Cryogenic Liquid Jets for High Repetition Rate Discovery Science

The following protocol details the assembly and operation of a 5 µm diameter cylindrical cryogenic hydrogen jet operated at 17 K, 60 psig as an example case. An extension of this platform to other aperture types and gases requires operation at different pressures and temperatures. As a reference, working parameters for other jets are listed in Table 1. Sections 1-3 and section 7 are performed at ambient temperature and pressure, while sections 4-6 are performed at high vacuum.

1. Installation of the cryostat in the vacuum chamber

CAUTION: A vacuum vessel can be hazardous to personnel and equipment from collapse, rupture due to back-fill pressurization, or implosion due to vacuum window failure. Pressure relief valves and burst disks must be installed on vacuum vessels within a cryogenic system to prevent over-pressurization.

1. Carefully insert the cryostat into the vacuum chamber. Vibrationally isolate the cryostat from the vacuum chamber using a stabilization platform.
2. Perform a vacuum test to determine the baseline vacuum pressure which, we have found, must be better than ~5 x 10^-5 mBar. A residual gas analyzer (RGA) is often helpful to identify moisture and contaminant gases present in the system.
3. Connect the temperature controller and heater to the cryostat and confirm an accurate reading at ambient temperature.
   1. If an unexpected value is measured, verify continuity from the temperature sensor to the correct terminals on the temperature controller. Otherwise, replace the temperature sensor.
4. Connect the helium return line(s) to an adjustable flow meter panel.
5. Evacuate the insulating vacuum shroud on the transfer line to better than 1 x 10^-2 mBar using a turbomolecular pump backed by a dry scroll pump.
6. Apply a thin layer of cryogenic vacuum grease to the O-ring inside the head of the cryostat.
7. Slowly insert the transfer line refrigerator bayonet into the cryostat until the adjustment screw contacts the cryostat head. There should be minimal resistance. Tighten the adjustment screw to set the needle valve on the refrigerator bayonet to the desired position.
8. Conduct a cryostat performance test to verify the temperature sensor reliability by cooling down to the lowest attainable temperature. If unexpected temperatures are measured during the cool-down, visually inspect the temperature sensors for good contact with the cryostat. If necessary, reposition and apply cryogenic vacuum grease to improve contact.
9. Assemble the sample gas line according to the P&ID diagram in Figure 1. Use a high sensitivity leak detector to identify any leaks.
   CAUTION: Hydrogen, deuterium, and methane are extremely flammable gases. Use piping and equipment designed to withstand the pressures and physical hazards. Local exhaust or ventilation are required to keep the concentration below the explosion limit. Before applying this procedure with any other gases, consult the associated safety data sheet (SDS).
10. Purge the gas line according to the continuous flow purging technique to dilute contaminant gases and water vapor to the purity of the sample gas. The total time depends on the volume of the gas line and gas flow at a given backing pressure.
    ​CAUTION: While purging the line, ensure the vacuum chamber is adequately ventilated or maintained under vacuum to prevent accumulation of flammable gases.
11. After the initial purge is complete, maintain constant positive pressure (e.g., 30 sccm at 50 psig) on the line to mitigate the risk of contaminant gases entering the line when the vacuum chamber is at ambient pressure.

2. Installation of the cryogenic source components

NOTE: All preparation and assembly of the cryogenic source components should be performed in a clean environment with the appropriate cleanroom clothing (i.e., gloves, hairnets, lab coats, etc.).

1. Use indirect ultrasonic cleaning to remove contaminants (e.g., residual indium) from the cryogenic source components.
   1. Fill a sonicator with distilled water and add a surfactant to reduce the surface tension of the water.
   2. 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 to prevent particle contamination.
   3. Place the beakers in the cleaning basket or a beaker stand in the sonicator to maximize cavitation. Beakers should not touch the bottom of the sonicator.
   4. Activate the sonicator for 60 min.
   5. Inspect the isopropanol using a bright white light for suspended particles or residue.
   6. If particles are visible, rinse the parts with clean isopropanol, and replace the isopropanol bath. Sonicate in cycles of 60 min until no particles or residue are visible.
   7. Place the parts on a covered, clean surface to desiccate for a minimum of 30 min before assembly.
2. Repeat section 2.1 for the stainless-steel filter, source cap, ferrule, and assembly screws.
3. Cut a piece of indium to maximally cover the junction between the cryogenic source body and cold finger of the cryostat.
4. Place the indium 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.
5. Screw the threaded stainless-steel filter onto the cryogenic source flange.
6. Place an indium gasket on the source flange. Attach the source flange to the cryogenic source body using the flange screws. Tighten the screws diagonally instead of sequentially around the circumference.
7. Connect the sample gas line on the cryostat to the cryogenic source. Check for leaks using a high sensitivity leak detector.

