laser wakefield was performed by irradiating 100-TW laser pulses on a supersonic helium gas flow by a 4-mm-long gas nozzle. The characteristics of accelerated electron bunches were investigated under different laser beam focusing conditions (f/2.5, f/10, f/14) [70]. The formation of a stable plasma channel and the generation of stable electron beams with several tens of MeV were related to the optimal f-number (f/10) of a focusing optic. On the other hand, with a small f-number (f/2.5), the instabilities such as filamentation and breakup were observed with a large deflection of the electron beam. This idea was adapted to demonstrate the generation of high-quality and reproducible GeV-class quasimonoenergetic electron beams [71]. In that experiment, 35-fs laser pulses having intensities ranged from 4.0 × 1018 W/cm2 to 8.0 × 1018 W/cm2 were focused on 4-mm-long and 10-mm-long gas get plasmas having densities of 3.0 × 1018 cm−3 to 7.0 × 1018 cm−3 to produce stable GeV-class quasimonoenergetic electron beams. In particular, the stable electron beams having a peak energy of 237 ±12 MeV (average ± standard deviation for 10 successive shots) were produced at a plasma density of 7.0 × 1018 cm−3 and an intensity of 6.5 × 1018 W/cm2.

Now, the electron acceleration using PW laser pulses is being performed to generate multi-GeV electron beams by employing the injector and accelerator scheme [72]. Electron beams with energy of a few hundred MeV generated from a 4-mm-long injector were accelerated to 3 GeV in a 10-mm-long accelerator (see Fig. 7). Electron densities for the injector and accelerator were optimized for the highest electron energy. Energy and beam-pointing stabilities at a higher electron energy are under investigation with PW laser pulses.

image

Figure 7. Multi-GeV electron generation when the injector and accelerator scheme was applied with the PW laser pulse. (a) Electron energy spectrum when the 4-mm-long injector only was used. (b) Electron energy spectrum when the 10-mm-long accelerator was only used. (c) Electron energy spectrum when electrons from the 4-mm-long injector were accelerated in the 10-mm-long accelerator.

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Proton/ion acceleration

The generation of ions with an energy range of 0.2 MeV to 1.3 MeV was observed when a picosecond high-power laser was irradiated on Myler targets [73]. Later, the generation of the energetic protons with a maximum energy of 55 MeV was first demonstrated using a high-power laser pulse by Hatchett et al. [74]. The proton acceleration by an ultraintense laser pulse (∼1020 W/cm2) was interpreted by the target normal sheath acceleration (TNSA) model [75]. In this model, the electrons in a µm-thick solid target (metal or plastic) can be pushed by the ponderamotive force and penetrate the whole target thickness. The penetrated electrons form a strong electric field with the remaining heavy ions in the target. The protons in the target can be accelerated by the strong electric field between the electron and heavy ion. This model successfully explained the proton/ion acceleration from a µm- or thicker solid target [76-78]. In the TNSA scheme, the maximum proton energy (Ep,max) is determined by the thermal electrons, and thus the maximum proton energy is dependent on the peak intensity of a laser pulse in a form of I1/2 [79]. Recently, a bilayered target with microstructures in the back and front sides was proposed and tested for generating high-quality proton beams with monoenergetic feature in the TNSA regime [80-83].

On increasing the peak intensity up to > 1023 W/cm2, proton beams can be directly accelerated by the laser field and highly efficient relativistic proton/ion acceleration was theoretically proposed in the laser-piston regime, known as the radiation pressure acceleration (RPA) [84]. In 2005, Macchi et al. showed that the intensity required for the RPA could be lowered to ∼1021 W/cm2 by focusing a circularly polarized (CP) laser pulse on a nanometer-thick target, because of a continuous drive by the ponderamotive force of a laser field [85]. This acceleration mechanism (so-called light-sail RPA) uses coherent electron nanosheet to produce monoenergetic proton beams. Alternative RPA-like schemes including breakout afterburner (BOA) [86], Coulomb explosion [87], and RPA-dominated regime [88] were proposed for the monoenergetic proton/ion generation. The main purpose of these ideas is to generate monoenergetic proton beams at a lower intensity (1020–1021 W/cm2) even with the linear polarization. As a first experimental demonstration for RPA-dominated scheme, Henig et al. showed a monoenergetic feature in the C6+ ion spectrum and a significant reduction of thermal electrons with CP laser pulses [89].

