Target Development

Our cryogenic liquid target system delivers a high-velocity, continuous, micron-sized liquid jet, allowing high-rate operation with rapid target insertion (up to kHz), alignment, and minimal production of debris. It can produce various target geometries, from cylindrical (5 - 10μm), spherical droplet (10 - 20μm) to planar sheet (0.1-7 μm x 10-40 μm) and can produce pure samples from a variety of low-Z gases such as hydrogen, deuterium, and higher-Z gases like neon, argon and krypton.

At LCLS, cryogenic jets have been used with moderate laser intensities at the MEC and CXI end stations, and PW class laser facilities in the US and around the world, including the Texas Petawatt Laser facility at UT Austin, the DRACO laser at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the PHELIX laser at the GSI Helmholtz-Institute in Darmstadt, Germany. These experiments allow us to study particle acceleration mechanisms for intensities above 10²¹ W/cm².

Frequently, the jet hardware is damaged by the high intensity laser-target interactions, either instantaneously or over the course of a beamtime. To address this, the interaction point can be moved farther away from the aperture, but this decreases target pointing stability and hence laser-target overlap. Instead, our collaborators at HZDR have developed a mechanical chopper which blocks line-of sight between the interaction zone and the nozzle when the laser pulse hit the target, while not affecting the jet in the interaction region.

The design, development and testing of the cryogenic liquid jet platform, shown to the right, is performed at our in-house Sector 10 laser laboratory. Using our  1 kHz laser system we can study any effects of high repetition rate experiments on the jet and chopper performance. Since the stability of the cryogenic jet system is heavily influenced by the quality of the vacuum, a jet-catcher system has been developed to remove most of the liquid shortly after injection. Testing in our  laboratory has shown that this design improves the vacuum chamber pressure during operation by an order of magnitude. 

The image to the right shows the chopper, jet and catcher in operation. This involves the aperture, the rotating blade chopper, and the catcher system at the bottom. The momentum transfer from the blade causes a small curl at the edge of the jet resulting in a deflection of the target within the first 0.5mm below the cutting point.  This gap is directly determined by the speed of the jet which typically decreases for jets with higher atomic charge number, such as argon and krypton. In all cases, the target is unaffected further down the jet, at the laser interaction point, allowing continuous operation.