Multiterawatt Ti:Sapphire/KrF laser GARPUN- MTW as a test bench facility for verification of combined amplification of nanosecond and subpicosecond pulses

The possibility of the same large-aperture KrF laser driver to amplify simultaneously both nanosecond pulses for thermonuclear target implosion and picosecond ones for fuel ignition is discussed relative to KrF-based Fusion Test Facility. In this way experiments were performed at hybrid Ti:Sapphire/KrF GARPUN-MTW facility on amplification of subpicosecond pulses. 2-TW, 330-fs pulses were produced with beam divergence 20 μrad in direct double-pass amplification scheme. Peak power as high as 30–40 TW can be achieved in 50-fs pulses, being combined with long pulses (of a few ns to 100 ns) of ~1 GW power. 1. KrF laser as a prospective driver for the Inertial Fusion Energy A single-shot 1.8-MJ Nd glass National Ignition Facility has been successfully completed and Thermo Nuclear ignition experiments were started at the LLNL, USA [1] whilst the other facility Laser Mega Joule is under construction in France, being scheduled for operation in 2014 [2]. Against this background the research program to produce Inertial Fusion Energy is underway considering Diode Pumped Solid State Laser and e-beam-pumped Krypton Fluoride laser as candidates for rep-rate, highly-efficient and durable IFE drivers [3]. The ongoing research with KrF lasers aims at obtaining ≥3×10 shots in continuous operation with ≥6-7 % overall efficiency at a laser module scalable to 28kJ energy that is the bench mark for Fusion Test Facility design [4]. To obtain economically attractive IFE power plant two advanced approaches are considered, which allows one to increase TN target gain and to reduce laser driver energy. In a Fast Ignition scheme [5, 6] a powerful tens-ps pulse with intensity I ~ 10 W/cm generates electrons or protons, which offcenter ignite the TN fuel preliminary compressed by nanosecond pulse. Simultaneous amplification in the same large-aperture KrF amplifiers stacks of angular-multiplexed nanosecond and picosecond pulses was proposed earlier [7] which seems to be a promising way for the FI IFE. Short pulses of 1 to 20 ps duration can be focused into the same point or distributed over the compressed target [8, 9]. According to the scaling law ( ) 1 2 2 19 2 1 05μm MeV 10 W/cm / e I / . E λ   × =       [10] for ponderomotive acceleration of electrons up to energy Ee=1 MeV, which is optimal for electron coupling to the compressed target core The Sixth International Conference on Inertial Fusion Sciences and Applications IOP Publishing Journal of Physics: Conference Series 244 (2010) 032014 doi:10.1088/1742-6596/244/3/032014 c © 2010 IOP Publishing Ltd 1 [11], higher intensity I ~ 2×10 W/cm is required for KrF laser wavelength λ =248 nm. Fortunately, it is easy to achieve due to better beam focusability (~λ). In Shock Ignition scheme [12, 13] hundred-ps final spike with intensity ~ 10 16 W/cm 2 , ten times higher than the main part of the pulse, generates a convergent shock wave, which ignites compressed TN fuel in the central spot. Appropriate target design [14] and laser system analysis [15] have shown that, for a short KrF laser wavelength, target gain as high as 140 may be achieved with modest 0.5-MJ driver energy. A required SI pulse shape can be formed in a course of quasi-steady amplification of angularly multiplexed train of input pulses, which should be specially precompensated for heavily saturated amplifier during high-intensity spike [16]. Alternatively, the idea of simultaneous amplification of short & long pulses [7, 8] can be used to combine a complex SI pulse form directly on a target. On the path to its realization we have investigated amplification of subpicosecond pulses in multistage GARPUN-MTW KrF laser facility [8]. 2. Amplification of short and long pulses in KrF laser Particular features of KrF lasers operating at B→X bound-free transitions (see [8] and references therein) are short lifetime of the excited state of the molecule (radiative τr ~ 6 ns; with account for collisional quenching it is τc ~ 2 ns), large induced emission cross section σ = 2.5⋅10 -16 cm 2 , small value of saturation energy fluence Qs=hν/σ =2 mJ/cm 2 (where hν = 5 eV is the energy of radiation quantum), and significant nonsaturable loss in a gain medium (typical ratio of small-signal gain to absorption coefficient g0/α =10–20). Because of rapid recovery time of the population inversion in the gain medium (τc ~ 2 ns), each short pulse (with duration τsh≤τc) does not affect the subsequent pulse if those follow with time interval ∆t≥τc. Also short pulses will be effectively amplified after termination of a stack of long pulses (with τlong ≥ τc, typically τlong ~ 5 ns), which are amplified in a quasi-steady manner. On the other hand, during quasi-steady amplification at optimal intensity (corresponding to maximum efficiency) Iopt=I[(g0/a) -1] the energy stored in the gain medium is not completely extracted. The rest can be used for simultaneous amplification of short pulses, though with less gain g=(g0α) . This also might be useful to reduce Amplified Spontaneous Emission, which decreases a contrast of the short pulse. 3. Experimental setup Hybrid Ti:Sapphire/KrF multiterawatt laser facility GARPUN-MTW combines the previous version of e-beam-pumped multistage GARPUN KrF laser with recently constructed Ti:Sapphire front-end “START-248 M” [8]. The final large-aperture GARPUN amplifier with a gain volume of 12×18×100 cm 3 is pumped by two counter-propagating 350-keV, 60-kA (50 A/cm 2 ), 100-ns e-beams guided by magnetic field of ~0.08 T. When operating in free-running oscillation mode with Ar/Kr/F2 gas mixture at 1.4-atm pressure and with specific pumping power Wb = 0.7–0.8 MW/cm , it provides up to 100 J in 100-ns pulse. Another 8×8×110-cm BERDYSH module pumped by a single-side magnetic field-guided 350-keV, 50-kA (50 A/cm), 100-ns e-beam with Wb = 0.6–0.7 MW/cm 3 produces up to 25 J at 1.8atm gas mixture. Both amplifiers are synchronized with KrF master oscillator (Lambda Physik EMG TMSC 150 model) producing 200-mJ, 20-ns pulses, which fire laser-triggered switches of HV pulsed power supply of e-beam guns of both Berdysh and GARPUN amplifiers. Frequency tripled Ti:Sapphire front-end “START-248 M” (Avesta Project Ltd.) was designed to generate 10-Hz train of 0.5-mJ, 60-fs pulses at wavelength λ=248 nm matched with KrF gain band. The layout of experiments on amplification of short pulses is shown in figure 1 with appropriate equipment for measuring pulse energy, duration, spectral and spatial distribution. A single pulse was cut out of a continuous Ti:Sapphire front-end train and synchronized with KrF amplifiers within accuracy of ±5 ns at the top of 100-ns pumping pulse. Double-pass amplification was used for both preamplifier and final amplifier with a spatial filter placed between them. Initial 8-mm front-end beam diameter was successively expanded by convex and concave mirrors to fit amplifier apertures. The only transmissive optics was CaF2 amplifier windows inclined to prevent parasitic oscillations. The Sixth International Conference on Inertial Fusion Sciences and Applications IOP Publishing Journal of Physics: Conference Series 244 (2010) 032014 doi:10.1088/1742-6596/244/3/032014