Petawatt Class Lasers Worldwide
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High Power Laser Science and Engineering, (2015), Vol. 3, e3, 14 pages. © The Author(s) 2015. The online version of this article is published within an Open Access environment subject to the conditions of the Creative Commons Attribution licence <http://creativecommons.org/licenses/by/3.0/>. doi:10.1017/hpl.2014.52 Petawatt class lasers worldwide Colin Danson1, David Hillier1, Nicholas Hopps1, and David Neely2 1AWE, Aldermaston, Reading RG7 4PR, UK 2STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK (Received 12 September 2014; revised 26 November 2014; accepted 5 December 2014) Abstract The use of ultra-high intensity laser beams to achieve extreme material states in the laboratory has become almost routine with the development of the petawatt laser. Petawatt class lasers have been constructed for specific research activities, including particle acceleration, inertial confinement fusion and radiation therapy, and for secondary source generation (x-rays, electrons, protons, neutrons and ions). They are also now routinely coupled, and synchronized, to other large scale facilities including megajoule scale lasers, ion and electron accelerators, x-ray sources and z-pinches. The authors of this paper have tried to compile a comprehensive overview of the current status of petawatt class lasers worldwide. The definition of ‘petawatt class’ in this context is a laser that delivers >200 TW. Keywords: diode pumped; high intensity; high power lasers; megajoule; petawatt lasers 1. Motivation of instabilities and perturbations on timescales where hydrodynamic motion is small during the laser pulse The last published review of high power lasers was con- (τcs λ, where cs is the sound speed of the plasma, ducted by Backus et al.[1] in 1998. At this time there τ is the laser pulse length and λ is the laser wavelength) was only one petawatt class laser, the NOVA petawatt[2], pushed the development of lasers with pulse durations τ in existence. The field has moved on a long way since of less than tens of picoseconds. The development and then with over 50 petawatt class lasers currently operational, delivery of chirped pulse amplification (CPA)[10] in large under construction or in the planning phase. The possibility aperture laser systems enabled a rapid push towards even of using focused high intensity laser beams to achieve shorter pulses (from picoseconds to attoseconds) and higher previously unobtainable states of matter in the laboratory intensities, where relativistic and field effects associated with gained much attention after the demonstration of the first the laser pulse dominate the interaction physics[11]. Under- pulsed laser[3] in 1960. Potential applications, such as standing and learning to control and manipulate the complex generating the conditions for fusion in the laboratory, be- interactions taking place at the laser/matter interface led to a came a major driver for the early development of high power wide variety of experiments and potential new scientific[12] lasers in the 1960s to 1980s. It was realized that although and industrial applications being pursued and necessitating matter could be heated[4] to hundreds of electron volts using the development of matching laser capability, some of which ∼ns pulses and directly compressed using light pressure are outlined below. (∼I/c, where I is the intensity and c is the speed of light), The production of quasi-coherent VUV/soft x-ray sources spherical compression using laser driven ablation could for biological imaging or plasma probing was investigated achieve much higher pressures and densities[5], suitable for using ‘recombination pumping[13]’ where, after heating a the achievement of fusion conditions[6]. First estimates plasma, it was allowed to expand, usually into vacuum, and for laser driven fusion[7] proposed lasers delivering 20 ns rapidly recombine, ideally creating a population inversion shaped pulses of megajoule energies, operating at 100 Hz, in an ionized state such a hydrogen-like carbon[14].To eventually leading today to megajoule scale projects such as achieve higher gain[15], shorter duration laser pulses were NIF[8] and LMJ[9]. required, and by 1995, high power ∼terawatt pulses of The potential to interact with hot plasmas (greater ∼20 ps duration had been developed. Collisional excitation than hundreds of electron volts) and probe the growth soft x-ray laser pumping using high (∼kilojoule) energy, Correspondence to: C. Danson, AWE, Aldermaston, Reading RG7 4PR, nanosecond pulses was first demonstrated at high gain with UK. Email: [email protected] neon-like selenium[16] and subsequently and more efficiently 1 Downloaded from https://www.cambridge.org/core. 30 Sep 2021 at 22:12:47, subject to the Cambridge Core terms of use. 2 C. Danson et al. in neon-like germanium[17]. The highest possible brightness has attracted a significant amount of research effort since the