3. Installation of aperture

1. Select an aperture according to experimental needs.
   1. Inspect the aperture using brightfield and darkfield microscopy techniques to identify imperfections in the aperture, physical obstructions, or residual photoresist.
   2. Some physical obstructions can be removed easily when rinsed with isopropanol. Otherwise, discard the aperture.
   3. If there is residual photoresist from the nanofabrication of the aperture, use an acetone bath or piranha solution to remove it.
      CAUTION: Piranha solution, consisting of 3:1 sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂), is extremely corrosive to organic material, including the skin and respiratory tract. The reaction of Piranha with organic material releases gas, which may become explosive. Never seal containers containing Piranha. A full-face shield, chemical resistant apron, lab coat, and neoprene gloves are required.
2. Rinse the aperture with electronics-grade isopropanol to remove any debris or surface contamination. Allow the aperture to dry on a clean and covered surface for 10 min before installation.
3. Place the ferrule inside the cap.
4. Use clean, soft-tipped tweezers to place the aperture inside the ferrule. Tap the cap to center the aperture in the ferrule.
5. Drop an indium ring on top of the aperture. Again, tap the edge of the cap to center the indium ring on the aperture.
6. Hand-tighten the cap onto the source flange until minimal resistance is detected.
7. Derestrict the flow rate on the mass flow controller by increasing the setpoint to 500 sccm and set the gas pressure to ~50 psig on the pressure regulator.
8. Tighten the aperture delicately by a few degrees at a time using a wrench until the flow rate begins to decrease.
9. Finish tightening the cap by checking the leak rate at the top of the cap with the high-sensitivity leak detector instead of the mass flow controller. Stop when tightening no longer decreases the measured leak rate.
10. If the flow rate does not drop below approximately 50 sccm, proceed with the following steps.
    1. Use the leak detector to check for leaks around the source flange and cap. Retighten the screws on the source flange and remeasure the leak rate.
    2. Remove the cap and inspect the aperture and tip of the source flange.
    3. If the aperture is damaged, clean the cap according to step 2.2 and repeat section 3.
    4. If the indium ring is fixed to the aperture, discard the aperture and repeat section 3.
    5. If the complete indium ring is fixed to the flange, use a clean plastic razor blade to scrape off residual indium, then repeat steps 3.2-3.10.
    6. Over time, indium may accumulate on the tip of the source flange preventing subsequent apertures from sealing. In this case, remove the source flange and repeat sections 2.1-2.2 followed by steps 2.5-2.7.
11. As a safety precaution, change the setpoint on the mass flow controller to 10 sccm higher than the final flow determined by the dimensions of the aperture.

4. Cool-down procedure

1. Verify vacuum chamber pressure has reached the expected baseline for a given sample gas flow. To ensure the absence of contaminant gases, which will deposit on the cryogenic source during cool-down, the vacuum chamber is typically pumped for at least 1 h after reaching baseline pressure. This duration varies with local humidity levels and the vacuum system.
2. Turn on the cryostat exhaust heater to prevent frosting of the cryostat head from the return flow of helium gas.
3. Derestrict the gas flow on the mass flow controller by increasing the setpoint to 500 sccm.
4. Fill the open-cycle cold trap with liquid nitrogen. Ensure that the level of liquid nitrogen is above the in-line filter at all times. Monitor and refill as required during cool-down and jet operation.
   CAUTION: Contact with cryogenic liquids, such as liquid nitrogen or liquid helium, will burn the skin, face, and eyes. When handling large volumes of cryogenic liquids (multi-liter), wear a face shield, safety glasses, thermally insulated cryogenic gloves, cryogenic apron, long pants without cuffs, and close-toed shoes. Such liquids may displace oxygen and cause rapid suffocation.
5. Set the adjustable flow meter(s) on the helium return line(s) to fully open.
6. Depressurize the liquid helium dewar using the vent valve.
7. Close the ball valve to the low pressure relief valve on the liquid helium dewar. The recommended dewar pressure during cool-down is 10 psig. An angle valve on the dewar adapter allows the operator to reduce the dewar pressure if there is surplus cooling power after sample liquefaction.
8. Insert the supply dewar bayonet into the liquid helium dewar in one smooth motion. The dewar should pressurize to 10 psig when the bayonet contacts the liquid.
   CAUTION: Keep all exposed skin away from the neck of the dewar at all times.
9. Check for helium gas leaks between the dewar and dewar adapter once the connection has been tightened using a leak detector.
10. Activate the heater on the temperature controller and set the temperature setpoint to 295 K.
11. Once the transfer line fills and cools, the cryostat temperature will drop from ambient temperature to 295 K, at which point the heater will activate to prevent a further drop in temperature. Note that the time required for the initial drop in temperature depends on the dewar pressure and total transfer line and cryostat length.
12. Set the ramp rate on the temperature controller to 0.1 K/s and the setpoint to 200 K. Regulate the helium flow to follow the ramp so the heater does not turn on. Hold at 200 K for a brief dwell segment (e.g., 5 min) to allow the cryostat to thermalize. Repeat for two additional ramp-dwell segments to 120 K then 40 K. A conservative cool-down procedure is used to avoid strong temperature gradients along the system and allows the system parameters to be closely monitored. The dwell temperatures are selected away from sublimation temperatures for contaminant gases.
    1. If the gas flow increases unexpectedly, the indium seal on the source flange or aperture may have failed. Abort the cool-down procedure by proceeding to step 6.4. Once the vacuum chamber has been vented, inspect the seals and refer to section 3.10 to retighten and check for leaks.
13. At 40 K, manually tune the temperature controller P-I-D parameters following the Ziegler-Nichols method²² until the temperature stability is better than ±0.02 K.