Proton acceleration is one of major applications utilizing 100-TW (LiFSA) and PW (PULSER) laser beamlines. By irradiating 100-TW laser pulses on metal (Cu and Al) and plastic (polyimide) targets, proton beams with an energy range of several MeV were produced in the TNSA regime, and the accelerated proton beams were applied for the development of imaging technology [90, 91]. The proton acceleration from a thin target (from µm to several tens of nm) was also investigated with 100-TW laser pulses [92]. Recently, the generation of protons with a maximum energy of 45 MeV was demonstrated from a 10-nm-thick polymer (F8BT) target irradiated by a laser intensity of 3.3 × 1020 W/cm2. Interestingly, the change of the intensity scaling was observed for a target thinner than 30 nm and this phenomenon was explained by a hybrid acceleration mechanism including TNSA (dominated by thermal electrons) and RPA (by collective electrons) (see Fig. 8 and Ref. [93]).

image

Figure 8. (a) Proton and carbon ion spectrum obtained with Thomson parabola and microchannel plate. 10-mm- and 100-nm-thick polymers were used as targets. Targets were irradiated by an laser intensity of 3.3 × 1020 W/cm2. (b) Intensity scaling for the maximum proton energy. 10-nm-, 30-nm-, and 100-nm-thick targets were investigated. Targets thinner than 30 nm showed the change in the intensity scaling. The change of the intensity scaling can be explained by the thermal and collective electrons.

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4 Ultrashort secondary light source generation

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Femtosecond petawatt laser
  5. 3 Particle acceleration
  6. 4 Ultrashort secondary light source generation
  7. 5 Conclusion
  8. Acknowledgement
  9. References
  10. Biographies

The generation of ultrashort high-energy photons (XUV, X-ray, and γ-ray) is another important topic using femtosecond high-power laser pulses. Many physical mechanisms including plasma-based X-ray lasers [94], nonlinear Thomson scattering [95], betatron radiation [96], relativistic flying mirrors [97], X-ray-free electron lasers [98], and high-order harmonics (HOH) [99], Compton scattering of laser pulse from energetic electron bunches [100] were proposed and tested for producing ultrafast high-energy photons. In particular, the generation of coherent high harmonics was introduced as a way to generate attosecond light pulses for studying ultrafast phenomena in molecules and atoms [101, 102]. Although the production of harmonic spectrum is possible from gas and solid targets [99, 103], we only focus on the high-order harmonic generation from a solid target. In principle, the high-order harmonic generation from a solid target has no limitation on laser intensity and thus an intense harmonic spectrum should be possible at a higher order. The high-harmonic generation from a solid was first demonstrated using CO2 laser by Burnett et al. [103] and later second- and higher-order harmonics were soon generated using femtosecond laser pulses [99]. In 1996, Norreys et al. could produce efficient (10−6) 68th harmonic spectrum from solid irradiated by picosecond 1-µm laser pulses [104]. Two different mechanisms, relativistic oscillating mirror (ROM) [105] and coherent wake emission (CWE) [106], have been currently proposed to explain HHG from solid targets at relativistic intensities.

In the experiments using 100-TW laser pulses, we could markedly extend the high-harmonic order up to the 164th, which corresponds to a wavelength of 4.9 nm [107]. In our case, the contrast ratio of the laser pulse used was optimized to have a long underdense plasma on a solid target. The optimized contrast ratio was 7.0 × 10−8, which was adjusted by the switch-on time of the UPC at 500 ps before the main pulse. In the experiment using high-contrast laser pulses (better than 10−11), the cut-off of the harmonic spectrum was determined to be at the 43rd order, which corresponded to 18.7 nm. The remarkable extension of the harmonic order with the optimized contrast laser pulse was interpreted by the self-induced oscillatory flying mirror (OFM) model including a long underdense plasma with a density scale length of ∼100 µm. Other interesting features, such as spectral shift and modulation, related to the OFM mechanism were described elsewhere [107]. The generation of higher-order harmonics in the water-window range is expected with PW laser pulses.

The OFM mechanism is briefly described below. The ASE intensity produces a long underdense plasma on a target surface. When a laser pulse propagates through the long underdense plasma, background electrons are pushed forward by the ponderomotive force of the laser pulse and form a copropagating high-density electron nanosheet in the leading edge of the laser pulse. A part of the laser pulse is reflected by the copropagating oscillatory electron nanosheet, resulting in a spectral redshift at low harmonic orders. Then, at some point, the copropagating electron nanosheet is stopped by the space-charge force and begins a backward (counterpropagating) oscillatory motion due to the ponderomotive force with the reverse sign. Within a short time period, the electron nanosheet attains a speed close to the speed of light in vacuum. Consequently, the incident laser pulse is reflected by the relativistically oscillating and flying electron nanosheet, which is now known as the OFM. After a few oscillation periods, the counterpropagating electron nanosheet smears out into the background plasma, while at the same time a portion of the incident laser pulse propagates into the higher-density region and initiates the creation of another electron nanosheet at a different position. The difference between ROM and OFM was described in Fig. 9 with experimental results.

image

Figure 9. (a) Description of relativistic oscillating mirror (ROM) model. (adapted from Tsakiris et al. [97]. (b) Harmonic spectrum obtained with the high-contrast laser pulse. ROM mechanism can be applied for the harmonic spectrum of (b). (c) Description of self-induced oscillatory flying mirror (OFM) model. (d) Harmonic spectrum obtained with the optimized contrast laser pulse. OFM mechanism can be applied for the harmonic spectrum of (d).

