P. Velarde

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

X-ray free-electron lasers1, 2 delivering up to 1 × 1013 coherent photons in femtosecond pulses are bringing about a revolution in X-ray science3, 4, 5. However, some plasma-based soft X-ray lasers6 are attractive because they spontaneously emit an even higher number of photons (1 × 1015), but these are emitted in incoherent and long (hundreds of picoseconds) pulses7 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 emission8. 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 × 1014 fully coherent soft X-ray photons in 200 fs pulses and with a peak power of 20 GW.

Atomic and Plasma Physics Software and Databases for the Simulation of Short Wavelength Sources

A large variety of software has been developed for numerical simulations of plasma radiation emission and transport and the design of optics. Many of these codes are not specifically for short wavelengths but they can be helpful in the design of X-ray and extreme ultraviolet (XUV) sources. The astrophysics community has written codes to study the spectra and evolution of stars and gas clouds while civilian and military applicants of atomic processes were aware very early of the need for atomic, plasma and hydrodynamic software to simulate nuclear fission and fusion and their effects. Of more relevance to low cost EUV sources, the large synchrotron community has also generated efficient tools for the simulation of radiation from electrons and the design of short wavelength optics. The present intention is not to present all the available software, since resources such as Computer Physics Communications and Plasma Gate give much more comprehensive, if never complete, reviews. Instead, some of the more popular programs will be introduced, as they are proven and tested assistance is available from experienced users. The software may be downloaded from the web sites listed in the references; registration is sometimes required.

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

Ultra-intense X-ray sources have opened new avenues by creating new states of matter or probing and imaging living or inert matter. Free-electron lasers have a strong leadership by delivering pulses combining femtosecond duration and 10s of microJoules to milliJoule energy. However, these sources remain highly expensive limiting their number to a few worldwide. In parallel, laser-pumped soft X-ray lasers hold outstanding promises having demonstrated the most energetic monochromatic soft x-ray pulse and being intrinsically fully synchronized with any secondary source of the pump laser. Since the first successful demonstration of amplification of a high harmonic pulse in a plasma from gas in 2003 and from solid in 2008, we have developed an extensive numerical study. 2D hydrodynamic simulations showed that optimized Transient Collisional Excitation plasma amplifiers, may store up to 0.4 mJ in the population inversion. If carefully seeded, pulses of 80 fs and 20 μJ might be generated with table-top lasers (10J). As the energy extracted is far from the milliJoule requirements of most exciting applications, we studied the seminal experiment of Ditmire et al who seeded a plasma emitting milliJoules in the form of Amplified Spontaneous Emission (ASE).We retrieved and explained for the first time the experimental result (ASE 1,000 times stronger than amplified seed). We thus proposed and fully modeled the transposition of the so-called Chirped Pulse Amplification (CPA) in the soft X-ray range, showing that 6 mJ, 200 fs, fully coherent soft X-ray pulse is accessible with compact pump lasers.

Architecture and Bloch-Maxwell modelling of multi-mJ 100 fs fully-coherent soft X-ray laser based on X-ray CPA

By seeding amplifying plasmas pumped with the so-called Transient collisionnal excitation scheme, the amplified pulse seems to be limited to an energy of several 10s of μJ. Aiming to attain several mJ, we study the seeding of plasma pumped by long laser pulse. Thanks to our time-dependent Maxwell-Bloch code, we demonstrate that direct seeding with femtosecond pulse is inefficient. We also study the amplification of pulse train with the drawback of re-synchronizing the pulses. We proposed and studied the amplification of high harmonic seed stretched by a grating pair, amplified finally compressed. We consider off-axis diffraction on the gratings for maximizing their efficiency. Considering the phase deformation induced by the amplification and the spectral narrowing the final pulse is 230 fs in duration and 5 mJ.

Bloch-Maxwell modelling of multi-mJ 100 fs fully coherent amplified high harmonic pulse

By seeding amplifying plasmas pumped with the so-called Transient collisionnal excitation scheme, the amplified pulse seems to be limited to an energy of several 10s of μJ. Aiming to attain several mJ, we study the seeding of plasma pumped by long laser pulse. Thanks to our time-dependant Maxwell-Bloch code, we demonstrate that direct seeding with femtosecond pulse is inefficient. We also study the amplification of pulse train with the drawback of re-synchronizing the pulses. We proposed and studied the amplification of high harmonic seed stretched by a grating pair, amplified finally compressed. We consider off-axis diffraction on the gratings for maximizing their efficiency. Considering the phase deformation induced by the amplification and the spectral narrowing the final pulse is 230 fs in duration and 5 mJ.

Optimization of soft x‐ray amplifiers by controlling plasma hydrodynamics

Several coherent soft x‐ray lasers are available for applications nowadays. Among them, plasma‐based soft x‐ray lasers promise to generate high‐energy, highly coherent, short pulse beam. Solid target based amplifiers, due to the fact that his density is higher, should store a higher amount of energy. However, to‐date output energy from seeded solid amplifiers remains as low as 60 nJ. We demonstrated that the extraction of the energy stored in the plasma is enhanced by carefully tailoring the plasma shape, to inhibit deleterious hydrodynamical effects. With 1 mm wide plasma, energy as high as 22 μJ in sub‐ps pulse is achievable. Not only the energy extracted is higher in these tailored plasmas but also gain and pumping efficiency are increased by nearly a factor of ten as compared to the narrowest plasma amplifiers studied previously and here.

Optimized soft x-ray amplifier by tailoring plasma hydrodynamics

Plasma-based seeded soft x-ray lasers have the potential to generate high-energy, highly coherent, short pulse beam. Due to their high density, plasmas created by interaction of intense laser with solid target should store the highest amount of energy density among all plasma amplifiers. However, to-date output energy from seeded solid amplifiers remains as low as 60 nJ. We demonstrated that careful tailoring of the plasma shape is crucial for extracting energy stored in the plasma. With 1 mm wide plasma, energy as high as 22 μJ in sub-ps pulse is achievable. With such tailored plasma, gain and pumping efficiency has been increased by nearly a factor of 10 as compared to the narrower plasma amplifiers studied previously and here.

Soft X-ray laser of second generation

This paper explores the second generation soft X-ray laser both numerically and experimentally. Numerical studies have been achieved by successively using two numerical codes. The first code (hydrodynamic code) describes the spatio-temporal evolution of the plasma while the second code (ray-trace) models the amplification and propagation of the seed along the plasma amplifier. Preliminary numerical study tends to show that mJ level, fully coherent, soft X-ray laser with 100 fs in duration is achievable for wavelengths ranging from 30 to 4 nm, and with a pump laser of several tens of joules as the driver. Moreover, experimental studies have been performed using a 2 J, 20 fs Ti:sapphire laser operating at 10 Hz. About 100 mJ has been used to generate the seed i.e. high harmonics of the fundamental laser. Amplification as high as 1000 have been observed leading to 3 /spl mu/J, 500 fs, highly coherent, polarised soft X-ray laser.