5. Liquefication and jet operation

1. Confirm that the liquid nitrogen level is above the in-line filter.
2. Disable the temperature ramp and change the setpoint temperature to well below the theoretical vapor-liquid phase transition temperature (e.g., 20 K for hydrogen).
3. At the onset of liquefaction, the gas flow will increase up to the maximum and a mixture of gas and liquid will spray from the aperture. Increase the helium flow(s) to provide additional cooling power to quickly pass through the phase transition.
4. Use high magnification shadowgraphy with pulsed, sub-nanosecond illumination to visualize the jet stability and laminarity²³.
5. Optional: If an application or experiment has a pre-determined location for the sample (e.g., detectors aligned to the same position in space), translate the cryogenic source using a multi-axis manipulator on the cryostat flange or motorized push-pin actuators in the vacuum chamber.
6. Translate the catcher to maximize the pressure in the catcher foreline.
7. Optimize the P-I-D parameters and helium flow to improve the temperature stability to better than ±0.02 K. Note that the overall stability of the jet strongly depends on the vacuum chamber pressure, gas backing pressure, and temperature. For example, a change in as little as 1 x 10^-5 mBar may require reoptimization.
8. Scan in temperature and pressure to optimize the jet stability and laminarity. Sample jet parameters are listed in Table 1.
   1. If the jet breaks up into a spray, the pressure and temperature in phase space may be too close to the vaporization curve.
   2. Large amplitude temperature or helium flow oscillations will result in periodic spatial perturbations, which (in the extreme case) result in driven breakup of the jet. Reduce the helium flow and reoptimize P-I-D parameters to damp the oscillations.
   3. If the jet exhibits transverse (i.e., first-wind regime) or longitudinal waves (i.e., Plateau-Rayleigh instability), decrease the temperature to increase the viscosity, thereby reducing the Reynolds number.
   4. If laminarity cannot be achieved and the jet characteristics are independent of changes in temperature and pressure, there may be a physical obstruction (e.g., physical debris or ice) in the aperture. Before aborting the test, follow steps 6.1-6.5 and closely monitor the vacuum pressure and cryostat temperature. If a contaminant gas or water has sublimated on the aperture causing a partial or full blockage, it can be identified by the boil-off temperature. Repeat steps 4.11-4.12 and 5.1-5.6 to determine if the jet stability improves.

6. Warm-up procedure

NOTE: If the aperture is damaged during operation, immediately limit the sample gas flow to 10 sccm and reduce the sample gas pressure to 30 psig. Then, proceed directly to step 6.5.

1. Change the setpoint to 20 K and decrease the gas pressure from operating pressure to approximately 30 psig.
2. Increase the temperature setpoint in steps of 1 K while monitoring the pressure on the gas regulator. As the liquid in the cryogenic source vaporizes, the pressure in the gas line will rapidly increase and the flow across the mass flow controller will read 0 sccm.
   NOTE: Do not allow the gas pressure to exceed the maximum operating pressure of the components on the sample gas line. If this occurs, wait until the line depressurizes to a safe value through the aperture or pressure relief valve before increasing the setpoint further.
3. Repeat step 6.2 until increasing the temperature setpoint by 1 K does not result in an increase in gas line pressure.
4. Enable the temperature ramp, change the temperature setpoint to 300 K, and regulate the helium flow as required to maintain a temperature increase of 0.1 K/s.
5. Once the source temperature is above 100 K, close the adjustable flowmeter(s) on the helium return line(s). Depressurize the dewar and open the ball valve to the lowest pressure relief valve.
6. Wait until the cryostat thermalizes at 300 K before venting the vacuum chamber. This will prevent water vapor from condensing on the cryostat and cryogenic source components.
7. Depressurize the dewar, then remove the supply dewar bayonet.
8. Remove the liquid nitrogen cold trap.
9. Limit the gas flow on the mass flow controller to 30 sccm.
10. Turn off the exhaust gas heater.
11. Deactivate the heater on the temperature controller.
12. If the aperture is damaged or an obstruction is suspected from a change in flow, proceed to section 7. Otherwise, the aperture does not need to be replaced.

7. Replacement of aperture

1. Remove the cap and inspect the aperture and tip of the source flange.
2. If the indium ring sticks to the flange, use a clean plastic razor blade to scrape it off using moderate pressure.
3. If the aperture remains sealed to the source flange when the cap is removed, limit the gas flow to 10 sccm and confirm the gas backing pressure has dropped to 30 psig. Remove the aperture carefully with a plastic razor blade. If removed prematurely, over-pressurization in the line may damage or eject the aperture.
4. Repeat section 3 to install a new aperture.