A proposal for multi-tens of GW fully coherent femtosecond soft X-ray lasers

Abstract

X-ray free-electron lasers 1,2 delivering up to 1 X 10 13 coherent photons in femtosecond pulses are bringing about a revolution in X-ray science 3 ' 5 . However, some plasma-based soft X-ray lasers 6 are attractive because they spontaneously emit an even higher number of photons (1 X 10 15 ), but these are emitted in incoherent and long (hundreds of picoseconds) pulses 7 as a consequence of the amplification of stochastic incoherent self-emission. Previous experimental attempts to seed such amplifiers with coherent femtosecond soft X-rays resulted in as yet unexplained weak amplification of the seed and strong amplification of incoherent spontaneous emission 8 . Using a time-dependent Maxwell-Bloch model describing the amplification of both coherent and incoherent soft X-rays in plasma, we explain the observed inefficiency and propose a new amplification scheme based on the seeding of stretched high harmonics using a transposition of chirped pulse amplification to soft X-rays. This scheme is able to deliver 5 X 10 14 fully coherent soft X-ray photons in 200 fs pulses and with a peak power of 20 GW.Over the past 10 years, the emergence of hard (A ~ 0.1 nm) and soft (A ~ 4-30 nm) X-ray free-electron lasers (FEL) with intensities rapidly increasing to 1 x 10 W cm has led to scientific breakthroughs in a diverse number of fields 3 " 5 . This race has led to renewed interest in a specific class of soft X-ray lasers (XRLs) that use a plasma amplifier created by the interaction of a nanosecond, high-energy laser with a solid target. These XRLs routinely produce an extreme number of photons per pulse 7 , up to 1 x 10 15 (that is, 10 mj), which compares favourably with soft X-ray FELs, which emit a maximum of 1 x 10 13 photons per pulse 1 ' 2 . However, because these plasma-based XRLs are running in amplification of spontaneous emission (ASE) mode, they demonstrate weak coherence and long pulse durations (100 ps). Seeding these plasmas, which naturally emit up to 10 mj, holds the greatest promise of producing fully coherent, femtosecond, multi-milljoule soft X-ray pulses. Such an experiment was carried out in 1995 8 , but this showed an as yet unexplained weak amplification of the seed (output, 100 nj) and strong amplification of the ASE (reaching several millijoules). Plasmas created by femtosecond 9 to picosecond 10 infrared lasers have been seeded, but continue to be limited by the generation of long (picosecond) and low-energy (1 |xj) pulses. Numerical studies have shown that these schemes could amplify the seed to generate pulses of up to 70 fs (ref 11), but with energy restricted to <40 |xj (ref. 12). It is only an indepth study of Ditmire's seminal experiment 8 that holds the key to unlocking the path towards millijoule, femtosecond soft X-ray lasers.The modelling parameters of our Maxwell-Bloch model 13 were adjusted on the basis of experiments 8 with a 200 fs, 0.5 nj, 21.2 nm seed, and plasma with an electron density of 4 x 10 20 cm 3 and temperature of 550 eV. In the absence of seed, gain peaks at 25 cm 1 were...

X-ray Chirped Pulse Amplification: towards GW Soft X-ray Lasers

“…The full architecture is displayed in Figure 7 showing the seed HHG, a pre-amplifier followed by a soft X-ray stretcher (grating pair + telescope) the main plasma and the soft X-ray compressor (grating pair). Compressor efficiency as high as 50% is realistic if using a single-pass compressor and off-axis gratings [12,29]. Figure 7.…”

“…Using a Maxwell-Bloch time-dependant code, we have previously shown [12] that the key to extracting the energy from long pulse X-ray lasers is to stretch the seed, to accommodating the fast gain recovery time in higher density plasmas. Noticing that a high level of seed was necessary, we anticipated that plasma pre-amplifier, producing a µJ-level pulse with a bandwidth matched to the long pulse amplifier, should yield the highest final energy at the shortest pulse duration (>10 mJ, sub 200 fs at 21 nm).…”

Multi-tens of GW peak power plasma-based soft x-ray laser

“…By including all these effects, we calculated the temporal pulse shape after compression (figure6.B) and observe a main pulse of 215 fs containing 97% of the energy (5 mJ) followed by a weak pulse. 21 Finally, since bandwidth of a single HOH is estimated to be about 100 times larger than line width of XRL, 1% of the energy is seeded into XRL line at the plasma entrance. Considering a minimum seeding level of 0.1 µJ in-band, HOH energy has to be about 10 µJ, orders of magnitude above the most energetic HOH emitted around 20 nm.…”

Influence of non-maxwellian electron distribution in low-Z elements iluminated by a high intensity x-ray lasers

“…Our interest is to study the influence of this assumption under conditions when the electron distribution is far from equilibrium when modeling laser-matter interaction in the x-ray regime, especially the free electron (XFEL) 4,5 and plasma based high intensity x-ray lasers. 6,7 In this regime materials are iluminated by a monochromatic x-ray source with photon energies up to several keV, able to photoionize inner shells. These photoionized electrons may have high energies (depending on the ratio between photon energy and the ionization potential of the ionized shell) and, if inner shells are ionized, may produce auger electrons with energies in the order of hundreds of eV.…”