The system described in this protocol has led to a high impact results in proton and ion beam acceleration. Future applications of this system will explore laboratory astrophysics, material science, and the next generation of particle accelerators, leading to peak powers of the order of gigawatts High repetition rate laser has created a need for continuous target. Hydrogen target, in particular, have attractive properties for laser ion acceleration, leading to energetic, pure proton beams.
Christopher Schoenwaelder, a PhD student for my laboratory, will be demonstrating in the procedure All preparation and assembly of the cryogenic source components should be performed in a clean environment, with the appropriate clean room clothing. Begin by using indirect ultrasonic cleaning to remove contaminants from the cryogenic source components. Fill a sonicator with distilled water and add a surfactant to reduce the surface tension.
Place cryogenic source parts in individual glass beakers, fully submerge them in electronics grade isopropanol, and loosely cover the beakers with aluminum foil to reduce evaporation and prevent contamination. Place the beakers in the cleaning basket or a beaker stand in the sonicator, making sure that they do not touch the bottom of the sonicator. Activate the sonicator for 60 minutes, then inspect the isopropanol using a bright white light for suspended particles or residue.
If particles are visible, rinse the parts with clean isopropanol and replace the isopropanol bath. Sonicate in cycles of 60 minutes until no particles or residue are visible. When finished, place the parts on a covered, clean surface to desiccate for a minimum of 30 minutes before assembly.
Repeat the cleaning process for the stainless steel filter, source cap, ferrule, and assembly screws. Cut a piece of indium to maximally cover the junction between the cryogenic source body and the cold finger of the cryostat. Then, place it on the cryogenic source and hold it flush with the cold finger of the cryostat.
Tighten the retaining screws, ensuring the indium remains flat to establish a thermal seal between the components. Do not overtighten, as the copper threads are easily damaged. Screw the threaded stainless steel filter onto the cryogenic source flange.
Place an indium gasket on the source flange and attach the source flange to the cryogenic source body using the flange screws. Then, tighten the screws diagonally instead of sequentially around the circumference. Place the ferrule inside the cap.
Then, use clean tweezers to place the aperture inside the ferrule. Tap the cap to center the aperture in the ferrule. Drop an indium ring on top of the aperture.
Then, tap the edge of the cap again to center the indium ring on the aperture. Hand-tighten the cap onto the source flange until minimal resistance is detected. Derestrict the flow rate on the mass flow controller by increasing the set point to 500 standard cubic centimeters per minute and setting the gas pressure to approximately 50 PSI gauge on the pressure regulator.
Use a wrench to tighten the aperture a few degrees at a time until the flow rate begins to decrease. Check the leak rate at the top of the cap with the high sensitivity leak detector and stop tightening when the measured leak rate stops decreasing. Disable the temperature ramp and change the set point temperature to well below the theoretical vapor liquid phase transition temperature.
At the onset of liquefaction, increase the helium flow for additional cooling power to quickly pass through the phase transition. As the reservoir becomes fully liquified, a mixture of gas and liquid will initially spray from the aperture and sequentially transition into a continuous jet. Use high magnification shadowgraphy with pulsed sub nanosecond delumination to visualize the jet stability and laminarity.
If the experiment has a predetermined location for the sample, translate the cryogenic source using a multi-axis manipulator on the cryostat flange or motorized push pin actuators in the vacuum chamber. Optimize the PID parameters and helium flow to ensure that the temperature stability is better than plus or minus 0.02 Kelvin. Note that the overall stability of the jet strongly depends on the vacuum chamber pressure, gas backing pressure, and temperature.
High magnification shadowgraphs are used to assess laminarity, positioning jitter, and long-term stability during jet operation. It is critical to use pulsed sub nanosecond illumination to record an instantaneous image of the jet so that the jet motion does not blur surface irregularities or turbulence. A study of the spatial jitter of a 2 by 20 micrometer hydrogen jet as a function of distance from the aperture was performed during a single test over several hours.
The positioning jitter for each data point was calculated from 49 images recorded at 10 hertz. The jet position was determined relative to a fixed reference position. The normalized histograms of the jet position at 23 millimeters are shown as an example.
Typical system observables during liquefying and jet operation of a 4 by 20 micrometer cryogenic deuterium jet are shown here. Careful monitoring of the temperature, flow, sample backing pressure, and vacuum pressures allows the operator to quickly identify any irregularities and react accordingly. Typical operation parameters are summarized here.
Once the optimal parameters are identified for a given gas and aperture type, the resulting jet is highly reproducible. Any deviations in the aperture require reoptimization starting from the previously identified values. When following this protocol, is important to notice that sudden changes in pressure or temperature can rapidly vaporize the liquid hydrogen in the reservoir.
To prevent vacuum system failure, the operator should restrict the gas flow and isolate sensitive hardware. The system can also be extended to other aperture geometries and sample gases. Among other things, heavier elements can be used to study fundamental plasma processes such as ion acoustic waves or the transition to one phase of matter, a state that is expected to exist in the interior of gas giants and exoplanets.
High brightness ion beams from this target system are used to generate a directional high flux neutron beam. This technique has paved the way for non-destructive inspection of materials and may contribute to studies of nucleosynthesis in the laboratory.