with a soft x-ray laser is obtained when it is operated in turn of the century. ‘Target normal sheath acceleration’ using saturation, and this was initially achieved in 1992 using a petawatt class picosecond laser[45] was used to accelerate neon-like germanium[18] at 23 nm with a 500 ps pump, a population of electrons through a metallic foil, creating and later in 1997 with nickel-like samarium[19] at 7 nm a large sheath field on the rear side which resulted in a using a 50 ps pump. To improve the efficiency of these highly laminar ion beam containing large fluxes (>1013)of devices[20], shorter pulse pumping using a mode of operation high energy (>10 MeV) ions. In subsequent experiments termed ‘transient collisional excitation’ helped to push the using tens of terawatt drivers, it was demonstrated that development of shorter pulse laser drivers. High transient improved efficiency could be achieved by reusing the laser collisional excitation gain was demonstrated using an 800 ps driven electrons[46] as they bounce back and forth in the foil low energy pulse to pre-form a large plasma volume and then target, termed ‘recirculation’. As electron recirculation[47] a 5 J ps pump to generate the transient collisional excitation experiments pushed to thinner and thinner targets (<50 nm − to deliver gain at 14.7 nm[21] in nickel-like palladium. thick foils by ∼2007) at intensities of >1019 Wcm 2, laser The generation of quasi-coherent VUV/soft x-ray sources system contrasts of >109 were routinely required. Currently, using high harmonics[22] rather than soft x-ray lasers was the new contrast enhancing techniques described earlier will given a significant boost in the mid-1990s by the observation need to used in combination with enhanced picosecond of the 68th harmonic of a 1.05 μm driving laser at 15.5 nm, cleaning schemes to achieve picosecond intensity contrasts using a 2.5 ps pulse focused to an intensity of 1019 Wcm−2 of >1011, which are essential to explore new mechanisms[48] > 21 −2 on a solid target[23]. This was extended into the keV of ion acceleration at intensities of 10 Wcm . regime using petawatt power pulses focused to 1021 Wcm−2 intensities[24] by 2007, and into the attosecond region[25] 2. The road to petawatt class lasers using even shorter few femtosecond optical driving pulses. The production of high currents of MeV electrons[26] and From the first demonstration of the laser, attempts have associated gamma-ray production[27] brought significant been made to increase the peak power and focused intensity attention to the scale length of the interaction. At such in order to reach extreme conditions within the laboratory. intensities, any illumination of the target above the ionization Initial jumps in peak power came with the invention of threshold (1011–1012 Wcm−2) can generate a pre-plasma Q-switching then mode locking, but progress slowed until which expands and dramatically changes the scale length the late 1980s and the dawn of CPA. The original use of of the interaction. The ability to control the scale length of CPA was in radar systems where short, powerful pulses that were beyond the capabilities of existing electrical circuits the interaction[28] led to significant effort in improving the were needed. By stretching and amplifying the pulses prior laser contrast and developing pre-pulse mitigation strategies to transmission, then compressing the reflected pulse, high (frequency doubling of high power short pulses[29, 30]; peak powers within the amplifier circuitry could be avoided. [31] [32] plasma mirrors ; saturable absorbers ; XPW tech- These ideas were first applied in a laser amplification [33] [34] [35] niques ; low gain OPA ; short pulse OPA ). scheme at the Laboratory for Laser Energetics at the The concept of using the electric field associated with a University of Rochester, USA by Strickland and Mourou[10]. laser driven plasma wave to accelerate electrons was given Here, the output from a mode-locked Nd:YAG oscillator a major boost in the late 1970s when it was realized that was stretched and spectrally broadened by 1.4 km of [36] GeV/cm fields could be potentially achieved . Subsequent optical fibre, amplified in a Nd:YAG regenerative amplifier work utilizing the then available lasers investigated exci- then compressed using a Treacy grating pair[49] which [37] tation of instabilities and driving of the plasma at two compensated for the second order spectral phase imposed different wavelengths using either 2 ns pulses from a carbon by the fibre. dioxide based laser[38] or Nd:glass lasers[39] to generate Due to the limitations of mode-locked lasers operating suitable beat-wave plasma modulations. By the mid 1990s, at 1064 nm, early high power/energy CPA lasers[50–53] all wave-breaking[40] generated electron beams with thermal- relied on the use of self-phase modulation to generate enough like spectra up to 45 MeV using a 25 TW picosecond driver bandwidth to support sub-few-picosecond pulses[54].