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Because the PULSER facility has two beamlines delivering PW laser pulses, it is highly beneficial to carry out experiments on the interaction between an intense light field and energetic particle (specifically electron) beams. The inverse Compton scattering and relativistic flying mirror can be tested with two PW beamlines. The PULSER, an international user facility [108], will also play a key role in producing an ultrashort high-energy photon source, which will be a unique source for research on ultrafast phenomena in biomedical science, nano-science, and nuclear science.

5 Conclusion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Femtosecond petawatt laser
  5. 3 Particle acceleration
  6. 4 Ultrashort secondary light source generation
  7. 5 Conclusion
  8. Acknowledgement
  9. References
  10. Biographies

The PW fs laser systems for relativistic laser–matter interaction research in the intensity range of 1019 W/cm2 to 1022 W/cm2 are going to become commercially available sooner or later. The PW fs laser system can accelerate electrons to the multi-GeV level and protons to several hundreds of MeV. The deep understanding of physical mechanism responsible for the charged particle acceleration will be possible from experiments using PW fs lasers. To date, energetic electron beams with a maximum energy of 3 GeV and energetic proton beams with a maximum energy of 45 MeV were demonstrated using the PULSER, a PW fs laser system. The photon–particle scattering in the relativistic regime will be helpful to produce ultrashort high-energy photon sources in the X-ray and γ-ray range.

Several recent efforts in laser development will bring us 10-PW laser systems by utilizing the XPW technique to enhance the temporal contrast ratio and hybrid CPA-OPCPA technique to reduce pulse duration. Also, the higher-gain amplifiers with larger Ti:sapphire crystals to suppress parasitic lasing will be continually developed to obtain higher powers than 10 PW. Therefore, the 10-PW or higher-power laser systems will allow us to enrich our understandings of fundamental physics in the ultrarelativistic regime.

Acknowledgement

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Femtosecond petawatt laser
  5. 3 Particle acceleration
  6. 4 Ultrashort secondary light source generation
  7. 5 Conclusion
  8. Acknowledgement
  9. References
  10. Biographies

We are grateful to a number of our colleagues: J. H. Sung, S. K. Lee, T. J. Yu, I J. Kim, H. T. Kim, I. W. Choi, K. H. Pae, C. M. Kim, C. W. Lee, S. W. Kang, J. H. Jung, J. M. Yang, J. H. Lim, S. Y. Kim, and W. H. Cho. This work was supported by the Ministry of Knowledge and Economy of Korea through the national long-term R&D program of the Ultra-short Quantum Beam Facility.

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Femtosecond petawatt laser
  5. 3 Particle acceleration
  6. 4 Ultrashort secondary light source generation
  7. 5 Conclusion
  8. Acknowledgement
  9. References
  10. Biographies

Biographies

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Femtosecond petawatt laser
  5. 3 Particle acceleration
  6. 4 Ultrashort secondary light source generation
  7. 5 Conclusion
  8. Acknowledgement
  9. References
  10. Biographies
  • Image of creator

    Tae Moon Jeong received his Ph.D. degree in physics in 1999 from the Korea Advanced Institute of Science and Technology (KAIST). From 2000 to 2004 he worked as researcher for the Korea Atomic Energy Research Institute, Samsung Electronics, and University of Rochester. Since 2005 he works at the Gwangju Institute of Science and Technology (GIST) as a senior research scientist. His research interests include the development of ultrashort high power laser system, charged particle acceleration under super-intense light fields, and high energy photon generation.

  • Image of creator

    Jongmin Lee received his Ph.D. degree in physics in 1980 from the Korea University. From 1973 to 2002 he worked at the Agency for Defense Development as Director of Department of Electro-Optics and the Korea Atomic Energy Research Institute as Vice President. Since 2003 he is Professor at Gwangju Institute of Science and Technology (GIST). In 2013 he also became Distinguished Professor and Director of Global Institue of Laser Science and Technology at Handong Global University. His work concentrates on development of ultra-short petawatt/exawatt laser systems and research of laser-plasma acceleration and ultra high field science.