Within all the developments achieved so far on satellite facilities, short-wavelength sources cover a wide range of photon energy, from typically 30 eV to tens of keV, with a total emitted energy per pulse from a few nanojoules to a few millijoules and pulse durations ranging from a few tens of attoseconds to 10 nanoseconds.
The proposed activities using the APOLLON-10P laser facility will have the clear mission to fully characterize the sources and extend their limit, especially in term of photon energy, duration and total energy. We will also produce a reliable beam of X-rays intrinsically synchronized with the laser beam, for access to users.
Indeed, an essential goal of this research topic is to find ways to generate very intense light pulses at short wavelengths (ideally down to the x-ray range), with ultra-short durations down to the attosecond range. The major interest of such short wavelength light pulses is to considerably extend (i) the range of experimental techniques that can be exploited in a pump-probe scheme with the opportunity to perform experiments (e.g. single shot x-ray diffraction, multiphotonic processes in the XUV,…), using different kinds of pumps (laser, ions, electrons), and (ii) the range of dynamical processes that become accessible experimentally (e.g. the attosecond dynamics of electrons in matter).
However, this research subject does not only aim at producing new light sources for the benefit of other research fields. It also aims at gaining a fundamental understanding of light-matter interaction in extreme regimes of intensity and duration, the produced radiation being itself an extremely valuable probe of the dynamics of the system.
Three main types of objectives can be identified for this research subject:
- Fundamental understanding: studying the basic physical processes involved in high-intensity laser-matter interaction and leading to the generation of short-wavelength radiation.
- Source properties: studying the properties of the light sources obtained by these processes.
- Applications: using the light sources obtained in this way for novel and original application experiments.
We are going to study and push at their limits different laser induced x-ray sources, a domain where our community already has very strong skills:
Flying mirrors
Laser-accelerated particles can produce relativistic “flying-mirrors”, which can be used to frequency-up-shift in the X-ray domain and temporally compress intense laser pulses to attosecond domains [19]. When a relativistic short pulse propagates in underdense plasmas, it can generate a wake. Near the wave-breaking threshold, the electron density in this wake exhibits sharp spikes. These spikes move at a phase velocity equal to the group velocity of the driver pulse, which is close to the velocity of light in vacuum. If one sends a second beam in the opposite direction, it will be reflected back from what is known as the relativistic flying mirror, which is nothing but a double Doppler shift. The reflected pulse will present a much higher frequency (4g2) and a much shorter duration.
This experiment will also be done using a thin solid target which will generate, after focusing high contrast pulse on the surface, a sheath of free electrons which will be used as relativistic reflector for the second counter propagative beam whose frequency will be 4g2 up-shifted through the Doppler effect.
Harmonics from plasma-mirror
Another very promising approach to produce ultrashort coherent XUV or X-ray light pulses consists in using reflection of high intensity laser light on a sharp surface. Upon reflection, the non-linear response of this so-called “plasma-mirror” induces the generation of high-order harmonics of the incident frequency in the reflected light [20], associated in the time domain to trains of intense attosecond pulses [21].
The measured conversion efficiencies are extremely significant (10-4) it would be feasible to use this extreme-UV radiation for time-resolved and imaging experiments in chemistry and biology and also to extend the scope of multi photonic processes in the XUV region.
At higher intensities, the XUV pulse duration is predicted to decrease significantly, giving access to better time resolution in the attosecond domain.
Betatron radiation
In underdense plasmas, under appropriate conditions, electrons accelerated to relativistic energies undergo transverse oscillations due to the presence of a strong periodic transverse electric field. This so-called “betatron-radiation” leads to synchrotron-like collimated x-ray emission [22,23], with a « white » high-energy spectrum. A detailed understanding of the characteristics of betatron–radiation is also useful for electron acceleration as a diagnostic and potential means to improve beam quality. At the high powers accessible on APOLLON, the energy of the electrons and the distance over which they will oscillate can be greatly enhanced, thus producing harder x-rays at higher intensities.
Non-linear Thomson scattering & Compton scattering
Classical Thomson scattering is the scattering of light by electrons. It is a linear process such that the process does not change the frequency of the radiation, except by the Doppler effect on a moving electron, or the magnetic field component of the photon. However, for light intensity of at least 1018 W/cm2, the electrons in the electromagnetic field of the laser will oscillate at velocities close to the velocity of light during the scattering process. In this relativistic regime, the electrons are predicted to quiver nonlinearly, moving in figure-of-eight patterns rather than in straight lines. Scattered photons are therefore emitted at, Doppler-shifted, harmonics of the frequency of the incident light [24], with each harmonic having a unique angular distribution. Ultrahigh-peak-power lasers offer a means of creating the relativistic intensities required to study Thomson scattering in a very nonlinear regime [25].
Xray lasers
Photo-pumped soft X-ray lasers, not demonstrated until now, require an ultra-short and ultra-intense flash of x-rays that will become available with the future sources described above. This new type of pumping will allow extending soft-x-ray lasers to the keV, femtosecond domain [26].
Multi-stage, seeded soft x-ray lasers will be implemented. This will allow gaining several orders of magnitude in the output energy (typically from µJ to mJ), while shorter pulse duration will be obtained through seeding by a coherent femtosecond XUV pulse [27].
