Single-shot electrons and protons time-resolved detection from high-intensity laser–solid matter interactions at SPARC_LAB Download: 801次
1 Introduction
The interaction between solid-state matter and very intense lasers in the relativistic regime (
During this process, beams in the multi-MeV range[2–4], tightly confined in time (ps scale) and space (few
At SPARC_LAB[11], the high-power laser FLAME[12] is currently employed in pump-and-probe experiments, in which it is made to interact with solid-state targets. The main purpose of this is to carry out temporal characterization of the charged particles emitted during the interaction. For this reason two main diagnostic lines have been set up in the target area: an electro-optic sampling (EOS)[13, 14] line to measure, with sub-picosecond resolution, the temporal distribution of fast electrons leaving the target, and a time-of-flight (TOF) detector to measure the longitudinal profile of the emitted protons/ions, and their energy spectrum and charge after the acceleration process[15–19]. In this work, preliminary results obtained with these two online temporal diagnostics will be presented.
2 Experimental setup
Fig. 1. Experimental setup. The FLAME laser is sent to a stainless steel target. The charged particles emitted during this interaction are revealed by two single-shot time-resolved measurements: an electro-optical sampling diagnostic, able to measure the electric field carried by relativistic fast electrons, and a time-of-flight diamond detector, used to measure the temporal distribution of protons arriving on it and retrieve their energy spectra.
The high-power laser FLAME[12] has been employed for this experiment. It consists of a CPA chain[1] delivering more than 6 J at the final cryo-amplifier output at a 10 Hz repetition rate. After optical compression down to 25 fs, the laser beam is focused by means of a
The experimental setup is shown in Figure
After the fast electrons, protons are also emitted in the MeV energy range, thanks to the extremely high (
Fig. 2. Time-of-flight detector geometry. Schematic representation of the device layer structure (left) and picture of the surface Al interdigitated electrodes (right). The metal fingers were processed to $20~\unicode[STIX]{x03BC}\text{m}$ in width, with a spacing between the electrodes of $20~\unicode[STIX]{x03BC}\text{m}$ . The detector active area was approximately $2~\text{mm}^{2}$ .
3 Experimental results
The EOS diagnostic has been employed to study the fast electrons emitted during the interaction. In particular, from the longitudinal profile of the electric field carried by fast electrons, their temporal charge distribution has been retrieved, within a 8 ps temporal window with
Fig. 3. Typical 2D electric field carried by fast electrons as seen by our EOS diagnostic tool. The signal thickness is related to the temporal duration of the electric field. The typical shape is a direct consequence of our setup geometry[27].
Fig. 4. Line profile traced along the signal in Figure 3 . The measured electric field shows a peak value $E_{0}=1.5~\text{MV}/\text{m}$ and a temporal duration $\unicode[STIX]{x1D70F}=0.5$ ps FWHM. The fast electron charge also has been retrieved with a value of $Q=6$ nC.
Simultaneously, the proton energy spectra have also been recorded, thanks to the sub-ns resolution TOF diamond detector installed in our setup. Placed 1 m behind the target, with respect to the incoming laser beam, and along the same laser direction, it can provide temporal measurements with 800 ps resolution, thanks to the superficial interdigital structure shown in Figure
Fig. 5. Typical signal provided by the TOF detector as seen by our 2 GHz Lecroy 620ZI oscilloscope. As one can see, it is possible to distinguish between two different signals arriving at different times: the first is associated with X-rays and low-energy electrons coming at the early stage of the interaction; the second is related to protons accelerated through the TNSA mechanism.
Fig. 6. Monte Carlo simulations by the SRIM code results for $10~\unicode[STIX]{x03BC}\text{m}$ of aluminium.
The TOF detector was covered by a 10-
Fig. 7. Proton energy spectra retrieved from the data in Figure 5 with (black line) and without (red line) taking into account the aluminium filter. Tolerances present in the black curve are only due to the energy spread of the incoming proton beam caused by crossing the foil.
4 Conclusions
In this work, we have shown typical experimental results provided by our temporally resolved diagnostics, detecting, simultaneously, both fast electrons and protons generated during FLAME laser–solid matter interactions. In this case, the target was made from 10-
F. Bisesto, M. Galletti, M. P. Anania, M. Ferrario, R. Pompili, M. Botton, A. Zigler, F. Consoli, M. Salvadori, P. Andreoli, C. Verona. Single-shot electrons and protons time-resolved detection from high-intensity laser–solid matter interactions at SPARC_LAB[J]. High Power Laser Science and Engineering, 2019, 7(3): 03000e53.