Target development for relativistic laser-plasma generation


Microscopic liquid jets are very promising candidates for novel studies on intense laser-driven plasma generation. The interaction of ultrashort laser pulses with solid targets allows producing extreme conditions that are relevant to tabletop particle accelerators and laboratory astrophysics. Here, the laser energy is initially transferred to the target via the generation of relativistic electrons. However, the usually large dimensions of the employed targets, typically flat thin foils of mm2 to cm2size, allow the hot electrons to spread transversely leading to a significant reduction of the energy density in the target. This precludes the efficient heating of the target material and thus has immediate consequences for fundamental applications such as ion acceleration. Recent efforts have provided evidence for very efficient bulk heating with the use of nearly mass-limited targets whose transverse dimensions are comparable to the laser focus such as microspheres  or microdots, but the invariable presence of target holders still leads to a significant spreading of the electrons and hence to a rarefaction of the energy density. The use of levitated spherical targets in a Pauli trap has been demonstrate, but these experiments still suffer from the major drawback that the employed target must be replaced after each laser shot. This, in turn, greatly precludes detailed parametric studies requiring the collection of a large amount of data, and the use of laser-driven ion sources operating in the (quasi-) continuous mode that is mandatory for most potential applications such as ion-based cancer therapy. Rayleigh droplet beams are extremely attractive with respect to the above applications because, when the Rayleigh instability is induced by an intentionally applied excitation, the “triggered” breakup process delivers a perfectly periodic stream of identical, isolated droplets at a production rate of up to 1 MHz, thereby enabling detailed scaling studies under highly reproducible conditions by employing intense laser pulses in a wide range of repetition rates.

Experiments employing liquid water jets have clearly demonstrated the advantages of Rayleigh droplet beams for relativistic laser-plasma generation, yet for many potential applications hydrogen and nobel gases represent the most scientifically relevant target systems. For example, it has been shown numerically that the use of a pure hydrogen target characterized by a higher plasma density would significantly increase the efficiency of the proton acceleration process, which would be further enhanced by the small droplet size. Hydrogen is also of central importance as model system for studies of the equation of state under high-density plasma conditions that are expected in the interior of giant planets such as Jupiter. Liquid droplets of rare gases such as argon, on the other hand, are ideally suited for K-shell X-ray spectroscopy studies of the heating mechanisms of the bulk target material by providing direct access to the energy distribution and relaxation of the hot electron population.

Whereas a variety of microscopic Rayleigh droplet beams, which include water, metallic and various organic solvent liquid jets, have been routinely produced in the laboratory for a decade, the stable generation of periodic droplet beams of cryogenic elements such as hydrogen and argon proves challenging. The high vapor pressure at the triple point of liquid argon and hydrogen results in very efficient evaporative cooling upon vacuum expansion. The expanding liquid filament thus rapidly cools below its normal melting point and freezes well before Rayleigh breakup can take place. The jet freezing can be circumvented by expanding the liquid into an atmosphere of the respective gas before injecting the resulting droplet stream into vacuum, a scheme that has enabled the production of hydrogen droplet beams for applications in nuclear physics research in a storage ring. However, the droplet sources employed in these studies are characterized by extended dimensions and a substantial loss of the spatial synchronization of the triggered droplet beam on the vacuum side, crucial features that preclude their use in experiments in which pulses from a high-power laser are focused to a micro-scale spot. In our group we have developed a novel concept for an injection source that addresses the above drawbacks delivering stable, periodic droplet beams of the cryogenic gases hydrogen and argon ideally suited for novel studies on relativistic laser-plasma generation. At the heart of our droplet injector is the use of a glass capillary that is inserted into an outer glass capillary tube. As the liquid jet emerges from the inner capillary it expands in an axially co-flowing gas plenum that suppresses evaporative cooling. The use of a co-flowing gas sheath has been demonstrated by Ganan-Calvo as a method of generating columnar liquid jets of much reduced diameter as a results of gas dynamic forces, and this approach has been adapted recently to liquid water jets as a means to deliver proteins into vacuum for serial crystallography. A novel and important aspect of the source described here is the additional challenge of working at cryogenic temperatures. In our design the typical distance between the inner capillary orifice and the outer tube exit hole is adjusted down to 1 mm.  This feature thus allows significantly reducing the interaction time of the droplets with the co-flowing gas, which is held responsible for the degradation of the droplet spatial stability observed in previous studies. The compact size of our source would greatly facilitate the droplet beam operation in a vacuum environment characterized by the presence of many delicate optical components as typically encountered in laser-plasma generation experiments. The Fig. shows periodic beams of monodisperse argon and hydrogen droplets of diameter of 21 mm and 13 mm, respectively, jetting from the outer tube exit hole into vacuum. Beam imaging is obtained by using a 4M pixels CCD camera and a long-distance microscope. The function generator driving the piezoelectric actuator also triggers a 10 Hz Nd-YAG laser emitting 10 ns pulses for stroboscopic backside illumination, as verified by observation of droplets that appear stationary at the piezo driving frequency.

In order to test our droplet injector, especially regarding the synchronization between the droplets and the laser pulses, we performed a proof-of-principle experiment at the PHELIX laser facility by employing the argon droplet beam as target. A preliminary analysis of the experiments (to be published) evidences a spherical plasma that remains close to the initial droplet size during the thermal emission stage, thus supporting the conclusion that the droplets are heated isochorically to high-density plasma conditions. In addition, the estimated conversion efficiency of the laser energy into K-a radiation of 105 is in good agreement with previous experiments employing nearly mass-limited targets, indicating indeed an efficient coupling to the relativistic electrons.

These preliminary results clearly demonstrate the potential of our novel cryogenic droplet beam injector as a means to deliver ideal mass-limited target samples in vacuum. In particular, the compact design combined with a high spatial droplet stability makes the injection system described here ideally suited for relativistic laser-plasma applications in which a precise control on the overlap between the laser beam focus and the droplets is mandatory. Our droplet injector thus opens up new possibilities for tabletop proton accelerators and studies of matter under extreme conditions relevant to astrophysical phenomena.