Description
Liquid targets are well-suited for high-repetition-rate laser experiments due to their unique combination of fast renewability and excellent shot-to-shot reproducibility.
We present a compact deionized (DI) water target system and a proof-of-principle demonstration of ion acceleration and X-ray emission up to 1 kHz repetition rate [1].
The target is based on a high-pressure chromatography pump that delivers a continuous liquid flow into a vacuum chamber through a capillary, forming a stable microjet. Passing the interaction point, the liquid is collected by a specially designed catcher system equipped with temperature control to prevent freezing and to drain the water from the chamber for recycling. In combination with a liquid-nitrogen cold trap, this setup enables operation under high-vacuum conditions. When supplied with DI water, the system is debris-free and provides precise dimensional stability and positional reproducibility. Plasma generation was driven by a commercial ultrafast laser delivering ~45 fs (FWHM), ~6 mJ pulses at 1 kHz. Using a high-quality off-axis parabolic mirror, the on-target intensity reached 10¹⁷ W/cm². The accelerated ion beam was characterized using time-of-flight diamond and SiC solid-state detectors, revealing a cutoff energy of up to 170 keV with good shot-to-shot stability. The emitted X-ray spectrum was measured with a silicon-drift detector in the 2–40 keV range.
Within the class of liquid targets, cryogenic systems offer a continuous, debris- and contaminant-free ion source, which is essential for secondary beam quality and protecting sensitive optical components [2]. Their high material purity also makes them well-suited for numerical simulations.
The cryogenic target system developed at the ELI Beamlines Cryogenic Laboratory is designed for kHz laser operation. It is based on a closed-cycle, helium-free cryocooler coupled to a custom target cell capable of maintaining temperatures as low as 4 K. A cold head, located inside a helium exchange gas volume, ensures efficient, low-vibration thermal coupling to a cold plate supporting a brass or copper condensation cell. The cell is equipped with an exchangeable nozzle featuring apertures ranging from 5 to 25 μm, which defines the liquid jet diameter and enables the long-term, stable extrusion of high-purity cryogenic flows. Cryogenic jets of nitrogen, argon, and krypton have been successfully tested. For example, a stable 10 μm N₂ jet with adjustable velocity (5–20 m/s) was achieved at 70 K and 0.5–7 bar, while maintaining a chamber pressure of 10⁻⁴–10⁻³ mbar suitable for high-repetition-rate laser-driven ion acceleration. Ongoing work focuses on expanding the range of liquefied gases [3] and reducing spatial jet fluctuations.
[1] N. Gamaiunova, M. Tryus, F. Grepl, A. Velyhan, S. Stancek, V. Kantarelou, G.A.P. Cirrone, D. Margarone, L. Giuffrida, T. Chagovets, "Liquid jet target system for laser-plasma interactions at kHz repetition rate," Proc. SPIE 12579, Laser Acceleration of Electrons, Protons, and Ions VII, 1257908 (2023)
[2] T. Chagovets, J. Viswanathan, M. Tryus, F. Grepl, A. Velyhan, S. Stancek, L. Giuffrida, F. Schillaci, J. Cupal, L. Koubikova, D. Garcia, J. Manzagol, P. Bonnay, F. Souris, D. Chatain, A. Girard, D. Margarone, “Cryogenic Hydrogen Ribbon for Laser Driven Proton Acceleration at Hz-Level Repetition Rate”, Front. Phys., Sec. Interdisciplinary Physics, V.9 - 2021
[3] Timofej Chagovets; Hydrogen targetry in laser-plasma physics. Low Temp. Phys. 1 August 2022; 48 (8): 645–650.
